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Materials for Electronic Packaging

This Page Intentionally Left Blank

Materials for Electronic Packaging Edited by Deborah D. L. Chung

Butterworth-Heinemann Boston Oxford

Melbourne Singapore Toronto Munich New Delhi Tokyo

Copyright 9 1995 by Butterworth-Heinemann. A member of the Reed Elsevier group All rights reserved.

All trademarks found herein are property of their respective owners. No part of this publication may be reproduced, stored in a retrieval system, or transmitted in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise, without the prior written permission of the publisher. @

Recognizing the importance of preserving what has been written, Butterworth-Heinemann prints its books on acid-free paper whenever possible.

Library of Congress Cataloging-in-Publication Data Materials for electronic packaging/edited by Deborah D. L. Chung p. cm. Includes bibliographical references and index. ISBN 0-7506-9314-2 1. Electronic packaging--Materials. I. Chung, Deborah D. L. TK7870.15.M38 1995 621.381'046--dc20

British Library Cataloguing-in-Publication Data A catalogue record for this book is available from the British Library. The publisher offers discounts on bulk orders of this book. For information, please write: Manager of Special Sales Butterworth-Heinemann 313 Washington Street Newton, MA 02158-1626 109 8 7 6 5 4 3 2 1 Printed in the United States of America

94-49204 CIP

Contents

Contributors Preface xiii

xi

PART I Overview Overview of Materials for Electronic Packaging D. D. 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9

L. Chun9 Introduction 3 Printed Circuit Boards 16 Substrates 19 Interconnections 27 Die Attach 30 Encapsulation 31 Interlayer Dielectrics 33 Heat Sinks 34 Electromagnetic Interference Shielding References 36

35

PART I I Joining Solderability Fundamentals: Microscopic Processes J. A. 2.1 2.2 2.3 2.4 2.5 2.6

43

Clum, T. J. Singler Introduction 43 Background 44 A Microscopic Mass Transfer Model 48 Observations of Limiting Mass Transfer 50 Solder Alloy Selection and Process Design 54 Conclusion 55 References 56

vi

Contents

Determining the Damaging Strains which Cause Failure in Pb-Sn Solders 57 A. I. A ttarwala, B. C. Hendrix, J. M. Sanchez 3.1 Introduction 57 3.2 Test Methodology and Data Analysis 58 3.3 Deformation Behaviour of Pb-Sn Solders under Static and Cyclic Loading 64 3.4 Effect of Anelastic Strains on Accelerated Test Results 70 3.5 Lifetime Predictive Equation for Pb-Sn Solders 70 3.6 Summary 75 Acknowledgment 75 References 76

Fluxless Soldering for Microelectronic Applications D. R. 4.1 4.2 4.3 4.4 4.5

79

Frear, F. M. Hoskin9, D. M. Keicher, H. C. Peebles Introduction 79 Fluxless Laser Soldering 81 Activated Acid Vapor Fluxless Soldering 86 Laser Ablative Fluxless Soldering 93 Summary and Examples 101 Acknowledgments 102 References 103

The Effect of Microstructure on Fracture of Metal/Ceramic Interfaces 105 Ivar E. Reimanis 5.1 Introduction 105 5.2 Fracture Behavior 107 5.3 Grain Distributions in the Metal Layer 5.4 The Interface Pore Distribution 115 5.5 Interface Roughness 119 5.6 Summary 123 Acknowledgments 123 References 123

110

III Composites The Future of Advanced Composite Electronic Packaging Carl Zweben 6.1 Introduction 127 6.2 Status of Composite Packaging Materials 6.3 Applications 135

128

127

Contents

6.4 Future Directions 139 6.5 Summary and Conclusions References 142

142

Low Thermal Expansion Composite Materials for Electronic Packaging 145 D. D. 7.1 7.2 7.3 7.4 7.5

L. Chun9 Introduction 145 Heat Sinks, Backboards, and Substrates Brazes and Solders 149 Die Attach 150 Interconnections 152 Acknowledgment 152 References 152

147

Conducting Polymer-Matrix Composites 153 D. D. 8.1 8.2 8.3 8.4 8.5 8.6 8.7 8.8 8.9

L. Chung, Lin Li Introduction 153 Particles as the Filler 155 Flakes and Fibers as Fillers 156 Three-Dimensional Networks as Fillers 161 Slug as the Filler 164 Effect of the Polymer Viscosity 164 z-Axis Conductors 165 Electrically Insulating but Thermally Conducting Composites Conclusion 169 Acknowledgment 170 References 170

P A R TI V Metal Films Thick Film Technology 175 Renb E. Cotb 9.1 Introduction 175 9.2 Overview of Materials and Processes 9.3 Resistors 178 9.4 Conductors 185 9.5 Dielectrics 194 9.6 Vehicles 199 9.7 Thick Film Processing 202 9.8 Conclusion 220

175

168

vii

viii Contents 10 Electroless Copper for Micropackaging and Ultralarge-Scale Integrated Circuit Applications 221 Y. Shacham-Diamand

10.1 10.2 10.3 10.4 10.5 10.6 10.7

Introduction 221 Electroless Copper Deposition 224 Copper Nanoline Processing 227 Electrical Properties 233 Electroless Copper Oxidation 235 Hydrogen in Electroless Copper 236 Conclusions 239 Acknowledgments 239 References 240

11 Vacuum Metallization for Integrated Circuit Packages 241 K. J. Blackwell, P. C. Chen, A. R. Knoll, J. J. Cuomo 11.1 Introduction 241 11.2 Vacuum Processes 241 11.3 Coating Vessels 246 11.4 Physical Vapor Deposition by Evaporation 247 11.5 Evaporation Methods and Sources 251 11.6 Sputtering 254 11.7 Heat Transfer in Physical Vapor Deposition Processes 11.8 Roll Coater Metallization 265 11.9 Coating Material Properties 266 11.10 Evaluating Deposited Films 267 11.11 Conclusions 276 References 277

263

PARTV Polymers and Other Materials 12

Silicone-Based Polymers in Electronic Packaging

281

C. P. Wong

12.1 12.2 12.3 12.4 12.5 12.6 12.7

Introduction 281 Why Do Devices Need Encapsulation? 281 General Chemistry of Silicones (Elastomers and Gels) Results and Discussion 286 Temperature Humidity Bias (THB) Testing 288 Temperature Cycle Testing 288 Conclusion 289 References 289

284

Contents ix 13 Dielectric Films for High Temperature, High Voltage Power Electronics 291 Javaid R. Layhari, Jayant L. Suthar 13.1 Introduction 291 13.2 Experimental 292 13.3 Results and Discussion 295 13.4 Summary 300 Acknowledgment 301 References 301

14 Electrically Conducting Polymers and Organic Materials 303 M. J. Naughton 14.1 Introduction 303 14.2 Organic Conductors and Superconductors 305 14.3 Conducting Polymers 312 14.4 Potential Applications of Conducting Polymers 314 References 315 15 Diamond in Electronic Packages 319 D. J. Pickrell, D. S. Hoover 15.1 Introduction 319 15.2 Background on Diamond 319 15.3 Chemical Vapor Deposition of Diamond 321 15.4 Fabrication of Electronic Substrates 331 15.5 Package Design Considerations 333 15.6 Conclusion 335 References 335

PARTV l Materials Testing 16 Measurements of Properties of Materials in Electronic Packaging Joseph A. Carpenter, Jr. 16.1 Introduction 341 16.2 Electrical Properties 344 16.3 Thermal Properties 346 16.4 Mechanical Properties 352 16.5 Physical Properties 355 16.6 Manufacturability Properties 355 16.7 Summary 356 Acknowledgments 357 References 357 Index

361

341


Contributors

A. I. Attarwala Strategic Materials R&D Laboratory The University of Texas Austin, Texas

Ren6 E. Cot6 DuPont Electronics Fort Worth, Texas

K. J. Blackwell

IBM Technology Products Endicott, New York

J. J. Cuomo IBM Research Yorktown, New York

Joseph A. Carpenter, Jr. Ceramics Division National Institute of Standards and Technology Gaithersburg, Maryland

D. R. Frear Center for Solder Science and Technology Sandia National Laboratories Albuquerque, New Mexico

P. C. Chen IBM Technology Products Endicott, New York Deborah D. L. Chung Department of Mechanical and Aerospace Engineering State University of New York at Buffalo Buffalo, New York J. A. Clum Department of Mechanical Engineering Watson School of Engineering and Applied Science State University of New York Binghamton, New York

B. C. Hendrix Strategic Materials R&D Laboratory The University of Texas Austin, Texas D. S. Hoover Diamonex, Inc. Allentown, Pennsylvania F. M. Hosking Center for Solder Science and Technology Sandia National Laboratories Albuquerque, New Mexico xi

xii

Contributors

D. M. Keicher Center for Solder Science and Technology Sandia National Laboratories Albuquerque, New Mexico A. R. Knoll IBM Technology Products Endicott, New York Javaid R. Laghari Department of Electrical and Computer Engineering State University of New York Buffalo, New York Lin Li Department of Mechanical and Aerospace Engineering State University of New York Buffalo, New York M. J. Naughton Department of Physics State University of New York Buffalo, New York H. C. Peebles Center for Solder Science and Technology Sandia National Laboratories Albuquerque, New Mexico D. J. Pickrell Diamonex, Inc. Allentown, Pennsylvania

Ivar E. Reimanis Max Planck Institut fiir Metallforschung Stuttgart, Germany J. M. Sanchez Strategic Materials R&D Laboratory The University of Texas Austin, Texas Y. Shacham-Diamand School of Electrical Engineering Cornell University Ithaca, New York T. J. Singler Department of Mechanical Engineering Watson School of Engineering and Applied Science State University of New York Binghamton, New York Jayant L. Suthar Department of Electrical and Computer Engineering State University of New York Buffalo, New York C. P. Wong AT&T Bell Laboratories Princeton, New Jersey Carl Zweben GE Astro Space Division King of Prussia, Pennsylvania

Preface

Electronic packaging refers to (1) the packaging of integrated circuit chips (dies); (2) the interconnections (both on and off the chips) for signal transmission, power, and ground; (3) the encapsulations for protecting the chips and interconnections from moisture, chlorides, and other species in the environment; (4) the heat sinks or other cooling devices needed to remove heat from the chips; (5) the power supply; and (6) the housing for electromagnetic interference (EMI) shielding. Semiconductor technology has made tremendous progress in the last few decades, and the present problems in the electronics industry lie mainly in electronic packaging, which is critical to the reliability and performance of electronic systems. The technology behind electronic packaging involves both systems and materials considerations. Systems considerations pertain largely to the packaging schemes, whereas material considerations pertain to the development of improved materials that allow more demanding packaging schemes to be possible. An example is the development of materials of high thermal conductivity for dissipating heat from the package. Another example is the development of materials of low thermal expansion for avoiding failure due to thermal stresses during thermal cycling. Yet another example is the development of conducting adhesives to replace solders, which are undesirable due to the ozone-depleting tendency of the defluxing chemicals. Although materials play a critical role in electronic packaging, the vast majority of attention has been given to the systems aspect. This situation is partly due to the fact that the training of most workers lies in electrical engineering rather than materials science/engineering. Another reason is that most materials experts shy away from the field because they feel that they do not know its needs well enough. Most advanced materials have been developed for structural applications rather than electronic packaging applications. A goal of this book is to alleviate this situation by viewing the field from a materials perspective. This perspective is in contrast to the systems perspective offered by other books on electronic packaging. In contrast to conference proceedings, this book consists of self-contained chapters which are review or tutorial in nature and are broad in scope. The 16 ooo

xm

xJv

Preface

chapters are grouped into six parts: (1) overview, (2)joining, (3) composites, (4) metal films, (5) polymers and other materials, and (6) materials testing. The chapters are contributed by a variety of active researchers from industrial, academic, and governmental sectors. This book is suitable for use as a textbook or a reference book for students (senior undergraduate or graduate) or professionals interested in electronic packaging. Although it adopts a materials perspective, the book requires only basic knowledge of materials science at the junior undergraduate level. It assumes no knowledge of electronic packaging and only a small knowledge of electronics. The subject matter is relevant to materials scientists/engineers, electrical engineers, mechanical engineers, physicists, and chemists. With the exception of Chapter 9, each chapter contains a list of up-to-date references. I hope this book will help increase research into electronic packaging materials, a field full of scientific excitement and technological relevance. Deborah D. L. Chung

PART I

Overview


CHAPTER

1

Overview of Materials for Electronic Packaging D. D. L. Chung

1.1 Introduction Electronic packaging refers to (1) the packaging of the integrated circuit chips (dies); (2) the interconnections (both on and off the chips) for signal transmission, power, and ground; (3) the encapsulations for protecting the chips and interconnections from moisture, chlorides, and other species in the environment; (4) the heat sinks or other cooling devices needed to remove heat from the chips; (5) the power supply; and (6) the housing for electromagnetic interference (EMI) shielding. Its conventional hierarchy has the following levels [1], which are illustrated in Figures 1.1 [-1] and 1.2 [2]: Level O" bare chip as removed from the finished wafer. Level 1. bare chip mounted on a chip carrier (or substrate) and encapsulated as a packaged chip (category I of level 1); bare or packaged chip(s) mounted on a module, called multichip module (MCM), together with discrete components (category II of level I). Level 2" printed circuit board (or card) with packaged chips, modules, and other components. Level 3: backplane (or mother board) into which printed circuit boards (or cards) are inserted. Level 4: electronic module formed by integration of backplane and power supply with an outer housing. Level 5: system formed by integration of electronic module. During the last 20 years, most of the attention of the electronic industry was directed to level 0, which constitutes the heart of the electronics. This effort, which was centered on semiconductors, resulted in a rapid increase in the packing density of devices on a chip, as indicated by the rapid miniaturization of electronics in the last 20 years. However, this miniaturization is accompanied by large increases in the amount of interconnections associated with each chip and in the amoun! of heat generated by each chip. Therefore, the key to further miniaturization currently lies on levels 1, 2, and 3. The immediate goal is to package the chips

4

MATERIALS FOR ELECTRONIC PACKAGING

Wafer

Level 0

Level 1

\

/ (

I il /

V

/ Y

i

'\ llllllllllllllllllll

Level 4

Level 3

i

Level 2

Figure 1.1 Hierarchy of electronic packaging. (Reprinted by permission of Chapman & Hall Ltd. from W. Eakin, K. Gardiner, and J. Nayak, Journal of Electronics Manufacturing 1, 13-22 (1991).

2

Scale of

Electronic Equipment [C Card and Ultra Small Size

I (Daughter Board )

3rd Level

!

I

Board _] (MotMer Board,Back Board )

I

[

4th L e v e l

~

5 t h Level .

,,

(EM)

1

~

'~

Medium Size Large Size Large Size

!Equipment L

Small Size

System

Ultra Large Size

Figure 1.2 Printed circuit board mounting classes and scale of equipment. (Reprinted by permission from K. Takagi and S. Yasufuku, IEEE Electrical Insulation Magazine 7(2), 9-16, 19-27 (1991) 9 1991 IEEE.)

Overview of Materials for Electronic Packaging

5

and the associated interconnections in a compact way that allows for sufficient heat removal, that can withstand the thermal cycling associated with the turning on and turning off of the electronics, that protects the electronics from environmental attack, and that allows the electronics to operate at high speeds. As the power of electronics increases, the heat dissipation problem becomes even more difficult. As the speed of electronics increases, the signal delay caused by the capacitive effect of dielectric packaging materials becomes more intolerable. It is anticipated that levels 4 and 5 will start to dominate the picture in about 10 years. This book is mainly concerned with levels 1, 2 and 3, which are of current importance. The solution of the electronic packaging problem involves the devising of packaging schemes and the use of advanced materials. Both aspects of the work are important and must take place coherently. Materials are intimately tied to processing, which is directly affected by the packaging scheme. Certain packaging schemes may not be possible unless advanced materials are used. For example, a packaging scheme may require so much heat dissipation that an advanced thermal conductor must be used. Although materials play an important role in electronic packaging, most of the work on electronic packaging is concerned with packaging schemes rather than materials. Examples of packaging schemes are wafer-scale integration (WSI) [3]; power hybrid packaging [4]; three-dimensional interconnection [5]; high density interconnect (HDI), which uses an interconnect overlay [6,7]; and others [8,9]. This book therefore focuses on materials for electronic packaging. The actual applications of materials in electronic packaging include interconnections, printed circuit boards (Figs. 1.3 [10], 1.4 [2], and 1.5 [ 11]), substrates (Fig. 1.5), encapsulations (Figs. 1.6 [2] and 1.7 [12]), interlayer dielectrics, die attach, electrical contacts, connectors, thermal interface materials, heat sinks, solders, brazes, lids, housings, and so on. In general, the integrated circuit chips (dies) are attached to a substrate or a printed circuit board on which the interconnection lines have been written (usually by screen printing) on each layer of the multilayer substrate or board. In order to increase the interconnection density, another multilayer involving thinner layers of conductors and interlayer dielectrics may be applied to the substrate before attachment of the chip. By means of soldered joints, wires connect between electrical contact pads on the chip and electrical contact pads on the substrate or board. The chip may be encapsulated with a dielectric for protection. It may also be covered by a thermally conducting (metal) lid. The substrate (or board) is attached to a heat sink. A thermal interface material may be placed between the substrate (or board) and the heat sink to enhance the quality of the thermal contact. The whole assembly may be placed in a thermally conducting (metal) housing. There are numerous variations to the packaging described above. In the most conventional variation, one or more chips are attached to a ceramic substrate via soldered joints, while the substrate is in turn mounted via soldered joints to a printed circuit board (also known as a card). In another variation, the chip is attached directly on the card, resulting in a direct chip attach module (DCAM).

b MATERIALSFOR ELECTRONICPACKAGING

Figure 1.3 Printed circuit board shown in cutaway to reveal inner-interconnection layers and vias; a surface mount device (left) and a through-hole device (right). Surface mount devices can be mounted on both sides of the board and do not consume valuable inner layer space with the through-hole. From [10]. Via Hole Interstitialvia 1 hole(~uried)

Interstitialvia hole(Blind)

Componentlead Insertiinhole

[== L,., .......

l== Figure 1.4 Structure of plated-through hole multilayer board (eight layers). (Reprinted by permission from K. Takagi and S. Yasufuku, IEEE Electrical Insulation Magazine 7(2), 9-16, 19-27 (1991) @ 1991 IEEE.) In yet another variation, the chip is attached via a cardlet, one of many small cards, attached to a large card, resulting in a multichip module laminate (MCML). An M C M L obviates the need for a sophisticated mother card, allows denser packaging than DCAM, and is less expensive than a multichip module involving ceramic substrates. The conventional packaging process involves putting the interconnections on a flat substrate before putting on the chips--a process known as chip last. A new process, chip first, saves the total number of processing steps by putting the chips in wells of chosen depths in a substrate before putting the interconnections on the plane of the welltops.


1.700 SQ

.O32-.050 "]w AIRFLOWv ~

e~

7

6061, Aluminum

~-.soo --~ "

l:

9176

Epoxy 2 mil thick

t-- .030

--T-

t__.o5o

.095 .015 PC Board

.550 SQ _ _ ~

A!203 90%

;ilicon

,800 9 SQ

Figure 1.5 Cutaway view of a pin grid array (PGA) package. (Reprinted by permission from R.J. Schnipke, D. Hayward, and J.G. Rice, in Proceedings of the 5th IEEE Semiconductor Thermal and Temperature Measurement Symposium, 1989, pp. 81-87 9 1991 IEEE.)

A printed circuit board (Figs. 1.3-1.5) is a sheet for the attachment of chips, whether mounted on substrates, chip carriers, or otherwise, and for the drawing of interconnections. It is a polymer-matrix composite that is electrically insulating and has conductor lines (interconnections) on one or both sides. Multilayer boards have lines on each inside layer so that interconnections on different layers may be connected by short conductor columns called electrical vias (Figs. 1.3 and 1.4). Printed circuit boards (or cards) for the mounting of pin-inserting-type packages (Fig. 1.6(a)) need to have lead insertion holes punched through the circuit board (Fig. 1.3, right; Fig. 1.4, right). Printed circuit boards for the mounting of surface-mounting-type packages (Fig. 1.6(b)) need no holes. Surface-mounting-type packages, whether with leads, leaded chip carriers, or without leads, leadless chip carriers (LLCCs), can be mounted on both sides of a circuit board (i.e., a card), whereas pin-inserting-type packages can only be mounted on one side of a circuit board (Fig. 1.3). In surface mounting technology (SMT), the surfaces of conductor patterns are connected together electrically without employing holes. Solder is typically used to make electrical connections between a surface-mounting-type package (whether leaded or leadless) and a circuit board. A lead insertion hole for pin-inserting-type packages is a plated-through hole, a hole on whose wall a metal is deposited to form a conducting penetrating connection. After pin insertion, the space between the wall and the pin is filled by solder to form a solder joint. Another type of plated-through hole is a via hole (Fig. 1.4, left), which serves to connect different conductor layers together without the insertion of a lead. Holes become difficult to drill and plate as the ratio of board thickness to hole diameter, called the aspect ratio, increases. Via holes that do not go all the way through a circuit board are called interstitial via holes (IVHs), which include buried holes and blind holes (Fig. 1.4, center). A buried hole connects the internal conductor layer

8


Name

Appearance

Remarks Material

Lead Pitch, etc

DIP

DUAL INLINE PACKAGE

P C

2.54 mm (100 mil) Lead Pin No. 4 --- 64 Pin

S-DIP

SHRINK DIP

p

1.778 mm (70 mil) Lead Pin No. 4 --- 64 Pin

SKINNY

SKINNY DIP

P

2.54 mm (70 mil) Widthwise Pitch 1.2 Size Lead Pin No. 8 --- 28 Pin

PGA

PIN GRID ARRAY

2.54 mm/1.27 mm (100 mil)/(50 mil) Lead Pin No. 64 --- 240 Pin

(a) Pin Inserting Type Package

Name SOP

SMALL OUTLINE PACKAGE

QFP

QUAD FLAT PACKAGE

LCC

LEADLESS CHIP CARRIER

PLCC

PLASTIC LEADED CHIP CARRIER

SOJ

JBEND SOIC

PGA

PIN GRID ARRAY

Appearance

Remarks Material

,,,~r ~

Lead Pitch, etc 1.27 mm (50 mil) Two-way Lead 16 --- 36 Pin 1.0mm

0.65ram

9

Four-way Lead 20 --- 128 Pin 1.27 mm (50 mil) 1.00 mm (40 mil) 0.975 mm (30 mil) Contact No. 16 --- 36 Pin

/ ~ ~

j-/~",,,~ ~

1.27 mm (50 mil) J-Shaped Lead 16 --- 124 Pin

~

1.27 mm (50 mil) Two-way Lead 16 --- 124 Pin

,,,,c~~ ~-'~ 400~ low dielectric constant (e ~ 3.2) and ease of planarization. However, polyimide suffers

34


from a high processing temperature and a high modulus. Therefore, polyimide siloxane is being considered as a compromise. To further decrease the dielectric constant of a polymer, hollow glass spheres or pores (10-100 nm, obtained from a thermally decomposing polymer particulate phase) can be incorporated in the polymer. Polymer films tend to exhibit CTE anisotropy, due to the preferred orientation of the polymer chains in the film. The degree of anisotropy varies with the curing conditions. Because an interlayer dielectric is used together with metal interconnections in a multilayer, the adhesion between a dielectric and a metal is relevant [81,82]; in most cases the adhesion is poor. It may be improved by surface treatment (e.g., plasma treatment) of the polymer and by judicious choice of the functional groups in the polymer.

1 . 8 H e a t Sinks Material requirements for heat sinks include the following: 9 high thermal conductivity 9 low coefficient of thermal expansion (CTE) 9 low density (for aerospace electronics) Copper is the traditional heat sink material, but it suffers from a high CTE and a high density. Aluminum is also a traditional heat sink material, but it suffers from a high CTE and it is not as thermally conductive as copper. Composite materials are increasingly being used because they provide properties which most

Table

1.12

Properties ofcomposite heat sink materials compared to conventional metals.

Composite

85Mo-15Cu 60AI-40Si Cu-Invar-Cu a Cu-Mo-Cu a Cu/Cf b

Thermal conductivity ( W m -1 K -1)

0.44 0.3 164 208 737-750

CTE (10 -6 ~

7.0 13.0 6.4 5.7

Density (g/cm 3)

Reference

10.01 2.53 8.40 9.84 5.6

83 83 85 85 85

1.78-1.82 1.78-1.82 1.8 8.96 2.70

84 84 85 85 85

C/Cf: b

x-direction z-direction PI/Cf Cu A1

400-483 46-50 550 400 220

16-17 22-24

"Sandwich composite. bTwo-dimensional laminate with carbon fibers (Cf)of thermal conductivity 1 100 W m -1 K-1.

Overview of Materials for Electronic Packaging 35

single-component materials cannot provide. Composite heat sink materials include metal-matrix composites (e.g., Cu-W [83,84], C u - M o [84-1, and AI-Si [84]), carbon-matrix carbon fiber composites [85], and polymer-matrix carbon fiber composites [86]. Carbon fibers are an attractive filler because highly graphitic fibers exhibit a thermal conductivity as high as 1 100 W m-~ K-~, while having a low CTE and a low density. Table 1.12 lists the properties of some composite heat sink materials. Some of these composites exhibit thermal conductivities exceeding that of copper, while having densities much lower than that of copper. Better even than composites, diamond's outstandingly high thermal conductivity (2 000 W m-~ K-~) finds it a number of electronic applications [87,88-1, but its high price does limit their number. The thermal contact resistivity between the heat sink and the circuit board should be small. This is achieved by the use of a thermally conductive adhesive, a thermal grease, or an elastomeric conductor at the interface. For this purpose, it also helps to use a heat sink material that is not covered by an insulating film (e.g., an oxide). Metals are usually covered with an insulating oxide film, whereas carbon is not. This gives another benefit for the use of carbon fiber composites for heat sinks.

1.9 Electromagnetic Interference Shielding With the increasing sensitivity and abundance of electronics, electromagnetic interference (EMI) shielding is receiving more attention. The shielding material is an electrical conductor used in the form of an enclosure or housing for a module or a system, and in the form of a lid for a chip package. Metals in the form of sheets (e.g., A1) or plated coatings (e.g., Ni and Cu [89,90]) are traditionally used for enclosures. However, metals are not easily moldable, in contrast to polymers. Leaks in the shielding may occur at the joints between the metal sheets. Coatings are prone to damage by scratching and wear. Therefore, electrically conducting polymer-matrix composites are increasingly used for enclosures [91]. By using metallized ceramic microballoons as a conductive filler in a polymer, the composite has the added attraction of a very low density [92]. In general, the conductive filler can be in the form of particles, flakes, short fibers, or continuous fibers. Long fibers are more effective than short fibers. Metal fillers (e.g., A1 and Cu) are most commonly used [93-95], but metal-coated carbon fibers are increasingly used [96]. The mechanism of shielding includes absorption and reflection; it depends on the material and the frequency of the radiation. Increasing the volume fraction of the conductive filler the shielding effectiveness of the composite. However, a high filler content may result in a low tensile strength in the composite, so there is a compromise between its electrical and mechanical properties [97]. For the lid of a chip package, low thermal expansion alloys are used for the purpose of shielding. The most commonly used alloy is Kovar, which has a low coefficient of thermal expansion compatible with the low coefficient of the chip.

36


References

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60. C.G. Kuo, S.M.L. Sastry, and K.L. Jerina, in Proc. 1st Int. Symp. Microstructures and Mechanical Properties of Aging Materials, 1993, pp. 417-424. 61. J.W. Roman and T.W. Eagar, ISHM S),mp. Proc., Oct. 1992, pp. 1-6. 62. A.A. Shores, in Proc. 39th Electronic Components Cop!f., IEEE, 1989, pp. 891-895. 63. C.J. Lee and J. Chang, in Proc. 1991 hit. Symp. Microelectronics, ISHM, Reston VA, 1991. 64. C.J. Lee and K. Sherman, in Proc. 6th Int. SAMPE Electronics Conf., 1992, pp. 500-507. 65. C. Dominic, Electronics 33(4), 48-49 (1987). 66. E. Razon and Y. Tal, Hybrid Circuit Technol. 6(12), 17-20 (1989). 67. N.M. Davey and F.W. Wiese, Jr., Proc. 1986 Int. Symp. Microelectronics, ISHM, Reston VA, 1986, pp. 665-674. 68. C.P. Wong, J. Mater. Res. 5(4), 795-800 (1990). 69. C.P. Wong, J.M. Segelken, and J.W. Balde, IEEE Trans. Components, Hybrids, and Manuf. Technol. 12(4), 421-425 (1989). 70. C.J. Lee, G. Fabinowski, R.J. Jaccodine, S.P. Murarka, and F. Sun, in Microelectronic Packagin9 Technology, Proc. 2nd A S M Int. Electronic Materials and Processing Congress, Philadelphia PA, Apr. 1989, edited by W.T. Shieh, ASM International, Materials Park, Ohio, 1989, pp. 359-367. 71. C.J. Lee, in Proc. 39th Electronic Components Conf., IEEE, 1989, pp. 896-900. 72. C.J. Lee, in Proc. 6th Int. SAMPE Electronics Conf., 1992, pp. 521-527. 73. T.G. Tessier, G.M. Adema, and I. Turlik, in Proc. 39th Electronic Components Cot!f., IEEE, 1989, pp. 127-134. 74. B.T. Merriman, J.D. Craig, A.E. Nader, D.L. Goff, M.T. Pottiger, and W.J. Lautenberger, in Proc. 39th Electronic Components CoJ!f., IEEE, 1989, pp. 155-159. 75. C.P. Wong, in Proc. 6th Int. SAMPE Electronics Conf. 1992, pp. 508-520. 76. K.K. Chakravorty, C.P. Chien, J.M. Cech, L.B. Branson, J.M. Atencio, T.M. White, L.S. Lathrop, B.W. Aker, M.H. Tanielian, and P.L. Young, in Proc. 39th Electronic Components Conf., IEEE, 1989, pp. 135-142. 77. H.S. Cole, Y.S. Liu, R. Guida, and J. Rose, in Proc. SPIE, hit. Soc. Opt. Eng., g77 (Micro-Optoelectron. Mater.), 92-96 (1988). 78. T. Studt, R&D Magazine, Aug. 1992, pp. 30-34. 79. A.W. Lin, in Proc. 39th Electronic Components Conf., IEEE, 1989, pp. 148-154. 80. W.C. Shumay, Jr., Adv. Mater. Proc., Feb. 1989, pp. 42-47. 81. S.T. Chen, F. Faupel, and P.S. Ho, in Microelectronic Packaoin9 Technology, Proc. 2nd A S M Int. Electronic Materials and Processing Congress, Philadelphia, PA, Apr. 1989, edited by W.T. Shieh, ASM International, 1989, pp. 345-350. 82. A.L. Ruoff, E.J. Kramer, and C.-Y. Li, IBM J. R&D 32(5), 626-630 (1988). 83. P.H. Dawson, GEC Rev. 2(3), 168-170 (1986). 84. Sumitomo Electric Industries brochure on CMSH. 85. W.H. Pfeifer, J.A. Tallon, W.T. Shih, B.L. Tarasen, and G.B. Engle, in Proc. 6th Int. SAMPE Electronics Conf., 1992, pp. 734-747. 86. A.M. Ibrahim, in Proc. 6th Int. SAMPE Electronics Conf, 1992, pp. 556-567. 87. H. Eisele, Solid-State Electronics 32(3), 253-257 (1989). 88. J. Doting and J. Molenaar, in Proc. 4th Annual IEEE Semiconductor Thermal and Temperature Measurement Symp., IEEE, New York, (1988), pp. 113-117. 89. B. Chuba, Platin9 and Surface Finish#79 76(9), 30-33 (1989). 90. F. Matsui, T. Okada, T. Kawkubo, and T. Otaka, in Proc. AESF Annual Tech. Conf. 1990, pp. 1391-1403. 91. D.S. Dixon and J.V. Masi, SAMPE J. 25(6), 31-37 (1989). 92. D.W. Radford and B.C. Cheng, in Proc. American Socieo'for Composites, 6th Tech. Conf, Technomic Publishing, Lancaster, 1991, pp. 430-439.


39

93. Z. Osawa and S. Kuwabara, Polymer Degradation and Stability 35, 33-34(1992). 94. T. Takatani and Y. Saito, Sumitomo Keikinzoku Giho/Sumitomo Light Metal Technical Reports 28(3), 39-43 (1987). 95. N. Hayashi and K. Tanaka, Sumitomo Keikinzoku Giho/Sumitomo Light Metal Technical Reports 29(2), 56-63 (1988). 96. J.J. Glatz, R. Morgan, and D. Neiswinger, in Proc. 6th hTt. SAMPE Electronics Conf., 1992, pp. 131-145. 97. L. Li and D.D.L. Chung, Composites 25(3), 215-224 (1994).


~., I I

Joining


CHAPTER2

Solderability Fundamentals: Microscopic Processes J. A. Clum, T. J. Singler

2.1

Introduction

In the microelectronics industry the packaging of components is a multidisciplinary exercise requiring that electromagnetic, manufacturing, materials, mechanical, and thermal analyses be integrated to produce a cost-effective reliable product. The interconnections between components and circuitry represent an interesting microcosm of these varied concerns. Yet for nearly all joining of electronics packages we place our solders and soldering, mere modifications of materials and processes invented centuries ago [1]. And even after hundreds of years still there are many who study the behavior of solder and the control of soldering [2,3]. Especially useful and comprehensive books about solders and soldering, full of applications, are Klein Wassink [4] and Lea [5]; Lea focuses on surface mount technologies. As indicated in the excellent analysis of research directions in the physical metallurgy of soldering, presented by Romig et al. [6-1, most of the analytical work has fallen into two general categories. One group of workers has emphasized the relationships between laboratory measures of empirically related process control parameters: contact angle measurements and wetting force measurements as indicators of solderability [2,3,6]. Another emphasis has been on microscopic analyses of soldering reactions [5,7-12] including intermetallic formation kinetics, elemental redistributions between solder and substrate, and most recently the application of high resolution, in situ observations of solder spreading rates and reactions. We intend to argue that microscopic analyses are central to developing a fundamental understanding of solderability. Furthermore, we will attempt to show that development of a suitable solderability model will allow for selection of solder alloys to deal with certain soldering process problems, reduction of the use of unnecessary soldering reagents (lead, fluxes, environmentally undesirable cleaning agents) and/or controlling soldering reactions at the submicron scale for improved fine-pitch joining reliability. 43

44


2 . 2 Background The fundamental problem in solderability has often been expressed [4] in the simplest analytical terms as being the interfacial energy relation for spontaneous wetting, 7LV "~- ~SL - - ])SV < 0

(2.1)

where 7LV,7SL,and 7sv are the liquid-vapor, solid-liquid, and solid-vapor interfacial energies, respectively. Equation (2.1) is a special case of Young's equation, 7LV COS 0 + 7SL = 7SV

(2.2)

where 0 is the contact angle. Equation (2.2) can be derived by minimizing the energy of a system comprised of a liquid and vapor in contact with a solid in a state of equilibrium. Each 7 in equation (2.2) may be considered as a thermodynamic property provided that the effects of curvature on the interfacial energies are negligible. Comparison of equations (2.1) and (2.2) suggests that as the value of the contact angle tends to zero, spontaneity of wetting is approached. Consequently, the emphasis on attaining "good" wetting translates into observing a small contact angle. Similarly the relationship between wetting force and contact angle as expressed in [4] shows that the force is maximized as 0 tends to zero: Force = (TSL)(COS0)(specimen periphery)

(2.3)

However, the implication of the value of the contact angle is rather more illusory when one factors it into a wetting process model. The liquid solder components generally react strongly with the substrate to form a variety of intermetallic products, e.g., MxNy,where M = Au, Cu, or Ni; and N = In or Sn. So should the contact angle be expected to remain constant while the interfacial compositions are changing? For an example of the complexities which these reactions present consider the case of Pb-Sn solder on Cu substrates. In the case of a low Sn solder, for example 95Pb-5Sn, the only intermetallic product observed [13] is Cu3Sn. However, with higher Sn content we have observed Cu3Sn next to the Cu surface and Cu6Sn 5 next to the solder [7]. The latter observation is not unique [4-6], but we have noted [7] that the relative amounts of the two intermetallics change with time. This indicates a diffusional process normal to the original soldersubstrate interface, a process that alters ~'SL. This noted variability of the Cu-Sn intermetallics with Sn content of the solder follows a pattern similar to the observations of Yost [15] for Pb-In solders on Au. In that case the Au-rich phase (Au9In4) was found to dominate when there was an excess mass of gold ("... thin solder or thick gold...") [15] as was the case in the low Sn solder which produced only the Cu3Sn, copper-rich phase [13]. By analogy the observations of a preliminary Cu6Sn 5 layer followed by formation of a Cu3Sn layer can be rationalized as being due to an effective Sn deficiency at the Cu-Cu3Sn interface. Subsequent formation of Cu3Sn occurs only by consumption of Cu6Sns, due to kinetic diffusion control [16]. We might suppose the same behavior occurs in the

Solderabilitu Fundamentals: Microscopic Processes 4 5

analogous case of Pb-In on Au, reported by Yost [15]; we might even suppose it is a general phenomenon that affects the 7SL parameter. Furthermore, there is evidence on the microscopic scale [5,7,8,11,14,15] that, during spreading, transients in the wetting front morphology develop that may be attributed to a variation in solder composition, hydrodynamic instability, or to chemical heterogeneity of the solid surface, and which might induce similar transients in contact angle. These occurrences are manifest during a period in which most contact angle measurements are not recorded due to the perceived morphological variations. Such variations appear to be quite common, and are displayed by a wide variety of material systems undergoing wetting [17,18]. In particular, Williams [17] has shown morphological instabilities for organic fluids spreading on AI thin films, and they are very similar to our observations for solder films on Au [-8] (Fig. 2.1). The frequent incidence of wetting front morphological variations strongly suggests the possibility that contact angle measurements will also exhibit variability dependent on phenomena not accounted for in the derivation of equation (2.2). These two considerations dealing with changes in the substrate and the solder composition during soldering certainly imply that the 7SL term is likely to vary during spreading. Moreover, Lea has argued [5] that the 7LV term also varies as the overall solder composition varies during spreading. In that regard Lea notes that in the presence of fluxes there can be a significant reduction (~ 20%) in 7LV. The results of Shipley [12] on spreading time variations with and without flux for various solder compositions can be rationalized on the same basis. However, we would argue that near the substrate where dissolution of metallizations occurs, for example, Au or Cu, the 7LV effect can be counteracted. In the case of 63Sn-37Pb solder on an Au substrate, the surface energies, ~'LV in mJ/m 2 are 550 (Sn), 460 (Pb), and 1360 (Au). And only a small amount of Au dissolution results in a significant increase in the surface tension of tin or lead [8]. Lea [5] reports a particular observation of elemental surface segregation Auger analysis of a Pb-Sn-Cu wetting interaction. This is in general agreement with our own observations [7,8] as well as those of Prack [9,14] and Marshall [ 11]. However, Lea's conclusion that a Pb-rich film leads to the spreading process is not in agreement with our observations on precursor solder films [7,8]. We have observed the behavior for Pb-Sn, Pb-In, and Sn-In solders and we believe that In forms a precursor in Pb-In and that Sn forms a precursor in Pb-Sn and Sn-In. We also believe these precursor films spread preferentially along the substrate grain boundaries (Figure 2.2). It is worth noting here that the environmental scanning electron microscope (ESEM) micrograph of Figure 2.2 is remarkably similar to the observations reported by Marshall [11] (see Figs. 6-51 through 6-57 in that paper) of SEM examinations of reaction products taken postwetting for solder on A u. Rather than support the hypothesis for Pb-rich films created by 7LV effects, we would argue that the Pb-rich films are a residue from the Sn-Cu reactions which cause a Sn depletion to occur at the periphery of the spreading solder with the Sn actually being enriched in a peripheral reaction product. Such a notion is

46


Fig.re 2.1

Morphology of 63Sn-37Pb solder after spreading on Au film (on Cu substrate) at 300~ in air, no flux: (a) general morphology with evidence of precursor film and (b) higher magnification view of distinct precursor film from the upper center portion of (a).

Solderobi/ify Fundomenfo/s: Microscopic Processes 4 7

Figure 2.2 (a) ESEM micrograph at ambient temperature (25~ after flow experiment of 63Sn-37Pb on Au foil at 250~ 7.1 torr H20. Upper left shows unreacted Au; lower right shows Pb-rich primary crystals; the interface region is approximately AuSn (6) intermetallic mixed with primary Pb crystals. (b) ESEM of 50In-50Pb on Au foil taken at 90~ 6.6 torr H20, during cooldown from 250~ Au region at left; the shapes seen protruding from the solder mass on the right are crystals of approximately Auln2 (6) intermetallic composition and are growing up from the substrate through the solder mass.

48


consistent with the observations by Marshall [11], by Prack and Raleigh [14], and in our work [7,8] where a peripheral reaction product of the Sn (or In) intermetallic is observed with Pb-rich primary crystals remaining upon solidification in a subperipheral region (Figure 2.2).

2 . 3 A Microscopic Mass Transfer Model In order to rationalize these observations, a simple geometrically based kinetic model can be employed (Fig. 2.3). The conceptual model is based on a collapsing/ spreading disc of constant solder volume. The constraint of constant solder volume provides a relationship between the increment in solder/substrate contact in the x-direction and the decrement of disc height in the negative y-direction. During this solder rearrangement, diffusion of substrate material takes place mainly in the y-direction. The relationship between the height decrement, (Yo- dy), and the spreading front increment, dx; is expressed by g(R2)yo = z~(Ro + dx)2(yo - dy),

(2.4)

where R o is the initial disc radius and Yo is the initial disc height. The model is related to experimental observation through measures of the spreading rate, dx/dt. Additional information needed for the model implementation

Bulk Front ~

~

PrecursorFilm

Liquid

t

Y

Subst-rate

Adsorption Oxide Layer Cheatcal R~action

Volume Diffusion

Dissolution

Figure 2 . 3

Schematic geometry for solder wetting.

SolderabilityFundamentals.. Microscopic Processes 4r

is the diffusivity of substrate material in the molten solder and the composition of intermetallic(s) formed. For example, in the case of the P b - S n - C u reactions the Cu6Sn 5 phase is equivalent to 55 at.% Cu and the Cu3Sn phase is equivalent to 75 at.% Cu. The diffusivity of Cu in solder, Dcu, is of the order of 10-xo cm2/s at about 700 K (425~ or about 10-12 cm2/s near 500 K (225~ [13]. If we use a simple one-dimensional, constant surface concentration model for solute (Cu) diffusion [19] for this case, we can write

Ccu(y, t ) = Ccu(O)[1 -erf[y/(2(Dt)I/2)]]

(2.5)

Application of this model takes place in the following steps. First, we examine the time needed to achieve Ccu(Y, t)/Ccu(O)= 0.55 at y = 1.0 nm, that is the time to build an initial intermetallic layer at the interface. Note that the interface Cu concentration is Ccu(0)= 1.00--pure Cu. Second, using that increment of time, dt55, we evaluate the distance increment, dyol, for the location where Cc,(dyol, dt55) = 0.01. This location is arbitrarily chosen as a position where the Cu diffusion front may be detected. Third, using equation (2.4), with dy = dyol, we estimate the incremental distance of spread, dx55, that has occurred in the time increment dr55. Then if the measured dx/dt is greater than dx55/dt55 we can expect that unreacted Sn will be found at the spreading periphery. Alternatively, the Sn may be at least partially combined as intermetallic due to diffusion of Cu normal to the spreading direction. This simple analysis nominally ignores any diffusional mixing of Sn in the spreading solder as well as any convective transport of Cu in the direction of the spreading liquid. We assume the spreading solder liquid is translated from the unreacted portion of the solder disc. For T = 700 K (425~ Ccu and Dcu values noted above, an initial disc diameter of 1.0 mm, and an initial disc height of 1.0 mm, the first calculation yields dt55 = 1.397 x 10- 4 s. The second calculation yields dyo 1 = 4.3 x 10- 7 cm and the third yields dx55 = 1.065 x 10-Scm. The resulting dx55/dt55 is then 7.62 x 10-2 cm/s (762 #m/s). If the temperature is reduced to about 500 K (225~ the calculated spread parameter, dx55/dt55, decreases to 7.61 x 10 -4 cm/s--a reduction by two orders of magnitude (Dcu has decreased to about 10-12 cmZ/s). If the temperature is raised beyond 700 K, dx55/dt55 increases and the reactive solder component may be sequestered by the diffusing substrate material, as discussed below. Our high resolution ESEM observations suggest that spreading rates of the precursor films are on the order of micrometers per second. Consequently, this simple model would predict that Sn may be found at the spreading periphery. We have frequently observed Sn and In at the periphery for spreading on Au substrates in ESEM experiments [7]. However, for tests run using optical, hot-stage microscopy in air, with no flux, Pb was often found at the periphery [8]. The differences between these results are still being evaluated in terms of the experimental pressure variable, which distinguishes the ESEM from the optical hot-stage tests. ESEM has the capability to utilize a variety of atmospheres [7,9,14,20,21], albeit at total pressures lower than atmospheric. Work on a model to calibrate for these pressure effects is proceeding [21].

50


2 . 4 Observations of Limiting Mass Transfer This simple mass transport model is also applicable to two other behaviors we have observed in solder spreading experiments. One is a process which might be called dewetting [4,7]. In our observations of this behavior we have varied the thickness of Au films on glass. If the Au film is below a critical thickness for a given mass of solder, the solder will spread only a finite distance and upon solidification the solder will contract into globular shapes with minimum substrate contact and with the surrounding region of glass being scavenged of Au (Fig. 2.4). The solder will spread some distance consuming Au as it spreads by Au diffusion into the solder. If the mass of Au available is insufficient to saturate the solder~insufficient to form the Au-Sn or Au-In intermetallic--all the Au will be dissolved leaving no Au with which to bond. The glass is not wetted by the solder. This case represents the behavior where dx/dt is greater than dX~M/dt~M,the reference spreading rate for a particular intermetallic (IM), and where an unreacted solder component, such as Sn, is available for continued reaction at the spreading periphery but the Au supply has been exhausted. Another observation that may be rationalized by this simple model is the reaction between solder and substrate with insignificant spreading. In these observations the solder essentially melts through the substrate (pure Au foil) and little or no spreading is seen (Fig. 2.5). Such results imply a very high diffusive flux of substrate material (Au) into the solder; this effectively ties up the reactive solder component and reduces the spreading reactivity driving force. Using our simple model, the difference between these two behaviors might be rationalized on the basis of the diffusivity of substrate material in the liquid solder. A decrease of Dc~ of nearly two orders of magnitude occurs for a temperature drop from about 700 K to 500 K (425~ to 225~ The estimated dX~M/dt~M decreases by two orders of magnitude as the temperature decreases. But based on a A7 driving force, the spreading rates, through d(AT)/dT, are less sensitive to temperature. So dT/dT is about the same for each term in equation (2.1), and the spreading rates are still expected to be in micrometers per second. Nevertheless, higher temperatures could result in sufficiently high fluxes of substrate material into the solder to reduce the solder reactivity. There is an important geometrical effect in this model which was a factor in our observations of the melt through phenomenon. In those tests solder cylinders were placed base down on the substrate to provide a large area of contact throughout the temperature rise. Solder spheres have a reduced contact during the period of spheroid collapse and this greatly reduces the total flux of substrate material relative to the rate of spreading. Based on these simple arguments the observations noted above might be rationalized in terms of the temperatures used. For the case of "thin" substrates it is doubtful that a low enough temperature can be found to prevent dewetting in all cases. However, the model implies that the minimum thickness to prevent dewetting should be a direct function of soldering temperature, an important but often ignored process design consideration. At the other extreme it may also be prudent to avoid excessive temperatures as the diffusive flux into the solder can

$olderability Fundamentals: Microscopic Processes 51

Figure 2.4

SEM micrographs of 50Pb-50In flowed on Au substrates for 10 min at 250~ (a) solder on thin (0.5/~m) Au on pyrex, the solder mass has dewet and resolidified into a nearly spherical shape, the surrounding glass is almost free of Au except at the distance periphery (dark upper region); and (b) solder on 0.2 mm Au foil, the solder wetting is greater than in (a).

52


~"~~'!~IsilT~i~'i!i!!i!i!i!i!!!i i~/i~i~3i!iiii!qi,li!i!i!i!ii iiii~iiiiiiii!i!:!iii:~

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Figure 2.5 ESEM micrograph of 63Sn-37Pb on Au foil taken at 100~ 1.2 torr N2, during cooldown from 300~ Original solder mass at left has melted through foil with some precursor film shown as darker stain regions emanating to the right from the solder mass. X-ray spectra show Sn and Pb in all regions of stain. A strong Au signal is seen in all regions, even within the solder mass.

also apparently retard wetting. Another important factor at higher temperatures is the possible oxidation of solder, partly responsible for retarded wetting. In the worst cases an oxide skin encloses the solder mass (Fig. 2.5). Obviously this very simple model needs to be expanded to account for the more intricate details of fluid mixing and general fluid force distributions. But more elaborate modeling requires more extensive data on both wetting behavior (e.g., precursor film spreading rates and composition) and fluid properties (e.g., 7Lv, density, viscosity) as functions of temperature and composition. We have begun computational model development in conjunction with our experimental work on precursor films [21]. The dynamics of precursor films and their role in wetting are not completely understood. As late as 1974 [22], the existence of precursor films for certain liquid-solid systems was apparently in question. And as late as 1980 [23], there was little understanding of the role of precursor films in the wetting process of liquid-solid systems for which the existence of precursor films had been established. But significant understanding of the precursor film dynamics has recently developed [24a,b]; this understanding includes their genesis, their flow, and their role in wetting, with the tacit assumption that every wetting event involves precursor films. It is a theory that links the observable macroscopic wetting phenomena with the physicochemical submicroscopic phenomena of thin films. However, this theory

Solderabi/ity Fundamentals: Microscopic Processes 53

is not yet entirely satisfactory, and quantitative discrepancies between theoretical predictions and some wetting data are documented [24b]. The theory has, moreover, been developed for smooth surfaces and applies to liquids which are chemically inert. The term precursor film is a descriptive term for thin films which form and flow in advance of the macroscopic (bulk) wetting front. We can distinguish between primary thin films and secondary thin films. Primary thin films, or simply thin films, have thicknesses on the order of a few molecular diameters. Secondary thin films, or thick films, can be as thick as several micrometers. The two types of films spread by different mechanisms. The substrate is initially wet by the primary film. For volatile liquids, primary film spreading occurs by evaporation and condensation, for nonvolatile liquids, film spreading occurs by surface diffusion. Spreading of the secondary film can effectively be interpreted as the thickening of the primary film. This phase of the wetting process can be driven by both the disjoining pressure and surface tension gradients. The effect of the precursor film in wetting processes involving inert liquids is essentially to cover an otherwise dry solid surface with a thin liquid film, over which the bulk liquid eventually flows; one purpose of the theory is to predict the thicknesses of these films and the rate at which they spread. The rate of spreading of the precursor film can be the rate-limiting factor for bulk wetting. The current understanding of precursor film dynamics cannot be directly applied to the physicochemical systems that occur in soldering processes; this is because there are chemical reactions and their attendant bulk diffusion and because the liquid-fluid interface and the solid substrates undergoing soldering have nonideal surface properties (Fig. 2.3). All these factors are likely to influence the spreading mechanisms referred to above. While the low vapor pressures of molten alloys at typical soldering temperatures make surface diffusion the likely mechanism of primary film spreading, the diffusive driving force is probably augmented by the increased chemical potential gradient due to the high reactivity of the most reactive constituent of the alloy with the substrate. Verification of this assertion needs to be made in the reduced total pressures of the ESEM environment. The liquid-vapor and liquid-solid boundaries of the wetting flow are reactive surfaces. In particular, for oxidizing atmospheres, the liquid-vapor boundary is oxidized and the resulting oxide layer alters the character of the flow boundary condition at the surface. The oxide layer has characteristics of both solid and liquid interfaces; it appears capable of deforming in a direction normal to the interface without exhibiting relative tangential motion. It is clear that it suppresses to some extent internal motion within the wetting front. Because of the very low partial pressure of oxygen required to form an oxide layer, oxidation will likely be present in all commercial soldering systems. The liquid-solid boundary moves with time in a way determined by the local mass transfer driven by dissolution and modulated by chemical reaction, specifically the formation of intermetallics. While the thickness of the intermetallic layer remains small over the wetting timescale, the mass transfer coefficient at the boundary is significantly altered by the reduction in magnitude of the diffusion coefficients of the substrate metal and reactive component in the intermetallic. For

54


example, Yost [25] has shown that this mechanism greatly reduces the scavenging of Au substrates (coatings) by alloys containing In as compared to alloys containing Sn. The possibility of simultaneous oxide and intermetallic layer formation in the precursor film region greatly complicates the wetting front physics. The implications for the trailing bulk flow are very significant because the interfacial energies of intermetallics are in general higher than those of the unreacted substrate. If dissolution is not significantly attenuated before the substrate species is distributed throughout the bulk by volume diffusion, the presence of this higher free energy species will affect the surface tension. If as a result of nonuniform distribution of such species a concentration gradient arises at the interface, the associated surface tension gradient will induce Marangoni motions. In the precursor film region, particularly in surface capillaries (grain boundaries, where the large surface area to volume ratio amplifies dissolution effects) which intersect the film contact line, such Marangoni motions have been postulated to enhance the wetting process [8]. Obviously, a great deal of work remains to be done in order to clarify the physics of reactive wetting fronts.

2.5 Solder Alloy Selection and Process Design Now we are ready to propose a solder composition-solderability process design paradigm. As expressed in equations (2.1) through (2.3), the various surface energy terms appear to play nearly equal roles in the soldering process. However, it has become apparent to us that the VSLparameter is the major factor in controlling solderability. Nonetheless, in a typical analysis of the application of the energetic equations to soldering ~'SLis treated as an unknown, adjustable parameter. For example, in a practical process design one starts with the assumption of an oxidized surface (low Vsv) which is to be converted to higher 7sv by the action of fluxing reagents on the substrate, for example the breakdown of oxide films by chloride anions. By default the choice of a Pb-containing solder, with alloy liquids of typically low 7LV allows for the extension of the liquid-vapor surface required to achieve the desired spreading geometry of low contact angles. (Refer again to Lea [5] for the arguments regarding alloy surface segregation, i.e., adsorption, and the effect of flux on lowering 7LV.)Finally, 7SLis fitted into the energetics, mostly as an unknown, adjustable parameter to satisfy the observed wetting constraints of equations (2.2) and (2.3). However, we prefer to follow the arguments about the importance of controlling 7SL, before the fact, as put forth by Klein Wassink [26]. In that paper the author showed the need to separate the role of 7LV and 7SL in evaluating solderability. For example, in the case of solder spreading on a free surface the improvement of wetting with increasing Pb content occurs at the 60Sn--40Pb alloy composition, since at that composition reduction in VLV by increasing the Pb content begins to be outweighed by the increase in VSLby increasing the Pb content.

Solderability Fundamentals: Microscopic Processes 5 5

However, for the spreading of solder in a capillary, where 7LV plays little or no role, the best results occur with the most reactive Pb-Sn alloyml00% Snmand the role of Pb as a diluting carrier component is clearly seen. Our arguments not only support Klein Wassink, they go further. We say it is possible to compose solder alloys based on separate considerations of components' roles in 7LV and '~SL. We can construct an argument for replacement of Pb in solder alloys, if that is environmentally desirable, by choosing substitutions in terms of 7LV and ~'SL effects. Imagine a Pb substitute for Pb-Sn and Pb-In alloys; on solderability grounds we are drawn to consider Bi. Bi is chosen for other applications where Pb liquid and solid Cu interact, for example machinability of Cu alloys [27]. The value of 7LV is about 100 mJ/m 2 lower for Bi than for Pb; 7LV for Pb is 450 mJ/m 2 [27]. Compared to Pb there are no significant differences in the phase equilibria between the solder alloy components and Bi, and no significant differences in the solubility of Bi in substrate materials (Au and Cu) [28]. Consequently, as a first approximation we would expect solderability of Bi-Sn and Bi-In alloys to be equivalent to the corresponding Pb-Sn and Pb-In compositions, perhaps even slightly better for free surface applications. Preliminary results from tests with such materials shows this to be the case in simple binary alloys and Pb-Bi-Sn ternaries [21]. Results from more complex Bi-containing alloys do not appear as promising, although they do show large variations in the measurements [29]. Klein Wassink provides results on the doping of nonwetting liquid metals with reactive components, such as liquid Ag doped with Pd, in order to wet solid Fe. The concept they reflect evolved from an analysis similar to this chapter; it emphasizes the importance of solubility and reactivity between the solder components and the substrate for controlling solderability. The importance of this concept is also recognized for tailoring brazing alloys [30]. We believe it may be extended to the development of solder alloy systems which have reduced flux requirements; there we emphasize cation effects instead of anion effects on the creation of bonding on oxidized surfaces [21].

2 . 6 Conclusion

We have presented some observations from solder spreading experiments to illustrate our need for a more comprehensive and, we hope, more predictive model of the soldering process. Much work has been done on measurements of such parameters as contact angle, intermetallic growth kinetics, and wetting force but a few reports [-4,5,6] have attempted a unifying analysis, even qualitatively. Now that analytical tools such as ESEM can be used to make real-time, high resolution measurements of chemical changes in representative environments, we advocate the expansion of efforts to develop a more quantitative understanding of the liquid-solid interactions exemplified by soldering and brazing.

56


References 1. M.F. Ashby and D.R.H. Jones, Engineering Materials, Vol.2., Pergamon Press, Oxford, 1986, pp. 31-33. 2. D.R. Frear, W.B. Jones, and K.R. Kinsman (eds.). Solder Mechanics." A State of the Art Assessment, TMS-EMPMD Monograph Series, No. 1, Warrendale PA, 1991. 3. J.H. Lau (ed.), Solder Joint Reliability: Theory and Applications, Van Nostrand Reinhold, New York, 1991. 4. R.J. Klein Wassink, Soldering in Electronics, 2d ed., Electrochemical Publications, Ayr, Scotland, 1989. 5. C. Lea, A Scientific Guide to Surface Mount Technology, Electrochemical Publications, Ayr, Scotland, 1988. 6. A.D. Romig, Jr., Y.A. Chang, J.J. Stephens, D.R. Frear, V. Marcotte, and C. Lea, in Solder Mechanics." A State of the Art Assessment, edited by D.R. Frear, W.B. Jones, and K.R. Kinsman, TMS-EMPMD Monograph Series, No. 1, Warrendale PA, 1991, Ch. 2. 7. J.A. Clum and T.J. Singler, in New Technology in Electronic Packaging, Proc. 3rd A S M Electronic Materials and Processing Congress, San Francisco, 1990, pp. 175-186. 8. T.J. Singler, J.A. Clum, and E.R. Prack, ASME Winter Annual Meeting, Atlanta, GA, Dec. 1991, ASME Paper 91-WA-EEP-37. pp. 1-10, 1991. 9. E.R. Prack, in Proc. IEEE C H M T (1991). 10. D.R. Frear, F. M. Hosking and D. M. Keicher, this volume, pp. 79-103. 11. J.L. Marshall, in Solder Joint Reliability: Theory and Applications, edited by J. J. Lau, Van Nostrand Reinhold, New York, 1991, Ch. 6. 12. J.F. Shipley, WeMing Res. Suppl., pp. 357s-362s, Oct. 1975. 13. D. Grivas, D. Frear, L. Quan, and J.W. Morris, Jr., "The Formation of Cu3Sn Intermetallics on the Reaction of Cu with 95Pb-5Sn Solder," Lawrence Berkeley Lab., LBL-19416. 14. E.R. Prack and C.J. Raleigh, Ultramicroscopy 37 (1990). 15. F.G. Yost, Gold Bull. 10 (1977). 16. U. Gosele and K.N. Tu, J. Appl. Phys. 66, 2619-2626 (1989). 17. R. Williams, Nature 266, 153-154 (1970). 18. W.W. Mullins and R.F. Sekerka, J. Appl. Phys. 35, 444 (1964). 19. P.G. Shewmon, Diffusion in Solids, 2d ed., TMS, Warrendale PA, 1989, pp. 22-23. 20. N. Baumgarten, Nature 341, 81-82 (1989). 21. T.J. Singler, J.A. Clum, and E.R. Prack, unpublished research. 22. W. Radigan, H. Ghiradella, H.L. Frisch, H. Schonhorn, and T.K. Kwei., Colloid Interface Sci. 49(2), 241-248 (1974). 23. A. Marmur and M.D. Lelah, J. Colloid Interface Sci. 78(1), 260-265 (1980). 24(a). P.G. de Gennes, Reviews in Modern Physics 57, 827-863 (1985). (b) G.F. Teletzke, H.T. Davis, and L.E. Scriven, Chem. Eng. Commun. 55, 41-82 (1987). 25. F.G. Yost, Proc. Int. Soc. Hybrid Microelectronics, 1978, pp. 61-66. 26. R.J. Klein Wassink, J. Inst. Metals 95, 38-43 (1967). 27. J.T. Plewes and D.N. Loiacono, Adv. Mater. Proc. Oct. 1991, pp. 23-27. 28. A S M Metals Handbook, Vol. 8, 8th ed., Metallography, Structures and Phase Diagrams, ASM, 1973. 29. F.M. Hosking, P.T. Vianco, and D. R. Frear, in Materials Developments in Microelectronic Packaging." Performance and Reliability Proc. 4th A S M Electronic Materials and Processing Congress, Montreal, 1991, pp. 365-371. 30. R.R. Kapoor and T.W. Eagar, Metall. Trans. 20B, 919-924 (1989).

CHAPTER

3

Determining the Damaging Strains which Cause Failure in Pb-Sn Solders A. I. Attarwala, B. C. Hendrix, J. M. Sanchez

3.1 Introduction Solder joint reliability is an important concern in surface mount packages since the joints have to provide not only a good electrical connection but also a good mechanical connection. An electronic package is made of different materials which have different coefficients of thermal expansion. During operation, the thermal expansion mismatch between the chip carrier and the board subject the solder joints to cyclic stresses. The thermal cycles are caused by power on/off cycles, diurnal temperature variations, and seasonal temperature changes. Although thermal expansion mismatch between board and chip carrier can be reduced by matching their coefficients of thermal expansion, during power on/off cycles thermal gradients exist between them, therefore some mismatch is always present. Many approaches are being undertaken to predict how long a given solder joint will last before it fails under a certain set of conditions [1-26]. Testing has been performed on actual test packages, joint type specimens, and bulk solder material. One of the most common approaches used in industry is thermal cycling of test packages. The advantage of this approach is that it can provide good data for life prediction for that particular package. Most factors which interact to cause a solder joint to fail are taken into account, such as thermal gradients across the chip carrier and the board, warpage of the board, and thermal expansion mismatch between the carrier and the board. However, this approach is time-consuming and expensive since it requires extensive testing to develop every new package. The main drawback of this approach is that the data relates to the behavior of the individual package so the results are not universally applicable. Another approach for life prediction is to model the joint response based on constitutive equations [28-32]. Subhramanyan et al. [30,31] have developed a constitutive equation for solder subjected to thermomechanical cycling. Their life predictive equation is based on a fracture mechanics approach to model the propagation of a single crack through the solder joint. A different approach has been developed by Fox et al. [10] and Knecht et al. [32]; they use a constitutive 57

58


equation to separate the different strain components during each cycle. Their failure criteria is based on the accumulation of certain components of strain. Although this approach promises to decrease the effort expended on testing during package development, it lacks well-defined constitutive equations and failure criteria. Mechanical behavior of joints [-23-27], joint type specimens [10-12,22], and bulk specimens [8,9,13-20] are being studied by a number of researchers to establish a relation between the number of cycles to failure--the amount of damage being stored per cycle, and the loading condition. For example, Roger Wild [24,25] has done an extensive study of plated-through-hole solder joints, lap joints, and butt joints. He recognized that deformation behavior of solder depends on temperature and time through variables such as strain rate and frequency. Solder joints were tested under various conditions by Solomon [ 11,12], who has attempted to separate the plastic strains from the total strains and to relate them to the number of cycles to failure. Fine and Vaynman [8,9] have tested bulk solder specimens and have related the plastic strains to the number of cycles to failure. Extensive work on the characterization of the deformation behavior of eutectic Pb-Sn solders has also been carried out by Tien et al. [ 13-21 ]. These authors have shown that the presence of anelastic strains can suppress the storage of nonrecoverable creep strains during accelerated cycling. The microstructural evolution of solder material under mechanical loading has also been the subject of several investigations [33-43]. The aim of the microstructural studies is to understand how microstructure and microstructural changes affect the deformation and failure characteristics of solder. For example, Frear [39,40], Frost [36,37], and Raman [42,43] have focused on the effects of microstructure and dynamic recrystallization. Life prediction analysis requires knowing not only what processes are causing the deformation and failure but also how to measure the damage accumulation leading to failure. Although, a considerable amount of work has been done on studying the deformation behavior of solder under mechanical loading, there is still no consensus on what processes are primarily responsible for failure. Due to th~..,, cyclic ..~.n~ture of th~_,, ~'vv''~'~'""~nnl~'~ ti,-,n of stresses, many researchers characterize failure in terms of fatigue parameters, cycles to failure and plastic strain range. However, our work on bulk Pb-Sn solders tested under isothermal conditions at temperatures as low as - 4 0 ~ and at a frequency of 0.5 Hz show that failure occurs primarily by accumulation of damage due to creep. The sections which follow describe the deformation of Pb-Sn solder under static and cyclic loading conditions. This behavior is then related to the more tangible concepts of loading conditions and life.

3.2 Test Methodology and Data Analysis All tests were performed on bulk specimens cast from different compositions of lead and tin. Most of the specimens were tested in the as-cast condition but some were aged or directionally solidified to study the effects of different grain structures. Details of the specimen preparation and geometry can be found elsewhere [21].

Determining the Damaging Strains which Cause Failure in Pb-Sn Solders 59

Tests were performed on a computer-controlled servohydraulic testing machine with a feedback loop control system. The cyclic tests were conducted under uniaxial. tensile, load-controlled conditions. It is important to note that all of our tests were performed under load-controlled conditions as opposed to the more commonly used strain-controlled conditions. Under load-controlled conditions it is easy to separate and measure the contributions of the different strain components present during cycling.

3.2.1 Separation of Strains Four types of strain are stored during a cycle: elastic strain, anelastic strain, athermal plastic strain, and creep strain: (3.1)

~T--8el + ~an "[- 8pl + eerp

Anelastic strain and creep strain are time dependent; elastic strain and plastic strain are time independent. Elastic strain and anelastic strain are recoverable; creep strain and plastic strain are nonrecoverable and damaging. Table 3.1 shows the different types of strains. Figures 3.1 and 3.2 respectively show schematics of the strain response of a single load-controlled cycle and the stress response of a strain-controlled cycle. In the load-controlled cycle, the time-indepedent strains, which are the elastic and plastic strains, occur immediately the stress is applied during square wave cycling. The time-dependent strains, which are the anelastic and creep strains, are stored during the hold time on load. When the load is released, the elastic strain is recovered immediately, whereas the anelastic strain is recovered over a period of time. An example of actual anelastic strain storage and recovery is shown in Figure 3.3, which shows the strain versus time response of solder in a load-controlled test. Notice that during the off-load period, elastic strains are recovered immediately and then some more strain is recovered with time. This time-dependent recovered strain is the anelastic strain. The stress response of a strain-controlled cycle is more complex. During the ramp-up time, the stress is due to the sum of the elastic and instantaneous plastic strains. During the hold time at the maximum strain, some of the elastic strains get transformed into anelastic and permanent creep strains with a corresponding drop in load. During the ramp-down time, a compressive load has to be applied to bring the specimen to zero strain because of the time-dependent strains stored at the hold time at maximum strain. During the time at zero strain, the compressive load again relaxes toward zero as anelastic and creep relaxation occur. Table 3.1

Different components of the total strain.

Recoverable Nonrecoverable

Time independent

Time dependent

Elastic Plastic

Anelastic Creep

60


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Figure 3.1 Schematic showing a load-controlled cycle and the corresponding strain response. It is possible to separate the contributions of the different components of strain to the total strain.

I II

&

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el* = elastic strain which is converted to creep strains and anelastic strains during the hold time.

Figure 3.2 Schematic showing a strain-controlled cycle and the corresponding stress response. It is difficult to separate the different strain components because the driving force, the stress, is continually changing due to stress relaxation.

Determining the Damaging Strains which Cause Failure in Pb-Sn Solders 61

30

f C z_

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27 592

594

5 6

598

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Time (sec) Figure 3.3 Strain response of a load-controlled cycle. There is a large strain recovery during the off-load hold time.

The important advantage of the load-controlled test is that it allows each of the strain components to be measured separately in each section of the load cycle. 3 , 2 . 2 The Envelope Strain Curve The curve obtained by plotting the maximum strain seen during each cycle as a function of time spent on load is called an envelope strain curve. The envelope strain curve shows the average rate of storage of nonrecoverable strains, creep strain plus athermal plastic strain, obtained for a given loading condition. Figure 3.4 shows schematically how an envelope strain curve is obtained from the strain response during load-controlled cycling. Figure 3.5 shows actual test data of the strain response during load-controlled cycling and Figure 3.6 shows the envelope strain response of a cyclic load-controlled test. The envelope strain curve is similar to that observed during creep; each curve has primary, secondary, and tertiary strain regions. 3 , 2 . 3 Measurement of Nonre:overable Strain p e r Cy:le The creep strain curve and the envelope strain curve are direct measures of the accumulated nonrecoverable strains, the plastic and creep strains but not the elastic and anelastic strains. Thus, the strain rate for each curve gives a measure of the rate at which nonrecoverable strain is being stored in the specimen under a given set of conditions. Most of the time during load-controlled test is spent in the secondary region, as the strain rate in this region will be used to characterize the damage storage rate for a given set of conditions.

62

MATERIALS

FOR

ELECTRONIC

0

PACKAGING

"envelope" strain strain as a function of time

----

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Figure 3.4 Schematic representation of how an envelope strain curve is obtained from the strain response of a cyclic test. 0.12

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Time (sees) Figure 3.5 Strain response of a 63Sn-37Pb specimen subjected to cyclic loading between 0 and 12 MPa at 24~ and 0.002 Hz. In most creep situations, the minimum creep rate in the secondary region is used to characterize the secondary strain rate. In some of the solder materials we tested, the primary creep lasted less than one cycle and the strain rate varied during the secondary creep region, as shown in Figure 3.7. In cases where a minimum strain rate could not be characterized or the strain rate was varying during the secondary region, an average of the secondary region strain rate was used to characterize the response of the material.

Determining the Damaging Strains which Cause Failure in Pb-Sn Solders

63

0.3

region Secondaryregion = Tertiary region

A = Primary B = C

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Time (secs) Figure 3 . 6 Envelope strain curve of a eutectic specimen cycled at - 4 0 ~ between 0 and 45 MPa at a frequency of 0.5 Hz. The envelope strain curve shows the three distinctive creep regions primary, secondary and tertiary which are characteristic of a creep curve. 10-4

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10-7

0

_.2... ~*

9

i

10000

9

/

20000

9

I

30000

9

I

40000

9

50000

Time (sees)

Figure 3 . 7

Envelope strain rate response of a 90Pb-10Sn specimen cycled at 160~ between 0 and 8 MPa at a frequency of 0.002 Hz. The characteristic secondary strain rate, ~s, whether m i n i m u m or average, is multiplied by the time on load per cycle, th, to arrive at the nonrecoverable strain stored per cycle, enr: ~;nr =

~s t h

(3.2)

The strain per cycle, enr, can then be used to correlate the cyclic life of a specimen to the testing conditions.

64


3 . 3 Deformation Behavior of Pb-Sn Solders under Static and Cyclic Loading Deformation of materials due to creep processes is a function of time, temperature, and the applied stress. The dependency of the deformation on the various parameters is illustrated in the creep equation given below: g, = Aa" e x p ( - Q/k T)

(3.3)

where A is an experimentally determined constant, n is a stress exponent with a value between 3 and 5, Q is the activation energy, and k is Boltzmann's constant. Solder used in microelectronic applications experiences a wide range of operating temperatures, from 0.4 Tm to 0.75 Tm, where Tm is the melting temperature in kelvins. Figure 3.8 shows the operating temperatures experienced by the packages used in different applications. It is generally accepted that creep or thermally assisted deformation becomes significant at homologous temperatures, T/Tm > 0.5. Even at -0.40~ eutectic solder has a homologous temperature close to 0.5.

Homologous P b P nase

433K-rir-160~

i

0.9

i

_ 0.8

.0,6

333K-1-1-+60.C 313K-F-F+40"C

Environments

.0,7

373K-I-1-100,C 353K-FI--+80oc

Eutec "ic Pb-Sn

Typical Use

..O,8

413K I'rIlI140oc 393K-rT-120"C

Temperature

L Ua

E

.

0.7

o ;b

e-

E

g

L

--O.5

e~ O

293K-1-1--+20"C

i i

(-

_

273K I F r I o.c

0.6

e-

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0.

253K ITIII-20~ 233K-1-11-40" C

_>

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_

0.5

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E

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J3

r

~

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c~O c~ +.,.'

r

~

0

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86


narrow band of the remaining metallized layer and underlying Kovar was melted during the reflow experiment. Wavelength dispersive X-ray spectrometry identified AuSn 4 precipitates throughout the joint and a substantial quantity of feathery (Ni, Fe, Co)3Sn 2 precipitates in the solder where the base surface was melted. The AuSn 4 precipitates formed during the initial soldering operation and would affect the as-soldered joint properties. The (Ni, Fe, C0)3Sn2 precipitates formed on reaction between the base Kovar and Sn-Pb solder alloy and could influence the properties of the reflowed joint. 4 . 2 . 3 Summary

Laser heating can be applied to fluxless soldering of Au/Ni plated Kovar. Higher laser power, smaller laser beam focused spot size, and slower travel speed give the best wetting results. Some advantages of fluxless laser soldering are (1) higher joining temperatures and potentially stronger joints, (2) elimination of entrapped corrosive flux residues, (3) compliance with environmental restrictions on hazardous solvent use, and (4) automated processing and process control. Some disadvantages of laser soldering are (1) inhibited absorption of the laser energy by highly reflective materials, (2) conventional geometries may not be compatible with the process, and (3)overheating can produce unsatisfactory joint properties.

4.3 Activated Acid Vapor Fluxless Soldering Activated acid vapor fluxless soldering is a process in which liquid flux is replaced by a dilute solution of a vaporized acid in an inert carrier gas. By a strict definition, the acid v~/por is a flux in that its function is to reduce surface metal oxides in order to enhance solderability. However, the acid vapor leaves little residue that requires cleaning, as long as the acid concentrations are low. The process of acid vapor soldering involves passing the carrier gas and acid vapor over the metallized surface to be joined. The acid chemically attacks the surface oxide as the metal is heated, removing the oxide from the surface. In this way the surface is activated for wetting by the solder. The solder is introduced while the acid vapor flows across the joints and this protective atmosphere prevents reoxidation of the metallized surface. The process nominally involves the use of solder preforms but it can be applied to wave soldering, where assemblies are dragged through a molten solder bath. The parameters influencing the formation of solder joints using acid vapors have been investigated. Wetting was evaluated as a function of acid type and acid vapor concentration. The experimental techniques and solderability results discussed here show that acid vapor fluxless soldering is a promising production process.

Fluxless Soldering for/Hicroelectronic Applications

87

4.3.1 Experimental Procedure

4.3.1.1 Wetting Experiments Wettability here is defined as a measure of the degree to which a solid substrate can be covered by a molten metal (in this case solder). A way to quantify wettability is to measure the contact angle, 0, between the solder and the substrate. The angle 0 is defined in Figure 4.1. For a given volume of solder, the smaller the value of 0 the greater the extent of wetting. A straightforward way of measuring wetting is the sessile drop area-of-spread experiment. A small amount of solid solder, in the form of a sphere, is placed on a metallized substrate and heated to above the melting temperature of the solder. As the solder melts, the molten droplet spreads across the metallized substrate. The greater the amount of spreading the lower the wetting angle and the better the wettability. If the droplets are small their shape can be approximated as a spherical cap. If a solder drop is too large, gravity will flatten the top and the droplet will no longer be spherical. A numerical technique derived by Bashforth and Adams [ 161 can be used to determine droplet shape. The solder droplets in this study were kept small ( < 9 mg) so as to avoid the effects of gravity. An analytical solution for 0 can be derived geometrically. In a sessile drop, the area of spread of the molten droplet of solder can be used to calculate 0 using equation (4.1):

O(A) =

2b(A)[j(A) -

arctan< . . . . . . . . . .

k(A)]

( b ( A ) 2 - [ . j ( A ) - k(A)] 2

]

(4.1)

where

b(A) = ~ , j(A)-

+ b(A)6 +

3V l l/

V = volume of solder drop = mass/density A = area of spread of drop For this work V = (0.009 g)/(8.5 g/cc). The volume of the drop is known, and constant, and by measuring the area of spread the contact angle can be estimated. Additional data that can be derived from a sessile drop experiment are the wetting rate and the time to wetting. The wetting rate describes how quickly the solder droplet spreads and can be related to the metallurgical reaction between the solder and the substrate. The time to wetting is the time interval from solder melting to the onset of spreading. The time to wetting is a measure of how fast a surface oxide can be removed in a reducing (acid vapor) atmosphere, assuming no spreading occurs until all surface oxides are removed.

88


4.3.1.2 Fluxless Soldering Chamber A soldering chamber was used to evaluate the acid vapor soldering process and is shown schematically in Figure 4.5. The chamber had gas inlet and outlet ports along with a glass-covered viewing port. Evacuation was performed using a mechanical pump. Specimens were heated using a resistive plate inside the chamber. The temperature was controlled by digital feedback electronics. The sessile drop experiments were performed inside the vacuum chamber and viewed through the glass port. The spread of the sessile drop was recorded on videotape. The video images were digitized and analyzed for area of spread (which gives the wetting angle) as a function of time at temperature. 4.3.1.3 Acid Vapors To perform acid vapor fluxless soldering three requirements must be met: 1. The acid must be able to reduce the relevant surface oxides in a vaporized form at soldering temperatures (~ 220~ manufacturing concern. 2. The acid vapors must react quickly, on the order of a few seconds, manufacturing concern. 3. The vapors must be reactive enough to reduce metal surface oxides but without causing corrosion on any part of the soldered assembly, reliability concern. The reducing agents that best satisfy these requirements are shown in Table 4.2. Formic and acetic acids were chosen because they boil at low temperatures, which allows them to be in gas form during solder processing.

Laser Beam

(Laserl~ Window To Pump and Inert Gas Source

Solder Ball Manipulator

Sample

Figure 4.5 Schematic diagram of vacuum chamber configuration used for 60Sn-4OPb sessile drop wettability tests.

Flux/ess Soldering for/Hicroe/ectronic Applications

Table 4 . 2

89

Characteristics of reducing agents used for acid vapor fluxless soldering.

Reducing agent

Concentration (vol.%) Meltingpoint (~

Forming gas (H2) 4% H2 in argon Acetic acid (CH3COOH) 0.5-2.5% in nitrogen Formic acid (HCOOH) 1.5-7% in nitrogen

-259 4 8.4

Boilingpoint (~ -253 100.7 100.7

Hydrogen is gaseous at room temperature and can be easily introduced into the vacuum chamber for solderability tests. The acetic and formic acids had to be introduced via a bubbler. The inert carrier gas is aerated through a bath of formic or acetic acid. While passing through the bath the inert gas and the acid intermix to form a vapor. The concentration of the acid varies as a function of the vapor pressure which, in turn, is temperature dependent. Therefore, by varying the acid bath temperature, the acid vapor concentration varies. The acid vapor concentrations used are also listed in Table 4.2. The acid vapors pass over the heating stage and sessile drop specimen. Heating the stage and sample also heats the vapors; this increases the reactivity of the acid, which increases the oxide reduction rate and extent of reaction. Sessile drop tests were performed with 60Sn-40Pb solder in the vacuum chamber to explore the effect of time to wetting and wettability as a function of acid type, concentration, and metallization. The soldering temperature used was 220~ The metallizations characterized were plated layers of Ni, Cu, and Au/Ni on A1 coupons that had dimensions of 0.95 cm x 0.95 cm. 4 . 3 . 2 Results a n d Discussion

No wetting was observed in tests with forming gas. At 220~ hydrogen has a low reaction rate for reducing metal oxides and is consequently not suitable for soldering. At higher temperature (> 350~ the hydrogen reduces metal oxides rapidly but these temperatures could damage electronic components. Therefore, forming gas is not a suitable option for in situ fluxless soldering. However, if the metallized surfaces were cleaned by some other method prior to soldering, the forming gas could act as a blanket that displaces oxygen in the chamber and prevents oxidation during soldering. The formic and acetic acids provided better wetting on the Au/Ni and Cu metallizations. In the concentrations studied, the Ni metallizations could not be wet by formic acid vapor or by acetic acid vapor. The following is a discussion of test results for time to wetting, a measure of the reduction rate, and area of spread, a measure of the degree of wetting.

4.3.2.1 Time to Wetting The time to wetting results for formic acid are shown in Figure 4.6, and those for acetic acid vapor are shown in Figure 4.7. The data show the time between the solder melting and the onset of spreading recorded as a function of acid vapor concentration.

90

MATERIALS

~

20-

PACKAGING

60Sn-40Pb/Au-Ni

60Sn-40Pb/Cu

I

25

FOR ELECTRONIC

I

i

~. "i

I i

l

25

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20=

0

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Acid Concentration (%)

I

I

-

I

3 4 5 6 7 Acid Concentration (%)

l

8

Figure 4.6 Plot of the time to wetting as a function of formic acid vapor concentration for Au/Ni and Cu metallizations. 60Sn-40Pb/Au-Ni

60Sn-40Pb/Cu

20

g

|

i

15-

| il

| I

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Figure

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iz

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Acid C o n c e n t r a t i o n (%)

4.7

Plot of the time to wetting as a function of acetic acid vapor concentration for Au/Ni and Cu metallizations.

The formic acid vapor allowed the immediate wetting of the Au/Ni substrate by 60Sn-40Pb solder. This indicates that surface contamination on the Au is very thin and is easily removed. The oxide layer on the Cu is substantial and rapid wetting is observed only in acid vapor concentrations greater than 4%. At vapor concentrations less than 4%, there is insufficient acid to allow wetting. The results of time to wetting with acetic acid vapors exhibit similar behavior for both the Cu and Au/Ni metallizations, Figure 4.7. With increasing acid vapor concentration, the time to wetting decreases to a minimum at a concentration of 1.5% then increases as concentrations increase above 1.5%. Both Au/Ni and Cu have this minimum at an acid concentration of 1.5%, but the time to wetting for Au/Ni is an order of magnitude shorter than for Cu. This behavior can be explained as a two-stage process. At concentrations below 1.5%, there is insufficient acid vapor present to rapidly reduce the metal oxides. Above 1.5%, the acid vapors reduce the surface oxides rapidly but then recombine with the pure metal to form what appear to be acetates. The acetate layers are inhibitors that slow both wetting and oxide formation. However, as shown in the contact angle results, the acetate only influences time to wetting, not the degree of wetting.

Fluxless Soldering for/Hicroelectronic Applications

91

4.3.2.2 Wetting Rate The wetting rate is an indicator of the extent of metallurgical reaction between the molten solder and the oxide-free metal surface. This rate is measured as the area of spread per unit time. The wetting rates for the acid vapors and metallizations studied were similar and independent of vapor concentration. This indicates that once the surface oxide is removed (and before the acetate is deposited in the case of acetic acid) solder spreads at the same rate on both Au/Ni and Cu. 4.3.2.3 Contact Angle Contact angle is a measure of the degree of wetting. A small contact angle corresponds to a large area of spread and good wettability. Plots of contact angle as a function of acid vapor concentration are shown in Figure 4.8 for acetic acid and Figure 4.9 for formic acid. These measurements were made after 60 s at 220~ for all samples. For acetic acid, Figure 4.8, there is little change in wetting angle as a function of acid vapor concentration. The Au/Ni metallization was found to show slightly

60Sn-40Pb/Cu ~oo'

C3~ "10

l

l

l

60Sn-40Pb/Au-Ni

lOO

l

80 ........................................-.".......................................

-

iiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiii

80

6 0 ............. :.......................... -.".......................................

~

6o

40-

t450 ;> t60* Figure 9 . 1 9

Attack angle versus controls transfer mechanism. (Courtesy DuPont

Electronics.) 28

-

27

Resistor 1 Mfl/ll~ Squeegee Speed 20em/see (8 in/see) Squeegee Stroke 10cm (4 in)

26 E =1.

24

cn

22

(1) r

-~ o r.-o

(1)

:-

23 22 21 20

I 30 ~

I

Attack Figure 9.20

I

45 ~

60 ~

Angle

Attack angle versus thickness. (Courtesy DuPont Electronics.)

Downstop is important to the printing process to prevent such problems as coining, stretching, puncturing of the screen mesh and to prevent poor print resolution. The proper downstop should be 125-175/~m below the surface of the substrate being printed. Screen peel is the release of the mesh from the wet print and can lag behind the squeegee edge 0-5 cm depending on the tackiness of the composition being printed, the squeegee speed, the mesh count, the tension, the condition of the screen mesh, and the snapoff distance. It is important the squeegee speed is adjusted

206

MATERIALS FOR ELECTRONIC P A C K A G I N G

to insure the screen releases as close to the squeegee edge as possible. This means the squeegee must be slowed in some instances. Failure to do so may result in poor print quality, especially pinholes and pullouts. The best way to accomplish this is by observation. One should observe how the screen is releasing from the composition during the printing cycle and make proper adjustments to insure the print speed is right for the composition being printed and the screen peels immediately behind the squeegee edge. The screen printing process is basically a volume transfer of the composition to the surface being printed. The mechanism is controlled primarily by the screen mesh and the thickness of the emulsion. Figure 9.21 illustrates the percentage open area for various common screen meshes. Table 9.6 details the effect of screen mesh on print thickness assuming that a composition has been calibrated to yield a 25/~m dried print using a 200 mesh screen with 53 /~m (2.1 x 10 -3 mil) stainless steel wire diameter. The primary factors that affect print thickness are the percentage solids content of the composition, the screen mesh count, and the screen emulsion thickness. The percentage solids content is optimized by the thick film paste manufacturer and should not be altered by the user through the addition of excess solvent or vehicle. The reason is that most vehicle systems are complex and balanced

I I I

b

.

.

-t % open =

a

1_ - I ~ I--

Figure

9.21

I I I

200 m e s h

a = 1/200 = 0.005" b = 0.0021" or 0.0016"

b = 0.0021" b = 0.0016"

% o p e n = 3 3 .6 4 % % o p e n = 4 6 .2 5 %

250 m e s h b = 0.0014"

% o p e n = 4 2 .2 5 %

325 m e s h b = 0.0011"

% o p e n = 4 1 .2 8 %

Screen mesh selection. (Courtesy DuPont Electronics.)

(a-b) 2 a2

x 100

Thick Film Technology 2 0 7

Table

Mesh

200 200 250 325

9.6

Screen mesh versus thickness? B (pm)

% Open

Print thickness (llm)

53 41 36 28

33.6 46.2 42.3 41.3

25 26 21 16

Assumptions: 200 mesh (B = 53/~m) yields 25/~m dried print; any given composition has been normalized to give 25/~m at a given solids content with 200 mesh screen. a

for resin-to-solvent ratio. Improper or excessive dilution can cause separation of the vehicle and the inorganic solids, which at worst destroys the integrity of the films or at best causes rheology and printing problems. The best way for a user to alter print thickness is by the proper selection of screen mesh and emulsion thickness. Secondary factors which affect print thickness are attack angle, snapoff distance, squeegee durometer, squeegee pressure, squeegee speed and downstop. These factors are sometimes used to make minor adjustments in print thickness. It is very important to avoid excessive wiping of the screen. If a solvent is used to wipe the bottom of the screen it should be compatible with the composition being printed. It is usually desirable to use the solvent recommended for thinning the composition being printed. Emulsion wear, screen tension, and the general condition of the screen should be monitored. These directly affect both the thickness and the definition of the films. Paste should never be allowed to dry on the lid, and care should be taken to avoid composition drying on the lip of its container. Clogging of the screen mesh and void formation can result if dried paste particles inadvertently fall into the wet paste. The thick film screen printing process involves many variables which must be controlled for optimum results. The capabilities of the thick film composition play a key role in obtaining high production yields. Process control will be more effective if a high performance thick film composition is used properly. 9. 7. 2 Firing Process

During the drying and firing processes, the solvent evaporates at 25-150~ The polymer decomposes at 150-500~ the glasses begin to melt and other phases begin to sinter at 600~ and above. It is important to have adequate ventilation and exhaust during the evaporation phase. During the polymer decomposition phase, a sufficient amount of air is required to totally convert the organic polymer to gaseous phases which, coupled with a properly located exhaust, will totally remove the organics from the firing atmosphere. The rate of temperature rise from 300-600~ is typically limited to 50-85~ to assure that no carbon entrapment occurs. Thicker prints

208


require slower rates. The amount of air required for adequate burnout can be calculated from (9.1)

V = PLA WS

where V is the volume of airflow required in liters/min or standard cubic feet (SCF) per minute depending on whether metric or English units are used in the calculation; P is the ratio of printed paste area to total substrate area; L is the ratio of total substrate area in the furnace to total belt space area available; A is a constant representing the amount of air needed per unit area of printed paste being processed to completely burn out the polymer in the thick film composition (0.4 liter/cm 2 or 0~1 SCF/in.2); W is the belt width in cm or in.; and S is the belt speed in cm/min or in./min. For example, for a substrate which is 33% covered with paste, fired in a furnace which is 75% loaded, having a belt width of 20 cm (8 in.) at a belt speed of 10 cm/min (4 in./min) the required volume of burnout air is V = 0.33 x 0.75 x 0.4 x 20 x 10 = 20 liters/min (50 SCF/min)

(9.2)

The typical profiles used in firing most thick film compositions are shown in Figures 9.22 and 9.23. The airflow arrangement for a typical air-firing furnace is shown in Figures 9.24 and 9.25. Alternate air flow arrangements are shown in Figure 9.26. The location of the exhaust is very important. Ideally, the exhaust should be at 500~ to avoid passing any of the burnoff products over the fired films at higher temperatures, which could cause reduction of some of the glasses or oxides. Forcing of the burnoff products over cooler parts can cause precipitation of the organics or carbon; this may lead to entrapment later in the firing cycle. For the profiles shown, temperature should be controlled within __+2-3~ for a peak time of 9-11 min in all cases, except for encapsulant firing. For encapsulants the total cycle time is 20 min to a peak temperature of approximately 500~ with minimum soak time at peak temperature.

lOOOl-

90018OO O 70O 6OO 5OO E 4OO I--- 300 200 100 / 0 Figure 9 . 2 2

Riserate ..-50oC/min

/

Enlry 10

10min 850~

~

\

IBelttrav~I ~-20 30 40 Time(minutes)

Descent rate o~ %

1%Ex,t 50 60

Recommended firing profile, 60 min cycle. (Courtesy DuPont Electronics.)


~l

--

900

10 M i n u t e s at Peak ~

.- I

850 ~ C Peak

800

700

600 A

oo v

I.U rr :::)

I--

600~

Annealing

~1 ~7~176176 to 400~

Furnace airflow arrangement. (Courtesy DuPont Electronics.)

\

Belt

210


ii Da0er

Exhau

\m0er

I I

~ Q

Entrance--~air I Q ~ e e) curtain I Burnoutair

I

100~ (~

Firing air / (~ ~

I

400~

I

100oc

Air in Air flow direction 1. Burnout air PLAWS 2. Firing air 1.3 x PLAWS 3. Adjust exhaust vent damper

Figure 9.25

I

400oc

850~

Exit air curtain

4. Smoke test at exit 5. Set exit curtain to block room air 6. Smoke test at exit

Furnace air flow. (Courtesy DuPont Electronics.)

~.

Burnout Air Supply

Exhaust Venturi

Burnout Air Rake i( r

~,~ I , ] k l ' ~ % . ~ . ~ ~ ~ l ~ h ~ Firing Air Direction . . . . . . . . . . . --"---"-. . . . . . . . . . . . . . . . . . . . . . _-1-_e_.._______/ B t___D r____e i i___o_n____-~ ct . . . . . . =-.-----=-,

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~ ~l.._Exhaust Venturi

Burnout Air Supply

Im

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Im

m

m

m m

m m

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m

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m

Direction m m

m

m

m m m

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m

m

m

m

m

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ml

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J

Vent Stack l

Air Direction Burnout& Firing Air Direction

|__l__l__m.~im~l__l)__llm~l__l__l__l__l.~m__l__l__l)i

Figure 9 . 2 6

i

m.~l.._mmmm~

Alternate airflow arrangement. (Courtesy DuPont Electronics.)

Thick Film Technology 21 1

From 700-300~ on the descent slope of the profile, the glasses solidify and anneal. The rate of descent should be kept at 50-85~ to prevent stresses from accumulating in the fired films. These are most critical in resistor films since they can seriously affect stability and temperature coefficients of resistance. This has been a brief description of what happens during air firing. The precise shape of the profile is not important; it may vary from furnace to furnace. But most important is that once a profile has been established it must be duplicated as closely as possible to obtain the best and most reproducible results. The copper material system is fired in nitrogen. Its processing has already been covered briefly in the section on materials. Nitrogen firing is most common for the CMS systems, but other atmospheres and dopants have been found effective and in some cases superior to pure nitrogen. 9. 7. 3 Laser Trimming

Thick film resistors are usually printed and fired to values below the desired final value, with a fairly wide as-fired distribution. The resistors are then adjusted to value using a cutting process which usually reduces the width of the resistor, causes the resistance to increase, and the distribution to decrease. Laser trimming is a widely accepted method of adjusting resistors to their final value using a high intensity light beam which rapidly heats the resistor material and causes it to vaporize. The speed at which resistors can be laser trimmed increases in importance as more emphasis is placed on the high volume manufacture of resistor networks and hybrid circuits. Before beginning a discussion of laser trimming and faster processing, it is worthwhile to review some definitions, to examine specifications differences between laser trimming systems, and to establish some guidelines. A basic laser trimming system consists of a laser element, a computer control system, a resistor probing and measuring system, and a mechanism to move the laser beam from point to point across the substrate surface. Most of the trimming systems used in thick film processing are solid-state lasers using neodymium-doped yttrium aluminum garnet (Nd:YAG) laser rods. The rod is placed in a gold-plated, elliptical cavity with a DC krypton arc lamp. Normally, the laser produces a continuous wave of moderate to low power. To achieve the high peak power necessary to vaporize thick film resistors, giant pulse operation is required. This is achieved by adding an acoustic Q-switch to the system. A Q-switch is an electrically driven acousto-optical quartz block that interrupts the laser beam and causes large amounts of energy to build up. When the Q-switch is in phase with the laser beam path, it allows large energy pulses to be released at a frequency controlled by the RF power supply used to drive it. Pulses with peak power in excess of 25 kW can be produced at pulse rates of 1-50 MHz. A comparison of two commonly used lasers is given in Table 9.7. A block diagram of a laser system is shown in Figure 9.27; a cross section of the laser cavity is shown in Figure 9.28. The elliptical shape assures all the light emitted from the arc lamp will be concentrated on the laser rod for maximum

21~

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o

o

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~

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12-

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--

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Thick Film Technology 21 3

Table9 . 7

Comparison of two common lasers.

Average multimode power (W) Average TEMoo power (W) Multimode beam diameter (mm) Repetition rates (kHz) Rod size (mm)

Laser A

Laser B

30 4 4.0 1-50 4 x 50

50 6-8 4.0 1-50 4x50

Figure9.28 Cross section of elliptical laser pumping cavity; all light emitted from the lamp will focus on the rod. (Courtesy DuPont Electronics.)

TEMoo

TEMol

TEMol

TEMo3

TEMo4

8b TEMo2 Figure 9.2 9

Laser spot shapes.

efficiency. When the krypton arc lamp current is kept at reasonably low levels (16-18 A), the spot shape of the laser beam remains in its fundamental mode, TEMoo. Its diameter is 1.5-2 mm. Increasing the lamp current causes the beam to diverge into multimode operation TEMol, TEMo2, and so on. This increases the diameter of the beam and lowers its intensity. Figure 9.29 shows some of these laser spot shapes. The preferred mode of operation is TEMoo. In this mode the

214

MATERIALSFOR ELECTRONIC PACKAGING

energy of the beam has a Gaussian distribution. Although successful trims have been made using multimode operation, TEMoo mode is recommended for maximum control and precision. Two process variables have been introduced: lamp current influences average power produced by the laser beam and acoustic Q-switch frequency dictates pulse frequency or the number of pulses per second which will be produced. These are two of the three variables controlled during the laser trimming process. Figures 9.30 through 9.33 show the relationship of peak power, average power, and pulse width as a function of pulse frequency for 30 W and 50 W lasers. The 30 W laser produces approximately 4 W of TEMoo power and it decreases rapidly with increasing pulse frequency. Laser trimming thick film resistor material requires Peak Pulse Power

Peak Pulse Power 10.0

Pulse Width

Average Power

" (nsec)

(kW)

8.0 6.0

i ~ -

4.0

" q ~ Pulse Width

,.

2.0

9.30 Electronics.)

Figure

(W) 5.0

- 400

4.0

300

3.0

- 200

2.0

100

1.0

-

1.0 2.5 5.0 10 15 20 Repetition Rate kHz 30 watt laser, TEMo.o

Average Power

25

Repetition rate for 30 W laser, 2-10 kW peak power. (Courtesy DuPont ~~

~.~.s.

60 "~/.//"

50 L (D

~176176176

Average p~

- 600

30

1 500

25

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5.0

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10

I

15

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20

5

25

Repetitionrate kHz 30 watt laser,multimode Figure 9 . 3 1

Electronics.)

Repetition rate for 30 W laser, 10-50 kW peak power. (Courtesy DuPont

Thick Film Technology

Peak Pulse

21 '3

Pulse Average

Power (kVO

Average Power

Inset) (W)

10.0

50O 8.0

8.O

4O0 6.0

6.0

300

4.0

4.O

200

2.0

2.0

100

1.0

1.0 2.5 5.0 10 15 Repetition Rate kHz

20

25

50 watt laser, TEMo,0

Figure 9.32 Electronics.)

Repetition rate for 50 W laser, 2-10 kW peak power. (Courtesy DuPont

Peak

Pulse Power (kW) -

50 _ 40

~

Pulse Average l WK:IIh Power "1 (nsec) (W)

Peak Pulse ~ " Power

Average Power

500

50

400

40

300

30

30 20

Pulse Width

200 20 100

10 1.0 2.5 5.0 10 15 Repetition Rate kHz

20

10

25

50 watt laser, multimode

Figure 9.33 Electronics.)

Repetition rate for 50 W laser 10-50 kW peak power. (Courtesy DuPont

high peak power. For maximum resistor stability, the peak power should be maximized and the average power minimized. This assures that most of the energy is used to vaporize and remove the resistor material and less energy is lost in undesired heating of adjacent resistor and substrate material. Once the pulses exit the laser, they must be directed to the surface of the substrate. They must then be moved into position to adjust the resistors and cross the substrate from resistor to resistor. Two main methods are presently used. One method uses galvanometer driven x and y mirrors, very responsive and capable of

216


accelerations up to 1960 m / s 2 (2009). The other method uses linear accelerators which are directly coupled voice coil actuators; they have very high positional accuracy and repeatability, but since they also have considerably more mass their acceleration is approximately 196 m/s 2 (209). The galvanometer driven mirror system is therefore capable of higher trimmin9 speed, the third process variable. Incorporated into the beam positioner is a focusing device to concentrate the beam energy at the surface of the substrate. The focusing lens for thick film trimming optics has an optical rating equivalent to an f/30 setting on a camera lens. This allows the laser beam to be focused to a minimum theoretical waist of 30 #m. The practical minimum spot size achieved is 50 #m in diameter. The depth of field for these optics is approximately 400 #m. It is therefore important to keep the beam in sharp focus and to re-focus when changing from one substrate thickness to another. An aperture is generally used to filter the beam as it exits the laser; this facilitates focusing. The most commonly used aperture diameter is 1.5 mm. Focus and aperture are two setup variables of the laser trimming process. Producing clean laser cuts begins with proper processing of the thick film resistors to be cut. Film thickness is particularly important. Laser trimming is a physical process. Material is removed by vaporization. The thicker the film, the harder it is to vaporize cleanly with the available energy. If the film is too thin, resistance is difficult to control and the results are excessive resistance value distributions and erratic TCRs. In addition to the thickness of the resistor film, the film interacts with the substrate to a depth of 4-6 #m. This interaction zone must also be removed to achieve maximum resistor stability. The integrity of the remaining film and the stability of its resistance value are dependent on the cleanness of the kerf and the amount of damage which has been done. Excessive damage occurs when too much power, at less than vaporization level, is allowed to affect a given area. It is therefore important to optimize the peak energy for each pulse and the speed at which the pulses travel across the surface of the material being trimmed. Figure 9.34 illustrates the mechanics of the laser cutting process. The two conditions shown may be produced by trimming speed and/or pulse frequency. Figure 9.34(a) shows a cut with 50% overlap. Figure 9.34(b) shows a cut using less than 10% overlap. With less than 50% overlap, a ragged, irregular cut of insufficient depth often results. The minimum overlap should be 50%. Overlap is also important for accurate adjustment of thick film resistors. With 50% overlap and a focused pulse of 50 #m in diameter, each pulse takes a 25/~m bite out of the resistor. At this rate of removal, it is quite possible to overtrim a resistor. In Figure 9.35, two resistor geometries are compared. One is 2.5 mm x 2.5 mm and the other 1 mm • 1 mm. When the 1 m m 2 resistor is raised to a value 1.4 times its fired value, one extra pulse of 25 #m in length raises its value an additional 3.6%. Its value is increased 2.5 times its fired value, one extra 25 #m bite would raise its value an additional 10%; the overlap must be much less than 50% to achieve close tolerances. Bite sizes of 2.5-7.5 #m are most common in the industry. For this discussion, a maximum bite size of 8.25 #m or 120 pulses/mm is recommended.

Thick Film Technology

(a)

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N 88'.9 100

Figure

9.35 Trim characteristics of plunge cuts on one square resistor. (Courtesy DuPont Electronics.)

There are several ways of cutting resistors. Figure 9.36 illustrates six methods of trimming square or rectangular geometries. The single plunge cut is the fastest and the most often used for reasonable accuracy and stability. When greater initial precision is required the L cut is preferred. For maximum precision and stability, multiple cuts are recommended with a delay between the first and second cuts. The usual way to achieve the delay is to make all the first cuts on the circuit then all the second cuts. The first cut is made to 98% of value and the second cut to the desired cutoff value, usually the low tolerance of the final resistance value.

218


Plunge C u t "

Double Plunge Cut "L"-Cut. With Vernier '

"L"-Cut

t

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Laser cutting techniques. (Courtesy DuPont Electronics.)

The process control variables of speed, power, and frequency can be related by defining a new term, cutting power. It encompasses all three process variables and allows us to achieve maximum stability by relating the experimentally determined cutting power to speed, power, and frequency: P = average beam power in Q-switched mode Q = pulse frequency S = trimming speed

P/S = linear energy density (LED) (in J/mm) Q/S = linear pulse density (LPD) in pulses/mm Cutting power(CP) = LED • LPD in J pulses/mm 2 In a study using high reliability thick film resistor compositions, low ohm material (100 ~/sq.) and high ohm material (10 k~/sq.) were cut at various average beam powers, pulse frequencies, and trimming speeds. A 1 mm 2 resistor was cut completely through and insulation resistance (IR) was measured at 200 V. It was determined that an initial IR greater than 2 G ~ was necessary to produce cuts which would be stable after 100 h storage. Results of the study are shown in Figures 9.37 and 9.38. For the 100 ~/sq. material, a cutting power of 10 J pulses/mm 2 is necessary to produce stable resistors. The 10 ~/sq. material requires a cutting power of 6 J pulses/mm 2. Using the recommended 120 pulses/mm for linear pulse density and the cutting power determined experimentally, it is possible to calculate the pulse frequency and the average power necessary for any desired trimming speeds. Tables 9.8 and 9.9 show examples of these calculations. Laser trimming of thick film resistors requires an understanding of the


_.

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( C U R E D G E L ) - (I) * *

Figure 12~

Table 12.3

Excess Hydrides Reactive "Pr' Calalyst

Heat-curable silicone: hydrolyzation additional cure mechanism.

RTV silicone formulations.

hlgredients Base polymer: OH-terminated siloxane Crosslinker: (OMe)sSiMe Catalyst: titanate Filler(s): SiO 2

TiO2 Carbon black Solvent: xylenes

Concentration (PHR)

Impact on properties

100 8-12 0.5

Mechanical properties Mechanical properties Curing, shelf-life

10-12 2-5 0.2-0.4 50-100

Rheological control Light screening Light screening Rheological control

288


blends in with the higher viscosity vinyl resin to achieve a desired mixing ratio of part A (only vinyl portion) and part B (hydride plus some vinyl portion). The key to formulating a low modulus silicone is the deliberate undercross-linking of a silicone system with low reactive functional groups. A few ppm of a premixed platinum catalyst system (platinum coordinated with 2-methyl-3-butyn-2-ol) is used to formulate a one-component silicone gel that requires less mixing.

12.5 Temperature Humidity Bias (THB) Testing Temperature humidity bias (THB) testing was performed with a triple track resistance measurement. I grounded the two outer tracks, biased the center track, and measured the leakage current change of the center conductor line. Good encapsulants show very low leakage over long testing times. Typical test conditions were 85~ 85% relative humidity, and 10-180 V bias. Of all the tested encapsulants, silicone had one of the best THB electrical performances. However, other high purity encapsulants, such as silicone-polyimide, benzocyclobutene (BCB), polyimide, and Parylene, also have relatively good electrical performance (Fig. 12.7).

12.6 Temperature Cycle Testing To be an effective encapsulant, a material must possess excellent temperature cycling properties. The siloxane backbone of the silicone provides good thermal stability (continued use temperature ~ 150~ among all potential elastomers. ---

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WAVENUMBERS

i

1000

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1 3.3 Infrared spectra of PFA film at different temperatures: ( - - ) 150:~C, and (........ ) 250~C.

Figure

I

..

600

23~

(...... )

296


(a)

(b) Figure 13.4 Confocal laser microscopy assessment of PFA film at 10 pm depth from the specimen surface: (a) 23~ specimen and (b) 250~ specimen.

Dielectric Films for High Temperature, High Voltage Power Electronics 2 9 7

crucial. Microvoids in PFA and PPX were identified by confocal laser microscopy. Confocal images of PFA (Fig. 13.4) obtained at 10 pm below the surface confirmed the presence of submicrometer voids that expanded when the film was preheated to 250~ The growth in size of voids could contribute to a higher dielectric loss at high temperatures, as voids are usually the major reason for partial discharge inception and failure in dielectric materials [19]. 1 3 . 3 . 1 Electrical

Nondestructive dielectric characterization included relative permittivity and dielectric loss measurements as a function of frequency and temperature; destructive characterization included dielectric strength measurements at 60 Hz. Repeated measurements showed very little deviation, so for greater clarity in Figures 13.5 through 13.8 we report only a single data point at each frequency and temperature. Conversely, the statistical deviation of the breakdown measurements leads us to report the mean of seven data points. The variation in relative permittivity of the specimens with increasing frequency at room temperature is shown in Figure 13.5. It can be seen that none of the materials exhibits any noticeable change in its relative permittivity with frequency. Figure 13.6 shows the effect of temperature on the relative permittivity of the three films. PFA displays good stability with temperature but the other films exhibit modest changes in their permittivity. The permittivity of PBI seems to increase initially then to remain constant with increasing temperature. The permittivity of PPX remains unchanged up to 100~ then undergoes a slight

4.2 0 PFA [] PPX A PBI

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Frequency (Hz) Figure

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RelativepermittivityofPFA,PPX, andPBIasafunctionoffrequencyat22~

298


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100

150

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200

250

300

Temperature (oC) Figure 13.6 Dependence of relative permittivity of PFA, PPX, and PBI on temperature while stressed at 200 V, 60 Hz.

10 0

10-1

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Frequency (Hz) Figure 1 3 . 7 Comparison of dielectric loss of PFA, PPX, and PBI as a function of frequency at 22~C.

Dielecfric Films for High Temperature, High Voltage Power Elecfronics 2 9 9

10 0

A

A

[]

[]

10-1 (/) t/) 0 _J O

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I

50

I

100

I

150

I

200

I

250

3(X)

T e m p e r a t u r e (0C)

Figure 13.8 Dielectric loss of PFA, PPX, and PBI as a function of temperature while stressed at 200 V, 60 Hz.

increase as the temperature is further raised. Polymers in general tend to soften at high temperature and could undergo some degradation which may, in turn, contribute to a change in permittivity [18]. The presence of electrical stress might have contributed to the variation in this property [19]. The dielectric loss of the films as a function of frequency is plotted in Figure 13.7. PBI demonstrates the highest dielectric loss of the three films tested, but is found to be the most stable over a wide frequency range. The dielectric loss of PFA increases by about one order of magnitude with increasing frequency. And the dielectric loss of PPX decreases as the frequency rises to 10 kHz, then beyond this value it starts to increase modestly. The influence of temperature on the dielectric loss is shown in Figure 13.8. PPX displays the largest change in its dielectric loss with an increase in temperature, whereas PFA and PBI exhibit only slight increases. PBI shows the greatest loss of the three materials, whereas PFA exhibits the lowest loss at high temperatures. The increase in the dielectric loss is generally attributed to an increase in free carrier concentration, which often accelerates the breakdown phenomenon [20]. And, when the film is preheated, some of the voids expand and grow in size up to a few micrometers (Figure 13.4(b)). The presence of voids is also believed to be responsible for an increase in the dielectric loss at high voltages [21]. So for any film, the presence of voids appears to increase the dielectric loss at high voltages and high temperatures. The dependence of dielectric strength on test temperature is shown in Figure 13.9. The breakdown sites were randomly distributed over the surface of the test

300


300

E 250 :::I.

e--

200

-

A

-

[]

_

0

A

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a

1~

A []

c -.-.

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c

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5o 0

0

I

I

I

I

I

I

50

100

150

200

250

300

350

Temperature (oC) Figore 1 3.9 Temperature dependence of AC dielectric strength of PFA (25.0 l~m), PPX (25.0 ltm), and PBI (37.0 ltm). specimen at all temperatures. The data obtained show that PFA in particular undergoes a significant reduction in its breakdown voltage with an increase in temperature. The reduction in dielectric strength of PFA is about 50% at 250~ compared to its strength at room temperature; PPX and PBI do not exhibit such a large change. Most often, the decrease in dielectric strength can be attributed to the softening of the polymers when exposed to high temperatures [22]. A plausible explanation for the negative temperature dependence goes like this. When the applied voltage is raised, more energy is stored in the sample and more dielectric loss is converted into heat. This heat raises the sample temperature which cannot be dissipated outside due to the higher surrounding temperature. Consequently, at one instant, the so-called critical temperature, the leakage current increases rapidly and leads to the breakdown of the sample [23]. Therefore, thin dielectric films used in microelectronics are susceptible to runaway thermal breakdown. 1 3.4

Summary

The results obtained from the present studies on the three films (PFA, PPX, and PBI) indicate few changes in their key properties at high temperatures and high voltages. Their permittivities remain unaffected when the frequency is increased to 100 kHz. PBI displays the highest relative permittivity, 3.4, and PFA shows the lowest permittivity, 2.1. The permittivities of PPX and PBI films exhibit a modest positive temperature dependence when the temperature is raised. In comparing dielectric losses of the materials as a function of frequency,

Dielectric Films for High Temperature, High Voltage Power Electronics 301

PFA shows an increasing trend and PPX exhibits a little decrease. On the other hand, the dielectric loss of PBI remains constant when the frequency is increased, but its dielectric loss is the highest. Nevertheless, all three materials display different increasing trends for dielectric loss with temperature. The dielectric loss of PPX increases by one order of magnitude; and for PFA and PBI, it goes up by approximately half an order of magnitude. In PFA, dielectric strength, a key property for high voltage applications, shows strong negative temperature dependence. But in PPX and PBI, dielectric strength remains relatively stable with an increase in temperature up to 250~ It is interesting to note that PBI exhibits a higher dielectric strength than the other two materials at any test temperature in the temperature range 23-250~ Good dielectric properties, especially with a lower dielectric loss, could make PFA film more viable for low voltage, high temperature applications. However, the stability of the dielectric strength of PPX and PBI at high temperatures could make them more useful for high voltage, high temperature, power applications on earth and in space.

Acknowledgment This work was supported by the NASA Lewis Research Center under grant NAG3-1019.

References 1. A.K. Hyder, Jr., P.J. Turchi, and H.L. Pugh, in Proc. AFOSR Special Conf. Prime Power for High-Energy Space Systems, Norfolk IrA, 1982. 2. E. Sugimoto, IEEE Electrical Insulation Mag. 5(1), 15-23 (1989). 3. R.J. Jensen, in Chemical Engineering in Electronic Materials Processing, edited by D.W. Hess and K.V. Jensen, ACS, Washington, 1988. 4. S.D. Senturia, in Polymers for High Technology, edited by M.J. Bowden and S.R. Turner, ACS, Washington, 1987. 5. E. Kuffel and M. Abdullah, High Voltage Engineering, Pergamon Press, New York, 1981. 6. V.C. Truscello and H.S. Davis, IEEE Spectrum, Dec. 1984, pp. 58-65. 7. H.W. Brandhorst, "Power Technology DivisionBAn Overview of Industrial Reviews," a workshop at NASA Lewis Research Center, Cleveland OH, Feb. 1988. 8. A.N. Hammoud, E.D. Baumann, I.T. Myers, and E. Overton, in Trans. 1st International High Temperature Electronics Conf., Albuquerque NM, 1991, pp. 11-16. 9. F.M. Ott, S.P.S. Yen, and R.B. Somoano, IEEE Trans. Electrical Insulation, 20(1) 47-54 (1985). 10. F.J. Campbell, NRL Review, July 1989, pp. 117-118. 11. J. Van Laak, "Kapton Wire Concerns for Aerospace Vehicles, Wiring for Space Applications," a workshop at NASA Lewis Research Center, Cleveland OH, July 1991. 12. "Aircraft Wire Hazard Reported," Chicago Tribune, July 25, 1988. 13. "Teflon PFABFluorocarbon Resins," DuPont Properties Bulletin E80419, Dec. 1986.

302


14. "Abrasion Resistance of Parylene and other Conformal Circuit Board Coatings," Nova Tran, Wisconsin, Bulletin NTC#400-0114-00, 1986. 15. J.F. Jones, J.C. Waldrop, and R. Fountain, in Proc. 29th National SAMPE Symp., 1984, pp. 777-783. 16. E.J. Powers and G.A. Serad, in High Performance Polymers: Their Origin and Development, edited by R.B. Seymour and G.S. Kirshenbaum, Elsevier, New York, 1986. 17. J.R. Lagari et al., IEEE Electrical Insulation Mag. 2(6), 16-20 (1986). 18. P.J. Phillips, in Engineering Dielectrics, Vol. IIA, ASTM, 1983, Ch. 2. 19. R. Bartnikas, in Engineering Dielectrics, Vol. IIA, ASTM, 1983, Ch. 1. 20. J.L. Suthar and J.R. Laghari, in Proc. Electrical Insulation and Dielectric Phenomena Conf., Leesburg VA, 1989, pp. 495-502. 21. R. Bartnikas, in Engineering Dielectrics, Vol. I edited by R. Bartnikas, ASTM, 1979, Ch. 1. 22. J.L. Suthar, J.R. Laghari, and W. Khachen, Proc. Electrical Insulation and Dielectric Phenomena Conf., Knoxville TN, 1991, pp. 244-249. 23. J.J. O'Dwyer, The Theorv of Electrical Conduction and Breakdown in Solid Dielectrics, Oxford University Press, London, 1973.

14 Electrically Conducting Polymers and Organic Materials M. J. Naughton

14.1

Introduction

Organic and polymeric substances are no longer thought of simply as electrically insulating materials. In fact, some of the most exciting and innovative research in physics, chemistry, and materials science can be found in the fields of organic, molecular, and polymeric conductors. In the past few decades, there has been a steady increase in the number of highly conducting, synthetic organic materials, highlighted in 1980 by the discovery of superconductivity in the quasi-one-dimensional charge transfer salts (TMTSF)2X. Since then, organic conductors have been shown to possess an astonishing variety of electronic, magnetic, and optical properties, including some not observed in any other solidstate material. This chapter considers recent advances in the organic conductors and looks at several families of organic superconductors. As well as their novel electric and magnetic properties, organic systems essentially exhibit all the basic conducting states recorded for inorganic materials (elements, alloys, and compounds); organic systems can be insulating, dielectric, semiconducting, semimetallic, metallic, superconducting, and so on. We distinguish here between polymeric and organic conductors because most of the organic materials we discuss are crystalline dimer salts (charge transfer salts), and their conduction processes are more conventional. There has also been progress in the conducting properties of formal polymers, for example, the room temperature conductivity of doped polyacetylene, [CH]x, now approaches that of copper. As yet there are no polymer superconductors, but polymers are generally recognized to hold more promise than molecular crystals, as their properties can be tuned by doping and they are much easier to process. We include in the general category of conducting organic and molecular materials two forms of carbon known to be electrically conducting: intercalated graphite and buckminsterfullerene (buckyballs). In suitably doped forms, both systems are known to superconduct, and fullerene transition temperatures exceed 30 K. 303

304


In microelectronics and thin film technology, organic and polymeric conductors still lag behind their inorganic counterparts like gallium arsenide. Sensing the potential for significant contributions, theorists and experimenters are hard at work. The interested reader can find several volumes of various conference proceedings on low dimensional conductors, superconductors, organics, and polymers, and more recently, molecular conductors. Perhaps the most informative sources are the ongoing series from the International Conferences on Synthetic Metals, held biannually [ 1]. These conferences are evenly split between the physical properties of conducting polymers and the physical properties of conducting organic crystals. Other useful sources are the monographs and edited collections listed at the end of this chapter [2]. Several thousand years ago, the Greeks were aware that certain nonconducting materials could hold electrostatic charge. Many of these materials were in fact organic polymers, such as amber, from which the word electron was later derived. This characteristic of organic polymers remains the norm, as we all know from the many plastics we encounter and use in everyday life. There has traditionally been a rather clear distinction between conducting and nonconducting materials: metals do and plastics don't. A computer keyboard is made of little plastic keys in a plastic molded case. Every time you type a letter, this nonconducting key is pressed down to activate a switch, with information transmitted (in the form of electrons) to the central processor via metallic wires, probably copper. You don't get electrocuted when you type because the plastic exterior of the keyboard is electrically insulating. We now understand why an insulator holds a charge, rather than moves a charge; it has no available free electrons for motion. While metals have plenty of free electrons (on the order of 1022 per cubic centimeter), insulators can have 10-20 orders of magnitude fewer. This insulating property of organic polymers is exploited in your computer and countless other situations. So, organic polymers such as rubbers and plastics are most familiar to us as nonconductors of electricity; they usually have the ability to store electric charge, but not to move it. Such is not always the case. Early attempts at making polymers conduct electricity involved mixing metallic material with insulating polymer fibers, forming composites. Some polymers were found to have ionic properties, but none could be considered electronically conducting. Then in 1973 polysulfurnitride (SN)x was found to be metallic [3] and two years later superconducting [4], discoveries which played important roles in promoting efforts toward the creation of useful conducting polymers. Most of the successful materials are polyenes, in particular polyacetylene, the polymer of acetylene. Other related systems include polyphenyleni~, polypyrrole, and the polyimides. There is now a very large effort throughout the world in basic and applied research on conducting polymers, research which requires the combined efforts of chemists, physicists, materials scientists and, in many cases, biologists. The combined effort to develop polymeric conductors is many times larger than to develop organic crystals, due mainly to the perceived technological advantages and versatility of polymers.

Electrically Conducting Polymers and Organic Materials

305

14.2 Organic Conductors and Superconductors Some of the first reports of electrical conduction in organic solids appeared in the late 1940s. Semiconducting behavior was observed in the phthalocyanine molecule [5,6] and in aromatic carbon structures (perylene) [7]; photoelectric conduction was observed in organic dye films [8]. Since then, we have reached just about one milestone each decade: synthesis of TCNQ in the 1960s, synthesis of TTF in the 1970s, discovery of organic superconductivity in the 1980s, and the first organic superconductor with a transition temperature above 10 K in the 1990s. Though a landmark in its own way, polysulfurnitride is inorganic. The organic content in a large number of present-day organic conductors is a sulfur- or selenium-based donor molecule (cation); electrocrystallization with an appropriate anion acceptor forms a charge-transfer salt, which may be organic or inorganic. In contrast to polymers, which form large macromolecules, these molecular solids retain as building blocks their original donor molecules. In general, electrical conduction results from g-electron molecular orbital overlap along near-neighbor Se or S sites, rather than involving directly the C ~ C or C ~ C bonds (i.e. single or double carbon bonds). The delocalized orbitals are arranged along stacks (quasi-one-dimensional, quasi-l-D) or in planes (quasi-twodimensional, quasi-2-D), leading to anisotropic conductivity, conductivity that depends on measurement direction. In the past 20 years, starting with the salt TTF-TCNQ (tetrathiofulvalenium-tetracyanoquinodimethane), many organic salts with partially filled conduction bands, responsible for their large electrical conductivity, have been synthesized. The building blocks for all these organic conductors have long chemical names, which are routinely abbreviated for common usage. The best-known molecules are depicted in Figure 14.1. These salts can be rather easy to grow. With relatively inexpensive equipment, respectable quality (i.e. millimeter to centimeter size) single crystals can be synthesized in about a week. But there are difficulties with the exploitation of organic conductors for electronic packaging and other applications. It has proven quite difficult to design a route toward the fabrication of a processible material, such as a thin film. The nucleation process in the electrocrystallization is very poorly understood, and some methods employed for polymer films, such as the LangmuirBlodgett technique, have yielded little success. This is one obstacle which requires a concerted, interdisciplinary effort to overcome. To date, there are only a handful of families, or basic molecules, from which highly superconducting organic materials are synthesized. These are based on the donor molecules TMTSF, tetramethyltetraselenafulvalene; BEDT-TTF, bisethylenedithiotetraselenafulvalene, DMET, dimethyl(ethylenedithio)diselenadithiafulvalene, MDT-TTF, methylenedithio-tetrathiafulvalene, and the acceptor system M(dmit)2, metal-bis(dimercaptodithiolethione). The terms donor and acceptor refer to electron transfer, hence charge transfer salts. The first four of these molecules .form conducting solids by acting as electron donors (the cation), with an anion species acting as the acceptor. Collectively, there are nearly 50 organic supercon-

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ductors. There are several other organic-based materials which are not such good electrical conductors, instead they have special magnetic properties such as ferromagnetism. An example is a molecular magnet based on the TCNE (tetracyanoethylene) molecule [9]. A common property of all organic conductors is their low dimensional structure. For this reason, they have been a boon to both experimental and theoretical scientists, providing physicists with real physical systems in which to test existing theories concerning electron motion confined to one and two dimensions, encouraging chemists to experiment with bond arrangements in the molecular structure in order to obtain favorable physical or chemical properties, and enlisting materials scientists to consider the mechanical and processing properties of the new materials. Like nearly all endeavors in materials science, an interdisciplinary effort has developed in the recent past, and indeed is desperately needed in the immediate future, for the study and exploitation of organic conductors.

Electrically Conducting Polymers and Organic Materials 3 0 7

14.2.1 TMTSF Compounds The world's first organic superconductors were found in the TMTSF family, discovered almost two decades after William Little's 1964 prediction [10] of a mechanism for high temperature superconductivity in polarizable quasi-l-D polymers. This family of organic metals was first synthesized in 1980 by a Copenhagen team under Klaus Bechgaard [11] and the salts now bear his name. They are quasi-l-D, 2:1 charge transfer compounds, with (TMTSF)2 + combining with a monovalent anion such as AsF6-, PF6-, C104-, ReO4-, NO3-. In these crystals, the TMTSF molecules stack up like poker chips, providing a more or less linear path for n-orbital conduction along the selenium atoms. A n-orbital is a way of describing the shape of the electron trajectory (actually its probability density) with respect to the molecule. The nomenclature has to do with the angular momentum of the molecular orbital, which itself is formed from individual atomic orbitals. The n-orbitals in the TMTSF system look like airplane propellers projected out of the plane of the TMTSF molecule. If a portion of a propeller near molecule A overlaps with that near an adjacent molecule B, then the electron in orbital A can move to orbital B, and so on. In these materials, the overlap along the stack direction is large enough to facilitate metallic conduction in this direction (in fact, these n-orbitals form what are called a-type overlap, in that the propeller arms touch end to end, rather than side to side). The orbital overlap in the two perpendicular directions is significantly reduced compared to the stacking direction, hence the quasi-l-D description. However, one of these two perpendicular directions has approximately 10 times the conductivity of the other, as a result of significantly smaller Se-Se contact distances, so these materials can also be considered as quasi-2-D (the molecular stacks along the a-direction form sheets with repeat units along the b-direction, yielding 2-D a-b planes). This is an important consideration when these metals are placed in a large magnetic field at low temperature, where most of the interesting physics is observed. The overall conductivity anisotropy is given by aa:ab:ac= 105:103:1, which means that electrons flow 10000 times more easily along the a-axis than along the c-axis. While the room temperature conductivity of single crystals of the (TMTSF)2X conductors is metallic (103 to 104 S/cm, which is about a thousand times less conducting than copper), many members (i.e., salts of different anions X) undergo metal-to-insulator transitions at cryogenic temperatures, in the range 10-100 K. This is due to their quasi-l-D character, which leaves the system susceptible to electronic instabilities such as charge density waves or spin density waves. Known as Peierls transitions, they can be described rather naively by considering a real one dimensional chain. If you break one link in a chain, the continuity is lost. If each link represents a molecule, breaking a link is like breaking a bond; it prevents electrons hopping from one molecule to the next and it produces an insulator. In a two-dimensional lattice structure, several bonds or links can be broken before continuity is lost at a percolation threshold. It was only after increasing their 3-D character by the application of large hydrostatic pressure, on the order of several thousand bars (atmospheres), that Peierls transitions were suppressed and super-

308


C

4-

44Figure 14.2 The a - c plane and unit cell of (TMTSF)2CIO 4, the first ambient pressure organic superconductor.

conductivity was discovered in (TMTSF)2PF6 [ 12]. This significant discovery, also in 1980, occurred at Orsay, in the laboratory of Denis Jerome. The superconducting transition temperature (T~) was 1.3 K at 6 000 bar. The highly anisotropic crystal structure of this material is shown in Figure 14.2. Since that seminal discovery, which finally demonstrated the possibility of organic superconductivity, half a dozen other TMTSF superconductors have been found. All have superconducting transition temperatures in the vicinity of 1 K, but this is far from the original prediction of T~ near room temperature (~ 300 K). As it has turned out, the existence of superconductivity may not even be the most interesting property of the Bechgaard salts; they exhibit many other electronic and magnetic phenomena, all of them due to low dimensionality. (TMTSF)EPF 6 and (TMTSF)2CIO4 are the first and only bulk, crystalline materials to exhibit the quantized Hall effect, elsewhere observed only in very thin, in fact two-dimensional, artificially grown semiconductor systems such as GaAs and Si metal oxide semiconductor field effect transistors (MOSFETs). As it produces such constant plateaux in the Hall resistance of these semiconductors, the quantized Hall effect is now used as the world standard for resistance, the ohm, as well as for the most accurate determination of the fine structure constant ~, which plays an important role in the quantum theory of matter. Unique to TMTSF conductors, and perhaps their most interesting phenomenon, is the so-called magnetic field-induced spin density wave (FISDW). Similar to when a normal metal changes into a superconducting metal, this is an electronic phase transition induced by a large magnetic field aligned perpendicular to the highly conducting quasi-l-D chains and along the least conducting c-direction. The magnetic field causes a complete destruction of the nonmagnetic, metallic Fermi surface, yielding an antiferromagnetic SDW state of alternating spin alignment, T$ T~T$ T,L. In a low magnetic field the spins are oriented randomly, so the system is nonmagnetic. The field strengths for this transition are on the


309

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order of 10T (105 gauss, the field on the earth is about 0.5 gauss), and the temperature regime is below 10 K. Bechgaard salts show several other novel effects but we mention them only briefly. One is the mysterious destruction of this FISDW at yet higher fields, fields above 25 T [ 13]. It appears that the very same mechanism responsible for the creation of the field-induced spin density wave leads to its demise. The very high field state is thought to be a purely one-dimensional material, with each electron literally confined to a single chain. The experiments which led to the discoveries of these electronic states required the world's largest magnetic fields and the National Magnet Laboratory at MIT. Another new feature is the so-called commensurability resonance, or magic angle effect, in which a series of dramatic changes in the conductivity and magnetization occur as a specimen is rotated in a fixed magnetic field [14]. Much theoretical effort has been invested in understanding the many novel electronic phenomena seen in the Bechgaard salts, largely because physicists realize their fundamental origin is low dimensionality. This system thus provides a real physical testing ground for interactions between electrons, which are not easily observable in ordinary metals. Figure 14.3 shows a cumulative phase diagram of one of these organic conductors in magnetic field-temperature space. 1 4 . 2 . 2 BED?'-TTF Compounds

In TMTTF, the sulfur analog of TMTSF (Fig. 14.2), methyl groups (CH3) at the corners of the molecule can be replaced by ethylene (CH2) if the fourth carbon bond in the ethyl is satisfied by closing the end rings, forming a pair of C H 2 m C H 2 bonds. This is the structure of BEDT-TTF, or [(CH2)212C688, first

310


synthesized in 1983 by G. Saito and coworkers in Japan. While related to TMTSF in many ways, compounds formed with BEDT-TTF are dramatically different from the Bechgaard salts. First, they rarely exhibit linear chain stacking, instead they form infinite two-dimensional sheet networks. In fact, this was the original intention: to increase the dimensionality with respect to the TMTSF salts, in the hope of increasing the superconducting transition temperatures. Second, they crystallize in a variety of morphologies, with different molecular packing arrangements and stoichiometric possibilities. These lead to a wide range of conducting properties, from insulating to superconducting. Perhaps more importantly, the superconductivity [15] which has been discovered in these materials occurs with transition temperatures an order of magnitude higher than in the TMTSF salts [ 16]. They have transition temperatures approaching 15 K, excluding the fullerenes, the highest of the organic superconductors. There have been over 20 BEDTTTF-based superconductors synthesized so far, with contributions coming from groups in the United States, France, Japan, Germany, and Russia. Besides the great excitement over the consistent rise in superconducting transition temperatures in organic conductors, mainly but not entirely due to progress in the BEDT-TTF salts, there are some quite interesting aspects to the normal metallic state properties in (BEDT-TTF)zX. Due to the fact that metallic organic crystals can be grown with exceptionally high purity, and the dimensionality is so very nearly 2, there are very clear Shubnikov-de Haas (SdH) resistance oscillations de Haas-van Alphen (dHvA) magnetization oscillations in large magnetic fields. These are quantum oscillations that result from magnetic energy levels crossing the constant energy surface (Fermi surface) as a magnetic field is increased. Such measurements provide invaluable information on electronic band structure and the shape of the Fermi surface. Magnetic quantum oscillations have been observed in several (BEDT-TTF)zX salts, including X=I3, AuBr2, and Cu(NCS)/. In addition to these SdH and dHvA oscillations, a new magnetotransport effect has been observed in high magnetic fields in the BEDT-TTF salts. As a magnetic field is rotated about the sample, the electrical resistance is found to oscillate, periodic in the tangent of the angle between the field, and normal to the most conducting layers, i.e. tan 0 oc n, where n is an integer. It has been shown that this new effect is a direct result of the nearly cylindrical Fermi surface, a nearly perfect two-dimensional electronic structure. A representative member of the BEDT-TTF family of organic conductors is depicted in Figure 14.4.

14.2.3 Other Organic Superconductors In addition to the two families of organic conductors discussed so far, chemists have recently succeeded in synthesizing combination, or hybrid, molecules based on TMTSF and BEDT-TTF. You may have noticed that the skeletal core of each of these molecules has the fulvalene structure, with a double carbon bond followed by either a pair of CruSe or CmS bonds. If you were to attach the left half of the TMTSF in Figure 14.1 to the right half of the BEDT-TTF in Figure 14.3, you would still retain the fulvalene structure, but in a new hybrid molecule, DMET.


31 1

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Figure

14.4

C

(BEDT-TTF)2Xcrystal packing and the BEDT-TTF molecule.

This new family of materials was first discovered in 1987 by Kikuchi and coworkers [17]. Like both its parent molecules, D M E T charge transfer salts exhibit a variety of electrical properties. Many highly conducting 2:1 salts (DMET)2X have been made, including some superconductors, with transition temperatures in the range 1-2 K. A fourth family of organic superconductors is based on another hybrid molecule, MDT-TTF, with a maximum transition temperature of 4.5 K in (MDT-TTF)2AuI 2. Other hybrid molecular arrangements are made possible as new conducting compounds are synthesized. One of the potentially great advantages of the electrocrystallization growth process in organic conductors is the ability to try out new hybrids, which perhaps will lead to still newer conductors and superconductors. Two important differences separate our final class of organic conductors from those we've already discussed. The building block is an electron acceptor molecule instead of an electron donor and the center of the molecule is a transition metal such as Ni, Pd, or Pt. These materials were first synthesized in France by a group led by P. Cassoux [18-1, who intentionally designed an organic conductor with increased dimensionality with respect to (TMTSF)2X, in a search for new families of organic conductors and superconductors. The design worked, in that there are now a handful of superconductors [ 19] based on the new molecule, dmit, of the form X[M(dmit)2]2, where dmit is bis(4,5-dimercapto-1,3-dithiole-2-thione), M is nickel, platinum, and so on and X is a donor molecule such as TTF or (CH3)4N. Many of these materials have room temperature conductivity on the order of 10 3 S/cm, and are metallic to low temperature, reaching 10 5 to 10 6 S/cm at 10 K. However, due to an admixture of intercolumn and intracolumn interactions, the dimensionality of these materials is not well understood, and remains a bit

312


controversial. Nonetheless, the successful design and production of an organic superconductor is encouraging to researchers and bodes well for the future. There are many other related families of organic conductors, and while not necessarily superconducting, they continue to be of great interest to materials scientists. The fact that one can create through chemistry a highly conducting, or even superconducting, material using completely nonconducting constituent elements is a fascinating idea, one that will continue to encourage innovation in the design of new materials based on organic molecules.

14.3 Conducting Polymers As stated earlier, polymers are sometimes categorized apart from the organic crystals discussed above because of distinctly different atomic wavefunction overlap, or molecular orbitals. The overlap in most conducting polymers is an order of magnitude or two smaller than in the Bechgaard salts, and the basic conducting mechanism appears to be different. Below are brief reviews of some of the developments which have taken place in the last decade in the field of polymer conductors.

14,3,1 Polyacetylene Unlike polysulfurnitride, most polymers are inherently nonconducting, dielectric materials, the result of having completely (rather than partially) filled electronic shells. Pure polyacetylene, (CH)x, also known as polyvinylene, is one such insulator, with ambient temperature conductivity around 10 -8 S/cm, about one hundred trillion times lower than that of a good metal! The critically important discoveries that polyacetylene can be produced in film form [20], and that it can be made to conduct by stripping electrons from the chains which make up the polymer [21], thereby forming an ion, (CH)x-, opened the door to the reality of chemically doping (CH)x to form a conducting material. Solid polyacetylene is made by polymerization of gaseous acetylene in the presence of a Zeigler-Natta catalyst [22]. Recent methods [23] allow it to be grown as a film instead of a powder. The x in (CH)x refers to the number of repeat units of t h e - - C H - - C H - - building block. This number can reach 1 000, such that the molecular weight of this entity is nearly 10000. We mentioned above that the pure form is electrically insulating. Actually, it is an intrinsic semiconductor, which means there is a small amount of conduction in the pure form. At a sufficiently low temperature, the distinction between semiconductor and insulator is meaningless, since all the carriers of electronic charge will have frozen out, leaving no conduction. Upon doping, somewhat more traditional semiconducting behavior is found, albeit of an anomalous nature. Like silicon, polyacetylene can make a p-type or an n-type semiconductor, by introducing donors or acceptors which ultimately lie between the CH chains. In elemental or compound semiconductors, like silicon or gallium arsenide, these electron donors or electron acceptors can enter the lattice either substitutionally or interstitially. A phosphorous atom can


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The two common forms of polyacetylene, trans-(CH)x and cis-(CH)x, from

[2(b)].

replace a silicon atom, donating an extra electron to the lattice (p-type) or it can fit in between two silicon sites. The are two structural forms of polyacetylene, trans-(CH)x and cis-(CH)x, are shown in Figure 14.5. The trans form is more stable; the cis form reverts to the trans form above 180~ This material grows in the form of a fibrous film, with chains of CH aligned parallel to each other. The fibers have a typical thickness of 10-20 nm, and are completely entwined. This leaves significant voids in the solid, such that only about half of the volume contains (CH)x. So far, it appears to be completely insoluble. In 1987, Naarmann and Theophilou were able to obtain copper-like conductivity (a ~ 105 S/cm) by doping polyacetylene with iodine [24]. This was an increase of several orders of magnitude over the previous best [25], and has been attributed to improved synthesis and processing, yielding a more tightly packed, denser material. This improvement could have far-reaching consequences. In this so-called new polyacetylene, dramatic effects are observed in the thermal and electrical properties upon stretching and aligning the polymeric chains. The thermal conductivity can approach that of a dirty metal or of metallic alloys, inviting applications geared toward electronic devices, including packaging. A variety of physical properties of the new polyacetylene are now being investigated with such applications in mind. One problem with electric conduction in (CH)x old or new, is the actual mechanism is unknown. It appears the conductivity is limited by barriers in the material, such as defects and impurities. Still unknown and somewhat controversial are the intrinsic conduction process and the maximum conductivity. One possibility is that solitons provide the conduction mechanism, a situation wherein conjugational defects are responsible for charge transport.

314


! 4 . 3 . 2 Polyaniline

Another heavily studied conducting polymer system is the oxidized form of aniline, polyaniline, (C6Hr with 0 < y < x (y = hydrogen content). The most attractive property of this polymer is its relative stability in air and water. In addition, it can be prepared chemically or electrochemically. Lithium rechargeable batteries employing polyaniline are already available. Bridgestone [26], a Japanese company, makes them as secondary or backup power sources for Seiko watches. Recent studies of this old material [27] have shown that, by adding protons to the base polyaniline structure, the conductivity can change by nearly 10 orders of magnitude, without actually altering the electronic content [28,29]. This type of insulator-to-metal transition has generated much interest from materials scientists. With this added degree of control, polyaniline stands as the only polymer whose conductivity can reversibly be controlled by two independent variables: oxidation and protonation. There are three interchangeable forms of polyaniline: the leucoemeraldine base (fully reduced, y = x), the emeraldine base (y = x/2), and pernigraniline (fully oxidized, y = 0). Ultimately polyaniline systems are expected to be an easier path toward conducting polymers for electronic applications, but there remain serious difficulties and uncertainties. Foremost among them are the actual conduction mechanism and the structure. Polaron, bipolaron, and glass-like conduction mechanisms have been proposed, but the question is yet to be answered. Another interesting property of polyaniline is its electrochromicity; thin films can be reversibly switched from clear to blue, to green, or to purple [30]. Although the switching speeds are slow (~0.1 s), there appears to be significant potential for this application, but before they can be realized, like all other polymers, improvements in processibility are required. 1 4 . 3 . 3 Molecular Conductors C60

The exciting 1991 discovery of high temperature superconductivity in doped C6o, buckminsterfullerene, has opened a new door to molecular engineering. Less than one year after its isolation chemists are able to perform what amounts to molecular surgery on this molecule, attaching other organic species to the spheres, doping the lattice with inorganic elements to form metals, even placing helium atoms inside the C6o sphere. In fact, there exists a series of fullerene-type structures, including Cvo and Cv6. Besides the parent sphere, colloquially known as the buckyball, another possible structure is the buckytube. Applications may be somewhat distant, but C6o is an extremely attractive system in which to design and build new molecular materials.

14.4 Potential Applications of Conducting Polymers The potential for technical utility of polymer conductors is widely recognized but hardly realized in practice. Probably the most important feature to be exploited is the systematic variability of the conductivity, whether by doping pure polymers,


31 5

or by forming composite materials such as graphite-polyethlyene. Already, polymers span the entire region of electrical conductivity, from Teflon (PFTE) and Kapton (polyimide), with conductivities close to those of quartz or diamond, to polyacetylene, polypyrrole, and graphite, similar to silicon of dirty metals. The ultimate goal is to unite the technological advances in electrical and optical properties achieved in semiconductors with the mechanical and processing advantages perceived in polymers [31,32]. Recent advances [1,2] include: Fabrication of polythiophene transistors, metal insulator semiconductor field effect transistors (MISFETs), a p-n heterojunction device with interesting electro-optical properties Increases in solubility of various polymer blends and gels important for thin film processing Improvements in environmental stability of a wide variety of polymer conductors Fabrication of solid-state polymer batteries Applications toward industrial waste degradation employing polymers Fabrication of gas separation membranes with polyaniline Production of polymer glasses with new electro-optical properties Improved routes toward new photovoltaic energy conversion devices (solar cells) and electric power distribution conductors Conducting polymer electromagnetic interference (EMI) shielding devices Developments toward large area, color display panels using electroluminescent polymer dye films. Most of these advances have taken place in the past five years and have yet to reach the market. But it is quite likely, if not inevitable, that conducting polymers will permeate our culture just like other plastics. As materials scientists improve their manipulation of conducting properties, new science and technology will emerge. The distinction between what conducts and what doesn't is long gone. References

1. International Conferences on Science and Technology of Synthetic Metals 1979 Dubrovnik, Springer Lecture Notes in Physics 95, 96, Springer (1979). 1981 Boulder, Mol. Crvst. Liq. Cryst. 77,79,81,83,85,86 (1982). 1982 Les Arcs, J. de Physique (Paris) 44, Colloque C3 (1983). 1984 Albano Terme, Mol. Crvst. Liq. Crvst. 117-119 (1985). 1986 Kyoto, Synth. Metals 13 (1986). 1988 Sante Fe, Svnth. Metals 27-29, (1988, 1989). 1990 Tubingen, Synth. Metals 41-43, (1991)i 1992 Gothenburg, Synth. Metals (1993). 2. (a) Handbook of Conducting Polymers, Vols. 1 and 2, edited by T.A. Skotheim, Marcel-Dekker, New York, 1986. (b) T. Ishiguro and K. Yamaji, Organic Superconductors, Springer Series in Solid-State Sciences 88, Springer, Berlin, 1990. (c) Low Dimensional Conductors and Superconductors, edited by D. Jerome and L.G. Caron, Plenum, New York, 1987.

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3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19.

20.

(d) J.R. Ferraro and J.M. Williams, Introduction to Synthetic Electrical Conductors, Academic, New York, 1987. (e) The Physics and Chemistry of Organic Superconductors, edited by G. Saito and S. Kagoshima, Springer, Berlin, 1990. (f) Electronic Properties of Conjugated Polymers, edited by H. Kuzmany, H. Mehring, and S. Roth, Springer Ser. in Solid-State Sciences 91, Springer, Berlin, 1991. (g) Electronic Properties of Polymers and Related Compounds, edited by H. Kuzmany, H. Mehring, and S. Roth, Springer Series in Solid-State Sciences 63, Springer, Berlin, 1985. (h) Electronic Properties of Polymers, edited by J. Mort and G. Pfister, Wiley, New York, i982. (i) Advances in Polymer Technology, Vol. 11, edited by M. Xanthos, Wiley, New York, 1991. (j) Polymers for Advanced Technology, Vol. 3, edited by M. Lewin, Wiley, New York, 1992. (k) Lower-Dimensional Systems and Molecular Electronics, NA TO ASI Series B." Physics 248, edited by R. Metzger, P. Day, and G. Papavassiliou, Plenum, New York, 1990. V.V. Walatka, M.M. Labes, and J.H. Perlstein, Phys. Rev. Lett. 31, 1139 (1973). R.L. Greene, G.B. Street, and L.J. Suter, Phys. Rev. Lett. 34, 577 (1975). A.T. Vartantan, Zh. Fiz. Khim. 23, 769 (1948) [J. Phys. Chem. (USSR) 22, 769 (1948)]. D.D. Eley, Nature 162, 819 (1948). H. Akamatu and H. Inokuchi, J. Chem. Phys. 18, 810 (1950); H. Akamatu, H. Inokuchi and Y. Matsunaga, Nature 173, 168 (1954). A.T. Vartanyan, J. Phys. Chem. (USSR) 24, 1361 (1950). O. Kahn, D. Gatteshi, J.S. Miller, and F. Palacio (eds.), Proc. Con[i Molecular Magnetic Materials, (NA TO ARWE198), Kluwer Academic, Amsterdam, 1991. W. Little, Phys. Rev. A 134, 1416 (1964). K. Bechgaard, C.S. Jacobsen, K. Mortensen, H.J. Petersen, and N. Thorup, Solid State Commun. 33, 1119 (1980). D. Jerome, A. Mazaud, M. Ribault, and K. Bechgaard, J. de Phys. (Paris) Lett. 41, L95 (1980); see also, K. Bechgaard and D. Jerome, Scientific American, July 1982. M.J. Naughton et al., Phys. Rev. Lett. 61, 621 (1988). A.G. Lebed, JETP Lett. 43, 174 (1986); T. Osada, et al., Phys. Rev. Lett. 66, 1525 (1991); M.J. Naughton, et al., Phys. Rev. Lett. 67, 3712 (1991). S.S.P. Parkin, E.M. Engler, R.R. Schumaker, R. Lagier, V.Y. Lee, J.C. Scott, and R.L. Greene, Phys. Rev. Lett. 50, 270 (1983). H. Urayama (Mori), H. Yamachi, G. Saito, K. Nozawa, T. Sugano, M. Kinoshita, S. Sato, K. Oshima, A. Kawamoto, and J. Tanaka, Chem. Lett. 55 (1988). K. Kikuchi, K. Murata, Y. Honda, T. Namiki, K. Saito, T. Ishiguro, K. Kobayashi, and I. Ikemoto, Jpn. J. Appl. Phys. 55, 3435 (1987). M. Bosseau, L. Valade, M.F. Bruniquel, P. Cassoux, M. Garbauskas, L. Interrante, and J. Kasper, Nouv. J. Chim. 8, 3 (1984); M. Bosseau, L. Valade, J.P. Legros, P. Cassoux, M. Garbauskas, and L. Interrante, J. Am. Chem. Soc. 108, 1908 (1986). L. Brossard, M. Ribault, M. Bosseau, L. Valade, and P. Cassoux, C.R. Acad. Sci. Ser. B 302, 205 (1986); J. Schirber, D.L. Overmeyer, J.M. Williams, H.H. Wang, L. Valade, and P. Cassoux, Phys. Lett. A 120, 87 (1987); K. Kajita, Y. Nishio, S. Moriyama, R. Kato, H. Kobayashi, W. Sasaki, A. Kobayashi, H. Kim, and Y. Sasaki, Solid State Commun. 65, 361 (1988). T. Ito, H. Shirakawa, and S. Ikeda, J. Polvm. Sci. Polvm. Chem. 12, ll (1974).


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21. C.K. Chiang, C.R. Fincher, Y.W. Park, A.J. Heeger, H. Shirakawa, E.J. Louis, S.C. Gau, and A.G. MacDiarmid, Phys. Rev. Lett. 39, 1098 (1977). 22. G. Natta, G. Mazzanti, and P. Corradini, Accad. Nazi. Lincei. Sci. Fis. Mat. Nat. 25, 3 (1958). 23. H. Shirakawa, M. Sato, A. Hamano, S. Kawakami, K. Soga, and S. Ikeda, Macromolecules 13, 457 (1980). 24. H. Naarmann and N. Theophilou, Synth. Metals 22, 1 (1987); N. Basescu, Z.-X. Liu, A. Heeger, H. Naarmann, and N. Theophilou, Nature 327, 4032 (1987). 25. K. Soga, S. Kawakami, and H. Shirakawa, Macromol. Chem. Phys. 71,4614 (1979). 26. A.G. MacDiarmid, J. Chiang, A. Richter, and A.J. Epstein, Synth. Metals 18, 285 (1987). 27. A.G. Green and A.E. Woodhead, J. Chem. Soc. 97, 2388 (1910). 28. A.G. MacDiarmid, J.C. Chiang, M. Halpern, W.S. Huang, S.L. Mu, N. Somasari, W. Wu, and S.I. Yaniger, Mol. Crvst. Liq. Crvst. 121, 173 (1985). 29. E.W. Paul, A.J. Ricco, and M.S. Wrighton, J. Phys. Chem. 89, 1441 (1985). 30. P.M. McManus, S.C. Yang, and R.J. Cushman, J. Chem. Soc. Chem. Commun. 1156 (1985). 31. A.J. Heeger, P. Smith, A. Fizazi, J. Moulton, K. Pakbaz, and S. Rughooputh, Synth. Metals 41, 1027 (1991). 32. J.E. Ellis, in [2a] Vol. 1, p. 489; see also, J.R. Ellis and R.S. Schotland, Market Opportunities jbr Electrically Conductive Poh'meric Systems, Princeton Polymer Laboratories, and Schotland Business Research, Princeton NJ, 1981.


15 Diamond in Electronic Packages D. J. Pickrell, D. S. Hoover

1 5.1

Introduction

As the densities of integrated circuits and the power levels of discrete devices increase, rapid heat dissipation will be essential to maintain or increase operating performance and device lifetimes. To accomplish this, packages of improved thermal design will be required along with the use of high performance materials. Traditionally BeO substrates have been used in electronic substrate applications where an electrical insulator with high thermal conductivity is required. More recently, A1N is being evaluated as a substitute material for such applications because of the toxicity problems associated with BeO. Diamond has the highest thermal conductivity of any material (at room temperature), more than six times greater than A1N or BeO and five times greater than pure copper. It is also an excellent electrical insulator with high electrical resistivity, high dielectric strength, low dielectric constant, and low dielectric loss. It is therefore an extremely attractive material for use in electronic packages where rapid removal of heat from circuit components is required. Single crystals of natural diamond and synthetic diamond, made by a high pressure process, have for many years been sold as substrates for high power discrete devices. However, their use in electronic packaging has been severely limited because of their high cost and small size. Polycrystalline diamond can now be synthesized by a chemical vapor deposition process as a coating or in thick, freestanding form over large areas. It has a comparable performance to single crystal diamond but at a fraction of the cost. The availability of this polycrystalline form is greatly expanding the use of diamond in electronic packaging. This chapter will discuss the synthesis of diamond by chemical vapor deposition and its incorporation into electronic packages. 15.2

B a c k g r o u n d on D i a m o n d

The carbon atom has an electron configuration of lsZ2s22p2, essentially a helium noble gas core with four outer electrons for bonding. These outer electronic orbitals (2s and 2p) hybridize to various extents, allowing pure carbon to assume 319

320


a number of crystalline and amorphous structures. The two most common crystalline forms of carbon are graphite and diamond. In graphite, each carbon atom has a n s p 2 electronic configuration and forms strong covalent a-bonds with three other carbon atoms in a plane. The remaining electrons, one for each carbon atom, form weaker g-bonds with each other above and below the planes. The planes are stacked in an ABAB... fashion and held together by van der Waals forces. The properties of graphite are highly anisotropic due to the difference in chemical bonding within and between the carbon planes. For example, graphite has a high electrical conductivity parallel to these planes but not perpendicular to them. This is because the g-bonded electrons are delocalized and can move easily within the planes. Delocalized electrons also absorb electromagnetic radiation throughout the visible range causing graphite to appear black. Since the carbon planes are held together only by weak van der Waals forces, they can easily slip past one another; this makes graphite a soft material with high lubricity. In diamond, each carbon atom is s p 3 hybridized and forms strong a-bonds to four other carbon atoms arranged in the form of a tetrahedron. Consequently, a strong three-dimensional covalent network is produced. Since all four outer electrons of each carbon atom participate in covalent bonds, in its pure form carbon is an electrical insulator and is transparent throughout the visible and infrared spectrum, except for two-phonon absorption [1]. The strong bonding, dense packing of atoms, and high cohesive energy, make diamond the hardest, stiffest, and least compressible material ever known and are responsible for its very low thermal expansion coefficient. The strong, short, stiff bonds also give rise to diamond's most important property for electronic packaging applications, its extremely high thermal conductivity, which at room temperature is five times greater than pure copper. Because of its unique properties and rarity in nature, people have tried to synthesize diamond for over a hundred years [2]. Three distinct syntheses have emerged. First to gain commercial importance was the high pressure/high temperature (HPHT) growth process. At atmospheric pressure, graphite is the thermodynamically stable form of pure carbon and diamond is metastable. High pressures, on the order of 104 atm, are required to make diamond the stable form of carbon. To convert graphite to diamond at these high pressures, high temperatures are needed to overcome the activation energy barrier. High pressures and high temperatures can be generated statically, with a heated hydraulic press, or dynamically, by propagating a shock wave in graphitic material [3]. In the static HPHT process, the solvent/catalyst approach is used to lower the temperature and pressure from that required for direct conversion of graphite to diamond. In this technique, a metal such as nickel or iron is mixed with the graphite before it is placed in the die cavity. The mixture forms a eutectic melt around 1 300-1 500~ and under the high pressure (30000-40000 atmospheres) diamond precipitates out. Dissolution into the molten metal reduces the activation energy barrier, which lowers the necessary processing temperatures and significantly improves the kinetics for conversion of graphite to diamond. Diamond produced by HPHT processes is primarily used for industrial cutting and grinding applications. It has

Diamond in Electronic Packages

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also been marketed for electronic substrates, but the crystals are expensive and have areas limited to a few square millimeters. Syntheses based on physical vapor deposition of carbon species at low pressures [4] can produce diamond thin films. These processes involve some form of ion bombardment to achieve conditions necessary for the formation of s p 3 bonded carbon. The films formed are typically amorphous networks with variable ratios of sp 2 t o s p 3 bonded carbon and various amounts of hydrogen. They are generally termed diamond-like carbon films because they have properties which can approach that of true crystalline diamond. Although these films are usually amorphous, some fine diamond crystals have been reported [5,6]. Diamond-like carbon films are deposited at room temperature; they are hard, transparent, and chemically impervious; they have a range of potential applications but will not be discussed in detail in this chapter. The third class of syntheses is based on chemical varpor deposition (CVD). Diamond is crystallized via a chemical reaction with the gas phase under low pressure conditions, where it is the thermodynamically metastable form of carbon. Though last to gain commercial importance, synthesis of diamond in the laboratory by CVD actually predates HPHT [7]. CVD diamond has great potential for use in electronic packaging because it can be grown relatively inexpensively in thin films or thick slabs over large areas; it is therefore the focus of discussion in this chapter.

1 5.3 Chemical Vapor Deposition of Diamond 15.3.1 Deposition Techniques There are numerous types of CVD systems for synthesizing diamond, differing mainly in the manner in which the gas phase is activated. Figure 15.1 shows two of the most common, the hot filament technique and the microwave plasma assisted technique. In the hot filament process, a refractory metal wire heated in excess of 2 000~ is used to activate diamond growth [8]. A gas mixture composed of 2% or less of CH4 in H2 flows into a chamber held at around 30-50 torr pressure. On a substrate positioned about 1 cm from the filament, and heated in the range 700-900~ diamond grows at a rate of a few micrometers per hour. Many minor variations of the basic hot filament growth system have been described. Included in these are systems using different filament materials, such as W, Ta, and Re [9,10]; using the metal in different forms, such as a tube [11] or spiral ribbon [12]; RF induction [13] instead of resistance heating of the metal; and biasing of the substrate relative to the filament to enhance growth rates [14]. In the low pressure, microwave plasma CVD system [15] (Fig. 15.1) microwaves at 2.45 GHz are directed into a tubular, fused silica reaction chamber to create a plasma in the gases flowing through the tube. Again a mixture of 2% or less of CH4 in H2 flows into the chamber and the pressure is held at around 50--100 torr. The substrate is placed on a susceptor, positioned in the plasma, and heated to around 800-1000~ by the microwaves and plasma. Diamond grows on the substrate at a rate of a few micrometers per hour. Minor variations on this

322

MATERIALS FOR ELECTRONIC PACKAGING FEED GAS -- l GHz): (a) coaxial transmission technique, (b) microstrip resonator method, and (c) free-space bridge transmission. See [18] and [19] for details.

346


method, the specimen is loaded into or made part of a resonant cavity or structure and the resonant frequency and quality factor are measured to yield the dielectric constant and dissipation factor [10, 2520-86, D3380-90] [11, 2.5.5.5] [16] [17]. In a transmission line method, changes of the amplitude, phase and/or shapes of waves propagated through or reflected by a sample are measured to yield the same parameters [ 18] [ 19]. Another possibility is the use of pulse propagation delay and shape analyses. In these approaches, the time delay and degradation of a pulse along a conductor surrounded by an insulator might be used to measure the dielectric constant and loss factor of the insulator. This approach is similar to time domain reflectometry commonly used in industry for assessing the electrical quality of packaging. These approaches also offer the possibility of using the conductors in the packaging for in situ measurements. Recent work [20] [21] suggests the exploration of such techniques may have begun. 16.3

Thermal Properties

The thermal properties of main interest in modern electronic packaging are thermal conductivity/diffusivity, thermal expansion, specific heat, and maximum use temperature. Representative values are given in Table 16.2. References [22] through [28] are recommended, especially the more recent ones, [27] and [28]. Thermal conductivity [ 10, C 168-90] is the major material property determining the ability of packaging to dissipate heat. The overall thermal resistances of packages are experimentally assessed by means of thermal test chips or other devices of known heat output mounted in the package and measurements of the overall temperature drop between the test chip or heating device and the exterior of the package. The temperature profiles in the package are then calculated via models using values of the thermal conductivity [29] [30] [31]. Following Touloukian et al. [22], the methods for measuring thermal conductivity can be categorized as either steady-state or nonsteady-state. In a steady-state method the measurements are made after thermal equilibrium has been reached whereas in a nonsteady-state method measurements are made while the temperature is changing. The nonsteady-state methods determine the thermal diffusivity from which the thermal conductivity may be calculated by multiplying the thermal diffusivity by the material density and specific heat. Techniques for measuring density and specific heat are discussed below. A thermal conductivity value derived from such a calculation may not agree with the measured value because of such factors as variable anisotropies in the specimens and differences in the mechanisms of heat transfer, specific heats, and densities over the temperature ranges of the two methods. Unknown impedances of interfaces may also lead to large uncertainties in the test configuration. The majority of the thermal conductivity data in the literature is derived from either the guarded hot plate method [10, C177-85] or the longitudinal bar method [10, E1225-87]. Both are steady-state methods in which heat flows in one

Measurements of Properties of/Hateriab in Electronic Packaging

347

direction through the short thickness of a disk or plate (guarded hot plate) or down the long axis of a bar or rod (longitudinal bar) and the resulting temperature gradients are measured to yield the thermal conductivity knowing the heat flow and the cross-sectional areas of the specimens [32]. Though most such measurements have been made on bulk specimens, Decker et al. [33] employed thin film thermocouples to measure the thermal conductivities of dielectric films less than about a micrometer thick; the results indicated thermal conductivities as much as one order of magnitude lower than those of bulk specimens. A thermal comparator technique, in general, is one in which the thermal property is derived by comparing the thermal response of the unknown specimen with the responses of similar specimens with known thermal properties. The expected uncertainties in thermal conductivities measured by a thermal comparator method are typically twice those measured by the guarded hot plate or longitudinal bar methods [28, Chs. 1, 3-1.In a novel, steady-state, thermal comparator technique, a probe tip heated to one temperature is touched to the surface of the unknown specimen held at another temperature and the intermediate temperature of the probe at equilibrium is determined. The probe tip is similarly touched to specimens of known thermal conductivities to provide a calibration curve of intermediate equilibrium temperatures versus thermal conductivity from which the thermal conductivity of the unknown can be determined. Lambropoulos et al. [34] report results, using the thermal comparator shown in Figure 16.3, which indicate thermal conductivities of thin films two orders of magnitude lower than those of bulk specimens. Reference [34] is an excellent review of other techniques for making measurements of the thermal conductivities of thin films.

Probe

Substrate_ ~

Sample

/

Film Layer ~ t / ~ i 1 1 1 1 1 1 1 1 1 1 1 1 1 1 ~ (_ __Sensing.Tip-~ ConslantanTubing u.z:)-mm alam.)x~ll~-~,~/ ~N~lll~~Constantan Block CopperHeating / / ~ ~ 1 1 : ~ ~ Block :

Heat

Figure 1 6,3 Schematic of thermal comparator probe used to measure the thermal conductivities of thin films. For details see text and references listed in [34] and [35].

348


The nonsteady-state methods are further categorized [21] as either transient or periodic. In a transient method a single application of heat is made and the rate of change of temperature is measured. In a periodic method the heat is applied at some point on the specimen in pulses of fixed period, and the amplitude, velocity, and phase of resulting pulses at other points on the specimen are measured with reference to those of the pulses at the point of application. The flash diffusivity method [32] [36] [37] [38] is a transient method in which a short burst of radiant energy is applied to one face of a thin specimen and the temperature profile on the opposite face is measured as a function of time, usually by means of a thermocouple of small mass or an infrared sensor. The thermal diffusivity is calculated from a time characteristic of the rise (usually the half-rise time, tso) to a maximum temperature and the square of the measured specimen thickness. In early versions of this method, the heat pulse was applied uniformly to the specimen face, typically by means of a flash lamp, but more commonly now it is applied by a laser or sometimes an electron beam, both of which permit micrometer-scale resolution probing of any variations of the diffusivity at various points on the specimen. In the case above where the temperature is measured directly opposite from the point of heat application, the diffusivity parallel to the specimen thickness is measured. In another version of the flash diffusivity method, the temperature-time profile is monitored at a point on the back of the specimen, somewhat removed from the location of the point directly opposite the location where the heat pulse is applied, so the diffusivity parallel to the plane of the specimen can also be measured. For highest accuracies, this requires calibration by means of standards of known diffusivity. Other possible transient techniques that may be applied in the future to packaging materials include the forced Rayleigh scattering method and the use of thin film thermocouples. In forced Rayleigh scattering [39], two laser beams are crossed at a point on the surface of a specimen to create fringes. These fringes produce a standing heat wave pattern, which serves as a temporary diffraction grating as result of the change of refractive index with temperature. A third laser beam is directed through this grating, and the change of the diffraction angle with time after the two crossed beams are turned off yields the thermal diffusivity. Of course, the specimen must be transparent to the third (probing) laser beam. Thin film thermocouples [40] deposited onto a specimen may be pulse heated, and the fall in temperature with time may be used to determine the diffusivity of the substrate. Since metals such as copper and platinum, used as conductors in packaging, are also used in thermocouples, it is possible that thin film thermocouples might be formed within the packaging multilayer structures, thus providing measurements to verify the thermal models used today or monitor process and service conditions. Tye et al. [36] [37] have described a periodic method termed the AC calorimeter. A portion of the face of a long, thin, rectangular specimen is irradiated with chop-modulated light from a lamp or a defocused laser while the other portion is shielded from the heat by a mask. The temperature of the specimen at a position

Measurements of Properties of Materials in Electronic Packaging

349

shielded by the mask is measured by a thermocouple. The thermocouple measures the temperature amplitudes with the mask edge located at various positions along the long side of the rectangular specimen. The logarithms of the amplitudes are plotted versus distance between the mask edge and the thermocouple. The slope of the line is proportional to the ratio of the frequency divided by the diffusivity, from which the diffusivity can be calculated since the frequency is known. This method is a measure of the diffusivity in the plane of the specimen. In another technique [41], somewhat similar to the AC calorimeter and the flash techniques, periodic heating pulses are applied to one face of a thin specimen and the temperature-time profiles are measured on the other face. The frequency is varied and the temperature amplitudes of the pulses on the back are measured. The amplitudes are plotted as functions of the square root of the frequency to yield a straight line with a slope proportional to the thickness of the specimen divided by the square root of the diffusivity, which can be calculated knowing the thickness. For this method to work, the specimen must be thinner than the thermal diffusion length, meaning that for most of the polymers and ceramics in packaging, the specimen must be no more than a few to a few hundreds of micrometers thick. In thermal wave techniques [42] [43], a thermal pulse or wave is introduced at a point on the surface of a specimen and the resultant temperature is measured at a point on the same surface, either by pyrometry or an effect on the atmosphere just above it. The penetration of the thermal energy into the surface can be varied by varying the frequency of the incident pulse, thus providing a way of measuring the diffusivity at various thicknesses near the surface. This method has the advantage that access to only one free surface is required. The relative thermal expansions of the various materials in an electronic package determine the extent of thermally induced stresses in the packaging and the chips it contains. Though expansion occurs in three dimensions, the expansion in only one dimension is usually measured. Data are most often reported as linear coefficients of thermal expansion (CTE) assumed constant to a reasonable degree of approximation over a limited range of temperatures. The main methods available for measuring thermal expansion are [24] dilatometry, telemicroscopy, interferometry, capacitance, and (for crystalline materials) X-ray diffraction. Dilatometry remains by far the commonest way to measure most packaging materials. Telemicroscopy and X-ray diffraction methods are hardly ever seen in the packaging literature. In a dilatometric technique, one end of a long, thin probe rod made of a material of known thermal expansion characteristics touches the specimen, held in a heating or cooling chamber. The other end of the rod protrudes from the chamber and the expansion of the specimen upon heating or cooling is determined by measuring the movement of the protruding end of the probe rod by some means, usually electromechanical or optical [10, E228-85, D696-91] [11, 2.4.41, 2.4.41.1]. A thermomechanical analyzer (TMA) [10, D3386-84] [11,2.4.24] is a form of dilatometer typically used in bulk polymers measurements. The precision of dilatometry is on the order of + 1 ~m which, while adequate for many applications, is not adequate for high accuracy or applicable to thin films, which are themselves

350


only a few micrometers thick. In these cases other capacitance or interferometric techniques are required. Capacitance [44] and laser-based interferometric [45] techniques have been used for measuring CTE in the direction of film thicknesses. An example of a recently developed capacitance method is given in Figure 16.4. Strain gages have been used to measure the inplane thermal expansion of ceramics and metals used in packaging [46]. Resistive thin films used as strain gages are another possibility provided compensation can be made for the effects of temperature on the resistance of the films; films with low thermal coefficients of resistance (TCRs) appear most promising. Moir6 interferometry has been used [47] to follow the thermally induced distortions of packaging, but not to measure values of the expansion properties. Because the specific heat determines the temperature to which a material will rise for a given heat input, it is especially important in analyses to determine the maximum temperature the packaging will reach as a result of the input of a sudden, transient heat spike. It is also needed in deriving the thermal conductivity from the nonsteady-state methods discussed above. The most basic calorimetric methods [25] [27] [28] involve dropping a specimen heated to a known temperature into an adiabatic chamber containing another known substance at some other known temperature, and determining the final equilibrium temperature achieved by the specimen and the other substance. It can also be determined by differential scanning calorimetry (DSC) [ 10, E 1269-90] and the AC calorimeter technique discussed above for use in determining the thermal diffusivity [36] [37]. In a DSC, an unknown specimen and a standard are subjected side-by-side to a predetermined rate of temperature increase in an apparatus in which the heat flow into the unknown and standard can be determined. Analyses of the heat flows into the unknown versus the standard determine the heat capacity of the unknown. In the case of modern packaging, the m a x i m u m use temperature of concern is usually determined by a relevant property of the polymers. The glass transition temperatures of the thermoplastic polymers are often reported as rough indicators of their maximum use temperatures; in practice, maximum use temperatures are usually set by the degradation of some property to a specified value in a given time. The glass transition temperature is determined by TMA [11, 2.4.24], DSC [10, E1356-91] [11, 2.4.25] and dynamic mechanical thermal analysis (see stress-strain response in the next section). The glass transition temperature of a polymer is the temperature, really a narrow range of temperatures, at which the material changes from a rigid to a rubbery solid. It is most important in considering possible deleterious effects arising from the various higher temperature procedures involved in fabrication and assembly. The melting temperatures of the metals and ceramics in packaging are usually provided as rough indicators of their maximum use temperatures, but maximum use temperatures are actually set with reference to the temperature at which the materials will degrade in a certain time due to such factors as oxidation, corrosion or reactions with other materials. This is true for polymers and especially true for solder.


351

CAPACITANCE METER m

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FUSED QUARTZ PLATE

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POLYMER FILM

BOTTOM ELECTRODE PLATE

Figure 1 6 . 4 Recently developed capacitance method for measuring the coefficient of thermal expansion of polymer films. The C-shaped film establishes the thickness of the air gap between the top and bottom electrodes and the thickness is gaged by measuring the capacitance. See [44] for details.

352


16.4

Mechanical Properties

The mechanical properties are important as they determine the ability of the packaging to resist stresses imposed thermally or mechanically during processing or in service. The main properties of interest are the stress-strain responses, adhesion, and residual stress. Representative values of some stress-strain response properties are given in Table 16.3. The stress-strain responses of main interest in packaging are the elastic responses and time-dependent properties of creep, stress relaxation, and fatigue. Uniaxial tensile testing of cylindrical or fiat specimens [10, E8-91, E345-87, D638-90, D882-90] [11, 2.4.18, 2.4.19] has been used mostly for measuring the elastic responses of the metals and polymers used in packaging, whereas three- and four-point flexure testing has been used mostly for measuring ceramics [48] [49] and many polymers, especially laminates [10, D790-91-] [11, 2.4.4, 2.4.4.1]. Techniques for tensile testing of specimens (Fig. 16.5) with micrometer dimensions typical of features in modern packaging are under development [50] I-511. Biaxial tests are sometimes seen [52] [53] [54]. In addition to tension, solders are often tested in shear, double-lap shear [55], and torsion [56]. The elastic strength

Figure 16.5

Scanning electron micrograph showing enlargement of the miniature arrays of four tensile specimens, each 0.25 mm wide, 1 mm long and 2.2 #m thick, prepared using deposition, patterning and etching processes common to the semiconductor industry. Each array of four specimens is carried on and protected by a 6 mm • 8 mm rectangular silicon frame, similar in size and shape to IC dies. See [50] and [51] for details.


353

properties are usually reported as the elastic (Young's) modulus and stresses and strains where the material departs significantly from elastic behavior under the conditions of the test. The most significant test variables are temperature and rate of loading. Dynamic mechanical thermal analysis is used to measure the complex (measuring both the parts of elastic response in phase and out of phase with the applied stress) elastic modulus of polymers in oscillating flexure or shear [57]. This technique is often also used to detect the glass transition temperatures. Acoustic analyses [58] and microindentation [59] [60] can be used to measure the tensile and shear moduli of materials. Poisson's ratio is determined by measuring a dimension of a specimen in a direction perpendicular to the axis of applied stress. Creep and stress relaxation are of concern to the long-term reliability mainly of polymers and solder, which in packaging often operate at temperatures near to their maximum use temperature. In a creep test, a specimen is quite rapidly loaded to a stress significantly below the point at which it no longer behaves elastically then its extension at constant load or its stress is measured. A stress relaxation is performed in a similar way but measures the specimen stress as constant extension. The loading may be in tension, compression, flexure, or shear [55] [10, E328-86, D2990-90, D2991-84]. Tension is most appropriate for surface mount solders and for polymers, which tend to be in tension because their CTE is high with respect to other materials. Compression and shear are appropriate for solder used in die attach. Fatigue is the number of cycles required to fracture a specimen that has been repetitively cycled between two elastic stresses. It is of primary concern to the mechanical reliabilities of the thin film conductor stripes and the solder. The electrical conductor stripe material is almost always measured in alternating tension [10, E796-88] [11, 2.4.2.1], whereas the solders are measured in tension [61], compression [62], and shear [63]. Adhesion is the property that assesses how well two parts, properly termed adherends, resist being separated at their junction. Adhesive separations truly occur at the interface; cohesive separation occurs within the adherends. Of the hundreds of approaches for testing adhesion that have been developed [64] over the years, only a few have been used in packaging; excellent recent reviews are available [65] [66] [67]. These approaches include direct pull-off, direct shear, peel, scratch, stretch, laser spallation, and microindentation [ 11, 2.4.1, 2.4.1.2, 2.4.8, 2.4.9, 2.4.20, 2.4.21 ]. Three popular techniques are shown in Figure 16.6. Each yields measurements dependent in complicated ways on the properties of the two adherends, the strength of the interface, and the geometry of the bond. Since the true strength of the interface is virtually impossible to measure quantitatively, these approaches are best categorized as semiquantitative screening tests rather than measurements of basic material properties. Intrinsic residual stress results from defects introduced in the processing of packaging materials, whereas extrinsic residual stress results from the mismatch of their CTEs. Materials having residual stresses of the same nature (tension, compression, or shear) and the same sign as an externally imposed stress can fail at a much lower value than expected because much of their strength has already

354


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SUBSTRATE

COsuBSTRATE

(a)

(b)

(c)

Three adhesion tests: (a) direct pull-off, (b) peel, and (c) scratch. Reprinted by permission, from M. Ohring, The Materials Science of Thin Fihns, Academic Press, New York, 1992. Figure 1 6 . 6

SIDE VIEW

P

Au-Sn BRAZE OR Pb-Sn SOLDER

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TOP VIEW

T 2C

-

r

Figure 1 6 . 7 Side and top views of microindentation determinations of the residual stresses generated in a ceramic substrate near an input/output (I/O) bonding pad. The presence of the residual stress is indicated by the different lengths of the cracks generated by the indenter. See [71] for more details.

been consumed by the residual stress. X-ray diffraction has been used to measure residual stress in packaging metallization and ceramic substrates [68]. The most popular methods for measuring polymers are based upon the bending of a substrate on which the polymer has been deposited [69]; commercial instruments are available. Indentation [59] [70] has recently been used [71] to map the residual stresses in ceramic substrates near bonding pads (Fig. 16.7). Laser-based methods, making use of infrared or Raman lines, have been used to measure stress in surfaces of ceramics and polymers [72], and cathodoluminescence lines, induced by electron


355

beams, have been shown to exhibit shifts with stress. Because of their potential for high spatial resolution, methods based on indentation and spectral line shifts should find wider use in the future.

16.5 Physical Properties The physical properties of most interest are density and moisture absorption. Representative values are given in Table 16.3. Density determines the designed weight of the packaging and is needed to calculate many of the electrical, thermal, and mechanical properties, for example, to calculate the thermal conductivity from the thermal diffusivity measured by a nonsteady-state test. It can be determined by simply measuring the specimen dimensions and the specimen weight or by displacing the specimen in a liquid or gas of known density [10, D792-91]. Moisture absorption is of great concern in polymer applications, in fact so great is the concern there are current efforts to formulate polymers with low absorption and to provide other means of protecting the chips from moisture while still using the polymers to reduce costs associated with ceramic and metallic packaging that is completely hermetic. Moisture can result in swelling of the polymer and concentration of ionic species which can corrode the metallization. It can also cause changes in many of the properties mentioned above and even the popcorn phenomenon, catastrophic puffing or delamination during soldering [73]. The moisture absorption test method most commonly seen in the packaging literature is simply to weigh a sample as a function of time in a controlled humidity environment [74] [11, 2.6.2]. More sophisticated techniques are used, such as coulometry [10, D4019-88] and change of frequency of a vibrating substrate (e.g., a quartz crystal) covered with an absorbing polymer film [75].

16.6 Manufacturability Properties Manufacturability properties have an important bearing on the ease, efficiency, degree of success, or costs of processing the materials. The manufacturability properties of most interest are solderability, degree of polymer cure, and viscosity. Solderability is a term used to describe a combination of subjective and semi-quantitative factors determining the relative ease with which solder joints can successfully be made in an industrial setting. It is most often characterized by the solder wettability, the ease and tenacity with which the molten solder spreads on the substrate. The simplest test is the dip-and-look test in which a substrate is dipped into a solder bath and examined visually to determine how much of the substrate area is covered [11, 2.4.12, 2.4.14, 2.4.14.1). The most common quantitative method is the wetting balance or meniscograph method in which a test specimen is dipped into a solder bath and the force it experiences measured as a function of time [76]. The principle is depicted in Figure 16.8. The deyree of cure is important because it determines the desired achievement of many of the other properties and the degree of stability of the polymer itself. For example, incomplete cure can leave unreacted chemicals which, in the presence

356

MATERIALS

FOR

ELECTRONIC

PACKAGING

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=

0.5

z

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Materials for Electronic Packaging

Materials for Electronic Packaging

Materials For Advanced Packaging

Advanced Materials for Thermal Management of Electronic Packaging

Electronic Materials

Advanced MEMS Packaging (Electronic Engineering)

Global Legislation for Food Packaging Materials

Electronic properties of materials

Electronic structure of materials

Electronic Materials Science

Smart electronic materials

Electronic properties of materials

Electronic materials science

Electronic Materials Science

Electronic properties of materials

Adhesives Technology for Electronic Applications, Second Edition: Materials, Processing, Reliability (Materials and Processes for Electronic Applications)

Nano-Bio- Electronic, Photonic and MEMS Packaging

Mechanical Analysis of Electronic Packaging Systems

Nano-Bio- Electronic, Photonic and MEMS Packaging

The Electronic Packaging Handbook (Electronics Handbook Series)

Electronic materials: the oligomer approach

Electronic Materials: The Oligomer Approach

Micro- and Opto-Electronic Materials and Structures: Physics, Mechanics, Design, Reliability, Packaging: Volume 1 Materials Physics Materials ... Physical Design Reliability and Packaging

Pneumatic Conveying (Materials handling & packaging series)

Encapsulation Technologies for Electronic Applications (Materials and Processes for Electronic Applications)

Adhesives Technology for Electronic Applications: Materials, Processing, Reliability

Coating Materials for Electronic Applications: Polymers, Processing, Reliability, Testing

coating materials for electronic applications polymers processes reliability testing

Electronic basis of the strength of materials

Active Packaging for Food Applications

Springer Handbook of Electronic and Photonic Materials

Materials for Electronic Packaging