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SOLDER JOINT RELIABILITY
SOLDER JOINT RELIABILITY
Theory and Applications
Edited by lohn H. Lau
~ SPRINGER SCIENCE+BUSINESS MEDIA, LLC
Copyright © 1991 by Springer Science+Business Media New York Originally published by Van Nostrand Reinhold in 1991 Softcover reprint ofthe hardcover lst edition 1991
ISBN 978-0-442-00260-2
Ali rights reserved. No part of this work covered by the copyright hereon may be reproduced or used in any form by any means---graphic, electronic, or mechanical, including photocopying, recording, taping, or information storage and retrieval systems---without written permission of the publisher.
1 (T'i p Van Nostrand Reinhhold is a division of International Thomson - e-1 Publishing. ITP log o is a trademark under license.
16 15 14 13 12 Il 10 9 8 7 6 5 4 3 2
Library of Congress Cataloging-in-Publication Data
Solder joint reliability--theory and applications 1 [ edited by] John H. Lau.
p. cm. lncludes index. ISBN 978-0-442-00260-2 ISBN 978-1-4615-3910-0 (eBook) DOI 10.1007/978-1-4615-3910-0 1. Welded joints. 2. Solder and soldering. 1. Lau, John H.
TA492.W4S63 1991 671.5'2042--dc20 90-12968
CIP
Foreword Preface Acknowledgments
Contents
xv xvii' xix
1. Flux Reactions and Solderability 1 1.1 Flux History 1 1.2 Solderability Tests 2
1.2.1 Visual Assessment 3 1.2.2 Area of Spread Test 3 1.2.3 Edge Dip and Capillary Rise Tests 3 1.2.4 Globule Test 3 1.2.5 Rotary Dip Test 4 l.2.6 Surface Tension Balance Test 4
1.3 Flux Action from Solderability Measurements 4 1.4 Flux Types 5
1.4.1 Mechanistic Studies for Inorganic Fluxes 5 1.4.2 Mechanistic Studies for Rosin-based Fluxes 11 References 36
2. Solder Paste Technology and Applications 38
2.1 Chemical and Physical Characteristics 40 2.2 Fluxing and Fluxes 42
v
vi CONTENTS
2.3 Solder Alloys 44 2.4 Solder Powder 46 2.5 Paste Fonnulation 51 2.6 Paste Rheology 54 2.7 Rheology Behavior Characterization 56 2.8 Viscosity and Measurement 63 2.9 Printing Technique 64 2.10 Dispensing Technique 68 2.11 Soldering Principle 69 2.12 Solderability 70 2.13 Soldering Methods 72 2.14 Controlled Atmosphere Soldering 73 2.15 Solvent Cleaning 80 2.16 Aqueous Cleaning and Aqueous Cleaning Paste 84 2.17 No-clean Paste 85 2.18 Fine Pitch Paste 87 2.19 Quality 90 2.20 Conclusion 90
References 90
3. Technical Considerations in Vapor Phase and Infrared Solder Reftow Processes 92 3.1 Introduction to Surface Mount Reftow Soldering 92 3.2 Type I 93 3.3 Soldering Requirements for Surface Mount
Technology 94 3.4 Reftow Process Phases 95 3.5 Reflow Equipment 98
3.5.1 Infrared 98 3.5.2 Vapor Phase 101 3.5.3 Convection 105 3.5.4 Conductive Belt 105 3.5.5 Laser Soldering 105
3.6 Prereflow Solder Paste Bake 106 3.7 Maximizing Solder Joint Yield 106 3.8 Reflow Processing 109
3.8.1 Vapor Phase 110 3.8.2 Infrared 112 3.8.3 Cost Comparison 113
3.9 SMT Reliability 113 References 116
CONTENTS vii
4. Optimizing the Wave Soldering Process 117 4.1 Basic Wave Soldering Process Overview 117 4.2 Wave Soldering Process Hardware 119
4.2.1 Fluxing 119 4.2.2 Fluxers 119 4.2.3 Fluxer Measurement Parameters 124 4.2.4 Fluxer Optimization 124 4.2.5 Preheating 125 4.2.6 Preheaters 126 4.2.7 Preheat Measurement Parameters 128 4.2.8 Preheat Optimization 130 4.2.9 Wave Soldering 131 4.2. to Solder Waves 131 4.2.11 Solder Wave Measurement Parameters 133 4.2.12 Wave Soldering Optimization 134 4.2.13 Solidification 135 4.2.14 Conveyors 135
4.3 Wave Soldering Process Parameter Optimization 136 4.3.1 Optimization Procedure Test Study 137
4.4 Results 141 4.5 Conclusion 141
References 142
5. Post-Solder Cleaning Considerations 143 5.1 Purpose and Chapter Description 143 5.2 Environmental Concerns 143 5.3 Definition of Soldering Flux 144 5.4 Specifications 144
5.4.1 Test Methods 144 5.4.2 Institute for Interconnecting and Packaging Electronic
Circuits (IPC) 153 5.4.3 U.S. Military 153 5.4.4 Telecommunications 154
5.5 Flux Materials and Associated Cleaning 155 5.5.1 Rosin 157 5.5.2 Water Soluble 158 5.5.3 Synthetic Activated 159 5.5.4 Low Solids (No-Clean) 160 5.5.5 Controlled Atmosphere Soldering 162
5.6 Flux Application Methods 163 5.6.1 Wave 163
viii CONTENTS
5.6.2 Foam 164 5.6.3 Spray 164 5.6.4 Rotating Drum Spray 166 5.6.5 Application Issues for Low Solids Fluxes 166
5.7 Process Issues Associated with Reliability 166 5.7.1 Flux Residue 167 5.7.2 Solder Ball Formation 167 5.7.3 Top-Side Fillet Formation 167 5.7.4 Conformal Coating Compatibility 168
5.8 Non-Liquid Fluxes 168 5.8.1 Core Solder Material 168 5.8.2 Solder Paste Material 169
5.9 Trends 170 References 170 Additional Readings 172
6. Scanning Electron Microscopy/Energy Dispersive X-Ray (SEMlEDX) Characterization of Solder-Solderability and Reliability 173 6.1 Scanning Electron Microscopy/Energy Dispersive X-ray
Analysis 173 6.2 Other Methods-WDX 173 6.3 Detection Modes 174 6.4 Sample Preparation 174 6.5 Different Phases in Alloys 175 6.6 Intermetallics 175 6.7 Scope of the Chapter 176 6.8 SEMlEDX Characterization-General 176
6.8.1 Tin-Lead Solders 176 6.8.2 Two Percent Silver Solder 178 6.8.3 Gold- and Silver-Based Solders 178 6.8.4 Indium Solders 180 6.8.5 Bismuth Solders 183 6.8.6 Miscellaneous 186
6.9 Solderability Issues 186 6.9.1 Maintaining Solderability 186 6.9.2 Inadequate Tin Protective Coatings 189 6.9.3 The Dangers of "Forcing" Poor Solderability 189
6.10 Reliability Issues-Leaching of Substrate 194 6.11 Reliability Issues-Gold Embrittlement 205 6.12 Reliability Issues-Fatigue 213
References 222
CONTENTS ix
7. The Role of Microstructure in Thermal Fati2ue of Pb-Sn Solder Joints 225
7.1 Experimental Details 226 7.2 Eutectic Microstructures 229
7.2.1 Lamellar Eutectics 230 7.2.2 Degenerate Eutectics 231 7.2.3 Solder Joint Microstructures 233 7.2.4 Effects of Composition 238 7.2.5 Recrystallized Pb-Sn Microstructure 241 7.2.6 Coarsening Behavior 242
7.3 Mechanical Properties 243 7.3.1 Eutectic Structures 243 7.3.2 Deformation Mechanisms 246
7.4 Microstructural Evolution under Thermal Fatigue 248 7.4.1 Thermal Fatigue in Shear 249 7.4.2 Microstructural Mechanisms of Thermal
Fatigue 252 7.4.3 Other Microstructures 255
7.5 Conclusion 261 7.6 Acknowledgments 262
References 262
8. Microstructure and Mechanical Properties of Solder Alloys 266 8.1 Thermal Cycling Fatigue 267 8.2 Precipitation and Dissolution in Pb-Sn Alloys 268 8.3 Discussion 276
References 277
9. The Interaction of Creep and Fatigue in Lead-Tin Solders 279
9.1 Current Approaches to Accelerated Testing 9.2 Damage by Fatigue and Creep Mechanisms
279 280
9.3 Assessing Actual Joint Damage 283 9.3.1 In-service Testing 285
9.4 Understanding the Damage Mechanisms 9.4.1 Creep and Tensile Test Results 9.4.2 Cyclic Creep 287 9.4.3 Hold Time Effects 293
286 287
x CONTENTS
9.5 Interpretation for Packaging Applications 301 9.5.1 Deformation 301 9.5.2 Thermomechanical Test Guidance 302
9.6 Concluding Remarks 303 References 304
10. Creep and Stress Relaxation in Solder Joints 306
10.1 Ideal Expansivity of a Substrate 307 10.1.1 No Temperature Gradients, No Transients 307 10.1.2 Power Dissipation in the Component 309 10.1.3 Z-Gradients in the Substrate 309 10.1.4 In-Plane Gradients 311 10.1.5 Temperature Shock 311 10.1.6 Solder-Substrate Expansivity Mismatch 312 10.1.7 Overall Judgment 313
10.2 Creep and Stress Relaxation 313 10.3 Solder Properties 318 10.4 Constitutive Relations 319 10.5 Temperature Cycling 322
10.5.1 Small Temperature Range Cycling 323 10.6 Larger Temperature Cycles 326 10.7 Acknowledgments 330
References 330
11. Effects of Strain Range, Ramp Time, Hold Time, and Temperature on Isothermal Fatigue Life of Tin-Lead Solder Alloys 333 11.1 Definition of Failure, Specimen Design, and Mode of
Loading 333 11.2 Effect of Strain Range on Fatigue Life 334 11.3 Effect of Frequency on the Fatigue Life 338 11.4 Effect of Hold Time on Fatigue Life 343 11.5 Effect of Temperature on Isothermal Fatigue of
Solders 355 11.6 Conclusion 357
References 357
12. A Damage Integral Methodology for Thermal and Mechanical Fatigue of Solder Joints 361
12.1 Inelastic Deformation and Stress Calculation 363 12.1.1 Governing Equation for Solder Stress 363
CONTENTS xi
12.1.2 Inelastic Defonnation Behavior and Constitutive Relations 365
12.1.3 Stress Calculation 370 12.2 Damage Rate Fonnulation 372
12.2.1 Damage Mechanisms 372 12.2.2 A Phenomenological Fonnulation for Crack Growth
Rates 374 12.3 Damage Integration and Failure Criterion Effects 377
12.3.1 Thennal Fatigue Life Estimation 377 12.3.2 Failure Criterion Effects 379
12.4 Discussion and Conclusions 380 12.5 Acknowledgments 380
References 380
13. Modern Approaches to Fatigue Life Prediction of SMT Solder Joints
13.1 Mechanical Testing 385 13 .1.1 Detennination of Elastic Properties 385 13.1.2 Mechanical Properties 386
13.2 Life Prediction Techniques 388 13.2.1 Fatigue Models 388
13.3 Hybrid Life Prediction Techniques 395 13.3.1 Strain Range Partitioning Rule 397
13.4 Model Joints 398 13.4.1 Quality Control 398 13.4.2 Lap Joint Specimens 401 13.4.3 Straddle Board Specimens 401
13.5 Expert Systems 404 13.6 Conclusions 404 13.7 Acknowledgments 405
References 405
14. Predicting Thermal and Mechanical Fatigue Lives from
384
Isothermal Low Cycle Data 406 14.1 Low Cycle Fatigue (LCF) 411 14.2 Low Cycle Fatigue of Solders-Influence of the Definition
for Failure 413 14.3 Influence of the Temperature 423 14.4 Influence of Hold Times and Cycling Frequency 425 14.5 Influence of the Environment 436 14.6 Microstructural Changes 437
xii CONTENTS
14.7 Determination of the Displacement and Strain Distribution in a Solder Joint 438
14.8 Prediction of the Fatigue Life of Solder Joints 443 14.9 Inherent Limitations to Fatigue Life Predictions 446 14.10 Necessary Further Work 447 14.11 Acknowledgments 449
References 449
15. Static and Dynamic Analyses of Surface Mount Component Leads and Solder Joints 455
Stiffness of Gull-Wing and J Leads and Solder Joints for Surface Mounted Chip Carriers 456
15.1 Boundary-Value Problem 458 15.2 Finite Element Methods 460 15.3 Stiffness of Gull-Wing Lead and Solder Joint 464 15.4 Stiffness of J Lead and Solder Joint 472
15.4.1 Unit Displacement (0.0001 in.) in the I-Direction 472
15.4.2 Unit Displacement and Rotation in Other Directions 477
15.4.3 Comparison of the Stiffness Matrices between the PQFPs and PLCCs 477
Solder Joint Reliability Under Shock and Vibration Conditions 478
15.5 Free Vibration of Soldered and Unsoldered Leads 478 15.5.1 Vibration Results for Wide SOICs 480 15.5.2 Vibration Results for Narrow SOICs 481 15.5.3 Vibration Results for PLCCs 483 15.5.4 Vibration Results for PQFPs 483 15.5.5 Experimental Verification 484
15.6 Free Vibration of a Constrained PCB with a SMC 486 15.7 Acknowledgments 493
References 493
16. Integrated Matrix Creep: Application to Accelerated Testing and Lifetime Prediction 508 16.1 General Form of the Constitutive Relation 510 16.2 Development of the Constitutive Relation 511
16.2.1 Description of Data 511 16.2.2 Steady-State Creep Strain Component 511
CONTENTS xiii
16.2.3 Elastic Strain Component 514 16.2.4 Time Independent Plastic Strain
Component 515 16.3 Summary of Constitutive Equation 517 16.4 Comparison of the Steady-State Creep Equation to
Published Data 517 16.5 Application of Constitutive Equation to Data of
Reference 2 520 16.5.1 Description of Numerical Procedures 520 16.5.2 Results 522
16.6 Multiaxial Stress States 525 16.6.1 Derivation of Constitutive Equation in Three
Dimensions 525 16.7 Fatigue Calculations and Mechanical Shear Tests
16.7.1 Correlation of the Data of Reference 2 16.7.2 Correlation of the Data of Wild and
526 528
Solomon 529 16.8 Analysis of Leaded Solder Joints 532
16.8.1 Extension of Matrix Creep Failure Indicator to General Case 534
16.8.2 Description of Model 536 16.8.3 Results 540
16.9 Conclusions 542 16.10 Acknowledgments 543
References 543
17. Solder Joint Reliability, Accelerated Testing, and Result Evaluation 545 17.1 The Reliability of Electronic Assemblies and Solder Joint
Reliability 545 17. 1. 1 "Bathtub" Reliability Curve-Hazard Rate
Model 546 17.2 Solder Joint Loading Conditions and Reliability 549 17.3 Reliability and Accelerated Tested-Overview 551 17.4 The Thermal Expansion Mismatch Problem 553
17.4.1 Solution 1: CTE-Tailoring to Reduce Expansion Mismatch 554
17.4.2 Solution 2: Attachment Compliancy to Accommodate Expansion Mismatch
17.5 Analytical Model of Solder Shear Fatigue 17.5.1 Solder Joint Fatigue 556 17.5.2 Leadless Solder Attachments 559
555 556
xiv CONTENTS
17.5.3 Leaded Solder Attachments 564 17.5.4 Acceleration Transform 567 17.5.5 Failure Statistical Considerations 569
17.6 Accelerated Fatigue Reliability Testing 571 17 .6.1 Testing Considerations 571 17.6.2 Accelerated Test Conditions 578 17.6.3 Test Vehicle Design 581 17.6.4 Sample Statistics and Solder Joint Defects 581 17.6.5 Test Vehicle Assembly and Conditioning 582 17.6.6 Failure Definition and Detection 583
17.7 Prediction of SM Solder Joint Reliability 583 17.7.1 Simple Cyclic Load Histories 583 17.7.2 Multiple Cyclic Load Histories 584
17.8 Acknowledgments 585 References 585
18. Surface Mount Attachment Reliability and Figures of Merit for Design for Reliability 588 18.1 Mechanics and Fatigue of SM Solder Joints 589
18.1.1 SM Leadless Attachments 589 18.1.2 SM Leaded Attachments 591 18.1. 3 Failure Distribution 594
18.2 Figures of Merit for Attachment Reliability 596 18.2.1 Derivation of FM Formulas 598 18.2.2 FMs for Multiple Thermal Fluctuations 18.2.3 Graphical Interpretation and Reliability
Charts 600 18.3 Examples 601
599
18.3.1 Example 18-1: Chip Components on FR-4 Printed Wiring Boards 602
18.3.2 Example 18-2: 50-Mil Pitch Ceramic Leaded Devices on FR -4 603
18.3.3 Example 18-3: 25- and 50-Mil Pitch Plastic Leaded Components on FR-4 604
18.4 Concluding Remarks 607 18.5 Acknowledgments 607 Appendix 18-A FM Formulas and Scaling Constants Appendix 18-B FMs for Multiple Thermal Fluctuations References 611
Authors' Biographies 615
Index 627
607 610
Foreword
Solders have given the designer of modern consumer, commercial, and military electronic systems a remarkable flexibility to interconnect electronic components. The properties of solder have facilitated broad assembly choices that have fueled creative applications to advance technology. Solder is the electrical and mechanical "glue" of electronic assemblies.
This pervasive dependency on solder has stimulated new interest in applications as well as a more concerted effort to better understand materials properties. We need not look far to see solder being used to interconnect ever finer geometries. Assembly of micropassive discrete devices that are hardly visible to the unaided eye, of silicon chips directly to ceramic and plastic substrates, and of very fine peripheral leaded packages constitute a few of solder's uses.
There has been a marked increase in university research related to solder. New electronic packaging centers stimulate applications, and materials engineering and science departments have demonstrated a new vigor to improve both the materials and our understanding of them. Industrial research and development continues to stimulate new application, and refreshing new packaging ideas are emerging. New handbooks have been published to help both the neophyte and seasoned packaging engineer.
A critical element in the continued widespread use of solder is the reliability of the solder joint. Will solder provide the characteristics necessary to allow the world to depend on it in the future? This book, Solder Joint Reliability-Theory
xv
xvi FOREWORD
and Applications, edited by Dr. Lau and written by experts in the field, provides a focal point of current understanding. This will help all participants in the soldering world better plan for the future. It is an exciting time for solder, and reliability will be a key element in its development.
Donald W. Rice
Hewlett-Packard Company
Preface
Soldering has a history of several thousand years. It was considered an art until the electronic age, when it was recognized as a technology. Until very recently, however, research efforts into soldering technology have not been particularly aggressive because solder joints in plated-through hole technology do not cause serious reliability problems.
The 1980s have witnessed an explosive growth in the research efforts devoted to soldering science as a direct result of the rapid development of surface mount technology and the growing interest of miltichip module technology. (For a list of papers published after 1985 see references I through 202 in chapter 15; for a list of solder-joint papers published before 1985 see reference 185 in chapter 15.) Soldering is the joining method of choice for attaching component to printed circuit board or chip to substrate. In both cases the solder-joint functions simultaneously as the electrical and mechanical attachment medium. Thus, solderjoint reliability is one of the most critical issues in the development of these technologies.
We have now begun to obtain useful insights into the mechanical properties and microstructure of bulk solders and joints under fatigue, creep, and stressrelaxation conditions. Some effective methods of accelerated testing, failure analysis, and life prediction of solder joints have also been reported. We are also achieving a new understanding of how solder-joint reliability is influenced by flux reactions, solder paste, solderability, reflow methods, wave soldering, and cleaning. These results have been disclosed in diverse journals or more
xvii
xviii PREFACE
incidentally in the proceedings of many conferences, symposia, and workshops whose primary emphasis is material science or electronic packaging and interconnection. Consequently, there is no single source of information devoted to the state of the art of soldering science and technology. This book aims to remedy this deficiency and to present, in one volume, a timely summary of progress in all aspects of this fascinating field.
This book begins with the concerns of solder joint formation and ends with concerns over long term solder joint reliability. The book is divided into four basic parts. Chapters 1 and 2 describe the application of flux, solder paste, and solderability to solder joints. Chapters 3 through 5 review the various soldering and cleaning methods and their effects on solder joint reliability. Chapters 6 through 12 explain the failure mechanisms of bulk solders and joints under fatigue, creep, and stress-relaxation conditions. Chapters 13 through 18 provide the data and methods for the test, design, analysis, and life prediction of solder joints sUbjected to mechanicals thermal, shock, and vibration conditions.
Some duplication of material among chapters has been necessary if each chapter is to offer the reader all the information essential for understanding the subject matter. An attempt has been made to provide a degree of uniformity in perspectives, but diverse views on certain aspects of solder joint reliability are a reality. I hope that their inclusion here may be seen as an accurate reflection of the state of the art and a useful feature of the book.
For whom is this book intended? Undoubtedly, it will be of interest to three groups of specialists: (1) those who are active or intend to become active in research on soldering science and technology; (2) those who have encountered a practical soldering problem and wish to understand and learn more methods of solving such problems; and (3) students and professors at universities, in view of the fact that today's engineer receives on average less than one hour of instruction on soldering science during his or her university studies. I hope that this book will serve as a valuable reference to all those faced with the challenging problems created by the ever expanding use of solders in engineering practice, and that it will aid in stimulating further research on solder materials, testing and analytical methods, and the sounder use of solders. Reliability of the solder joint is limited only by the ingenuity and imagination of researchers, engineers, and management.
John H. Lau Hewlett-Packard Company
Acknowledgments
Development and preparation of the manuscript was facilitated by the efforts of a number of dedicated people at Van Nostrand Reinhold. I would like to thank them all, with special mention to Stefania Taftinska for her coordination of the publication process and to Stephen Chapman for his unswerving support and advocacy. My special thanks to Marjorie Spencer who made my dream of this book come true by effectively sponsoring the project and solving many problems that arose during the book's preparation. It has been a great pleasure and fruitful experience to work with them.
Most of the materials in this book have been presented at various ASMI ASMEI IEEElISHM conferences, symposia, and workshops in the past few years. I want to thank these respected societies for allowing me to organize technical sessions so I can have the privilege to invite the contributing authors to present their research results.
Each chapter of the book was reviewed by at least three individuals who are experts in soldering areas. According to their specialties, each individual reviewed at least three chapters of the book. These reviewers are: Dr. Birendra N. Agarwala of IBM, Professor Donald Barker of the University of Maryland, Dr. Kirk Bonner of Allied-Signal Inc., Professor Hans Conrad of North Carolina State University, Dr. T. Dixon Dudderar of AT&T Bell Laboratories, Dr. Darrel R. Frear of Sandia National Laboratories, Dr. Sung K. Kang oflBM, Dr. Joseph Kevra of Alpha Metals, Dr. Ken Kinsman of Intel Inc., Dr. Larry Moresco of Fujitsu Computer Packaging Technologies, Inc., Dr. Yi-Hsin Pao of Ford
xix
xx ACKNOWLEDGMENTS
Motor Company, Dr. Donald Rice of Hewlett-Packard Company Dr. Charles G. Schmidt of the Stanford Research Institute, Professor Karl K. Stevens of Florida Atlantic University, Dr. Boon Wong of Hughes Aircraft Company and Dr.Chee C.Wong of AT&T Bell Laboratories. I want to thank them for their many helpful comments and constructive suggestions that added significantly to this book.
I express my deep appreciation to all 30 contributing authors, experts in their respective fields, for their many helpful suggestions and cooperation in responding to requests for revisions. Their depth of knowledge, dedication, and patience have been demonstrated throughout the process of preparing this book.
I thank Dr. Donald W. Rice for bringing me to this wonderful world of electronic packaging and interconnection and for many fruitful discussions and much strong support in the past six years at Hewlett-Packard (first HP Laboratories and now HP Corporate Manufacturing). I also want to thank my daughter Judy and my wife Teresa for their consideration and patience by allowing me to work on this project on many weekends.
John H. Lau Hewlett-Packard Company
SOLDER JOINT RELIABILITY
