Mitigating Tin Whisker Risks Theory and Practice

Takahiko Kato, PhD, is a chief researcher in the Center for Technology Innovation – Materials, Research & Development Group at Hitachi Ltd, Tokyo, Japan. He is also a Guest Professor in the Center for Advanced Research of Energy and Materials at Hokkaido University in Japan. Dr. Kato has over thirty years’ research experience in the areas of practical materials technology and materials science related to boiling water reactors, fusion reactors, superconductors, electrical devices, and sustainable energy devices.

Carol Handwerker is the Reinhardt Schuhmann Jr. Professor of Materials Engineering at Purdue University, Indiana, US. Previously, she was chief of the Metallurgy Division at the National Institute of Standards and Technology (NIST), where she participated in the NCMS (National Center for Manufacturing Sciences) Lead-Free Solder Project, co-chaired the iNEMI Lead-Free Alloy Selection Team, and participated in the iNEMI Tin Whisker Fundamentals Project.

Jasbir Bath is the owner of Bath and Associates Consultancy LLC, California, US. He has more than twenty years' experience in research, design, development, and implementation in the areas of soldering, surface mount, and packaging technologies. Bath has been chair of various iNEMI lead-free consortia involving OEMs, EMS, and component and material supplier companies on alloy selection, assembly, and rework.

List of Contributors ix

Introduction xi

1 A Predictive Model forWhisker Formation Based on Local Microstructure and Grain Boundary Properties 1
Pylin Sarobol, Ying Wang, Wei-Hsun Chen, Aaron E. Pedigo, John P. Koppes, John E. Blendell and Carol A. Handwerker

1.1 Introduction, 1

1.2 Characteristics of Whisker and Hillock Growth from Surface Grains, 3

1.3 Summary and Recommendations, 17

Acknowledgments, 18

References, 19

2 Major Driving Forces and Growth Mechanisms for TinWhiskers 21
Eric Chason and Nitin Jadhav

2.1 Introduction, 21

2.2 Understanding the Mechanisms Behind Imc-Induced Stress Evolution and Whisker Growth, 24

2.3 Relation of Stress to Whisker Growth, 34

2.4 Conclusions, 39

Acknowledgments, 40

References, 40

3 Approaches of Modeling and Simulation of Stresses in Sn Finishes 43
Peng Su and Min Ding

3.1 Introduction, 43

3.2 Constitutive Model, 44

3.3 Strain Energy Density, 46

3.4 Grain Orientation, 46

3.5 Finite Element Modeling of Triple-Grain Junction, 48

3.6 Finite Element Modeling of Sn Finish with Multiple Grains, 55

References, 66

4 Properties and Whisker Formation Behavior of Tin-Based Alloy Finishes 69
Takahiko Kato and Asao Nishimura

4.1 Introduction, 69

4.2 General Properties of Tin-based Alloy Finishes (Asao Nishimura), 70

4.3 Effect of Alloying Elements on Whisker Formation and Mitigation (Asao Nishimura), 75

4.4 Dependence of Whisker Propensity of Matte Tin–Copper Finish on Copper Lead-Frame Material (Takahiko Kato), 89

4.5 Conclusions, 118

Acknowledgments, 118

References, 119

5 Characterization Techniques for Film Characteristics 125
Takahiko Kato and Yukiko Mizuguchi

5.1 Introduction, 125

5.2 TEM (Takahiko Kato), 125

5.3 SEM (Yukiko Mizuguchi), 140

5.4 EBSD (Yukiko Mizuguchi), 146

5.5 Conclusions, 154

Acknowledgments, 155

References, 155

6 Overview of Whisker-Mitigation Strategies for High-Reliability Electronic Systems 159
David Pinsky

6.1 Overview of Tin Whisker Risk Management, 159

6.2 Details of Tin Whisker Mitigation, 164

6.3 Managing Tin Whisker Risks at the System Level, 173

6.4 Control of Subcontractors and Suppliers, 183

6.5 Conclusions, 185

References, 185

7 Quantitative Assessment of Stress Relaxation in Tin Films by the Formation of Whiskers, Hillocks, and Other Surface Defects 187
Nicholas G. Clore, Dennis D. Fritz, Wei-Hsun Chen, Maureen E. Williams, John E. Blendell and Carol A. Handwerker

7.1 Introduction, 187

7.2 Surface-Defect Classification and Measurement Method, 189

7.3 Preparation and Storage Conditions of Electroplated Films on Substrates, 194

7.4 Surface Defect Formation as a Function of Tin Film Type, Substrate, and Storage Condition, 195

7.5 Conclusions, 209

Appendix, 209

Acknowledgments, 209

References, 213

8 Board Reflow Processes and their Effect on Tin Whisker Growth 215
Jasbir Bath

8.1 Introduction, 215

8.2 The Effect of Reflowed Components on Tin Whisker Growth in Terms of Grain Size and Grain Orientation Distribution, 215

8.3 Reflow Profiles and the Effect on Tin Whisker Growth, 216

8.4 Influence of Reflow Atmosphere and Flux on Tin Whisker Growth, 219

8.5 Effect of Solder Paste Volume on Component Tin Whisker Growth during Electronics Assembly, 220

8.6 Conclusions, 221

Acknowledgments, 222

References, 222

9 Mechanically Induced TinWhiskers 225
Tadahiro Shibutani and Michael Osterman

9.1 Introduction, 225

9.2 Overview of Mechanically Induced Tin Whisker Formation, 227

9.3 Theory, 228

9.4 Case Studies, 237

9.5 Conclusions, 245

References, 246

Index 249

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