SOLDER JOINT REALIABILITY

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 me­ chanical "glue" of electronic assemblies. This pervasive dependency on solder has stimulated new interest in applica­ tions 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 geo­ metries. 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.

1. Flux Reactions and Solderability.- 1.1 Flux History.- 1.2 Solderability Tests.- 1.2.1 Visual Assessment.- 1.2.2 Area of Spread Test.- 1.2.3 Edge Dip and Capillary Rise Tests.- 1.2.4 Globule Test.- 1.2.5 Rotary Dip Test.- 1.2.6 Surface Tension Balance Test.- 1.3 Flux Action from Solderability Measurements.- 1.4 Flux Types.- 1.4.1 Mechanistic Studies for Inorganic Fluxes.- 1.4.2 Mechanistic Studies for Rosin-based Fluxes.- References.- 2. Solder Paste Technology and Applications.- 2.1 Chemical and Physical Characteristics.- 2.2 Fluxing and Fluxes.- 2.3 Solder Alloys.- 2.4 Solder Powder.- 2.5 Paste Formulation.- 2.6 Paste Rheology.- 2.7 Rheology Behavior Characterization.- 2.8 Viscosity and Measurement.- 2.9 Printing Technique.- 2.10 Dispensing Technique.- 2.11 Soldering Principle.- 2.12 Solderability.- 2.13 Soldering Methods.- 2.14 Controlled Atmosphere Soldering.- 2.15 Solvent Cleaning.- 2.16 Aqueous Cleaning and Aqueous Cleaning Paste.- 2.17 No-clean Paste.- 2.18 Fine Pitch Paste.- 2.19 Quality.- 2.20 Conclusion.- References.- 3. Technical Considerations in Vapor Phase and Infrared Solder Reflow Processes.- 3.1 Introduction to Surface Mount Reflow Soldering.- 3.2 Type I.- 3.3 Soldering Requirements for Surface Mount Technology.- 3.4 Reflow Process Phases.- 3.5 Reflow Equipment.- 3.5.1 Infrared.- 3.5.2 Vapor Phase.- 3.5.3 Convection.- 3.5.4 Conductive Belt.- 3.5.5 Laser Soldering.- 3.6 Prereflow Solder Paste Bake.- 3.7 Maximizing Solder Joint Yield.- 3.8 Reflow Processing.- 3.8.1 Vapor Phase.- 3.8.2 Infrared.- 3.8.3 Cost Comparison.- 3.9 SMT Reliability.- References.- 4. Optimizing the Wave Soldering Process.- 4.1 Basic Wave Soldering Process Overview.- 4.2 Wave Soldering Process Hardware.- 4.2.1 Fluxing.- 4.2.2 Fluxers.- 4.2.3 Fluxer Measurement Parameters.- 4.2.4 Fluxer Optimization.- 4.2.5 Preheating.- 4.2.6 Preheaters.- 4.2.7 Preheat Measurement Parameters.- 4.2.8 Preheat Optimization.- 4.2.9 Wave Soldering.- 4.2.10 Solder Waves.- 4.2.11 Solder Wave Measurement Parameters.- 4.2.12 Wave Soldering Optimization.- 4.2.13 Solidification.- 4.2.14 Conveyors.- 4.3 Wave Soldering Process Parameter Optimization.- 4.3.1 Optimization Procedure Test Study.- 4.4 Results.- 4.5 Conclusion.- References.- 5. Post-Solder Cleaning Considerations.- 5.1 Purpose and Chapter Description.- 5.2 Environmental Concerns.- 5.3 Definition of Soldering Flux.- 5.4 Specifications.- 5.4.1 Test Methods.- 5.4.2 Institute for Interconnecting and Packaging Electronic Circuits (IPC).- 5.4.3 U.S. Military.- 5.4.4 Telecommunications.- 5.5 Flux Materials and Associated Cleaning.- 5.5.1 Rosin.- 5.5.2 Water Soluble.- 5.5.3 Synthetic Activated.- 5.5.4 Low Solids (No-Clean).- 5.5.5 Controlled Atmosphere Soldering.- 5.6 Flux Application Methods.- 5.6.1 Wave.- 5.6.2 Foam.- 5.6.3 Spray.- 5.6.4 Rotating Drum Spray.- 5.6.5 Application Issues for Low Solids Fluxes.- 5.7 Process Issues Associated with Reliability.- 5.7.1 Flux Residue.- 5.7.2 Solder Ball Formation.- 5.7.3 Top-Side Fillet Formation.- 5.7.4 Conformal Coating Compatibility.- 5.8 Non-Liquid Fluxes.- 5.8.1 Core Solder Material.- 5.8.2 Solder Paste Material.- 5.9 Trends.- References.- Additional Readings.- 6. Scanning Electron Microscopy/Energy Dispersive X-Ray (SEM/EDX) Characterization of Solder—Solderability and Reliability.- 6.1 Scanning Electron Microscopy/Energy Dispersive X-ray Analysis.- 6.2 Other Methods—WDX.- 6.3 Detection Modes.- 6.4 Sample Preparation.- 6.5 Different Phases in Alloys.- 6.6 Intermetallics.- 6.7 Scope of the Chapter.- 6.8 SEM/EDX Characterization—General.- 6.8.1 Tin-Lead Solders.- 6.8.2 Two Percent Silver Solder.- 6.8.3 Gold-and Silver-Based Solders.- 6.8.4 Indium Solders.- 6.8.5 Bismuth Solders.- 6.8.6 Miscellaneous.- 6.9 Solderability Issues.- 6.9.1 Maintaining Solderability.- 6.9.2 Inadequate Tin Protective Coatings.- 6.9.3 The Dangers of “Forcing” Poor Solderability.- 6.10 Reliability Issues—Leaching of Substrate.- 6.11 Reliability Issues Gold Embrittlement.- 6.12 Reliability Issues—Fatigue.- References.- 7. The Role of Microstructure in Thermal Fatigue of Pb-Sn Solder Joints.- 7.1 Experimental Details.- 7.2 Eutectic Microstructures.- 7.2.1 Lamellar Eutectics.- 7.2.2 Degenerate Eutectics.- 7.2.3 Solder Joint Microstructures.- 7.2.4 Effects of Composition.- 7.2.5 Recrystallized Pb-Sn Microstructure.- 7.2.6 Coarsening Behavior.- 7.3 Mechanical Properties.- 7.3.1 Eutectic Structures.- 7.3.2 Deformation Mechanisms.- 7.4 Microstructural Evolution under Thermal Fatigue.- 7.4.1 Thermal Fatigue in Shear.- 7.4.2 Microstructural Mechanisms of Thermal Fatigue.- 7.4.3 Other Microstructures.- 7.5 Conclusion.- 7.6 Acknowledgments.- References.- 8. Microstructure and Mechanical Properties of Solder Alloys.- 8.1 Thermal Cycling Fatigue.- 8.2 Precipitation and Dissolution in Pb-Sn Alloys.- 8.3 Discussion.- References.- 9. The Interaction of Creep and Fatigue in Lead-Tin Solders.- 9.1 Current Approaches to Accelerated Testing.- 9.2 Damage by Fatigue and Creep Mechanisms.- 9.3 Assessing Actual Joint Damage.- 9.3.1 In-service Testing.- 9.4 Understanding the Damage Mechanisms.- 9.4.1 Creep and Tensile Test Results.- 9.4.2 Cyclic Creep.- 9.4.3 Hold Time Effects.- 9.5 Interpretation for Packaging Applications.- 9.5.1 Deformation.- 9.5.2 Thermomechanical Test Guidance.- 9.6 Concluding Remarks.- References.- 10. Creep and Stress Relaxation in Solder Joints.- 10.1 Ideal Expansivity of a Substrate.- 10.1.1 No Temperature Gradients, No Transients.- 10.1.2 Power Dissipation in the Component.- 10.1.3 Z-Gradients in the Substrate.- 10.1.4 In-Plane Gradients.- 10.1.5 Temperature Shock.- 10.1.6 Solder-Substrate Expansivity Mismatch.- 10.1.7 Overall Judgment.- 10.2 Creep and Stress Relaxation.- 10.3 Solder Properties.- 10.4 Constitutive Relations.- 10.5 Temperature Cycling.- 10.5.1 Small Temperature Range Cycling.- 10.6 Larger Temperature Cycles.- 10.7 Acknowledgments.- References.- 11. Effects of Strain Range, Ramp Time, Hold Time, and Temperature on Isothermal Fatigue Life of Tin-Lead Solder Alloys.- 11.1 Definition of Failure, Specimen Design, and Mode of Loading.- 11.2 Effect of Strain Range on Fatigue Life.- 11.3 Effect of Frequency on the Fatigue Life.- 11.4 Effect of Hold Time on Fatigue Life.- 11.5 Effect of Temperature on Isothermal Fatigue of Solders.- 11.6 Conclusion.- References.- 12. A Damage Integral Methodology for Thermal and Mechanical Fatigue of Solder Joints.- 12.1 Inelastic Deformation and Stress Calculation.- 12.1.1 Governing Equation for Solder Stress.- 12.1.2 Inelastic Deformation Behavior and Constitutive Relations.- 12.1.3 Stress Calculation.- 12.2 Damage Rate Formulation.- 12.2.1 Damage Mechanisms.- 12.2.2 A Phenomenological Formulation for Crack Growth Rates.- 12.3 Damage Integration and Failure Criterion Effects.- 12.3.1 Thermal Fatigue Life Estimation.- 12.3.2 Failure Criterion Effects.- 12.4 Discussion and Conclusions.- 12.5 Acknowledgments.- References.- 13. Modern Approaches to Fatigue Life Prediction of SMT Solder Joints.- 13.1 Mechanical Testing.- 13.1.1 Determination of Elastic Properties.- 13.1.2 Mechanical Properties.- 13.2 Life Prediction Techniques.- 13.2.1 Fatigue Models.- 13.3 Hybrid Life Prediction Techniques.- 13.3.1 Strain Range Partitioning Rule.- 13.4 Model Joints.- 13.4.1 Quality Control.- 13.4.2 Lap Joint Specimens.- 13.4.3 Straddle Board Specimens.- 13.5 Expert Systems.- 13.6 Conclusions.- 13.7 Acknowledgments.- References.- 14. Predicting Thermal and Mechanical Fatigue Lives from Isothermal Low Cycle Data.- 14.1 Low Cycle Fatigue (LCF).- 14.2 Low Cycle Fatigue of Solders—Influence of the Definition for Failure.- 14.3 Influence of the Temperature.- 14.4 Influence of Hold Times and Cycling Frequency.- 14.5 Influence of the Environment.- 14.6 Microstructural Changes.- 14.7 Determination of the Displacement and Strain Distribution in a Solder Joint.- 14.8 Prediction of the Fatigue Life of Solder Joints.- 14.9 Inherent Limitations to Fatigue Life Predictions.- 14.10 Necessary Further Work.- 14.11 Acknowledgments.- References.- 15. Static and Dynamic Analyses of Surface Mount Component Leads and Solder Joints.- Stiffness of Gull-Wing and J Leads and Solder Joints for Surface Mounted Chip Carriers.- 15.1 Boundary-Value Problem.- 15.2 Finite Element Methods.- 15.3 Stiffness of Gull-Wing Lead and Solder Joint.- 15.4 Stiffness of J Lead and Solder Joint.- 15.4.1 Unit Displacement (0.0001 in.) in the 1-Direction.- 15.4.2 Unit Displacement and Rotation in Other Directions.- 15.4.3 Comparison of the Stiffness Matrices between the PQFPs and PLCCs.- Solder Joint Reliability Under Shock and Vibration Conditions.- 15.5 Free Vibration of Soldered and Unsoldered Leads.- 15.5.1 Vibration Results for Wide SOICs.- 15.5.2 Vibration Results for Narrow SOICs.- 15.5.3 Vibration Results for PLCCs.- 15.5.4 Vibration Results for PQFPs.- 15.5.5 Experimental Verification.- 15.6 Free Vibration of a Constrained PCB with a SMC.- 15.7 Acknowledgments.- References.- 16. Integrated Matrix Creep: Application to Accelerated Testing and Lifetime Prediction.- 16.1 General Form of the Constitutive Relation.- 16.2 Development of the Constitutive Relation.- 16.2.1 Description of Data.- 16.2.2 Steady-State Creep Strain Component.- 16.2.3 Elastic Strain Component.- 16.2.4 Time Independent Plastic Strain Component.- 16.3 Summary of Constitutive Equation.- 16.4 Comparison of the Steady-State Creep Equation to Published Data.- 16.5 Application of Constitutive Equation to Data of Reference 2.- 16.5.1 Description of Numerical Procedures.- 16.5.2 Results.- 16.6 Multiaxial Stress States.- 16.6.1 Derivation of Constitutive Equation in Three Dimensions.- 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 Solomon.- 16.8 Analysis of Leaded Solder Joints.- 16.8.1 Extension of Matrix Creep Failure Indicator to General Case.- 16.8.2 Description of Model.- 16.8.3 Results.- 16.9 Conclusions.- 16.10 Acknowledgments.- References.- 17. Solder Joint Reliability, Accelerated Testing, and Result Evaluation.- 17.1 The Reliability of Electronic Assemblies and Solder Joint Reliability.- 17.1.1 “Bathtub” Reliability Curve—Hazard Rate Model.- 17.2 Solder Joint Loading Conditions and Reliability.- 17.3 Reliability and Accelerated Tested—Overview.- 17.4 The Thermal Expansion Mismatch Problem.- 17.4.1 Solution 1: CTE-Tailoring to Reduce Expansion Mismatch.- 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.- 17.5.2 Leadless Solder Attachments.- 17.5.3 Leaded Solder Attachments.- 17.5.4 Acceleration Transform.- 17.5.5 Failure Statistical Considerations.- 17.6 Accelerated Fatigue Reliability Testing.- 17.6.1 Testing Considerations.- 17.6.2 Accelerated Test Conditions.- 17.6.3 Test Vehicle Design.- 17.6.4 Sample Statistics and Solder Joint Defects.- 17.6.5 Test Vehicle Assembly and Conditioning.- 17.6.6 Failure Definition and Detection.- 17.7 Prediction of SM Solder Joint Reliability.- 17.7.1 Simple Cyclic Load Histories.- 17.7.2 Multiple Cyclic Load Histories.- 17.8 Acknowledgments.- References.- 18. Surface Mount Attachment Reliability and Figures of Merit for Design for Reliability.- 18.1 Mechanics and Fatigue of SM Solder Joints.- 18.1.1 SM Leadless Attachments.- 18.1.2 SM Leaded Attachments.- 18.1.3 Failure Distribution.- 18.2 Figures of Merit for Attachment Reliability.- 18.2.1 Derivation of FM Formulas.- 18.2.2 FMs for Multiple Thermal Fluctuations.- 18.2.3 Graphical Interpretation and Reliability Charts.- 18.3 Examples.- 18.3.1 Example 18-1: Chip Components on FR-4 Printed Wiring Boards.- 18.3.2 Example 18-2: 50-Mil Pitch Ceramic Leaded Devices on FR-4.- 18.3.3 Example 18-3: 25- and 50-Mil Pitch Plastic Leaded Components on FR-4.- 18.4 Concluding Remarks.- 18.5 Acknowledgments.- Appendix 18-A FM Formulas and Scaling Constants.- Appendix 18-B FMs for Multiple Thermal Fluctuations.- References.- Authors’ Biographies.