The Mechanics of Solder Alloy Wetting and Spreading, 1993

In 1992 Congress passed the Defense Manufacturing Engineering Education Act with the intent of encouraging academic institutions to increase their emphasis on manufacturing curricula. The need for this incentive to integrate the academic and industrial communities was clear: gaps in manufacturing science were inhibiting the evolution of new manufacturing technologies that are required for the U.S. to maintain a competitive posture in the world marketplace. The Army Research Laboratory and Sandia National Laboratories sought to contribute to the congressional intent by initiating a new series of graduate level college textbooks. The goal was to focus next-generation scientists onto issues that were common to the needs of the commercial market, the affordability of DoD weapons systems, and the mobilization readiness of the U.S. Armed Forces. The textbook The Mechanics of Solder Wetting and Spreading was written in this spirit by nationally renowned scientists for academe and industry. Research­ ers using the book are encouraged to formulate programs that will establish scien­ tific correlations between manufacturing process controls and product reliability. Such correlations are essential to the building of a new electronics industry which is based upon the futuristic concepts of Virtual Factories, Prototyping, and Testing.

1. Introduction: The Mechanics of Solder Alloy Wetting and Spreading.- 1.1 Soldering in Electronics.- 1.2 The Wetting Problem.- References.- 2. Solderability Testing.- 2.1 Introduction.- 2.1.1 Definition of Solderability.- 2.1.2 Why Test for Solderability.- 2.1.3 The Obstacles to Progress in Testing.- 2.2 The “Numbers” Problem.- 2.2.1 Dip Tests.- 2.2.2 Dip Tests Versus Six Sigma Quality.- 2.2.3 Wetting Balance (Surface Tension Balance).- 2.2.4 Microwetting Balance.- 2.2.5 Configured Capillary.- 2.3 The Consistency Problem.- 2.3.1 Sample-to-Sample.- 2.3.2 Operator-to-Operator and Location-to-Location.- 2.4 The Requirements Problem.- 2.5 The Aging Problem.- 2.6 Conclusions.- 2.7 Appendix.- 2.7.1 A Descriptive and Practical Definition of Wettability and Solderability.- 2.7.2 Wettability.- 2.7.3 Solderability.- 2.7.4 Solder Process Yield.- References.- 3. Fluxes and Flux Action.- 3.1 Introduction.- 3.2 Flux History.- 3.3 Flux Requirements.- 3.4 Rosin-Based Fluxes.- 3.4.1 Rosin Flux Formulations.- 3.4.2 Flux Contents.- 3.4.3 Chemistry of Rosin.- 3.4.4 Synthetic Resin/Rosins.- 3.4.5 Mechanistic Inference.- 3.4.6 Wetting Balance Performance of Rosin Fluxes.- 3.4.7 An Active-Constituent, Concentration-Dependent Mechanism.- 3.4.8 Autocatalysis and Chain Reaction Equations.- 3.4.9 Implications of Alternative Mechanisms.- 3.4.10 Effect of Impurities in Solder with Rosin Flux.- 3.4.11 Effects of Fluxes on Surface Tension.- 3.4.12 Flux/Oxide Reactions for CuO.- 3.4.13 Effect of Atmosphere.- 3.4.14 Reaction Temperature Effects.- 3.4.15 Use of Other Oxides.- 3.4.16 A Flux Activity Number.- 3.4.17 Thermodynamics of Flux Action.- 3.4.18 Abietic Acid, Decomposition, and Interactions.- 3.4.19 Triethanolamine-Hydrochloride (TEA-HCI) and Decomposition.- 3.4.20 Materials Combinations: Building a Solder Cream.- 3.4.21 Fourier Transform Infrared Analysis.- 3.4.22 Flux Residues.- 3.4.23 Rosin Flux Formulations.- 3.4.24 Low Solids Fluxes.- 3.5 Gaseous Fluxes.- 3.5.1 Nitrogen Atmospheres.- 3.5.2 N2/H2 Atmospheres (including Ar(H2)).- 3.5.3 N2 Reactive Mixtures.- 3.5.4 Analysis of Residues.- 3.6 Inorganic Fluxes.- 3.6.1 Mechanistic Studies.- 3.7 Effect of Solder Impurities on Solderability.- 3.7.1 An Electrochemical Mechanism.- 3.8 Solderability Tests.- 3.8.1 Visual Assessment.- 3.8.2 Area-of-Spread Test.- 3.8.3 Edge Dip and Capillary Rise Tests.- 3.8.4 Globule Test.- 3.8.5 Rotary Dip Test.- 3.8.6 Surface Tension Balance Test.- 3.8.7 Flux Action from Solderability Measurements.- 3.8.8 Status of Flux Development and Flux Action Studies.- References.- 4. Reactive Wetting and Intermetallic Formation.- 4.1 Introduction.- 4.2 Analysis of Solder Spreading Kinetics.- 4.2.1 Heat Flow.- 4.2.2 Fluid Flow.- 4.2.3 Spreading of Spherical Droplets.- 4.3 Contact Line Motion Over Obstacles.- 4.3.1 Static Contact Angles.- 4.3.2 Effective Contact Angle.- 4.4 Metallurgy of the Moving Contact Line.- 4.5 Thermodynamic Calculation of the Ternary Pb-Sn-Cu System.- 4.5.1 General Thermodynamic Description.- 4.5.2 Binary Pb-Sn.- 4.5.3 Binary Cu-Pb.- 4.5.4 Binary Cu-Sn.- 4.5.5 Ternary System Cu-Pb-Sn.- 4.6 Wetting Balance Studies on Cu6sn5 and Cu3Sn.- 4.7 Conclusion.- 4.8 Acknowledgments.- References.- 5. Loss of Solderability and Dewetting.- 5.1 Introduction.- 5.2 Characterization of Dewetting.- 5.3 Wetting Stability Diagrams.- 5.4 Dynamics of Wetting Instabilities: Physical Mechanisms.- 5.5 Intermetallic Formation.- 5.6 Conclusions.- 5.7 Acknowledgments.- References.- 6. Oxidation of Solder Coatings.- 6.1 Introduction.- 6.2 Tin-Lead-Copper System Metallurgy.- 6.2.1 Tin-Lead Morphology.- 6.2.2 Intermetallic Underlayer.- 6.3 Role of Oxides in Solderability Loss.- 6.3.1 Wettability and Heat Transfer Effects.- 6.3.2 Copper Substrate Effects.- 6.3.3 Tin and Lead Oxides.- 6.3.4 Relative Effects for Tin-Lead-Copper System.- 6.3.5 Overall Degradation Factors.- 6.4 Tin-Lead Oxidation.- 6.4.1 Thermodynamic Considerations.- 6.4.2 Oxides Formed.- 6.4.3 Oxidation Rate.- 6.4.4 Electrochemical Oxidation.- 6.5 Solderability Assessment Via Oxides.- 6.5.1 Sequential Electrochemical Reduction Analysis.- 6.5.2 Solderability Loss Mechanism.- 6.5.3 Solderability Prediction.- 6.5.4 Solderability Degradation Factors.- 6.5.5 Intermetallic Effects.- 6.6 Accelerated Aging.- 6.6.1 Accelerated Aging Procedures.- 6.6.2 Predictive Capability.- 6.6.3 Oxides Produced by Steam Aging.- 6.7 Future Directions.- References.- 7. Surface and Interface Energy Measurements.- 7.1 Abstract.- 7.2 Introduction.- 7.3 Basic Concepts.- 7.3.1 Definition of the Interface.- 7.3.2 Surface or Interface Energy.- 7.4 Equilibrium Conditions for a Curved Interface.- 7.5 Capillarity Effect in Solids.- 7.5.1 Surface Energy and Surface Stress.- 7.5.2 Anisotropy of Surface Energy.- 7.5.3 Equilibrium Condition at a Triple Point.- 7.6 Experimental Techniques.- 7.6.1 Solid-Vapor Interface Energy.- 7.6.2 Liquid-Vapor Interface Energy.- 7.6.3 Solid-Liquid Interface Energy.- 7.7 Conclusions.- 7.8 Acknowledgments.- References.- 8. Advanced Soldering Processes.- 8.1 Impetus for Change.- 8.1.1 What Would the Ideal Soldering Process Be Like?.- 8.1.2 Possible Enabling Elements of Advanced Soldering Technology.- 8.2 Alternative Approaches to Promote Wetting.- 8.2.1 No-Clean Fluxes.- 8.2.2 Controlled Atmosphere Soldering.- 8.2.3 Laser Ablative Cleaning and Soldering.- 8.2.4 Fluxless Ultrasonic Pretinning.- 8.3 Alternate Heat Sources.- 8.3.1 Laser Heat Sources.- 8.3.2 Focused Microwaves.- 8.4 Solder Bump Technology.- 8.5 Future Directions for Solder Process Technology.- 8.6 Acknowledgments.- References.- 9.0 Reliability-Related Solder Joint Inspection.- 9.1 Abstract.- 9.2 Motivation.- 9.3 Inspection Criteria.- 9.4 Inspection Technologies.- 9.4.1 Visual Systems.- 9.4.2 2-D Inspection Systems.- 9.4.3 3-D Reflectance Systems.- 9.4.4 X-Ray Systems.- 9.4.5 Acoustic Systems.- 9.4.6 Thermal Systems.- 9.5 Mantech/ADSP Solder Joint Inspection Example.- 9.5.1 Mantech/ADSP Visual Inspection Results (Phase 3).- 9.5.2 Mantech/ADSP Automated Inspection Results (Phase 3).- 9.6 Inspection Automation.- 9.7 Advanced Solder Joint Inspection Techniques.- 9.8 Conclusions.- References.- 10. The Properties of Composite Solders.- 10.1 Introduction.- 10.2 Mechanics of Solder Joints.- 10.2.1 Origin of Thermomechanical Fatigue in Solder Joints.- 10.2.2 Origin of Vibration and Shock in Solder Joints.- 10.2.3 Failure Mechanisms in Solder Joints.- 10.3 Alloy Design for Thermomechanical Fatigue Resistance.- 10.3.1 Solder Joints with “Superplastic” Microstructures.- 10.3.2 Alloy Additions to Homogenize the Microstructure.- 10.3.3 Other Solder Alloys.- 10.3.4 Composite Solder Alloys.- 10.4 Methods of Producing Composite Solder Alloys.- 10.4.1 Powder Blending.- 10.4.2 Mechanical Alloying.- 10.4.3 In-Situ Composite Solders by Rapid Solidification.- 10.4.4 Fiber-Reinforced Composites.- 10.5 In-Situ Composite Solders by Rapid Solidification.- 10.5.1 Preparation of Rapidly Solidified In-Situ Composite Solder Powders.- 10.5.2 Microstructures of In-Situ Composite Solders.- 10.6 Properties of Composite Solder Alloys.- 10.6.1 Wetting and Solderability of Composite Solder Alloys.- 10.6.2 Mechanical Properties of In-Situ Composite Solders.- 10.6.3 Mechanical Property Testing.- 10.6.4 Tensile Properties.- 10.6.5 Creep Properties.- 10.6.6 High-Cycle Fatigue Properties (Stress Amplitude Controlled Fatigue).- 10.6.7 Low-Cycle Fatigue Properties (Strain Amplitude Controlled Fatigue).- 10.6.8 Fractography.- 10.6.9 Creep-Fatigue Interactions.- 10.7 Summary.- References.