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Microstructure Evolution of Pb-Free Solder Alloys in Mechanical Fatigue Testing
Keywords: Pb-Free Solder, Microstructure, Reliability
In spite of the wide application of Pb-free solder alloys in microelectronic products, lifetime prediction of Pb-free solder joints under various thermo-mechanical load conditions in service is far from perfect. Accelerated thermal cycling (ATC) tests are often used to evaluate or to qualify solder joint reliability. However, life prediction by extrapolating ATC results to service conditions is hampered by a lack of acceleration models. Common Pb-free solder alloys are microstructurally unstable under practical conditions, due to their high homologous temperatures. As solder joints are thermally aged or cycled, the microstructure evolves so that the mechanical behavior keeps changing. Coarsening of secondary intermetallic compound precipitates and recrystallization of beta-Sn grains are two microstructure changes often observed in ATC tests. The development of acceleration models requires good understanding of such microstructure evolution and damage accumulation during fatigue testing. In this study, precipitate coarsening and recrystallization are decoupled by utilizing the combination of thermal aging and subsequent joint level mechanical fatigue testing. The objective is to identify the correlation of the two microstructural features and its effect on the fatigue damage accumulation, and eventually the effect on the fatigue life of solder joints. SAC305, SAC105 and eutectic SnAg alloys are selected to evaluate the effect of Ag content on the microstructure evolution.
Luke A. Wentlent, Graduate Research Associate
Binghamton University
Greene, NY

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