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Mechanical Properties and Microstructural Fatigue Damage Evolution in Cyclically Loaded Lead-Free Solder Joints

Solder joints in electronic assemblies are typically subjected to cyclic loadings, either in actual applications or in accelerated life tests. Mismatches in the thermal expansion coefficients of the assembly materials lead to the solder joints being subjected to cyclic (positive/negative) mechanical shear strains and stresses when they are exposed to repetitive thermal cycles with fixed temperature extremes. This cyclic loading leads to thermomechanical fatigue damage that involves damage accumulation, crack initiation, crack propagation, and failure/fracture. The material damage that occurs during cyclic loading becomes immediately evident through the "load drop" and "widening" that occurs in the cyclic stress-strain curves as cycling progresses. Eventually, this damage leads to microcrack formation and recrystallization of the Sn grain structure. In the current work, a novel technique has been developed to characterize the fatigue damage evolution occurring in the lead free SAC305 and SAC_Q (SAC+Bi) solder joints subjected to shear mechanical cycling. The prime objective of this study was to better understand the damage accumulation and microstructural evolution in lead free solder joints subjected to cyclic fatigue loading. Specifically, both SAC305 and SAC_Q solder joints were fabricated, and then they were cycled for various durations up to values near failure. The fatigue life and accumulated inelastic work were also measured and correlated to the microstructural damage. In addition, nanoindentation tests were performed on the shear cycled joints to study the evolution of the joint creep properties occurring as a function of the duration of cycling and microstructural damage.