Experimental and numerical studies on fast cyclic loading of eutectic tin-lead solder and relevant micromechanical issues are presented. High-frequency twin-lap shear tests on solder joints show cracking inside the solder but often connecting the intruded tips of the intermetallic. Finite element modeling was carried out to study the effect of intermetallic morphology. Without the influence of local phase coarsening, the intrusion of intermetallic into the solder alloy is seen to trigger strain localization which promotes failure. The effect of local phase coarsening was also studied numerically, taking into account the individual phase arrangement. A coarser phase structure always shows a faster accumulation of local plastic strain, leading to early failure. Such results, in agreement with typical thermomechanical fatigue features, cannot be obtained from the traditional argument of strength vs. microstructural size. Modeling of the entire lap-shear specimen was also conducted for the purpose of quantifying the deformation behavior. The exact geometry of solder is found to play a dominant role in affecting the shear response.

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