Microstructure evolution in a Pb-free solder alloy during mechanical fatigue

Microstructural evolution in a Sn-rich eutectic Sn-3.8Ag-0.7Cu solder alloy has been investigated in low cycle mechanical fatigue. Inhomogeneity in deformation occurred on a grain scale (determined by grain orientation and plastic anisotropy of Sn) and on a subgrain scale where persistent slip bands were observed in Sn dendrites. Microcracks are formed predominantly on interfaces between the persistent slip bands and the eutectic regions within grains. Grain boundary decohesion or sliding was not observed, which is in sharp contrast to recent findings in unconstrained thermal fatigue of Sn-3.8Ag-0.7Cu.