Role of grain orientation in the failure of Sn-based solder joints under thermomechanical fatigue

Abstract

A small Pb-free solder joint exhibits an extremely strong anisotropy due to the bodycentered tetragonal (BCT) lattice structure of fl-Sn. Grain orientations can significantly in∞uence the failure mode of Pb-free solder joints under thermomechanical fatigue (TMF) due to the coe‐cient of thermal expansion (CTE) mismatch of fl-Sn grains. The research work in this paper focused on the microstructure and damage evolution of Sn3.0Ag0.5Cu BGA packages as well as individual Sn3.5Ag solder joints without constraints introduced by the package structure under TMF tests. The microstructure and damage evolution in cross-sections of solder joints under thermomechanical shock tests were characterized using optical microscopy with cross-polarized light and scanning electron microscopy (SEM), and orientations of Sn grains were determined by orientation imaging microscopy (OIM). During TMF, obvious recrystallization regions were observed with difierent thermomechanical responses depending on Sn grain orientations. It indicates that substantial stresses can build up at grain boundaries, leading to signiflcant grain boundary sliding. The results show that recrystallized grains prefer to nucleate along pre-existing high-angle grain boundaries and fatigue cracks tend to propagate intergranularly in recrystallized regions, leading