Heterogeneous Stress Relaxation Processes at Grain Boundaries in High-Sn Solder Films: Effects of Sn Anisotropy and Grain Geometry During Thermal Cycling

Four different types of stress relaxation responses have been observed in terms of local microstructural changes along grain boundaries (GBs) in large-grained high-Sn (Sn-3.0Ag-0.5Cu) solder films after thermal cycling. The grain boundaries were characterized using scanning electron microscopy, electron back scattered diffraction, and focused-ion beam (FIB) cross-section imaging. While the anisotropic coefficient of thermal expansion of Sn plays an important role in determining which boundaries have high local stresses relative to the film plane and trace of the grain boundary plane during thermal cycling, the four different relaxation behaviors of specific boundaries (no observable changes, surface defect/whisker formation, GB sliding, or a combination of GB sliding and whisker/defect formation) were determined by a combination of Sn anisotropy, the GB geometry, and crystallographic orientation relative to the film plane. The ability to separate GB sliding from surface defect formation provides insights into how long, straight whiskers form in polycrystalline thin films, particularly with respect to grain rotation at the early stages of their growth.