Effects of local grain misorientation and β-Sn elastic anisotropy on whisker and hillock formation

A whisker and hillock growth model based on local film microstructure, grain misorientation, and elastic strain energy density (ESED) as the driving force for growth was developed to predict preferred sites for growth. Local grain orientations and strains measured by synchrotron microdiffraction in nine regions containing whiskers or hillocks were compared with elastic finite element analysis simulations including Sn elastic anisotropy. Whisker and hillock grains were observed to have higher crystallographic misorientations with neighboring grains than generally observed in the microstructure. While elastic simulations predicted higher local out-of-plane elastic strains and ESEDs at those locations with high misorientations before growth, synchrotron measurements of out-of-plane strains of whisker and hillock grains after growth showed relaxation, with correspondingly low ESEDs calculated from measured strains. Hence, highly localized out-of-plane elastic strains and ESEDs of grains with high relative misorientations with their neighbors determined, at least in part, which grains became whiskers or hillocks.