Recrystallization as a nucleation mechanism for whiskers and hillocks on thermally cycled Sn-alloy solder films

Direct evidence was obtained for the nucleation of surface grains through recrystallization and their growth into whiskers and hillocks during thermal cycling of Sn-alloy films. Shallow grains with low dislocation densities nucleated at pre-existing film grain boundaries and grew at the expense of deformed parent grains. Micro-diffraction analysis showed that parent grains have a high grain orientation spread, subgrain formation, and high dislocation density. The whisker and hillock grains have different orientations from the parent grains. A crystallographic orientation relationship between a surface defect grain and one of the parent grains was demonstrated with the misorientation axis lying in the active slip planes of the parent grain.

► Recrystallization as a nucleation mechanism for whiskers and hillocks was proven.
► Micro-diffraction was used to follow orientation change, deformation, and nucleation.
► Parent grains (high dislocation density) had deformed, rotated, and formed subgrains.
► Whisker/hillock (low dislocation density) grew from nucleated surface grains.
► Whisker-parent grain orientation relationship and operating slip systems were found.