Recrystallized grain rotation behavior in a Pb-free BGA solder joint under electron current stress

Abstract

The grain orientations of a Pb-free ball grid arrays solder joint after thermomechanical fatigue (TMF) were characterized quantitatively using electron backscattered diffraction. Due to subgrain rotation, the small recrystallized grains evolved from the reflowed orientations appeared in a Pb-free solder joint subjected to thermomechanical stress. Also, these recrystallized grains rotated under electron current stress, indicating that the tin orientations of the Pb-free solder joints can significantly affect the response of the solder joints to service conditions such as TMF and electromigration. Meanwhile, two types of double twinning of tin in solder joints were observed in Pb-free solder joints. The change in orientation between the two groups of double twined orientations was in the range of 80°–90° (79.1°, 82.9° and 86.5° corresponding with 57.2° and 62.8°). Four orientations of tin grains, having the same twin grain with a [100] or [010] direction were observed in one of the systems, while the other one presented with two groups of tricrystals perpendicular to each other.