Recrystallization induced by subgrain rotation in Pb-free BGA solder joints under thermomechanical stress

- Further polishing of cross-sections was avoided after thermal shock.
- As-reflowed tricrystal joints often appear to be composed of two grains.
- Recrystallization occurred in strain concentration region by subgrain rotation.
- Recrystallization in solder joints was facilitated by the anisotropy of tin.
- Activated slip systems can be used to predict subgrain boundary formation.

In this paper, the grain orientation evolution of Sn3.0Ag0.5Cu ball grid array (BGA) solder joints under thermomechanical stress was characterized quantitatively using electron backscatter diffraction (EBSD). Further polishing of the cross-sections of as-reflowed solder joints was avoided after thermal shock treatment to accurately analyze the underlying recrystallization mechanism (no previous reports on this topic exist because of the inability of pattern indexing due to cross-section contamination after thermomechanical fatigue). The results showed that subgrains appeared before the formation of recrystallized grains in Pb-free solder joints under thermomechanical stress, and the orientations of the small recrystallized grains separated by high-angle grain boundaries evolved from the initial orientations via subgrain rotation. The localized recrystallization produced fine grains and grain boundary sliding in the solder joints during thermal shock treatment. Additionally, slip systems played an important role in recrystallization and could be used to predict the subgrain and recrystallized grain formation. Avoiding further polishing of the cross-sections of as-reflowed solder joints during thermal shock treatment is an effective method to accurately investigate the recrystallization behavior and grain orientation evolution (as a function of the subgrain and recrystallized grain rotation behavior) of Pb-free solder joints.