Early stages of localized recrystallization in Pb-free BGA solder joints subjected to thermomechanical stress

Abstract

This study quantitatively characterized the grain orientation evolution of Sn3.0Ag0.5Cu ball grid array (BGA) solder joints subjected to thermomechanical stress using electron backscatter diffraction (EBSD). Further polishing of the as-reflowed solder joint cross sections was avoided after thermal shock treatment to accurately analyze the underlying mechanism of recrystallization. No previous reports have been presented on this topic because of the inability to conduct pattern indexing due to cross-sectional contamination after thermomechanical fatigue. The results show that the pre-existing low-angle boundaries in the as-reflowed joints tend to gradually increase in misorientation and that low-angle boundaries inhomogeneously increase mainly by the cross-slip and climb of dislocations. Recovery tends to occur at the pre-existing low-angle boundaries in the as-reflowed joints and the regions in the solder joints near the package interface composed of a single grain, and there is a partial release of stored energy prior to recrystallization. However, the residual stored energy remains sufficiently high to induce subsequent recrystallization in the deformed solder alloy after subgrain formation. The transition process from subgrains to recrystallized grains is continuous, and subgrains appear before the formation of recrystallized grains in the Pb-free solder joints subjected to thermomechanical stress. The orientations of the small recrystallized grains separated by the low-angle boundaries evolve from the initial orientations by subgrain rotation. Additionally, the subgrain rotation behavior of Pb-free solder joints was investigated in terms of the rotation axis and angle. The results show that there is typically a single rotation axis for each subgrain in the early stages of recrystallization. This finding confirms the interrelationship between the activated β-Sn slip systems and the process of recrystallization in Pb-free solder joints.