Subgrain rotation at twin grain boundaries of a lead-free solder joint during thermal shock

A ball grid array specimen with cross sectioned edge row was thermally shocked to investigate subgrain rotation of recrystallized region in lead-free solder joints. Scanning electron microscopy (SEM) and electron backscattered diffraction were used to obtain the microstructure and orientations of Sn grains or subgrains in as-reflowed and thermally shocked conditions. Orientation imaging microscopy showed that several subgrains were formed at the tilted twin grain boundaries after 200 thermal shocks due to high mismatched coefficient of thermal expansion of twin grains. And four subgrains in the solder joint were selected and divided into two parts to research the grain rotation behavior in lead-free solder joint. The analysis of subgrain rotation indicated that there were three ways of subgrain rotation during localized recrystallization after 200 thermal shocks, which were about the Sn [100], [101] and [110] axes. There were three slip systems (010)[], (101)[010] and (110)[] which closely related with the subgrain rotation about Sn [100], [101] and [110] axes, so the three ways of subgrain rotation were possible. Furthermore, SEM showed that a concave region was generated at the top of the joint after 200 thermal shocks due to the different misorientation angles between nonrecrystallized region and subgrains.