Effects of impurities on double twinning nucleation and grain refinement of Sn-based solder joints

As-solidified Sn3.0Ag0.5Cu + 3.0POSS (polyhedral oligomeric silsesquioxanes), Sn3.0Ag0.5Cu + 0.05CNTs (carbon nanotubes) and Sn3.0Ag3.0Bi3.0In (SABI333) solder joint orientations were quantitatively characterized by electron backscatter diffraction observations. Unlike the typical Sn-based solder joints in which single grains, tricrystals or double tricrystals are normally observed, for the studied joints, the frequency of the single-grained joints was greatly reduced, and the Sn3.0Ag0.5Cu + 3.0POSS and SABI333 solder joints typically consisted of polycrystals. If the Sn dendrites deviated by more than 15° from the preferred [110] and \([1{\bar{1}}0]\) directions, then these polycrystals solidified from the Sn–Ag-based melt. Moreover, Bi and In atoms in the Sn–Ag-based alloys could significantly change the preferred [110] and \([1{\bar{1}}0]\) directions. In addition, two types of double twinning of tin were observed during reflow in the Pb-free solder joints. Different morphologies were observed between these twinned microstructures. However, the misorientations of the two groups of double twinned orientations ranged from 80° to 90°. In one of the double twinned systems, four Sn grain orientations had the same twin grain with a common [100] or [010] direction; in the other double twinned system, two groups of tricrystals were observed perpendicular to each other.