Mechanisms of beta-Sn nucleation and microstructure evolution in Sn-Ag-Cu solders containing titanium

The mechanisms by which Ti additions catalyse the nucleation of beta-Sn are studied in 550 mu m Sn-3Ag-0.5Cu (wt%) solder balls and joints on Cu and Ni substrates. It is shown that at least two new intermetallic compounds (IMCs), Ti2Sn3 and (Ti,Fe,Cu)Sn-2, form as a result of a 0.2 wt% Ti addition. The nucleation potential of each IMC was studied by electron backscatter diffraction (EBSD) of tin droplets solidified on the cross sectioned facets of each IMC. It is found that reproducible orientation relationships (ORs) form only between beta-Sn and Ti2Sn3 and that the two ORs generate good atomic matching between the Sn atoms in Ti2Sn3 and the closest packed plane in beta-Sn, {100}. beta-Sn cyclic twinning occurred in droplets on Ti2Sn3 where the twinning axis < 100 > Sn was always parallel with the lowest disregistry direction in the ORs. In solder balls and joints, the Ti2Sn3 addition triggered up to 12 independent beta-Sn grains, whereas Ti-free SAC305 always solidified with one independent grain. (C) 2018 Elsevier B.V. All rights reserved.