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 β-Sn are studied in 550 μm 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)Sn₂, 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 β-Sn and Ti₂Sn₃ and that the two ORs generate good atomic matching between the Sn atoms in Ti₂Sn₃ and the closest packed plane in β-Sn, {100}. β-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 β-Sn grains, whereas Ti-free SAC305 always solidified with one independent grain.