Controlling BGA joint microstructures using seed crystals

This paper overviews methods to catalyse the nucleation of tin in BGA solder joints and explores the potential of engineering tin nucleation to control joint microstructures. A range of heterogeneous nucleants for Sn is overviewed. It is shown that CoSn3, PtSn4, PdSn4 and IrSn4 can all catalyse Sn nucleation and substantially reduce the nucleation undercooling. The nucleation mechanisms are discussed in terms of a crystallographic lattice matching analysis, where each nucleant is shown to have good planar matching across the closest packed planes in each crystal structure. We then demonstrate an approach to incorporate the nucleants into solder joints as seed crystals to control the orientation of the tin nucleation event. With this approach, it is possible to generate single-crystal joints all with the same c-axis orientation.