Microstructural Effects on the Isothermal Coarsening Kinetics of Ag3Sn Particles in Sn-3Ag-0.5Cu Solder Joints

The isothermal coarsening kinetics of fine equiaxed Ag 3 Sn particles are investigated in Sn-3Ag-0.5Cu/Cu ball grid array (BGA) solder joints without thermal cycling. It is shown that the average global coarsening kinetics can be well described by a general Lifshitz–Slyozov–Wagner (LSW)-type coarsening law. However, the particle size distributions contain anomalously large Ag 3 Sn particles, which are shown to be due to particle coalescence and accelerated coarsening at β -Sn grain boundaries and β -Sn/Cu 6 Sn 5 phase boundaries within the bulk solder. Ag 3 Sn coarsening is heterogeneous at the level of the microstructure and varies from joint to joint. Ag 3 Sn coarsening is found to be faster in multigrain joints than in single-grain joints due to accelerated coarsening at β -Sn grain boundaries. Accelerated Ag 3 Sn coarsening also occurs near the Cu 6 Sn 5 reaction layer due to the stress gradient generated by Cu 6 Sn 5 layer growth.