Time-Lapse Imaging of Bismuth Precipitation and Coarsening on the Surface of Sn-Ag-Cu-Bi Solder Joints After Thermal Cycling.

Adding bismuth to Sn-Ag-Cu solder compositions can significantly improve reliability in thermal cycling, but there are uncertainties in how bismuth precipitates and coarsens in Sn-Ag-Cu-Bi solders containing > 3 wt.% Bi. Here we apply time-lapse imaging in a scanning electron microscope to study bismuth precipitation and coarsening at room temperature on the polished surface of Sn-2.25-0.5Ag-6Bi ball grid array solder joints after thermal cycling. It is shown that (Bi) precipitates on the surface within 2 h after polishing and then coarsens by a combination of Ostwald ripening, coalescence ripening, and competition between two orientation relationships. Time-lapse imaging revealed that coalescence causes an increase in the local bismuth particle size and the formation of anomalously large (Bi) particles. The accumulation of bismuth on the polished surface increases far beyond the equilibrium volume fraction for this alloy. The bismuth particle size distributions are significantly wider than expected from Lifshitz-Slyozov-Wagner (LSW) theory, that assumes only Ostwald ripening, which is shown to be because coalescence creates anomalously large particles. This study shows the important role of bismuth precipitate coalescence within the coarsening mechanism in Sn-2.25-0.5Ag-6Bi solder joints.