Some aspects of nucleation and growth in Pb free Sn-Ag-Cu solder

To investigate the minimum superheat necessary to solder components on printed circuit boards successfully using 95.5 wt-%Sn, 3.8 wt-%Ag, 0.7 wt-%Cu solder, experiments were carried out using separate solder balls of the type used in ball grid arrays. Significant differences in microstructure were observed depending on the peak temperature reached in the liquid on melting. On cooling, substantial undercooling was often observed, with values up to 18 K. Under some freezing conditions, the primary phase formed was Ag3Sn, while under other conditions the primary phase was Sn. The amount and type of eutectic microstructure formed was observed to vary with freezing conditions. The types of microstructure formed are illustrated. Nucleation phenomena and their effect on subsequent growth are discussed.