Nucleation and growth of Ag3Sn in Sn-Ag and Sn-Ag-Cu solder alloys

Large Ag3Sn plates in solder joints can affect the reliability of electronics, however, the factors affecting their nucleation and morphology are not well understood. Here, the faceted solidification of Ag3Sn was studied as a function of melt undercooling, revealing transitions from single crystal {001} plates to cyclic twinned plates and then to highly branched structures created by twin mediated branching. Real-time X-ray imaging proved that Ag3Sn cyclic twins come from a common point, indicating they initiate in the process of nucleation or in the very early stages of growth in the undercooled melt. Soldering to copper substrates significantly catalysed Ag3Sn nucleation. This is shown to be due to constitutional supercooling generated by Ag solute rejection into the liquid ahead of the Cu6Sn5 reaction layer, with additional contributions from geometrical catalysis in the grooves between Cu6Sn5 scallops and heterogeneous nucleation of Ag3Sn on Cu6Sn5. The relative ease of Ag3Sn nucleation on the Cu6Sn5 reaction layer is responsible for the large plates often reported in electronic solder joints.