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.