Experiments on the aging of Sn–Ag–Cu solder alloys

The microstructural stability of the Sn–3·8%Ag–0·7%Cu solder alloy was investigated by studying microstructural changes caused by heating small samples for various times, up to 1000 h, at 150°C. The first change, evident at high magnification after heating for 1 h, occurred from the as cast lamellar plus fibrous form of the Ag₃Sn and Cu₆Sn₅ interdendritic eutectic phases to a particulate form. With further heating, coarsening of the two compound phases occurred, gradually rendering the Sn dendrite pattern less distinct. Due to the very rapid diffusion of Cu in solid Sn, the Cu₆Sn₅ phase coarsened most rapidly, growing from its originally finely divided (200 nm) size in the ternary eutectic to form many particles up to 3 m m or more in size in a time of 100 h. At that time, nearly 50% of the total Cu was contained in these particles. The Ag₃Sn phase coarsened more slowly. Approximate measurements of average particle size as a function of time suggested that coarsening occurs by Ostwald ripening, controlled by diffusion in the Sn phase.