Experiments on the aging of Sn-Ag-Cusolder alloys

The microstructural stability of the 3-8Sn -0.7Ag - Cu solder alloy was investigated by studying microstructural changes caused by heating small samples for various times, up to 1000 h, at 150degreesC. The first change, evident at high magnification after heating for 1h, 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. Because of 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 pm 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.