The effect of small additions of copper on the aging kinetics of the intermetallic layer and intermetallic particles of eutectic tin-silver solder joints

The effects of small additions of copper to the aging kinetics of eutectic tin-silver solder joints were studied. Aging of joints made with tin-silver solder containing 0.5% and 0.7% copper was carried out at various temperatures ranging to 180°C, at various times ranging to 4000 h. The small differences in the compositions of the two types of solder alloys studied did not produce any significant differences in the aging kinetics of the intermetallic layer, although a noticeable difference in the aging kinetics of the Ag₃Sn particles present in the solder was observed. The growth rates of all intermetallics present in the solder joint were insignificant below 120°C. The results obtained were compared to earlier aging experiments involving eutectic tin-silver solder without any copper. It was found that the activation energy of the Cu₆Sn₅ intermetallic layer growth was about 10% less in the tin-silver solder joints with the small additions of copper added to the composition. Copper from the substrate tended to diffuse through the Cu-Sn intermetallic layer faster than tin from the solder.