Dissolution Rates and Reliability Effects of Au, Ag, Ni and Cu in Lead Base Solders

The rates at which Au, Ag, Cu, and Ni wires dissolved in 95wt%Pb-5%Sn and 92.5wt% Pb-5%In-2.57%Ag soft solders were measured between 360 and 500°C. The rates have an Arrhenius behavior with temperature and for both solders nickel has the lowest dissolution rate while gold has the highest. A model is discussed, using the analysis of Lommel and Chalmers, which allows prediction of relative dissolution rates into Pb-Sn solders with any composition. Small amounts of Au and Ag, purposely added to 95wt%Pb-5%Sn die bonds in test power transistors, were found to deleteriously affect the fatigue resistance of the solder as measured during power cycling. Analysis of fracture surfaces and cross sections indicated that most of the gold forms tin intermetallics at the die-solder interface while the silver dissolved into the molten solder.