The effects of substitution of Bi for Sn on the soldering properties of near eutectic Sn-Ag-Cu soldering alloys

We have followed the effects of substitution of Sn with Bi to the near eutectic SAC compositions. It is shown that Bi substitutions up to less than 2% substitution improved the joint strengths peaking at 50 – 70MPa. EDS studies confirmed that the improvement in strength is attributable to solid solution strengthening. The strengthening effect of Bi diminishes vastly beyond 2% limit and are characterised by brittle failure at higher Bi concentrations. Thermal analyses indicating much higher melting range suggest that SACbased compositions with more than 2% Bi had undergone hyper eutectic solidification that would have led to the evolution of primary brittle Bi phase in the microstructure. XRD studies confirmed the existence of primary Bi phase in the alloys with higher concentrations of Bi. This places a limit of maximum of 2wt% on substitution of Bi for Sn in the SAC-Bi alloy in order to retain the benefits of improvement in soldering properties