Lead contamination of a transient liquid-phase-processed Sn-Bi lead-free solder paste

The influence of Pb contamination on the solidification behavior of a transient liquid-phase powder-processed Sn-Bi solder paste has been studied using differential scanning calorimetry. The development of low-temperature ternary reactions was found to be very sensitive to both the Pb and Bi content of the solder. Solders with high Bi content favored the formation of the ternary eutectic reaction. Solders with high Pb contents favored the formation of a ternary peritectic reaction. These results agree very well with solidification predictions present in the literature for ternary Sn-Bi-Pb alloys. In particular, the dependence of ternary reactions on composition is due to a change in solidification path. Alloy compositions which mark the transition from one path to the next were identified.