Effects of a Third Element on Microstructure and Mechanical Properties of Eutectic Sn–Bi Solder

The effects of a third element, namely silver, copper, zinc, or antimony, on the microstructure and mechanical properties of eutectic tin-bismuth (Sn–Bi) solder were investigated. The investigation showed that, except for zinc, the addition of a trace amount of the third element improves the ductility of the Sn–Bi solder owing to the formation of a fine, homogeneous ternary eutectic microstructure. In particular, the antimony addition is the most effective in improving solder ductility. That is to say, the addition of 0.5 wt% antimony minimizes the grain size of the eutectic microstructure and increases the elongation up to about 40%. Moreover, an intermetallic compound, namely, SnSb, precipitated finely from the solid tin solution near the grain boundaries with bismuth. This fine precipitated intermetallic compound suppresses the coarsening of the eutectic structure and thus improves solder ductility.