NEW LEAD-FREE, SN-AG-ZN-CU SOLDER ALLOY WITH IMPROVED MECHANICAL-PROPERTIES

A new, lead-free solder alloy based on Sn-3.5%Ag-1%Zn-0.5%Cu is descri bed. The alloy exhibits mechanical properties superior to those in bot h the Sn-3.5%Ag binary and Sn-3.5%Ag-1%Zn ternary eutectic solder allo ys. The addition of small amounts of Cu, at less than 1% levels, is fo und to refine the effective grain size while retaining the uniform dis tribution of Ag3Sn precipitates in the solidification microstructure, thus significantly improving the ductility. The addition of excess amo unts of Cu or Zn beyond about 1% levels is not desirable as it causes precipitation of additional intermetallic phases that depletes the fin ely dispersed precipitates in the surrounding matrix and induces nonun iformities in the microstructure that consequently deteriorate the mec hanical properties.