THE PROPERTIES OF TIN-BISMUTH ALLOY SOLDERS

Tin-bismuth alloys may be an alternative to lead-based solders for low-temperature applications, but very little is known about their manufacturability and reliability. This article presents an overview of these issues. First, experiments to determine the wetting properties of the Sn-Bi solder are presented. The results show that Sn-Bi solders do not wet bare copper well, but that they do wet copper having a hot-dipped Sn-Bi coating. Next, the effects of aging on the microstructure of Sn-Bi solders are described. The results show that during aging, tin is depleted from the solder/base metal interface. The two-phase Sn-Bi microstructure coarsens during aging; the rate of coarsening can be slowed by adding 1.0 wt.% Cu to the solder. The aging also affects the shear strength of the solder joints, where aged joints show an increase in maximum shear stress and ductility at failure.