Correlation between microhardness and microstructure of low-temperature Sn-xBi-M(Ag, Sb) solders

The correlation between Vickers microhardness and microstructure of low-temperature Sn-xBi-M(Ag, Sb) solders was investigated. The β-Sn grains were refined with increasing Bi content, as well as the additions of Ag and Sb. Fine β-Sn grains in general should strengthen the mechanical properties of solders, however, the Vickers microhardness of Sn-xBi-M(Ag, Sb) solders decreased with increasing Bi content (i.e., fine β-Sn grains). It was shown that the microhardness of Sn-xBi-M(Ag, Sb) solders depended on the amount of Bi/β-Sn interfaces rather than the fine-grain strengthening effect. The smaller the amount of Bi/β-Sn interfaces was, the higher the Vickers microhardness of Sn-xBi-M(Ag, Sb) solders.