Sn-Bi-Zn-In 合金の微細構造と機械的特性に対する Bi 含有量の影響

In recent years, low-melting-temperature solders are required in electronics packaging. Among them, Sn-Bi alloys are expected to exhibit good wettability and strength. However, they have difficulty in ductility due to the brittle nature of Bi. In some research, to improve the mechanical property of Sn-Bi alloys, Zn and In are added. The modified alloy had better mechanical properties and the low-melting-temperature equivalent to Sn-58Bi. However, its ductility is not comparable to that of typical lead-free solder, Sn-3.0Ag-0.5Cu. Therefore, in this study, by reducing the content of Bi, we attempt to improve the ductility of Sn-Bi-Zn-In alloy. To investigate the effect of Bi content on the melting temperature, microstructure, and tensile strength were evaluated using Sn-xBi-2.6Zn-1.0In (x=45, 40, 35, 30, 25, 20 mass%) alloys. In addition, the fracture surfaces and nearby microstructures were observed to analyze the effect of the Bi content. As a result, the melting onset and offset temperatures increased with reducing the Bi content. In all alloys, the microstructures had Sn-rich phases, Bi-rich phases and Zn phases. As the Bi content was reduced, the morphology of Bi phase changed from layer-like eutectic phases to small round shape. Reducing the Bi content from 45 to 35 mass%, the ductility improved. However, with the decreasing Bi content from 35 to 20 mass%, the ductility was getting worse. The fracture surface analyses indicated that, their ductility was affected by the Bi-rich phase morphologies. Based on these results, the ductility enhancement with the enhancement with the 35 mass% Bi content would be understandable with the strain dispersion by the refined Bi-rich phases. The strength slightly increased decreasing the Bi content due to the decrease in the Sn-rich/Bi-rich phases interface.