Effects of Fe and Bi Minor Alloying on Mechanical, Thermal, and Microstructural Properties of Sn-0.7Cu Solder Alloy

The effects of adding 0.05 wt.% Fe and 1 wt.% or 2 wt.% Bi on the mechanical, thermal, and microstructural properties of Sn-0.7Cu solder alloy have been investigated. Addition of 0.05% Fe did not change the morphology of the microstructure, so the mechanical properties and melting behavior remained unchanged. Analyses showed that, besides inclusion of Fe in Cu 6 Sn 5 intermetallic compound (IMC), a few FeSn 2 IMCs formed in interdendritic regions. On the other hand, addition of 2% Bi to Sn-0.7Cu-0.05Fe alloy increased the yield strength by 97%, from 19.7 MPa to 38.84 MPa. This is because addition of Bi degraded interdendritic regions by reducing the amount of Cu 6 Sn 5 IMC particles and refining primary β-Sn dendrites. Furthermore, differential scanning calorimetry indicated that Bi reduced the melting temperature by 4°C from 232.2°C to 228.1°C for the 2% Bi alloy.