Effect of rare earth element additions on the microstructure and mechanical properties of tin-silver-bismuth solder

There are numerous intermetallic compounds (IMCs) with various shapes in a tin-silver-bismuth solder alloy. These IMCs can affect the mechanical properties of the solder and, therefore, the reliability of the joints. In this study, minimal rare earth elements added into the solder were adsorbed at the grain boundary of IMCs. This adsorption behavior changed the relationship between growth velocities of the various crystalline directions of the IMC polycrystalline, which resulted in finer particles and more uniform distribution of the IMC phase. The average size of IMC particles decreased from 0.20 μm to 0.12 μm, while their average distance decreased from 1.25 μm to 0.65 μm. These fine IMC particles made the alloy stronger and more ductile through dispersion strengthening.