Impact of cooling rate on microstructure and mechanical properties of lanthanum-doped tin-silver-copper lead free solders

Abstract

A detailed research study was carried out to suggest improvements in the properties of lanthanum-doped Tin-Silver-Copper (SAC) by eliminating the adverse effects caused by applying slow cooling rate during solidification. The effects of faster cooling rate during solidification on the microstructure and mechanical properties of lanthanum-doped SAC were studied. Scanning electron microscopic (SEM) images were studied and were further analyzed by ImageJ software to measure the average inter-metallic compounds (IMCs) size. These IMCs, present inside the bulk tin (Sn) matrix, are brittle in nature and have an adverse effect on the mechanical properties of the alloys. With faster cooling rate the IMCs formation was significantly controlled, both for the as cast and thermally aged samples and slow cooling rate led to undue IMCs growth. Optical microscopy with crossed polarized light was used to study the grains. The faster cooling rate resulted in much smaller grains and slow cooling rate caused an undue grain growth. MTS tensile machine was used to carry out quasistatic tensile tests to find out the mechanical properties. Yield stress and tensile strength were found to be greatly improved for the alloy with faster cooling rate. Finally, an optimum cooling rate was found to be 6°C/s.