Microstructure development during directional solidification of Sn–Ag–Cu ternary alloys

Abstract

Replace of Pb–Sn with Pb-free solders (LFS) is one of the most important issues in the electronic industry. Eutectic and near-eutectic Sn–Ag–Cu (SAC) alloys are recommended as lead-free replacement in welding processes of electronic devices. Close to the eutectic ternary composition, the solidification occurs with three distinctive phases: Sn-rich dendritic primary phase, and the intermetallic phases (Formula presented.) and (Formula presented.), showing a limited solubility into the Sn-rich phase. Given the key role of the solidification process in the microstructure development of the alloy, samples of Sn–Ag–Cu with a composition close to the ternary eutectic were directionally grown to analyze the mechanisms involved in the solidification process of these alloys. The typical microstructure agrees withe above description, but shows two different mechanisms, depending on the interface advance velocity, resulting in different distribution of the intermetallic phases.