Effect of Mg on the Microstructure and Properties of Sn-Ag-Cu Lead-free Solder

In this paper, a SnAgCuMg solder alloy system was developed considering its potential as a direct substitute for Sn-Pb solder for electronic assembly. Ingots with additive 1%, 3% and 5% (wt) Mg to an eutectic Sn-4.0Ag-0.5Cu alloy were cast by means of a resistance furnace. To reduce the oxidation tendency of magnesium during melting and casting process, a special device was employed. The melting characteristic of SnAgCuMg solders was evaluated and compared with Sn-4.0Ag-0.5Cu alloy by differential thermal analysis (DTA). The microstructure observation was carried out by means of optical microscopy (OM) and scanning electron microscopy (SEM). It showed that SnAgCu solder tends to melt at lower temperature with the addition of Mg. Without typical Ag3Sn/β-Sn eutectic microstructure, SnAgCuMg solders present different microstructures from eutectic Sn-4.0Ag-0.5Cu. Some Sn, Ag, Mg-rich polygons were found among β-Sn matrix of SnAgCuMg solders by energy dispersed spectrum (EDS).