Influence of gallium addition in Sn-Ag-Cu lead-fee solder

Sn-Ag-Cu based alloys are a series of important candidates of lead-free solders. However the melting temperatures are still higher than the traditional Sn-Pb solder. Gallium (Ga) is one of the candidate elements to be alloyed with the Sn-Ag-Cu solder. The present work aims at investigating the influence of Ga addition (<1.5 wt%) on properties of the Sn-Ag-Cu alloys, e.g. melting performance, wetting behaviour, formation of intermetallic compounds on the interface, and shear strength of the solder joints. When adding more Ga into the Sn-Ag-Cu alloy, the solder firstly shows poorer wettability (<2 wt%) while the wettability becomes better (>2 wt%). This phenomenon has been explained by change of interfacial energy qualitatively. It has also been found that addition of Ga can decrease the melting point of the Sn-Ag-Cu alloy after differential scanning calorimetry tests. As-cast microstructure of the Sn-Ag-Cu-Ga alloys has been analysed using optical microscopy. Thinner intermetallic compounds have formed when more Ga is added. The shear strength decreases slightly when Ga is added, but remains unchanged with more Ga addition.