Characterization of Co–Sn intermetallic compounds in Sn–3.0Ag–0.5Cu–0.5Co lead-free solder alloy

Abstract

The minor addition of Co into Sn–3.0Ag–0.5Cu lead-free solder alloy triggered the formation of Co–Sn intermetallic compounds. The Sn–3.0Ag–0.5Cu–0.5Co solder alloy was heated up to 300 °C or 400 °C and then cooled down to the room temperature at different rates. A new Co–Sn intermetallic phase, say, CoSn 3 containing small amount of Cu, were detected. Only CoSn 3 phase was formed in the solder alloy from 300 °C regardless of the cooling rate. However, during the solidification from 400 °C, the CoSn 2 + CoSn 3 cascade structures were illustrated after slow furnace cooling due to the peritectical reaction, i.e., CoSn 2 + L(Sn) → CoSn 3, while only CoSn 2 was observed after rapid quench. A novel DSC technique was employed herein to demonstrate the presence of this peritectical reaction. The mechanical properties of the individual phases of Co–Sn intermetallics were measured and compared with other sole phases in the solder alloy.