Effect of bonding time on the microstructure and mechanical properties of Co/Sn/Cu joint

The bonding process of Co/Sn/Cu couple has been conducted at 270 °C. The microstructure evolution was observed. The spreading test of Cu/Sn–xCo and Co/Sn–xCu was conducted to explain the microstructure evolution of Co/Sn/Cu couple. The results showed that the main interfacial intermetallic compounds (IMCs) layer of Cu/Sn side of Co/Sn/Cu couple was (Cu, Co)6Sn5, as the dissolution of Co from Co substrate to solder layer. The (Cu, Co)6Sn5 IMCs layer only thickened slightly during bonding process as large amount of the (Cu, Co)6Sn5 IMCs layer dissociated into solder alloy layer. As the adsorbing effect of Co substrate on the dissociating (Cu, Co)6Sn5 IMCs, there was a thick (Cu, Co)6Sn5 IMCs layer covered on the surface of Co substrate at the early stage of bonding process. For longer bonding time (> 30 min), the (Co, Cu)Sn and (Co, Cu)Sn3 IMCs generated on the Co/(Cu, Co)6Sn5 interface as the diffusion of Sn atom from liquid solder alloy to Co/(Cu, Co)6Sn5 interface. The Co/Sn/Cu joint bonded for 60 min could work at the temperature of 300 °C. The shear strength of joints bonded for 20 and 30 min was approximate 45 MPa. The shear strength of joints bonded for a time more than 30 min declined gradually as the generation of (Co, Cu)Sn3 IMCs.