Fabrication of joint based on Cu@Sn@Ag core–shell preform under ambient atmosphere for high-temperature applications

This paper presents a novel soldering material for stator winding lead wire of high-power generator based on Cu@Sn@Ag core–shell preform. The Cu@Sn@Ag preform can be reflowed at 280 °C for 10 min under a pressure of 0.5 MPa in the atmosphere and the resulting bondline can withstand high temperature up to 450 °C, which meets the ultimate service temperature for stator winding up to 370 °C. The results show that Sn phase was totally transformed into Cu3Sn and Ag3Sn phases, surrounding Cu particles after reflowing. The Ag3Sn phase surrounded outside the Cu3Sn layer and filled the voids caused by volume shrinkage of Cu3Sn phase and thus formed the dense interconnect interface of Cu3Sn/Ag3Sn. The elastic modulus and microhardness of the Cu/Cu3Sn/Ag3Sn bonding bulk were reduced by 15% and 2% respectively by forming the dense interconnecting interface compared to Cu/Sn system, indicating that low-circle fatigue resistance performance of Cu/Sn/Ag system is excellent. The electrical conductivity of bondline was 5.4 μΩ·cm and thermal conductivity was 144.182 W·m−1·K−1. The shear strength of the joints was 44.9 MPa and 40.3 MPa as reflow and after aging at 250 °C for 216 h, respectively. This novel preform shows great potential in high-temperature applications.