Enhancing the shear properties of SAC305/Cu solder joints via microstructure modification induced by Bi-Co co-addition

In this study, a solder alloy system of Sn-3Ag-0.5Cu (SAC305) with the co-addition of Bi and Co was developed. The melting characteristics and microstructures of these solder alloys were systematically studied. The shear properties and interfaces of these solder alloys welding on Cu substrate were analyzed. The melting points were reduced to 211.2 °C ∼ 212.1 °C by Bi and Co addition, the undercoolings were correspondingly reduced to 2.1 ∼ 3.1 °C. The microstructures were refined by Bi and Co co-addition through forming heterogeneous nucleation sites. In addition, the co-addition of Bi and Co led to an initial decrease in the thickness of the intermetallic compound (IMC) layer at the solder/Cu interface (0-0.04 wt.% Co), followed by an increase (≥0.06 wt.% Co). Co atoms tend to enter the Cu 6 Sn 5 IMC, thereby forming interfacial (Cu, Co) 6 Sn 5 IMC layer. The (Cu, Co) 6 Sn 5 IMC exhibits a larger lattice distortion and refined grains than Cu 6 Sn 5 . Thuse, the larger lattice distortion and grain refinement contributed to the improved shear strength of SAC305-3Bi- x Co/Cu solder joints.