Shear strength and fracture analysis of Sn-9Zn-2.5Bi-1.5In and Sn-3.0Ag-0.5Cu pastes with Cu-substrate joints under different reflow times

Sn-Zn solders have been considered as promising lead-free solders. This study investigated the shear strength and fracture mechanism of Sn-9Zn-2.5Bi-1.5In and Sn-3.0Ag-0.5Cu pastes with Cu-substrate joints. It is found that the intermetallic compounds (IMC) in Sn-Ag-Cu joints are Cu6Sn5 and Cu3Sn, while the IMC of Sn-Zn joints are particular Cu5Zn8, different from other Sn-based solders, which grow more slowly. Both solder joints fracture by a mixed mechanism. The cracks tend to form and propagate at the interface of the solder and IMC in the cases of too short or too long reflow times, resulting in more interfacial fracture and lower shear strength. An optimum soldering zone of reflow time will bring about more reliable joints, which fracture mainly in the solder. Sn-Zn joints have more solder fracture and less interface fracture, therefore have higher shear strength and reliability, compared with Sn-Ag-Cu joints. For Sn-Zn joints, the optimum reflow times are 20 s-120 s with the shear strength of 62 MPa-70 MPa, while for Sn-Ag-Cu joints, the optimum reflow times are 50 s-180 s, with the strength of 50 MPa-62 MPa.