Research on Reliability of SnAg1.8/Cu Solder Joints in Extreme Environment

In this paper, the interfacial microstructures and mechanical properties of SnAg1.8/Cu solder joint under different isothermal aging conditions and thermal shock have been studied. The interfacial microstructure of SnAg1.8/Cu solder joint under ­196 °C,100 °C, and 150 °C were studied. The results show that the growth of interfacial intermetallic compounds (IMC) during cryogenic temperature storage was mainly promoted by the stress gradient. The thickness of interfacial IMCs stored at ­196 °C, 100 °C, and 150 °C for 960 h were 3.842 μm, 3.345 μm, 7.244 μm respectively. It is found that there is a large stress gradient at the solder joint under ­196 °C, which leads to atomic diffusion and promotes the growth of IMC. The interfacial IMCs in SnAg1.8/Cu solder joints transformed from scallope-like type to column type at ­196 °C, and from scallope-like type to wave type at 100 °C and 150 °C. Moreover, the interfacial IMCs in the solder joints grows fastest at 150 °C. The mechanical properties of SnAg1.8/Cu solder joint were decreased after isothermal aging test and thermal shock test. After 960 h of storage at ­196 °C, 100 °C and 150 °C, the shear strength of the SnAg1.8/Cu solder joints decreased from 43.88 MPa to 34.88 MPa, 21.04 MPa, 19.67 MPa, respectively. With increasing cycles of thermal shock, crack initiation occurs in the Cu6Sn5 IMC layer. The research results provided some general design guidance and concepts for the optimal design of solder joints.