Interfacial intermetallic compound growth in Sn-3Ag-0.5Cu/Cu solder joints induced by stress gradient at cryogenic temperatures

During deep space exploration, electronic devices will be inevitably exposed to cryogenic temperatures. The effects of cryogenic temperature storage on the interfacial microstructure and mechanical behaviors of Sn-3Ag-0.5Cu/Cu (SAC305/Cu) solder joints were systematically investigated. The thickness of interfacial intermetallic compounds (IMCs) in the joints stored at −196 °C and −100 °C was found to gradually increase with the prolonging of storage time, and the morphology of the interfacial IMCs transformed from scallop type to column type. The growth rate of interfacial IMCs in the joints stored at −196 °C was faster than that stored at −100 °C. The stress gradient in the solder joints induced by thermal expansion mismatch was the primary driving force for atomic diffusion and interfacial IMC growth at cryogenic temperatures. The stress gradient in solder joints stored at −196 °C was higher than that stored at −100 °C, leading to a faster growth rate of interfacial IMCs at −196 °C. During cryogenic temperature storage, the interfacial IMC growth caused a volume shrinkage and thus generated residual stresses around the interface. As a result, the shear strength of the solder joints stored at −196 °C and −100 °C declined and the fracture location shifted towards the solder/IMC layer interface with the increase of storage time.