Mechanical properties and fracture mechanisms of Sn-3.0Ag-0.5Cu solder alloys and joints at cryogenic temperatures

For deep-space applications under cryogenic temperatures, the mechanical properties of Sn-3.0Ag-0.5Cu (SAC305) solder alloys and SAC305/Cu solder joints have been investigated over the temperature range 77-298 K. The fracture surfaces were analyzed in the plan view to identify the fracture locations in the solder joints, and the fracture mechanisms in solder alloys and solder joints were also explored. Results showed the tensile strength of the solder alloys firstly increased and reached its maximum value at 123 K, and then decreased with descending temperature. Meanwhile, the solder alloys displayed a shift in their fracture mechanism from dimple-ductile fracture to brittle fracture in a mixed model which was combined by transgranular and intergranular. It could be attributed to the asymmetric tetragonal structure of beta-Sn which is the main constituent of SAC305 solder. The tensile strength of SAC305/Cu joints showed increment at first and then decrement as the temperature decreased, while increasing the strain rate enhanced the tensile strength. In addition, the fracture location of the joints changed from the solder to the solder/IMC interface or the interfacial IMC layer with the decrement of temperature, resulting in the variation of the fracture mechanism from ductile fracture to brittle fracture.