Wen Jiang, Changwei Wang, Kangning Han, Kefeng Ye, Yaxin Zhu, Chuantao Hou, Ruisi Xing
The mechanical properties of solder alloys at extreme temperatures are critical for advanced aerospace and telecommunications applications. This study reveals an exceptional strength–ductility synergy in the low-Ag SAC0307 solder alloy at cryogenic temperatures, which contrasts with the typical brittle behavior of many metals. We systematically elucidate the effects of annealing (75 °C and 125 °C) on tensile properties and fracture behavior across a wide temperature range (120 °C to −150 °C). The results show that lowering the temperature to −150 °C simultaneously increases both ultimate tensile strength and ductility. Annealing further enhances this cryogenic ductility. Microstructural analysis indicates that the superior cryogenic performance arises from a shift in the deformation mechanism: from dislocation-dominated plasticity at high temperatures to a synergy of grain refinement, stacking faults, and deformation nanotwins at −150 °C. The interaction among dislocations, stacking faults, and twins forms a multi-level nanostructure that facilitates simultaneous strengthening and toughening. In addition, annealing promotes microstructural homogenization and increases the energy barrier for brittle fracture, suppressing the ductile-to-brittle transition. This work provides a mechanistic understanding of the cryogenic toughness in low-Ag solder alloys and offers guidelines for optimizing their thermomechanical reliability.