Study on ductile-to-brittle transition behavior and fracture mechanisms of Sn-based solder alloys under cryogenic impact loading

With the advancement of deep-space exploration and aerospace electronic devices, the reliability of solder joints under extreme cryogenic environments has become a critical challenge. This study systematically investigates the impact properties and fracture behaviors of three typical solder alloys, Sn-3.0Ag-0.5Cu (SAC305), Sn-37Pb and Sn-90Pb, over a temperature range of −196 °C to 50 °C. Charpy impact tests combined with macro/micro-fractographic analysis reveal that: (1) SAC305 experiences a ductile-to-brittle transition temperature (DBTT) between 0 °C and 10 °C, exhibiting complete brittle fracture at −196 °C with characteristic cleavage facets resembling “rock-candy” morphology; (2) Sn-37Pb shows a DBTT between −50 °C and −25 °C, displaying mixed ductile–brittle fracture features with quasi-cleavage planes and tear ridges at cryogenic temperatures; (3) Sn-90Pb maintains ductile fracture even at −196 °C due to the plastic superiority of Pb-rich phases, with no significant temperature dependence of the microstructure. Theoretical analysis demonstrates that the cryogenic brittleness of Sn (bct structure) dominates the fracture behavior of SAC305 and Sn-37Pb, while the high Pb content (fcc structure) preserves the plastic deformation capability of Sn-90Pb under extreme low temperatures. This work provides crucial guidance for solder selection and anti-embrittlement design in deep-space electronic devices.