Ductile-brittle transition during tensile tests of the general solder alloys at cryogenic temperatures

The ductile-brittle transition at low temperature behavior of the general solder alloys is important for the reliability of solder joints operating at a wide cryogenic temperature range. In this study, tensile deformation behavior of Sn37Pb, Sn-3Ag-0.5Cu and Sn90Pb solder alloys was investigated within the temperature range from -196 °C to room temperature, and the fracture morphologies were observed. In the range of -150 °C ~ 0 °C, the tensile deformation process of Sn37Pb and Sn-3Ag-0.5Cu solder alloys exhibited typical three distinct stages: the elastic deformation stage, the plastic deformation stage, the necking and fracture stage. At -175 °C and -196 °C, both alloys broke before necking. However, Sn37Pb still showed partial plastic deformation at -175 °C. Sn90Pb alloy showed ductile fracture in the range of -196 °C ~ 0 °C, and there was no phenomenon of ductile-brittle transition. The yield strength (YS) of the three alloys decreased with increasing tensile temperature, and the YS of Sn37Pb and Sn-3Ag-0.5Cu alloys were always higher than that of Sn90Pb, which was always maintained at about 25 ~ 30 MPa. The ultimate strength (US) of Sn37Pb and Sn-3Ag-0.5Cu alloys reached the maximum near the ductile-brittle-transition temperature. Since there was no ductile-brittle-transition in Sn90Pb, the highest US appeared at - 196 °C. For the ductile fracture samples, numerous dimples were distributed on the fracture surface. For the brittle fracture samples, the fracture basically presented cleavage characteristics or rock candy intergranular fracture characteristics.