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 degrees C to room temperature, and the fracture morphologies were observed. In the range of-150 degrees C similar to 0 degrees 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 degrees C and-196 degrees C, both alloys broke before necking. However, Sn37Pb still showed partial plastic deformation at 175 degrees C. Sn90Pb alloy showed ductile fracture in the range of 196 degrees C similar to degrees 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 similar to 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 degrees 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.