Comparative Mechanical Behavior of Sn-Bi based Low Temperature Solder Alloys under Different Pretest Aging Conditions

The use of Low Temperature Solders (LTS) as an alternative to high temperature Sn-Ag-Cu (SAC) alloys has gained increasing interest owing to the benefits associated with low reflow temperatures such as decrease in warpage during assembly, nontoxicity, and lower operating and energy costs. Sn-Bi alloy system, with reduced reflow temperature of approximately 183 degrees C, is a popular LTS alloy system. In addition to the existing literature on microstructural properties, there is a need to study and compare the mechanical properties of the new LTS solders. As part of this study, a baseline mechanical characterization of Sn3.0Ag0.5Cu and Sn58Bi solder compositions is first performed. Several alloying elements have been suggested to improve the reliability of Sn-Bi system, of which Ag, In and Sb have been studied in this work. Mechanical behavior of Sn57Bi1Ag, HRL3 (by MacDermid Alpha), L27 and L29 (by Senju Metal Industry) alloys are presented here. In addition, the effect of pretest aging (after reflow and aged to 10 days at 85 degrees C/125 degrees C) on the mechanical behavior is studied. A comprehensive comparison of the mechanical behavior of the four new LTS alloys against the baseline alloys is presented. A custom designed micro-precision mechanical tester is used to perform isothermal creep, monotonic and fatigue testing of the solder test specimen under shear at room temperature and an elevated temperature of 70 degrees C.