Evolution of the Creep Response of SAC+Bi Lead-Free Solders Subjected to Various Thermal Exposures

Thermal cycling of lead-free solders in electronic packages is prevalent during qualification testing or actual use in electronic assemblies. Thermal aging causes mechanical and microstructural evolution while at high-temperature extremes during different thermal exposures. Such changes can also occur during other types of thermal exposure profiles. In our prior studies [1]–[3], we have examined the degradation of stress-strain curve material properties (modulus and strength) that occurs for several types of thermal exposures. Both SAC305 and SAC+Bi solders were investigated, and cycling and ramping thermal exposures were found to be more detrimental to the material properties than pure isothermal aging. In addition, the same thermal exposures were all found to cause extremely large increases in the creep strain rate for SAC305 solder [4]. In the current study, we have extended our prior work to examine the effects of thermal exposures on the creep behaviors of SAC+Bi solders. A controlled temperature profile was used during the preparation of rectangular cross-section uniaxial test specimens by reflow solidification. After fabrication, the samples were exposed to various types of thermal pre-conditioning (aging, cycling, or ramping) for varying amounts of time (0–20 days) in a stress-free environment (no load). Finally, creep testing at room temperature was performed on the pre-conditioned specimens. The changes in the creep strain rate for the SAC+Bi solder alloys were measured for each type of thermal exposure and for different exposure times. Results for the SAC+Bi alloys were then compared to our prior results for SAC305, and it was observed that adding bismuth had the beneficial result of significantly reducing the creep rate degradations. Higher levels of bismuth led to increased mitigations of the thermal degradation effects.