Evolution of the Creep Behavior for SAC305 Lead Free Solder Exposed to Various Thermal Profiles

Abstract

In our prior study, the degradations of the effective elastic modulus (E) and ultimate tensile strength (UTS) of lead free solders were observed for samples subjected to isothermal aging and three different time dependent thermal exposure profiles. In the current paper, the evolutions of the creep behavior of SAC305 bulk solder under isothermal aging, slow thermal ramping, and slow thermal cycling exposures have been investigated. For creep testing, rectangular cross-section uniaxial test specimens were prepared using reflow solidification under a controlled temperature profile. After formation, the samples were subjected to either thermal aging, thermal ramping, or thermal cycling pre-conditioning exposures for various durations under stress-free conditions (no load). After the preconditioning was completed, creep testing was performed on the samples at room temperature.The three thermal profiles considered for preconditioning were (1) isothermal aging at 125 °C, (2) slow thermal cycling from -40 to +125 °C with 45 minutes ramps and 30 minutes dwells, (3) thermal ramping from -40 to +125 °C with 45 minutes ramps and no dwells. The thermal exposures times considered with the three profiles were 0, 1, 2, 5, and 20 days. Creep testing was performed on the exposed samples at three different stress levels (10, 12, 15 MPa). For each of the thermal profiles, the evolution (increase) of the secondary creep strain rate with exposure time was characterized. The evolutions were then compared. For long exposures, it was found that the creep rate degradation was highest for thermal cycling with a slow ramp rate. This effect was exacerbated for higher creep stress levels.