Evolution of the Mechanical Behavior of Lead Free Solders Exposed to Thermal Cycling

Lead free electronic assemblies are often subjected to thermal cycling during qualification testing or during actual use. During the dwells at the constant high temperature extreme, the lead free solders joints will experience thermal aging phenomena, resulting in microstructural evolution and material property degradation. Additional aging effects can also occur in the ramp periods from low to high temperature. In this study, we have investigated the evolution of the mechanical behavior of SAC305 lead free solder materials exposed to various thermal cycling exposures. Uniaxial test specimens were prepared by reflowing solder in rectangular cross-section glass tubes with a controlled temperature profile. After reflow solidification, the samples were placed into the environmental chamber and thermally cycled from -40 C to +125 C under a stress-free condition (no load). Several thermal cycling profiles were examined including: (1) 90 minute cycles with 15 minutes ramps and 30 minutes dwells, (2) air-to-air thermal shock exposures with 30 minutes dwells and near instantaneous ramps, and (3) 30 minute cycles with 15 minutes ramps and 0 minutes dwells, (4) no cycling (simple aging at high temperature extreme). For each profile, sets of samples were exposed to various durations of cycling (e.g. 48, 96, and 240 cycles), which were equivalent to various aging times at the high temperature extreme of T = 125 C. The thermally cycled samples were then subjected to stress-strain testing to measure the mechanical properties including the effective elastic modulus (E) and the Ultimate Tensile Strength (UTS). For each cycling profile, the evolutions of the mechanical properties were characterized as a function of the cycling duration, as well as the net aging time at the high temperature extreme. The results for the various profiles were compared, and it was found that the detrimental effects of aging are accelerated in a thermal cycling environment. Test results revealed that the mechanical properties degradation for SAC305 are higher in thermal cycling with the same time at the high temperature extreme when compared to pure aging. For example, the elastic modulus and UTS of SAC305 reduced by 41%, and 38%, respectively, after 5 days of isothermal aging; whereas these properties reduced by 63%, and 43.5%, respectively after 5 days of equivalent aging under thermal profile #1 (240 cycles).