Mechanical Behavior Evolution of SAC+Bi Lead Free Solder 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 SAC+Bi (SAC_Q) lead free solder material under different thermal cycling loadings. The nominal chemical composition of the SAC+Bi solder in this work is 92.5% Sn, 4.0% Ag, 0.5% Cu, and 3.0% Bi. A controlled reflow profile was used to prepare rectangular solder samples for uniaxial tensile testing. Then, all the samples were preconditioned by thermal cycling from -40 C to +125 °C inside an environmental chamber under no load condition (stress-free). Several thermal cycling profiles were examined including: (1) 150 minute cycles with 45 minutes ramps and 30 minutes dwells (slow thermal cycling), (2) airto-air thermal shock exposures with 30 minutes dwells and near instantaneous ramps (thermal shock), (3) 90 minute cycles with 45 minutes ramps and 0 minutes dwells (thermal ramping), and (4) no cycling with simple aging at high temperature extreme (aging).For each profile, sets of samples were cycled for various durations (e.g. 48, 96, and 240 cycles), which resulted in various aging times at the high temperature extreme of T = 125 °C. The cycled samples were then subjected to stress-strain or creep testing to measure the mechanical properties including effective elastic modulus, Ultimate Tensile Strength (UTS), yield stress, and the creep strain rate. This paper will report on the observed changes in the stress-strain curves, effective modulus, and UTS. The evolutions of the stress-strain curves and associated mechanical properties for each cycling profile were characterized as a function of the cycling duration, as well as the total dwell time at high temperature extreme. For the various thermal cycling profiles, only small changes in the mechanical properties were observed for the SAC+Bi solder. Also, the results for the mechanical property evolutions of SAC+Bi (SAC_Q) were compared to those for SAC305 measured in our previous studies [1]-[2]. Significantly higher changes were observed in the SAC305 solder relative to the SAC+Bi alloy.