Effect of Different Thermal Cycling Profiles on the Mechanical Behavior of SAC305 Lead Free Solder

Abstract

Lead free solder materials are used as interconnects in electronic assemblies due to their relatively high melting temperature, mechanical properties, and thermal cycling reliability, as well as their environmentally friendly chemical properties. Electronics devices can sometimes be subjected to harsh environmental conditions such as in aerospace engines, automotive power electronics, well drilling, and geothermal energy. Prior work has shown that the mechanical properties of lead-free solder materials are highly dependent on temperature, and also on their thermal exposure history such as aging and thermal cycling. In this work, the effects of several different thermal cycling exposures on the mechanical behavior evolution of lead-free solder materials have been investigated. In particular, the mechanical behavior variations with elapsed time have been measured for SAC305 lead free solder subjected to different thermal cycling profiles. The nominal chemical composition of the SAC305 solder in this work was 96.5% Sn, 3.0% Ag, and 0.5% Cu. The samples for uniaxial tensile testing were fabricated by carefully reflowing solder in rectangular shaped glass tubes with a controlled temperature profile. The prepared tensile samples were inserted into the environmental chamber under stress-free condition and cycled from -40 to +125 °C. Various thermal cycling profiles were investigated: (1) Slow Thermal Cycling: 90 minutes cycles with 45 minutes ramps and 30 minutes dwells, (2) Thermal Shock: air-to-air thermal shock exposures with 30 minutes dwells and near instantaneous ramps, (3) Fast Thermal Ramping: 30 minute cycles with 15 minutes ramps and 0 minutes dwells, (4) Slow Thermal Ramping: 90 minute cycles with 45 minutes ramps and 0 minutes dwells, and (5) Aging: isothermal aging at the high temperature extreme of T = 125°C. For each profile, sets of samples were exposed to cycling/aging for various durations of elapsed time (e.g. 0, 1, 2, and 5 days). Stress-strain tests at room temperature (T = 25°C) were performed on the cycled/aged samples, and mechanical properties such as the effective elastic modulus (E), Yield Stress (YS), and Ultimate Tensile Strength (UTS) were extracted. The evolutions of the stress-strain curves and associated mechanical properties for each thermal profile were characterized as a function of the cycling/aging duration (elapsed time) and then compared. After one day of thermal exposure, the degradations in elastic modulus and UTS were generally largest for the thermal aging exposure. However, after the first day of exposure, the degradation rates for the cycling exposures were typically higher than those for thermal aging. The largest degradations after 5 days of exposure were found in the specimens subjected to slow thermal cycling, followed by those subjected to thermal shock. In particular, the reductions in effective modulus and UTS were 59%, and 41%, respectively, for the slow thermal cycling exposure; while for thermal aging, the corresponding degradations were 41% and 38%. Further work is underway to understand the corresponding microstructural evolutions taking place in the samples for the various thermal profiles.