In the electronic assembly arena, lead is being targeted due to the concern regarding environmental pollutants. So, the lead-free solder and its reliability are getting highlighted. During qualification testing or actual use lead-free solders in electronic assemblies, they are often subjected to thermal cycling. In the lead-free solder material, microstructural evolution and material property degradation occurs due to the thermal aging phenomena during dwell periods at the high temperature extreme of thermal cycling. In addition, during ramping between low and high temperature extreme, lead-free solders can experience additional aging phenomena. In our prior work, we have compared material properties (stiffness and strength) degradation of lead-free solder materials in isothermal aging and five different thermal cycling exposures. Changes in material properties were higher for all the thermal cycling exposures compared to the aging. In addition, microstructural evolution and material property degradation were exacerbated as the ramp rate decreased in the thermal cycling.
In this study, the creep behavior evolutions occurring in SAC305 lead free solder subjected to isothermal aging and slow thermal cycling exposures have been investigated. Uniaxial test specimens were prepared by reflowing solder in rectangular cross-section glass tubes with a controlled temperature profile. Afterwards, the reflowed samples were exposed to either isothermal aging at 125 °C, or to thermal cycling from −40 to +125 °C, under a stress-free condition (no load) for various durations in an environmental chamber. A slow thermal cycling profile, e.g. 150 minutes cycle with 45 minutes ramps and 30 minutes dwells, was chosen for this study as it was found in our previous work that it caused the most detrimental effects on the mechanical behavior evolution. The thermally exposed samples were isothermally aged for 0, 1, 2, and 5 days; or were thermally cycled for 0, 48, 96, and 240 slow thermal cycles, which had the same aging times at the high temperature extreme of T = 125 °C.
After aging or cycling, creep testing was performed at room temperature on the thermally exposed samples at three different stress levels (10, 12, 15 MPa). The evolutions of the secondary creep strain rate were obtained as a function of the stress level, as well as the net aging time at the high temperature extreme, and then compared. Results showed that secondary creep strain rate increased dramatically with equivalent aging time, and that the degradation effects were larger for slow thermal cycling than for pure aging. For example, the creep rate increased by 3.0–3.4X for 5 days of pure aging at T = 125 °C; while they increased by 10.9–13.1X for 240 thermal cycles, when there had been 5 days of equivalent aging at the T = 125 °C high temperature extreme.