The Effect of Dwell Time on Thermal Cycling Performance of High Reliability Solder Alloys with a -55/ 125 °C Test Condition

As electronic assemblies in automotive and aerospace sectors face increasingly harsh operating environments, traditional Sn-Ag-Cu (SAC) solder alloys often fail to meet reliability requirements. This study, part of a multi-consortium effort, evaluates third-generation high-reliability Pb-free solder alloys modified with bismuth (Bi), antimony (Sb), and indium (In). Researchers compared these alloys against a SAC305 baseline using two ball grid array (BGA) components (192CABGA and 84CTBGA) under an aggressive -55/125 °C thermal cycling condition (TC7). A key focus was the effect of increasing dwell time from 10 minutes to 60 minutes, a variable known to accelerate solder degradation in less severe conditions.

The study reveals that high-reliability alloys consistently outperform SAC305 in thermal fatigue resistance. While literature typically suggests that longer dwell times reduce solder reliability due to increased creep and stress relaxation, this study found that the dwell time effect is significantly diminished or absent under the -55/125 °C test condition. Specifically, for the 84CTBGA, SAC305 itself exhibited no dwell time effect, which contradicts findings from milder test conditions. The authors hypothesize that at this high temperature range, the strain-assisted diffusion caused by the large coefficient of thermal expansion (CTE) mismatch dominates the degradation process, effectively masking the influence of dwell time.

These findings suggest that the reliability benefits of high-performance alloys are robust even under extended dwell times in extreme thermal environments. However, the results also caution researchers against assuming that dwell time sensitivity is a universal constant across all test conditions. The aggressive nature of the -55/125 °C profile may create a high-strain environment that alters the fundamental failure mechanisms, making it difficult to extrapolate these results directly to standard operational use. Understanding the interplay between strain and temperature-dependent diffusion is critical for accurate reliability modeling.