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

This study investigates how extending thermal cycling dwell times from 10 minutes to 60 minutes affects the reliability of third-generation high-reliability lead-free solder alloys. These alloys, based on the Sn-Ag-Cu (SAC) system with additions of bismuth (Bi), antimony (Sb), and indium (In), are designed for harsh automotive and aerospace environments. Researchers tested two ball grid array (BGA) components—the 192CABGA and 84CTBGA—under the aggressive -55/125 °C thermal cycling profile (TC7) defined in IPC-9701B. The study compares these results against a SAC305 baseline to determine if longer dwell times, which typically allow for more stress relaxation and creep, degrade performance as expected.

The high-reliability alloys consistently outperformed the SAC305 baseline. While literature typically suggests that longer dwell times reduce solder reliability due to increased creep and precipitate coarsening, this study found that high-reliability alloys maintained comparable performance between 10-minute and 60-minute dwells. Most notably, SAC305 itself exhibited no dwell time effect when tested with the 84CTBGA under the -55/125 °C condition. The authors hypothesize that at this high temperature range (ΔT = 180 °C), the strain-assisted diffusion caused by the coefficient of thermal expansion (CTE) mismatch between the component and the board becomes the dominant degradation mechanism, potentially overshadowing the time-dependent effects usually observed at lower temperature ranges.

Understanding the impact of dwell time is critical for predicting the lifespan of electronic assemblies in long-term, high-temperature service environments. The findings suggest that the industry-standard assumption—that longer dwell times always lead to significantly lower reliability—may not hold true for extremely aggressive thermal cycling conditions. This research provides essential data for engineers selecting materials for automotive and aerospace applications, while highlighting that failure modes are complex and often involve a mix of bulk fatigue and interfacial cracking.