Investigating the Evolution of Creep Properties During Thermal Cycling of Homogeneous Lead-Free Solder Joints

Electronic assemblies are continually subjected to thermal–mechanical stres, leading to failure mainly due to creep and fatigue mechanisms. The reliability of the assembly may be compromised by the coarsening of intermetallic compounds IMCs) and intergranular fractures caused by dynamic recrystallization within the bulk solder joint due to these mechanisms. To address these issues, manufacturers are developing solder alloys micro-alloyed with bismuth (Bi), indium (In), and antimony (Sb) to enhance fatigue life and mitigate thermal cycling effects. This study investigated the effect of newly developed solder alloys on the reliability and evolution of mechanical properties during accelerated thermal cycling (ATC) tests. Five different solder alloys were tested, including SAC305, SAC-3.3Bi, SAC-0.5Bi-6In, SAC-0.5Bi-1.4Sb-0.15Ni, and Sn63Pb37. During the thermal cycling tests, the temperature ranged from −40 °C to +125 °C with a ramp rate of 10 °C/min, and the maximum and minimum temperatures were maintained for a dwell time of 15 min. The resulting failure data were analyzed using a two-parameter Weibull distribution. The microstructure evolution and steady-state creep properties were studied for 0, 100, 250, 625, 1560, and 3900 cycles. The study found that Bi-based alloys performed the best among the different components, followed by SAC305. Three stress levels for distinct aging conditions were defined through preliminary micro-indentation tests. An empirical model was developed to study steady-state creep rate systematically, and a power dependency prediction model was developed to investigate the creep mechanisms based on stress levels. Results showed that SAC305 had a significantly higher steady-state creep rate than tin-silver-copper (SAC)-Bi alloys due to the presence of Bi in the solid solution.