Mechanical Property and Microstructure Evolution in SAC and $\text{SAC} +\mathrm{X}$ Lead Free Solders Exposed to Various Thermal Cycling Profiles

Solder joints in electronic packages are frequently exposed to thermal cycling environments where temperature variations occur from very low to high temperature. These exposures can occur in real life applications as well as in accelerated thermal cycling tests used for the characterization of thermal-mechanical fatigue behavior. Due to the CTE mismatches of the assembly materials, cyclic temperature leads to damage accumulation due to shear fatigue in the solder joints. In addition, the thermal cycling dwell periods at the high temperature extremes will cause thermal aging phenomena in the solder material. This leads to additional microstructural evolution and material property degradation. Further aging effects can occur during the ramp periods between the low and high temperature extremes of the cycling. While changes in solder materials during aging have been examined in detail in our prior studies published at ECTC, there have been limited studies examining material evolution occurring during thermal cycling. In our paper at ECTC 2020, the mechanical behavior evolutions in both miniature bulk SAC305 solder samples and SAC305 solder joints have been investigated under the same slow thermal cycling profile from to +125 °C for up to 5 days of exposure, and then the results were compared. Severe degradations in the mechanical properties (modulus, UTS, yield strength) were observed for both the bulk samples and the solder joints. In the current investigation, our prior study has been extended to examine both (SAC_Q, 92.5Sn-4.0Ag-0.5Cu-3.0Bi) and SAC305 (95.5Sn-3.0Ag-0.5Cu) solder alloys, as well as several additional thermal profiles (slow thermal cycling, thermal shock, thermal ramping, and thermal aging). Finally, longer exposure times up to 50 days have been considered in this new work. Uniaxial miniature bulk specimens and BGA solder joints were prepared for both solder alloys. A controlled reflow profile was used to prepare both types of solder samples. After preparation, the samples were then preconditioned by exposure to thermal cycling from to +125 °C or thermal aging at for various durations up to 50 days. The cycling was performed inside an environmental chamber under no load condition (stress-free). Several thermal cycling profiles were examined including: (1) 150 minute cycles with 45 minutes ramps and 30 minutes dwells (slow thermal cycling), (2) air-to-air thermal shock exposures with 30 minutes dwells and near instantaneous ramps (thermal shock), and (3) 90 minute cycles with 45 minutes ramps and 0 minutes dwells (thermal ramping). The results for the three cycling profiles were compared to those for pure isothermal aging at the high temperature extreme (aging). After preconditioning via thermal exposure, the samples were tested to characterize their mechanical behavior and microstructure evolutions. The uniaxial bulk specimens were subjected to stress-strain testing to measure their mechanical properties including effective elastic modulus, yield stress, and Ultimate Tensile Strength (UTS). The mechanical behavior of the BGA solder joint samples was studied using nanoindentation. Single grain solder joints were tested since they had large regions of solder material with equivalent mechanical behavior, which could then be indented several times after various durations of thermal cycling/aging. Using the nanoindentation technique, the mechanical properties (modulus and yield stress) were extracted. For both the miniature bulk and solder joint samples, the evolutions of the mechanical properties for each thermal profile were characterized as a function of the time the samples were preconditioned before mechanical testing. Several comparisons were then made including: (1) comparing bulk sample vs. joint behavior for each thermal cycling/aging profile, (2) comparing the observed evolutions for SAC305 vs. mechanical properties for each thermal cycling/aging profile, and (3) comparing the relative severity of degradations occurring for the four different thermal profiles. For all four of the thermal cycling/aging profiles, only small changes in the mechanical properties were observed for the solder in both the bulk and joint samples. Also, significantly higher degradations in the mechanical properties were observed for the SAC305 solder relative to the alloy. Reduced microstructural evolution in the alloy samples was found to be the major reason for the improved resistance to mechanical behavior changes. For both alloys, the slow thermal cycling profile resulted in the largest degradations relative to the other thermal profiles.