Correlation of Mechanical and Microstructural Evolutions in Lead Free Solders Subjected to Various Thermal Exposures

During application in the field, as well as during accelerated life testing, solder joints in electronic products are routinely subjected to different thermal exposures in which the temperature ranges from very low to high. Due to the CTE mismatches of the assembly materials, cyclic temperature causes shear fatigue in the solder joints as they accumulate damage. In addition, dwell times at elevated temperatures will lead to evolution of the microstructure and material property degradations. Such "aging effects" can also occur during the transition times between high and low temperature extremes.While solder material behavior evolution during isothermal aging at fixed temperature has been examined in detail in our prior studies published at ECTC, there have been only a few previous studies examining material property changes occurring during other thermal exposures such as thermal cycling and thermal shock. In our recent papers published at ECTC 2022 and ECTC 2023, the mechanical behavior evolutions occurring in SAC305 and SAC+Bi (SACQ) lead free solders have been characterized for up to 20 days of exposure to four different thermal profiles including isothermal aging, thermal cycling, thermal shock, and thermal ramping. Degradations in effective modulus, creep resistance, Ultimate Tensile Strength (UTS), and Yield Stress (YS) were noted for both bulk solder materials and small-scale solder joints, and the outcomes were then contrasted for various exposures. Slow thermal cycling exposures were found to cause the biggest changes in mechanical behavior for both bulk and joint samples.In the current investigation, we have characterized the lead-free solder microstructure evolutions resulting from various thermal exposures. In particular, thermal profiles including thermal aging, thermal cycling, and thermal ramping have been explored. The observed changes in microstructure were then correlated with our previously measured mechanical property and creep behavior evolutions. Using a controlled reflow profile, microstructure samples were prepared for SAC305 and a set of SAC+Bi alloys with 1%, 2%, and 3% Bi. The reflowed samples for each alloy were separated into groups, and then each group was preconditioned by one of the three thermal exposures under stress-free conditions. Several durations of exposure were considered including 0, 1, 5, and 20 days. After exposing the samples to the various thermal profiles, the samples were viewed in an SEM to depict their microstructures, and various attributes of the microstructures (e.g. IMC particle size, particle spacing, dendrite size, etc.) were measured as a function of the exposure time. For each type of thermal exposure, the largest coarsening of the IMC particles was found in the SAC305 alloy, while increase in the Bi percentage in the SAC+Bi alloys led to higher mitigation of microstructure evolution. For all 4 alloys, the largest changes in IMC particle size occurred with the thermal cycling exposure profile.Correlations of the observed microstructure evolutions with the corresponding changes in mechanical properties and creep behavior were performed for each alloy, and it was demonstrated that the Ag3Sn IMC particle size (diameter) was the most significant characteristic of the microstructure that controls the SAC lead free solder mechanical behavior. More importantly, it was observed that for given alloy, the mechanical property degradation depended on the IMC particle diameter in same manner irrespective of thermal profile that caused the mechanical and microstructural evolutions. This suggests that a single curve/relation can be used for each alloy to correlate mechanical behavior and microstructure, independent of the thermal history seen by the solder. We are further testing this hypothesis with additional thermal profiles and the results look promising. If true, one need merely look at the microstructure of the solder to know its mechanical behavior.