Investigation of Aging Induced Microstructural Changes in Doped SAC+X Solders

Aging effects are common in lead free solder joints within electronic assemblies that are exposed to isothermal environments for extended periods. Such exposures lead to evolution of the solder microstructure, which results in changes in the mechanical properties and creep behavior of the solder joints. These changes often lead to dramatic reductions in reliability of lead free electronic assemblies subjected to aging. In our recent investigations, we have been utilizing Scanning Electron Microscopy (SEM) to better understand aging induced degradations. In particular, our approach has been to monitor aging induced microstructural changes occurring within fixed regions in selected lead free solder joints and to create time-lapse imagery of the microstructure evolution. With such an approach, quantitative analysis of the microstructural changes can be performed, removing the limitations of many prior studies where aged and non-aged microstructures were taken from two different samples and could only be qualitatively compared. In our current work, we have used the SEM approach for small fixed regions in SAC305 and SAC_Q (SAC+Bi) solder joint samples, and provided aging data for up to 2000 hours of aging. In particular, microstructural evolution has been studied for several different regions from several different joints, and both short term (up to 240 hours) and long term (up to 2000 hours) aging of the joints have been performed. In all of the aging experiments, the microstructure evolutions were observed in solder joint samples exposed to isothermal conditions at $\text{T}=125\ {{}^{\circ}\text{C}}$. The microstructures in several fixed regions of interest were recorded after predetermined time intervals of aging, which were 1 hour and 10 hours for the short term aging samples; and 250 hours for the long term aging samples. Using the recorded images and imaging processing software, the area and diameter of each IMC particle was tracked during the aging process. As expected, the quantitative analysis of the evolving SAC_Q microstructure showed that the particles coalesced during aging leading to a decrease in the number of particles. This caused an increase in the average diameter of the particles of slightly more than 100% for long term aging of 2000 hours. For SAC305, the average particle diameter was found to increase at three times the rate (increase of 300% after 2000 hours of aging). Thus, coarsening of IMC particles was greatly mitigated in the SAC_Q alloy relative to that observed in SAC305. Immediately after reflow solidification, Bismuth rich phases were present in the SAC_Q joints. During aging at $\text{T}=125\ {{}^{\circ}\text{C}}$, the bismuth was observed to quickly go into solution both within the beta-Sn dendrites and in the intermetallic rich regions between dendrites. This resulted in solid solution strengthening of the lead free solder. It was also found that the aging-induced presence of bismuth in solution within the beta-Sn matrix provided an increased resistance to the Ostwald ripening diffusion process that coarsens the Ag3Sn IMC particles. The combination of these two effects in the SAC+Bi alloy lead to greatly improved resistance to aging induced effects relative to the SAC305 solder alloy.