Investigation and Comparison of Microstructural Changes in SAC and SAC plus X Solders Exposed to Short-Term Aging

The microstructure and mechanical properties of Sn-Ag-Cu (SAC) solder joints in electronic assemblies are constantly evolving when exposed to isothermal aging. 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 this work, we illustrated the SEM approach for microstructure evolution in SAC305 and SAC_Q (SAC+Bi) solder joint samples and created time-lapse imagery of the microstructure evolution for isothermal aging (up to 2000 hours) performed at T = 125 degrees C. In current study, we have extended this work for longer aging time and results for aging performed at T = 100 degrees C and 125 degrees C (up to 500 hours) have been added. The microstructures in several fixed regions of interest were recorded after predetermined time intervals of aging, which were 1 hour (up to 270 hours) and 250 hours (up to 500 hours). 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 analysis of the evolving SAC305 and SAC_Q microstructure showed a significant amount of diffusion of silver and bismuth in the beta-tin matrix during aging. The quantitative analysis of the microstructure showed that the particles coalesced during aging leading to a decrease in the number of particles. For SAC305 alloy, the average particle diameter was found to increase of 300% after 500 hours of aging at 125 degrees C and 30% after 500 hours of aging at 100 degrees C. For SAC_Q alloy, the average particle diameter was found to increase of 100% after 500 hours of aging at 125 degrees C. 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 T = 125 degrees 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 Ag 3 Sn 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.