Investigation of Microstructural Evolution in SAC Solders Exposed to Short-Term and Long-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. In particular, both the stress-strain behavior and creep behavior of SAC solders degrade significantly with aging exposures. These changes in solder mechanical behavior are a result of the evolution of the solder microstructure that occurs during aging. The most well-known and widely observed changes are coarsening of the Ag₃Sn and Cu₆Sn₅ intermetallic compounds (IMCs) present in the eutectic regions between beta-Sn dendrites. In our prior investigation, we have developed new procedures to study the coarsening of IMC Particles that occur in a small fixed region of a SAC305 solder joint sample after several durations of aging. Scanning Electron Microscopy (SEM) has been utilized to examine aging induced coarsening of IMCs occurring within lead free solders. To identify the fixed region of interest for subsequent SEM imaging, a nanoindentation system was used to make small markers at the corners of the studied region in the solder joint sample. The markers were used to position the sample and locate the same region of the microstructure precisely. In the current work, we have extended our prior study to examine microstructural evolution in SAC305 lead free alloys for both short and long term aging experiments. Samples were formed with reflowed cooling profiles and resulting initial microstructures, and then polished microscopy cross-sections were prepared. For short term aging experiments, the microstructures were observed and recorded in the selected regions after every one hour of aging exposures using Scanning Electron Microscopy (SEM). For long term aging experiments, the microstructures of the several fixed regions of interest were recorded every 250 hours. After each aging time, we were able to quantitatively analyze the evolution of Ag₃Sn and other IMCs with aging. Specifically, the aging induced changes in number of IMCs, total area of all IMCs, average area of a particle, average diameter of a particle and the normalized diameter were quantified in the fixed regions. Quantitative analysis results show that the number of IMC particles decreases while the average diameter of the particles increases significantly (more than 300%) with increasing aging time. With that being said, the most of the changes in the microstructure were found to occur within first 250 hours of aging. Finally, empirical models describing the growth of IMC particles with aging were examined and applied to our experimental data.