Visualization of Microstructural Evolution in Lead Free Solders during Isothermal Aging Using Time-Lapse Imagery

In this investigation, Scanning Electron Microscopy (SEM) and Scanning Probe Microscopy (SPM) have been utilized to examine aging induced microstructural changes occurring within lead free solders. Unlike many prior studies, fixed regions in the solder joint cross-sections were monitored throughout the aging process, rather than examining different samples and/or different regions after the various aging exposures. Sn-3.0Ag-0.5Cu (SAC305) lead free solder joint samples were formed with reflowed (RF) and water quenched (WQ) cooling profiles and resulting initial microstructures, and then polished microscopy cross-sections were prepared. Nanoindentation marks were added to the cross-sections at certain locations to facilitate locating the fixed regions of interest in subsequent microscopy observations. After preparation, the joints were then aged at T = 125 oC, and the microstructures were observed and recorded in the selected regions after various aging exposures using SEM and SPM. With this approach, time-lapse imagery of the microstructure evolution in a particular region of a solder joint has been recorded as a function of the aging time. When placed together sequentially, these images can be used as frames to create experimentally recorded movies of the microstructural evolution in SAC solder joints exposed to aging. This approach has allowed us to visualize phenomena such as the coalescence and migration of intermetallic compounds (IMCs) and the coarsening of beta-Sn dendrites. The results presented in this paper detail our findings for small aging times up to 10 hours, where the most rapid and dramatic changes in microstructure occur. In particular, this work has concentrated on observing the coalescence, change in shape, and migration of Ag3Sn intermetallic particles. The aging induced changes in number of IMCs, total area of all IMCs, average particle area, and average particle diameter have been quantified for several fixed regions in the joints.