Mechanical Behavior and Microstructural Evolution of SAC and SAC+Bi Solder Alloys in Harsh Environment Applications
Type of DegreePhD Dissertation
Restriction TypeAuburn University Users
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The microstructure and mechanical properties of lead free solder joints in electronic assemblies are constantly evolving when exposed to isothermal aging. These changes lead to dramatic reductions in reliability of lead free electronic assemblies subjected to aging. The root cause of the changes in solder joint mechanical behavior is the evolution of the SAC 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 the most prior studies, it has been impossible to measure the changes that occur in IMC size and spacing since observations for different aging conditions (e.g., different aging times) were made using different solder joints. Thus, the comparisons made were necessarily qualitative in nature since the two microstructures were from different samples, and thus could not be directly compared in a quantitative manner. This dissertation demonstrates new procedures that removed the limitations of prior studies on the effects of aging on lead free solder microstructure and addresses those changes in the properties of lead free solders by conducting four different projects. In the first and second part of this work, Scanning Electron Microscopy (SEM) has been utilized to examine aging induced microstructural changes occurring within lead free solders. Unlike many prior studies, fixed regions in the solder cross-sections were monitored throughout the aging process, rather than examining different samples and/or different regions after the various aging exposures. 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 samples were aged at T = 100 ii and 125°C, and the microstructures were observed and recorded in the selected regions after various aging exposures using SEM. 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 solders exposed to aging. This approach has allowed the visualization of phenomena such as the coalescence and migration of intermetallic compounds (IMCs) and the coarsening of beta-Sn dendrites. 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 solder samples. In the third project, it was found that adding Bismuth (Bi) to the SAC composition to form SAC+Bi alloys has been effective in reducing both intermetallic coarsening and the degradations in material properties. These studies have been performed with the well-known SAC_Q alloy that modifies SAC405 to include 3.0% Bi. There have been no prior studies to examine the performance of SAC+Bi alloys with other concentrations of Bi. In this investigation, the mechanical behavior, microstructural evolution, and reliability of several different SAC+Bi alloys with various levels of Bismuth (1.0%, 2.0%, and 3.0%) have been chemically analyzed and then mechanically tested. To examine base mechanical behavior, stress-strain tests were performed for SAC_Q and SAC305 alloy. The effects of aging were studied for the SAC305 and SAC_Q alloys using both mechanical testing and microstructure observations. For the solder mechanical response, the fabricated uniaxial specimens were aged (preconditioned) at T = 125 °C for several durations of aging including 0, 2, 6, 12, and 24 hours. Stress-strain tests on the aged iii specimens were then performed at a single strain rate of 0.001 (sec-1), at temperatures of 125 °C. In particular, aging induced coarsening of the IMCs was studied for each alloy using Scanning Electron Microscopy (SEM), and correlated to the corresponding material property evolution findings. The results have shown that all of the SAC+Bi alloys demonstrate superior resistance to aging effects relative to SAC305. During aging, the bismuth was observed to go into solution within the beta-Sn dendrites and in the intermetallic rich regions between dendrites. It was observed that the coarsening of the intermetallic compounds was greatly mitigated in the SAC+Bi alloys relative to that observed in SAC305. The resistance to aging and increases in mechanical properties were found to be directly proportional to the amount of Bi present in the SAC+X alloy.