Investigation and Comparison of Aging Effects in SAC+Bi Solders Exposed to High Temperatures

Isothermal aging of Sn-Ag-Cu (SAC) solder joints results in evolution of the solder microstructure and degradation of the mechanical properties such as modulus, strength, and creep resistance. Such changes can result in large reductions in reliability for 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. This includes coarsening of the Ag
₃
Sn intermetallic compounds (IMCs) present in the eutectic regions between beta-Sn dendrites. Our recent work has been aimed at mitigating aging effects in SAC solders. In our recent investigations, we have 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 present work, three SAC+Bi lead free solder materials recommended for high reliability applications have been chemically analyzed and then observed after aging. The alloys are referred to as SAC+3% Bi (SAC_Q), SAC+2.0% Bi, and SAC+1.0% Bi. Solder joints have been extracted from SAC+Bi assemblies and subjected to aging exposures up to 2000 hours at high temperatures (T = 100 and 125 °C). After each aging time, we were able to quantitatively analyze the evolution of Ag
₃
Sn and other IMCs with aging. Specifically, the aging induced changes in number of IMCs, total area of all IMCs, average area of particles, average diameter of particles and the normalized diameter were quantified in the fixed regions. In addition, the Energy Dispersive Spectroscopy (EDS) mapping was used to visualize the diffusion of Bi in SAC+Bi alloys after aging.The results have shown that all of the SAC+Bi alloys demonstrate superior resistance to aging effects relative to conventional SAC305. It was observed that during aging, the bismuth in the alloy will go into solution into the solder. This occurs within the beta-Sn dendrites and also in the intermetallic rich regions between dendrites. It was also observed that the presence of bismuth in solution results in reduced coarsening of the Ag
₃
Sn intermetallic compounds relative to that observed in SAC305. The resistance to aging and microstructure evolution were found to be directly proportional to the amount of Bi present in the SAC+Bi alloy. The aging induced degradation of the SAC+Bi alloy will be greatly reduced compared with those for SAC305 for all aging times if the level of Bi is at least 1%. For the same alloy, the aging induced degradation after at 125°C was much more significant than aging at 100°C. Focused Ion Beam (FIB) was also used to observe the IMC particles underneath the exposed surface. It is clear from the FIB images that the IMC particle size measured from the surface are similar to the IMC particle size underneath the surface for both aging and no aging condition.