Mechanical Behavior and Reliability of SAC+Bi Lead Free Solders with Various Levels of Bismuth

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. 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 papers at ECTC 2019 and ECTC 2020, 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 this investigation, we have studied 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%). To examine base mechanical behavior, stress-strain tests were performed for each SAC+Bi alloy with three strain rates (0.001, 0.0001, and 0.00001 (sec-1)), and five different testing temperatures ( $\mathrm{T}=25,50,75,100$ , and 125 °C). The Anand parameters were calculated for each alloy from the stress-strain data, and good correlation was observed between the experimental curves and the model predictions. In addition, the temperature dependent mechanical properties of the various SAC+Bi solders were measured and compared including initial modulus, and ultimate tensile strength. The effects of aging were studied for the various SAC+Bi alloys using both mechanical testing and microstructure observations. For the solder mechanical response, the fabricated uniaxial specimens were aged (preconditioned) at $\boldsymbol{T} =\mathbf{125}\ {}^{\circ}\boldsymbol{C}$ for several durations of aging including 0, 1, 5, and 20 days. Stress-strain tests on the aged specimens were then performed at a single strain rate of 0.001 (sec−1), and temperatures of 25, 50, 75, 100, and 125 °C. Using the measured data, the evolutions of the stress-strain behaviors and mechanical properties were determined for the SAC+Bi alloys as a function of aging time. Microstructural evolutions of the new solder alloys were also observed for aging at $\boldsymbol{T}=\mathbf{125}\ {}^{\circ}\boldsymbol{C}$ for the same durations of 0, 1, 5, and 20 days. In particular, aging induced coarsening of the IMCs was studied for each alloy using Scanning Electron Microscopy (SEM), and correlated to our 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.