Shear property and microstructure evaluation of Pb-free solder bumps under room temperature and multiple reflow/high temperature aging

In this study, bump shear and microstructure evaluations have been performed to characterize Sn-3.5Ag, Sn-3.8Ag-0.7Cu, and Sn-0.7Cu, along with eutectic Sn37Pb. Bump shear strengths were monitored during room temperature aging up to 1000 hrs, and during high temperature storage at 125, 150, and 170°C up to 1000 hrs after two and 10 reflows. All bump shear failures were through the solder, indicating that the bump structure interfaces all have good adhesion. Different shear strength was observed with respect to solder alloy composition. At room temperature, the shear strength of eutectic Sn37Pb decreased by 25% after 51 days aging after reflow while the shear strength of all the Pb-free alloys decreased slightly by 5-8%. Microstructure evolution with aging time was observed and correlated to shear strength change at room temperature. The shear strength of both Sn-3.5Ag and Sn-3.8Ag-0.7Cu solders became lower after two reflows but were not affected up to 10 reflows. The shear strength of Sn-37Pb and Sn-0.7Cu solder had no change after either multiple reflows or high temperature storage. Solder bump microstructure under these conditions was characterized. Chemical reactions between solders and under-bump metallurgy (UBM) interactions during multiple reflows and high temperature storage were also investigated. Sn-37Pb solder consumed less Cu UBM than all other Pb-free solders during reflow. However, it consumed more Cu after solid state annealing. An optimal Pb-free alloy (Sn-0.7Cu) is proposed based on this study and previously reported mechanical property and thermal fatigue reliability evaluations for flip chip solder joints