Investigating the Effect of Ag Content on Mechanical Properties of Sn-Ag-Cu Micro-BGA Joints

In order to evaluate the effect of Ag content on the mechanical property in micro-ball grid array (micro-BGA) solder joints, a shear test was carried out on Sn-3.0Ag-0.5Cu/Cu (wt.%) and Sn-4.0Ag-0.5Cu/Cu solder joints with a diameter of 90 μm. Corresponding microstructure before and after the shear test were investigated in this study. The shear test results demonstrated that the shear strength of Sn-4.0Ag-0.5Cu/Cu was greater than that of Sn-3.0Ag-0.5Cu/Cu. In Sn-4.0Ag-0.5Cu/Cu. The number of fine eutectic Ag 3 Sn particles in the solder matrix was higher, and the occurrence of plate-like Ag 3 Sn formation was much more frequent than in Sn-3.0Ag-0.5Cu/Cu. Moreover, by applying a fallback mode shear test and observing the fracture surface, broken plate-like Ag 3 Sn was found in the Sn-4.0Ag-0.5Cu/Cu joints with higher shear strength. These plate-like Ag 3 Sn acted as obstacles against shear force and enhanced the shear strength of joints. Therefore, both fine Ag 3 Sn precipitates and huge plate-like Ag 3 Sn improved the shear performance of micro-BGA joints. In addition, the shear behavior of 250 μm solder joints was also investigated and discussed, which was not identical to micro-BGA joints. The strengthening effect of plate-like Ag 3 Sn only occurred in micro-BGA joints, which had higher volume ratio of plate-like Ag 3 Sn in solder joints.