Shear strength and aging characteristics of Sn-Pb and Sn-Ag-Bi solder bumps

Ball shear strength and aging characteristics of the 63Sn-37Pb solder bumps were characterized with variations of UBM finish (Au/Ni/Cu and Ni/Cu) and solder ball size (0.35 to 0.76 mm). Also, tensile strength, ball shear strength, and aging characteristics of the Sn-3.5Ag-Bi solder alloys were investigated. When formed on Au/Ni/Cu UBM, ball shear strength of the 63Sn-37Pb solder bumps of 0.35 mm size was higher than those of the 0.62 and 0.76 mm sizes due to precipitation hardening of Au-Sn intermetallic compounds. When the bump size was identical, the 63Sn-37Pb solder bumps exhibited higher shear strength on Au/Ni/Cu UBM than on Ni/Cu UBM. Shear strength of the 63Sn-37Pb solder bumps formed on Ni/Cu UBM varied little with aging at 150°C up to 1000 hours. With increasing aging time, the failure mode of the 63Sn-37Pb solder bumps on Au/Ni/Cu UBM was changed from ductile fracture (solder failure) to brittle fracture (interface failure). The 63Sn-37Pb solder bumps on Ni/Cu UBM showed ductile fracture regardless of the aging time up to 1000 hours. Tensile strength of the Sn3.5Ag-Bi alloys increased with increasing Bi content up to 9 wt.%. Ball shear strength of the Sn-3.5Ag-Bi solder bumps increased with increasing Bi content and reached a maximum at 5-7 wt.% Bi. Shear strength of the Sn-3.5Ag-5Bi solder bumps increased substantially from 70 MPa to 84 MPa by aging at 150°C for 300 hours