Under bump metallurgies for a wafer level CSP with eutectic Pb-Sn solder ball

Six types of under bump metallurgies (UBM) were investigated in terms of ball shear strength, fracture surface analysis, the adhesion of sputter-deposited metal to dielectric polymer, and microstructure observation to optimize the UBM of a wafer level chip size package (WLCSP) with Pb-Sn eutectic solder ball. UBMs investigated in this paper include Ti(0.2 μm)/Cu(1.0 μm), Ti(0.2 μm)/Cu(5.0 μm), Ti(0.2 μm)/Ni(0.2 μm)/Cu(1.0 μm), Al(0.2 μm)/Ni(0.2 μm)/Cu(1.0 μm), Ti(0.2 μm)/Ni(1.0 μm), Ni(0.2 μm)/Cu(1.0 μm). The purpose of this research is to investigate the effect of reflow cycle and two reliability stress, i.e., temperature cycling and pressure cooker soaking, on the ball shear strength as well as interface microstructure evolution of UBMs