Eutectic Sn/Pb solder bump and under bump metallurgy: interfacial reactions and adhesion

In flip-chip interconnection on organic substrates using eutectic tin/lead solder bumps, a highly reliable under bump metallurgy (UBM) is required to maintain adhesion and solder wettability, Various UBM systems such as 1 mu m Al/0.2 mu m Ti/5 mu m Cu, 1 mu m Al/02 mu m Ti/1 mu m Cu, 1 mu m Al/0.2 m u m Ni/1 mu m Cu and 1 mu m Al/0.2 mu m Pd/1 mu m Cu, applied under eutecti c tin/lead solder bumps, have been investigated with regard to their interf acial reactions and adhesion properties. The effects of the number of solde r reflow cycles and the aging time on the growth of intermetallic compounds (IMCs) and on the solder ball shear strength were investigated, Good ball shear strength was obtained with 1 mu m Al/0.2 mu m Ti/5 mu m Cu and 1 mu m Al/0.2 mu m Ni/1 mu m Cu even after four solder reflows or seven-day aging at 150 degrees C. In contrast, 1 mu m Al/0.2 mu m Ti/1 mu m Cu and 1 mu m Al/0.2 mu m Pd/1 mu m Cu showed poor ball shear strength. The decrease of t he shear strength was mainly due to the direct contact between solder and n on-wettable metals such as Ti and Al, resulting in a delamination. In this case, thin 1 mu m Cu and 0.2 mu m Pd diffusion barrier layers were complete ly consumed by Cu-Sn and Pd-Sn reaction.