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 mu m Ni/1 mu m Cu and 1 mu m Al/0.2 mu m Pd/1 mu m Cu, applied under eutectic tin/lead solder bumps, have been investigated with regard to their interfacial reactions and adhesion properties. The effects of the number of solder 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 the shear strength was mainly due to the direct contact between solder and non-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 completely consumed by Cu-Sn and Pd-Sn reaction.