In the flip chip interconnection on organic substrates using eutectic Pb/Sn solder bumps 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 Pd/1$mu$m Cu, laid under eutectic Pb/Sn solder were investigated with regard to their interfacial reactions and adhesion proper-ties. The effects of numbers of solder reflow and 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 4 solder reflows or 7 day aging at 15$0^{circ}C$. In contrast 1$mu$m Al/0.2$mu$m Ti/1$mu$m Cu and 1$mu$mAl/0.2$mu$m Pd/1$mu$m 쳐 show poor ball shear strength. The decrease of the shear strength was mainly due to the direct contact between solder and nonwettable metal 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 layer were completely consumed by Cu-Sn and pd-Sn reaction.