A study on the electroless-deposited Ni-P under bump metallurgy (UBM) was performed for the fabrication of flip chip bumps. The interfacial reaction at the electroless Ni UBM/solder interface was investigated. The growth kinetics of the Ni-Sn intermetallic compounds (IMC) showed a rapid initial growth, followed by a reduced growth and a diffusion-controlled growth. Shear testing showed that the Ni UBM-Sn containing solder systems caused brittle bump failure, resulting in decreased bump adhesion strength.