In this study, UBM material systems for flip chip solder bumps on Cu pads were investigated using the electroless copper (E-Cu) and electroless nickel (E-Ni) plating methods; and the effects of the interfacial reaction between UBMs and Sn-36Pb-2Ag solders on the solder bump joint reliability were also investigated to optimize UBM materials for flip chip on Cu pads. For the E-Cu UBM, scallop-like Cu6Sn5 intermetallic compound (IMC) forms at the solder/E-Cu interface, and bump fracture occurred along this interface under a relatively small load. In contrast, at the E-Ni/E-Cu UBM, E-Ni serves as a good diffusion-barrier layer. The E-Ni effectively limited the growth of the IMC at the interface, and the polygonal-shape Ni3Sn4 IMC resulted in a relatively higher adhesion strength compared with the E-Cu UBM. As a result, electroless deposited UBM systems were successfully demonstrated as low cost IJBM alternatives on Cu pads. It was found that the E-Ni/E-Cu UBM material system was a better choice for solder flip chip interconnection on Cu pads than the E-Cu UBM.