Electroless Ni–P with a thin layer of immersion gold has been considered as a promising under bump metallization (UBM) for low-cost flip–chip technology. However, the presence of P in electroless Ni–P causes complicated interfacial reactions, which affect the reliability of solder joint. In this work, barrier properties of thin Au/Ni–P UBM between Cu substrate and Sn–3.5Ag solder were investigated during annealing at 160, 180, and 200 °C in terms of IMC formation. Multilayer Sn–3.5Ag/Au/Ni–P/Cu sample was prepared by electroless chemical plating and solder reflow for the investigation. Annealing results showed that electroless Ni–P acts as a good barrier for Sn diffusion at 160 and 180 °C. However, it fails to protect the Cu substrate from reacting with Sn at 200 °C. The reason is that the electroless Ni–P layer starts converting into a ternary Ni–Sn–P layer at 200 °C. Complete conversion of the Ni–P layer into Ni–Sn–P, results in the formation of two Cu–Sn intermetallics, Cu6Sn5 and Cu3Sn, at the Ni–Sn–P/Cu interface and the formation of (NixCu1−x)6Sn5 intermetallic at the Ni3Sn4/Ni–Sn–P interface.