Barrier properties of thin Au/Ni-P under bump metallization for Sn-3.5Ag solder

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.