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 degrees 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 Sri diffusion at 160 and 180 degrees C. However, it fails to protect the Cu substrate from reacting with Sn at 200 degrees C. The reason is that the electroless Ni-P layer starts converting into a ternary Ni-Sn-P layer at 200 degrees 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 fort-nation of (NixCu1-x)(6)Sn-5 intermetallic at the Ni3Sn4/Ni-Sn-P interface. (c) 2004 Elsevier B.V. All rights reserved.