TEM study on the interfacial reaction between electroless plated Ni-P/Au UBM and Sn-3.5Ag solder

This study examined the interfacial reaction between electroless plated Ni-P/Au under bump metallization (UBM) and a eutectic Sn-3.5Ag solder using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The chemical and crystallographic analysis using TEM provided important information on the microstructural evolution at the interface. In this study, UBM was prepared by the electroless plating of Au (0.15 mu m)/Ni-15 at.%P (5 mu m) on a bare Cu substrate and was then reacted with a Sn-3.5Ag eutectic solder at 260 degrees C for various amounts of time to examine the different sequential stages of the interfacial reaction. TEM analyses confirmed that beside the Ni3Sn4 layer, there were three more IMC layers at the interface: the Ni-Sn-P ternary layer, Ni3P layer, and the layer of phase mixture of the MY and Ni2SnP ternary phases.