Electromigration mechanism on interconnected Cu pillar in flip chip package

This paper describes an EM (electromigration) mechanism on interconnected Cu pillar in flip chip package. It will be clarified that the EM life depends on Cu pillar interconnection structure. Recently, it was reported that the EM life was deteriorated by applying ENEPIG (electroless Ni-P/Pd/Au) plating layers on a substrate pad used for solder connection [1]. It was because Ni diffusion into solder lead to connection failure forming Ni3P layer. In this study, we found that an optimized structure was realized by applying no Ni barrier on the Cu pillar. Diffusion of CuSn IMCs (Inter Metallic Compounds) from Cu pillar accumulated onto Ni-P layer of substrate pad and they blocked Ni atom diffusion. It improved EM life about 2 times longer than that of Cu pillar with Ni barrier. Kirkendall voids were slightly formed on an interface between Cu pillar and solder after 500 hours at 175 deg C, however, shear strength was stable and any crack on the interface was not observed.