Secondary IMC formation induced by Kirkendall voiding in Cu/Sn-3.5Ag solder joints

Abstract

In this investigation on the formation of multiple-layered Kirkendall voids at Cu/Sn-3.5Ag solder joints, Sn-3.5Ag solder balls were reacted with Cu under bump metallurgy (UBM), which was electroplated using bis-sodium sulfopropyl-disulfide, C(6)H(12)O(6)S(4)Na(2) (SPS) additive. The sequence of multilayer Kirkendall voids and Cu-Sn IMC (intermetallic compounds) formations are explained with the aid of cross-sectional scanning electron microscopy (SEM) micrographs and schematic diagrams. During the aging treatment at 150 degrees C, layers of Cu(6)Sn(5)/Cu(3)Sn formed at the solder joints and Kirkendall voids nucleated at the Cu(3)Sn/Cu interface as a result of the segregation of residual S originating from SPS. However, with Kirkendall void growth, the net section area of the Cu/Cu(3)Sn interface decreased and the Cu flux into Cu(3)Sn was inhibited. As the atomic ratio of Cu against Sn in the Cu(3)Sn dropped, transformation of Cu(3)Sn into Cu(6)Sn(5) ensued. Subsequent diffusion of Sn atoms into the remaining Cu UBM through the remaining ligament of the Cu(6)Sn(5)/Cu interface precipitated secondary Cu(3)Sn beneath the primary Cu(3)Sn/Cu interface, and the secondary Kirkendall voids formed at the new Cu(3)Sn/Cu interface and so on.