Secondary IMC formation induced by Kirkendall voiding in Cu/Sn-3.5Ag solder joints
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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, C6H12O6S4Na2 (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 C, layers of Cu6Sn5/Cu3Sn formed at the solder joints and Kirkendall voids nucleated at the Cu3Sn/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/Cu3Sn interface decreased and the Cu flux into Cu3Sn was inhibited. As the atomic ratio of Cu against Sn in the Cu3Sn dropped, transformation of Cu3Sn into Cu6Sn5 ensued. Subsequent diffusion of Sn atoms into the remaining Cu UBM through the remaining ligament of the Cu6Sn5/Cu interface precipitated secondary Cu3Sn beneath the primary Cu3Sn/Cu interface, and the secondary Kirkendall voids formed at the new Cu3Sn/Cu interface and so on. I. INTRODUCTION
Modern packages for commercial electronics are required to use Sn-based Pb-free solders where Sn reacts with under bump metallurgy (UBM) and forms intermetallic compounds (IMCs) at the solder joints. When Cu metallization is used, reactions between Sn and Cu usually result in the formation of layer-type Cu6Sn5 (Z phase) and Cu3Sn (e phase).1,2 Although Cu–Sn IMCs are essential for the formation of robust solder joints, solder joints are often susceptible to Kirkendall voiding, which induces premature failure of the package.2 Therefore, Kirkendall void formation at Cu–Sn solder joints has been the subject of extensive experimental and theoretical studies.1–9 Kirkendall voiding was often ascribed to the Kirkendall effect, i.e., nonreciprocal diffusion of Cu and Sn atoms in the Cu–Sn binary couple.10,11 However, experimental evidences on Kirkendall voids were not consistent, and the mechanisms of void nucleation and growth were not fully understood.12–14 Recent work by Yu and Kim15 showed that nucleation of Kirkendall voids were facilitated by the segregation of residual S, incorporated into the electroplated Cu film via SPS additive, to the Cu/Cu3Sn interface, which was a typical a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2010.0232
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http://journals.cambridge.org
J. Mater. Res., Vol. 25, No. 9, Sep 2010 Downloaded: 03 Jan 2015
embrittlement problem. It was suggested that tensile stress which drove the void growth evolved as a result of the Kirkendall effect.15 The Kirkendall void formation was significantly reduced by adding sulfide-forming elements to the solder such as Zn, Mn, and Cr (0.5 wt%), which effectively scavenged S atoms and suppressed S segregation to the Cu3Sn/Cu interface.16 Nucleation and growth of Kirkendall voids at the Cu/Cu3Sn interface reduces th
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