Microstructure and morphology of interfacial intermetallic compound CoSn3 in Sn–Pb/Co–P solder joints

• Interfacial IMCs in Sn–Pb/Co–P joints were studied with high-resolution TEM.
• CoSn3 with or without stacking fault in crystal grains was found.
• Morphology of CoSn3 depended on its growth rate.

A systematical microscopic analysis on structure, morphology, and growth of CoSn3 intermetallic compound (IMC) that formed at the interface between Sn–Pb alloy and Co–P films was carried out using scanning electron microscopy with back-scattered electron imaging and high-resolution transmission electron microscopy with energy dispersive spectrometry. CoSn3 IMC with two kinds of morphology was found out after Sn–Pb alloy reacted with Co–P films with different microstructures. One kind of CoSn3 with stacking fault was distributed densely on nanocrystalline and amorphous Co–P films, and the other kind of CoSn3 without stacking fault existed sparsely on the Co–P film with nanocrystalline/amorphous mixed structure. The stacking fault was caused by the fast growth of CoSn3 for the cases of Co-7 at.% P and Co-23 at.% P. Co-12 at.% P film with a nanocrystalline/amorphous mixed microstructure had the best diffusion-barrier property among Co–P films with different compositions, because the diffusion of Sn into Co–P was the least. Our study shed light on diffusion-barrier performance of Co-based metallization.

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