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

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.