Formation and characterization of intermetallic compounds in electroplated cobalt–tin multilayers

Intermetallic compounds (IMC) form during the metallurgical bonding processes when the interfaces involve solid metal and molten solder. Cobalt (Co) is a promising candidate to be used as under bump metallization (UBM) material, tin (Sn)-based solder alloying element, and transient liquid phase bonding base metal. This work aims at studying the intermixing reaction in Co–Sn system from electroplated Co and Sn multilayers. Co–Sn couples were sequentially electroplated and reflowed at 400 °C for 1 and 4 h. The microstructure and composition of the phases formed at different reflow duration were characterized by field emission scanning electron microscopy (FESEM) coupled with energy dispersive X-ray spectroscopy (EDX), and X-ray diffraction (XRD). Mechanical properties of the IMC layers formed in the samples were characterized using the nanoindentation technique under quasi-static and continuous measurement modes. FESEM/EDX and XRD analysis showed a mixture of CoSn+CoSn2 phases was formed in the 1 h reflow sample. As reflow time was increased to 4 h, only CoSn phase was found. Pure CoSn phase exhibits high nanohardness of 9.51 GPa while the region with CoSn+CoSn2 phases gives nanohardness value of 6.83 GPa