Rapid formation of full Cu-In intermetallic compounds (IMCs) joints under electric current

In this paper, we propose a novel interconnection method of rapid formation of full Cu-In IMCs joints under different electric current densities for 200ms at room temperature with the pressure of 0.16MPa. The “sandwich” structure of Cu/In(30μm)/Cu solder joints were designed. To make the experiment replicable, the Cu slices were polished by the uniform standard. The microstructure evolution, phase transformation and temperature were investigated and measured under different current densities ranging in 1.28×10⁴ A·cm^-2 to 1.84×10⁴ A·cm^-2. When the sufficient electric current flowed through the Cu/In/Cu solder joints, the temperature was sharply up to the In melting point (156.60°C). The electron wind force migrated the massive Cu atoms at cathode side to the In matrix, which enhanced the Cu/In inter-diffusion during the Cu-In solid-liquid reactions. Under the current density of 1.60×10⁴ A·cm^-2, the full Cu₂In solder joints were obtained. While Under the current density of 1.76×10⁴ A·cm^-2, the three layer IMCs, Cu₄In/Cu₇In₃/Cu₄In, solder joints were fabricated. The Cu₇In₃ phases were firstly formed at the Cu/In interface, and then they were transformed to Cu₄In under the electromigration (EM) force and thermal diffusion force. With the current density increasing to 1.84×10⁴ A·cm^-2, the nearly full Cu₄In joints were obtained. The maximal temperature was up to 504°C, which lowered the activation energy of Cu and In substantially. The electron wind force was predominant during inter-diffusion process. The Cu₄In phases kept growing and contacted with each other with consuming the Cu₇In₃ phases.