Preparation and atmospheric wet-reflow of indium microbump for low-temperature flip-chip applications

An urgent demand for lowering bonding temperature has been put forward by advanced flip-chip integration such as micro-LED packaging and heterogeneous integration of semiconductor devices. Indium microbump with low-melting point has attracted attention for its potential use as the interconnection intermediate, and the development of its fabrication process is therefore of great attraction. To reveal the critical process factors for successfully fabricating a high-density In microbump array, this paper investigated a simple process flow of In patterning and reflow and detailed the flux-assisted wet reflow process. Critical process conditions, including the patterned In volume, alignment accuracy, reflow reagent liquidity, and temperature profile, were described, with a particular emphasis on the role of surface tension of molten indium film during the formation of spherical microbumps. A high-density indium ball array with an overall yield greater than 99.7% can be obtained, which suggests that the In patterning and wet-reflow processes are robust and that a high-quality microbump array could be readily formed with low equipment requirements. Furthermore, the interfacial reaction characteristics between In microbump and Au adhesion layer were investigated under thermal aging conditions, which revealed lateral intermetallic growth of AuIn₂ compound and well-retained interfacial strength even after prolonged aging.