A fine pitch COG technique for a TFT-LCD panel using an indium alloy

A fine pitch chip-on-glass (COG) bonding technique for liquid crystal display (LCD) panels has been developed. An IC chip with gold bumps was dipped in a stirred indium alloy bath in a nitrogen atmosphere without flux. Shallow-bowl-shaped In alloy bumps were selectively formed on the Au bumps on the IC electrodes. The minimum bump pitch was 50 μm, and the bump size was 31 by 31 μm. The In alloy bumps whose minimum pitch was 100 μm were connected to molybdenum conductors without flux at low pressure (30 gf/bump or less) and low temperature (110°C or less). The temperature was lower than the alloy melting point. The mean contact resistance was 0.78 Ω. It was found that the calculation of the minimum bump pitch for the bump sizes and the In alloy bump height is useful for designing new ICs with fine pitch bumps. It has been demonstrated through a thermal shock test (TST), a high temperature and high-humidity storage test; and a high-temperature storage test that the contact resistance changes satisfied the specification. Prototype TFT-LCD panels with 80-μm pitch driver ICs were successfully developed