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 is described. An IC chip with gold (Au) bumps was dipped in a stirred indium-tin (InSn) alloy bath in a nitrogen atmosphere without flux. Shallow-bowl-shaped InSn alloy bumps were selectively formed on the Au bumps on the IC electrodes. The minimum examined bump pitch was 50 μm, and the bump size was 31×31 μm. The InSn alloy bumps, which had minimum pitch of 100 μm, were connected to molybdenum (Mo) 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. The technique allows easy repair. A thermal shock test, a high-temperature and high-humidity storage test, and a high-temperature storage test showed that the contact resistance changes satisfied the specification, which allows up to 10 Ω. Prototype thin-film-transistor (TFT) LCD panels with 80-μm pitch-driver ICs were developed using the new bonding method