A FINE-PITCH COG TECHNIQUE FOR A TFT-LCD PANEL USING AN INDIUM ALLOY

The authors have developed a fine pitch chip-on-glass (COG) bonding technique for liquid crystal display (LCD) panels. 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 onto the Au bumps on the IC electrodes. The minimum examined bump pitch was 50 mum, and the bump size was 31 x 31 mum. The InSn alloy bumps whose minimum pitch were 100 mum were connected to molybdenum (Mo) conductors without flux at low pressure (30 gf/bump or less) and low temperature (110-degrees-C or less). The temperature was lower than the alloy melting point. The mean contact resistance was 0.78 OMEGA. For designing the pad size and bump pitch of IC's, the calculation model of the minimum bump pitch for the bump sizes and the InSn alloy bump heights are useful. The developed COG technique allows easy repair. The maximum number of repair times was three through trial manufacturing. It has been proved 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 which allows up to 10 OMEGA. Prototype TFT-LCD panels with 80-mum pitch-driver IC's were successfully developed. This new bonding method can be applied to TFT-LCD panels and many other kinds or electronic equipment.