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

Abstract

The authors have developed a fine pitch chiponglass (COG) bonding technique for LCD panels. An IC chip with gold (Au) bumps was dipped in a stirred indium (In) alloy bath in a nitrogen atmosphere without flux. Shallow-bowl-shaped In alloy bumps were selectively formed onto the Au bumps on the IC electrodes. The average height for the Au core In alloy bump was 23.8 pm. The minimum examined bump pitch was 50 pm, and the bump size was 31 by 31 pm.The In alloy bumps whose minimum pitch were 100 pm were connected to molybdenum (MO) conductors without flux at low pressure (30 gilbump 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 R.It was been 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. The developed COG technique allows easy repair. 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. Prototype TFT-LCD panels with 80 pm pitch driver ICs were successfully developed. This new bonding method can be applied to TFT-LCD panels and many other kinds of electronic equipment.

Application of Al/PI composite bumps to COG bonding process

“…Among these, the COG process is the most favorable technology due to its capability of miniaturization, low cost, high density, high reliability, and ease of repair [1]. In COG process, the bonding of driver chips to the substrate can be achieved by using metal bonding [2], resin bonding [3], [4], conductive paste bonding [5], [6], and conductive particle bonding [7], [8]. Except for metal bonding, it is always necessary to deposit joint bumps on the bonding pad of driver chip.…”