Anisotropic conductive films (ACFs) for ultra-fine pitch Chip-On-Glass (COG) applications

Abstract

This paper describes the development of anisotropic conductive films (ACFs) for ultra-fine pitch Chip-On-Glass (COG) application. In order to have reliable COG interconnects using ACF at fine pitch, the number of conductive particles trapped between the bump and substrate pad should be enough and less conductive particle between adjacent bumps. The ACF in this paper has double-layered structure, in which ACF and nonconductive film (NCF) layer thickness is optimized, to have as many conductive particles as possible on bump after COG bonding. In ACF layer, non-conductive particles of diameter 1/5 times smaller than the size of conductive particles were added to prevent electrical short between the bumps of COG assembly. The conductive particles are naturally insulated by the non-conductive particles even though conductive particles are flowed into and agglomerated in narrow gap between bumps during COG bonding. Also, flow property of the conductive particles is restrained due to increased viscosity of ACF layer with non-conductive particles, and the number of the conductive particles is constantly maintained. To ensure the insulation property at the level of 10 μm gap, insulating coated conductive particles were used in ACF layer composition. The double-layered ACF using low temperature curable binder system was also effective in reducing the warpage level of COG assembly due to low modulus and low bonding temperature.