Anisotropic conductive films (ACFs) for ultra-fine pitch chip-on-glass (COG) applications

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 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 anisotropic conductive film is double layered structure, in which ACF and NCF layer thickness is optimized, to have as many conductive particle as possible on bump after COG bonding. In ACF layer, non-conductive particles of diameter 1/5 times smaller than the conductive particles are added to prevent an electrical short between the bumps of COG assembly. The conductive particles are naturally insulated by the nonconductive 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 nonconductive particles, and results the number of the conductive particles constantly maintained. To ensure the insulation property at 10 μm gap, insulating coated conductive particles were used in ACF layer composition. The double-layered ACF using low temperature curable binder system is also effective in reducing the warpage level of COG assembly due to low modulus and low bonding temperature.