ACF-COG interconnection conductivity inspection system using conductive area

The liquid crystal display (LCD) module package mainly adopts the chip-on-glass (COG) process, in which the driver IC is assembled onto a glass substrate with anisotropic conductive film (ACF). With the higher demand on the display capacity, the stability of COG interconnection under the finer pitch package becomes more important. Currently bump conductive particle counting is used to assess the interconnection quality. However, it cannot verify the interconnection conductivity promptly and would be labor intensive in case that all the bumps were required to inspect. In this paper, a conductivity inspection system based on optical microscope is presented. The resistance prediction model used in the system is based on particle conductive area and has been verified with the published experimental data, which revealed it could calculate the resistance value with the mean of particle conductive area and the number of conductive particles trapped by bump. The model is more accurate than the existed models. A visual inspection system for bump interconnection is developed in this study to compute the particle conductive area and the particle number. The system has been tested with 228 bump images of the ACF-COG modules, and the predicted resistance, 163 mΩ, is in the common range. The results show that the conductive area (CA) can be an integral index of bump conductivity and comply with a normal distribution. The resistance model and inspection system provide guidance for the COG interconnection conductivity online testing and will benefit the COG assembly reliability.