Study of several key reliability problems of COG/ACF interconnect in LCD module

Liquid crystal display (LCD) has been widely used in various electronic products. Chip-on-glass (COG)/anisotropic conductive film (ACF) interconnect technique is usually used to fabricate the LCD panels with high resolution and high quality. However, COG/ACF interconnect still has weakness that could cause severe reliability issues, e.g., no display. No display problem is considered to be caused by the increment of contact resistance of COG/ACF joints because of IC warpage, interface delamination and misalignment. In this study, a 3D global finite element (FE) model of whole LCD module was developed by implementing novel equivalent block method for the whole COG bonding process. The simulation results showed that IC warpage can be reduced by decreasing the temperature difference between bonding head and glass substrate, using low stress ACF as well as optimizing IC geometry. The global and local modeling indicated that von Mises stress of gold bump in the center region is much smaller than in the side region. The interface between ACF epoxy and glass would delaminate first and further cause the display failure. A slice of LCD module was modeled to investigate the misalignment solution with nonlinear elastic-plastic and sequential thermal-mechanical analysis. Numerical studies showed appropriate ACF with the properties of small elastic modulus, large coefficient of thermal expansion (CTE) and low glass transition temperature (T_g ) could reduce misalignment effectively. Finally, electrical conductive characteristics of cracked conductive particle were revealed using a novel 3D FE model with cohesive element to further understand the conduction mechanism of ACF particles