Design and understanding of anisotropic conductive films (ACF's) for LCD packaging

Anisotropic conductive film (ACF) composed of an adhesive resin and fine conductive fillers such as metallic particles or metal-coated polymer balls are key materials for fine pitch chip-on-film (COF) and chip-on-glass (COG) LCD packaging technologies. To understand and design better quality ACF materials, the theoretical electrical conduction model with physical contact mechanism was simulated and experimentally proven. To understand the contact area changes, two pressure dependent models (1) elastic/plastic deformation; (2) finite element method (FEM) model were developed, and experimentally proven by various ACF's fabricated in our laboratory. Experimental variables were applied bonding pressure, number, size, mechanical and electrical properties of nickel powders and Au-coated polymer conductive particles. It was found that the models were in good agreement with experimental results except at higher bonding pressures. In general, as bonding pressure increases, a sharp decrease of contact resistance followed by a constant value is observed after reaching the critical bonding pressure. However, an excessive bonding pressure rather increased the connection resistance of ACF interconnection. If more conductive particles-were added, the connection resistance rapidly decreased and then became constant. This is because the counter-effect of two opposing factors, the resistance increase caused by a decrease of contact area per one particle and the resistance decrease caused by increasing number of conduction path. In addition, environmental effects on contact resistance such as thermal aging, high temperature/humidity aging, and temperature cycling were also investigated. As a whole, better design of ACF materials can be achieved by understanding the ACF conduction mechanism.