The requirements for better resolution and color quality LCD panels need advanced LCD packaging technology because of higher numbers of pixels and I/Os to be interconnected. ACFs composed of an adhesive polymer 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. Understanding the conduction mechanism of ACFs will impact on the better performance and reliability of LCD packages by choosing right ACF materials, proper processing conditions and ACF materials development in the future.
To understand the efficiency of electrical conduction in ACF, the theoretical electrical conduction model with physical contact mechanism has been simulated and experimentally proved. The electrical resistance of ACF interconnection depends on the external pressure, mechanical and electrical properties of particles. Three pressure dependent models-1)elastic deformation Hertz contact model, 2)plastic deformation model and 3)FEM model-are developed, and experimentally verified using nickel powder and metal-coated polymer filled ACFs. Electrical contact resistance was measured on metallized flexible printed circuit / ITO glass joint using a four-point-probe method. Electrical conduction through the pressure engaged contact area between conductive particles and conductor substrate is the main conduction mechanism in ACF interconnection. Contact resistance of ACF is determined by the contact area change between particles and contact substrates. An increase of contact area between conductive particles and substrate is due to the plastic deformation of particles in ACFs. As bonding pressure increases, a sharp decrease of contact resistance followed by a constant resistance value are observed after reaching the critical pressure value which is the bonding pressure for good electrical conduction.
In addition, environmental effects on contact resistance such as thermal aging, high temperature/humidity aging and temperature cycling was also investigated. Increasing contact resistance after harsh environmental tests is mainly due to the thermal stress effect, elastic stress effect and partially due to the oxide formation of metal oxide on the conductive particles or metal substrate.