Design and understanding of anisotropic conductive films (ACFs) for LCD packaging

Anisotropic conductive films (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. To understand and design better quality ACF materials, a conduction model with a physical contact mechanism was simulated and experimentally proved. To understand the contact area changes, two pressure dependent models - (1) elastic/plastic deformation and (2) FEM - were developed and proved by testing various ACFs. Experimental variables such as bonding pressure, and the number, size, mechanical and electrical properties of Ni powders and Au-coated polymer conductive particles were applied. The models agreed well with experimental results, except at higher bonding pressures. In general, as bonding pressure increases, sharp decrease in contact resistance followed by a constant value is observed after reaching a critical bonding pressure. However, excessive bonding pressure inversely increased the ACF connection resistance. If more conductive particles were added, the connection resistance rapidly decreased to a constant. This is the counter-effect of two opposing factors: resistance increase by decrease in contact area per particle and resistance decrease by increased conduction path numbers. Also, environmental effects on contact resistance and adhesion strength such as thermal aging, 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.