ACF Curing Process Optimization for Chip-on-Glass (COG) Considering Mechanical and Electrical Properties of Joints

Abstract

In the field of flat panel displays (FPD), packaging technology has significant influence on display performance. The electrical and mechanical interconnect between the liquid crystal displays (LCD) and its driver integrated circuit (IC) is a key issue that needs improvement to achieve finer pitch, easier assembly and greater connection reliability. With the decrease of the pixel size and the increase of pixel count for high-density LCD, the overall trend of the driver IC is packaged closer and closer to the LCD itself, even is onto the backside glass of the LCD. Bonding the driver IC chips directly to the glass substrate of the LCD panel might be a better choice when the pitch becomes less than 70–100 μm (Helge & Liu, 1998). Since the announcement by Citizen back in 1983 of a chip-on-glass (COG) driver assembly process for their LC pocket TV, many different types of COG assembly processes have been developed (Helge & Liu, 1998). In COG technology, the driver ICs are bonded directly to the indium-tin-oxide (ITO) traces on the glass without increasing the size of the panel, except for the finer bump pitch and smaller contact resistance, which can bring a significant reduction in the size of the FPD module. Because LCD is particularly heat sensitive and cannot withstand normal soldering temperatures, conductive adhesives are widely used to connect the driver IC to LCD. Usually, there are two different mechanisms used to cure the conductive adhesives, heat curing for thermosetting adhesives and UV curing for thermoplastic adhesives. Among them, heat curing appears to be most common. Nowadays interconnection using anisotropic conductive film (ACF) is the most major packaging method for production of FPD modules, to provide electrical conduction and mechanical adhesion between the driver ICs and the glass substrate with a high resolution, light weight, thin profile, and low power consumption (Myung & Kyung, 2006). Figure 1 presents a schematic illustration of a typical COG connection process using ACF. ACF is a thermosetting epoxy impregnated with small amount of electrically conductive particles, which can be pure metals such as gold, silver, or nickel, or metal-coated ones with plastic or glass cores. During ACF curing, when the heat and force are applied, the conductive particles are trapped between the mating bumps of IC and substrate to provide electrical conductivity, and the adhesive matrix is used to provide the necessary electrical insulation, to protect the metallic contacts from mechanical damage, and to provide stable O pe n A cc es s D at ab as e w w w .in te ch w eb .o rg