Development and reliability of ultra-thin chip on plastic bonding for flexible liquid crystal displays

The development of portable and flexible liquid crystal displays (LCDs) has led to a change in the substrate panel material. In the driver integrated circuit (IC) packaging process, ultra-thin chip on plastic (UTCOP) bonding is required to impart flexibility to plastic LCDs. In his study, the performance of 50-μm-thick ultra-thin chips on transparent polyimide (PI) substrate was investigated. To study the temperature distribution effect of bonding structures on the contact resistance during the anisotropic conductive adhesive (ACA) bonding process, three kinds of structures were evaluated: (1) a chip thickness of 670 μm, (2) a chip thickness of 50 μm, and (3) a 50-μm-thick chip with a 420-μm carrier IC of attached by thermo-release film. To determine the relationship between daisy-chain resistance and ACA-chip interface temperature, in-situ daisy-chain resistance and temperature during the bonding process were measured. The effect of the bonding structures on the UTCOP temperature distribution was analyzed using 3-D finite element (FE) modeling. The reliability of UTCOP was evaluated by thermal humidity storage testing (THST) at 85°C and 85% RH, thermal cycle testing (TCT) from -55°C to 125°C, and static bending testing at 10 mm radius of curvature. Scanning electron microscopy (SEM) cross sections of ACA joints and profiles of chip backs after bonding were analyzed to determine the failure reasons. Our results indicate that the temperature distribution of the bonding structure affected the electrical conductivity and reliability of the ACA joints. Structures with the lowest thermal conductivity had the best electrical contact resistance and reliability because that the structure-induced temperature distribution affects the ACA flow during bonding process. Optimizing this process made it possible to develop a highly reliable UTCOP assembly with ACA interconnections, greatly enhancing its potential for use in flexible LCDs in the near future.