Advanced low-cost bare-die packaging technology for liquid crystal displays

Packaging technology for FPD (flat panel display) drivers has evolved from TAB (tape automated bonding) to COG (chip-on-glass) for size, cost, and performance reasons. COG supports demands for increased display sizes, pixel counts, and gray levels, as well as for reduced power consumption. It is also considered the most promising candidate for next-generation LCD packaging because the reduction in bonding steps lowers cost and increases reliability. COG packaging bonds bare IC driver chips directly to the panel electrodes, thus reducing materials requirements and production costs. The current advanced COG mounting technology for LCD (liquid crystal display) panels includes the pressure welding method, the conductive paste method, the conductive elastic particles method, and the anisotropic conductive adhesives (ACAs) method. Nowadays, two variants of the conductive paste method are in commercial use. The most common approach uses a Pd-Ag paste, which reportedly provides a bonding resistance of less than 5 Ω/electrode and a bond strength of 20 g/electrode. Heating the paste to approximately 150°C reduces its adhesive strength enough to allow the drivers to be removed and rebonded. The second commercial method uses bumps that are not plated but formed with a two-stage ball-bonding technique. This process covers the Cu/Au stud-bump with a conductive paste and uses a zigzag formation that improves connection efficiency and allows a bond pitch of 7 μm. In contrast to the approach using Pd-Ag paste, this technique typically uses silicon instead of epoxy for encapsulation