Silver Paste Transferability during Imprinting Process

Abstract

The minimum pitch for flip-chip bonding is limited by the difference in the thermal expansion coefficient between the chip and the substrate, it is approximately 40 µm when solder is used for the bumps. This constraint can be significantly alleviated by applying a conductive paste to the bumps and bonding the chips together at low temperatures. Bumps made of conductive paste are typically formed via screen printing. By using the imprinting method, bumps and wirings can be formed with a narrow pitch and high aspect ratio. In addition, replica molds can be used repeatedly to reduce processing costs. To use the replica mold repeatedly, the silver paste embedded in the recessed pattern of the replica mold during imprinting must be completely transferred to the substrate and the initial state of the replica mold must be kept constant. Therefore, we investigated the transferability of the imprint technology by changing the process conditions during imprinting. Using a UV-curable silver paste as an imprinting process, we confirmed that UV-curing the silver paste before separating the replica mold from the substrate improved the transferability.