Ultra-thin chip on flex by Solder-on-Pad (SoP) technology

CoF (Chip-on-Flex) packaging is widely applied in electronic products, especially in large scale LCD ( Liquid Crystal Display) manufacturing. A high quality flexible display product demanded the whole package structure to be flexible, which caught the attention of researchers to develop the UTC (ultra-thin chip) on flex technology. This paper focused on the interconnection method between ultra-thin chip and flexible substrate. A novel SoP (Solder-on-Pad) process was employed on the Cu pad of flexible substrate, where the patterned PI (Polyimide) film acted as a natural stencil for solder paste printing. Firstly, Cu trace was etched from the 2L-FCCL (Two-layer Flexible Copper Clad Laminate). Next, the PI film was laser drilled to form the pad opening and served as the stencil for solder printing. Solder caps took shape after reflow, which were also protected by the surrounding PI film. Finally, the Daisy-chain UTC (50µm) with gold stud on Al pad was aligned and attached to the flexible substrate with solder cap on Cu pad to accomplish electrical interconnection through Au stud-solder bump joint. Resistance of bonding joint was measured before and after TCT (thermal cycling test), results showed good electrical connection and no mechanical defect were observed.