Development of the wafer level compressive-flow underfill process and its involved materials

This paper describes a wafer level compressive-flow underfill process for a novel SMT transparent flip-chip technology. In this flip-chip technology, a liquid fluxable wafer level compressive-flow underfill (WLCFU) material is first coated on the active side of an entire patterned and bumped wafer. The WLCFU layer is dried at an elevated temperature to form a solid layer. The coated bumped wafer is then diced into individual chips, which are then placed on a carrier film with their active side to the tacky side of the carrier film. These diced individual chips are then picked from the tacky carrier film and placed on a substrate such as a PWB board using standard SMT equipment. At an elevated temperature (100-180°C) during solder reflow, the solid WLCFU layer can be re-melted and can easily fill in the gaps between chip and substrate. After solder reflow (190-200°C), the WLCFU material can be fully cured. A B-stage epoxy technology is used to develop this WLCFU material and the tacky material on the carrier film. A properly selected fluxing agent is added to both the WLCFU and the tacky materials to provide sufficient fluxing capability for good solder interconnection. A thermo-gravimetrical analyzer (TGA) is used to investigate the drying kinetics and the material weight loss during the reflow process. Differential Scanning Calorimetry (DSC) is used to study the curing kinetics of the prepared formulations. A thermo-mechanical analyzer (TMA) is used to investigate the heat distortion temperature (TMA Tg) and the coefficient of thermal expansion (CTE). A dynamic-mechanical analyzer (DMA) is used to measure the storage modulus (E´) and cross-linking density (ρ) of the cured materials. A rheometer is used to investigate viscosity (η) change with the temperature increase during the solder reflow process. Preliminary results demonstrate the feasibility of the proposed novel flip-chip technology with the developed WLCFU and tacky materials. The basic qualifications of the WLCFU material are examined. Some technical barriers related to this technology are also discussed