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

This paper described a wafer-level compressive flow underfill process and its involved materials for a novel SMT transparent flip-chip technology. In this flip-chip technology, a liquid fluxable wafer level compressive flow underfill (WLCFU) material is coated on the active side of an entire patterned and bumped wafer at first. The WLCFU layer is dried up at an elevated temperature to form a solid layer. The coated bumped wafer is then diced into individual chips. The diced individual chips are then placed on to a carrier film with their active side to the tacky side of the carrier film. These diced individual chips are 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 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 was used to develop this WLCFU material and the tacky material on the carrier film. A properly selected fluxing agent was added to both the WLCFU and tacky materials to provide sufficient fluxing capability for good solder interconnection. Thermo-gravimetrical analyzer (TGA) was used to investigate the drying kinetics and material weight loss during reflow process. Differential Scanning Calorimetry (DSC) was used to study the curing kinetics of the prepared formulations. Thermo-Mechanical Analyzer (TMA) was used to investigate the heat distortion temperature (TMA Tg) and the coefficient of thermal expansion (CTE). Dynamic-Mechanical Analyzer (DMA) was used to measure the storage modulus (E´) and cross-linking density (ρ) of the cured material. Rheometer was used to investigate viscosity (η) change with the temperature increase during solder reflow process. Preliminary results demonstrated the feasibility of the proposed novel flip-chip technology with the developed WLCFU and tacky materials. The basic qualifications of the WLCFU material were examined. Some technical barriers related to this technology are also discussed