Development of the wafer level compressive-flow underfill encapsulant

This paper describes a wafer-level compressive flow underfill (WLCFU) for a novel SMT transparent flip-chip technology. In this flip-chip technology, the liquid fluxable WLCFU is coated over a patterned and bumped wafer. The WLCFU layer is dried at elevated temperature to form a solid layer. The coated bumped wafer is then diced into individual chips. These chips can then be placed on a substrate such as a PWB using standard SMT equipment. At elevated temperature (100-180°C) during solder reflow, the solid WLCFU layer can be re-melted and easily fills the gaps between chip and board. After solder reflow (190-200°C), the WLCFU material can be fully cured. B-stage epoxy technology was used to develop the WLCFU. A fluxing agent was added to provide sufficient fluxing for solder interconnection. A thermo-gravimetric analyzer (TGA) was used to investigate the drying kinetics and material weight loss during reflow. Differential scanning calorimetry (DSC) was used to study the WLCFU curing kinetics. A thermo-mechanical analyzer (TMA) was used to investigate the heat flex temperature (TMA T_g) and the coefficient of thermal expansion (CTE). A dynamic-mechanical analyzer (DMA) was used to measure the storage modulus (E') and cross-linking density (ρ) of the cured material. A rheometer was used to investigate viscosity (η) change with temperature during solder reflow. Preliminary results showed the feasibility of the proposed novel flip-chip technology with the developed WLCFU material. The basic qualifications of the WLCFU material were examined. Some technical barriers related to this technology are also discussed