Compression flow modeling of underfill encapsulants for low cost flip-chip assembly

Abstract

Flip-chip technology represents a rapidly advancing area in commercial electronics. Flip-chip on board (FCOB) technology also called direct chip attach (DCA) involves the direct interconnection of integrated circuits to low cost organic substrates. In order to ensure adequate reliability, these flip-chip assemblies undergo an underfill encapsulation process in which a polymer material is placed between the chip and the substrate. Conventional underfill processing is achieved through chip site to chip site dispensing and underfill flow via capillary action, making it a costly and time consuming process particularly as device sizes increase and standoff gaps decrease. Extensive cost modeling of conventional flip-chip process technology has shown underfill processing, cleaning, and electroplating solder bumps and substrates to be the major cost driving factors. As part of the Low Cost Next Generation Flip-Chip Processing Program, an advanced flip-chip assembly process is being developed. This process eliminates the need for time consuming capillary flow processing using a compression flow technique where the underfill is applied prior to chip placement. The innovative process integrates the chip placement and polymer underfill processes using a compression or squeeze flow technique. It results in significantly lower assembly costs and reduced cycle time. In general, the compression flow of the underfill material governs assembly yield and reliability. This paper focuses on flow simulation studies of the compression flow chip placement process. It represents a fundamental advancement in compression flow simulation of polymers in its successful application to the complex geometries and surface topologies demanded by miniaturized flip-chip assembly. Here a simulation methodology is developed and simulation studies are conducted to characterize the compression flow of the underfill, estimate required chip placement forces, evaluate the effect of underfill geometry, and assess the potential formation of voids. Results yield design guidelines that give insight into process parameters such as the limits on underfill deposition geometry and underfill viscosity, and provided an initial process window.