Investigation of Characteristics of the Capillary-driven Underfill Flow

Abstract

Underfill is applied to fill the gap between the silicon chip and the organic substrate of the flip-chip to enhance its thermal-mechanical reliability. This paper focus on the traditional capillary-driven underfill process. Different capillary models including surface force model and volume force model, are verified with an experiment in predicting the movement of the melt-front. The characteristics of underfill flow are studied through 3D numerical analysis. Volume of fluid (VOF) method is used in tracking the melt-front of the underfill flow. The two models are calculated through a CFD code Ansys Fluent which is formulated with the finite volume method. It is found that the surface force model is more accurate in predicting the movement of the melt-front than the CSF model, which exaggerates the infiltration velocity of the underfill flow. An experiment has also been carried out to study how the melt-front advances with time. Complete filling for the PDMS specimen has been achieved with a commercial underfill material, and it is found that the underifill passage can accelerate the underfill flow.