A New Analysis of the Capillary Driving Pressure for Underfill Flow in Flip-Chip Packaging

Control of the capillary driven underfill flow process is a very important problem in flip chip technology in electronics packaging. The challenge here is to uniformly distribute fluids in the gap, which is composed of the chip and substrate and solder bumps. In this paper, we first present a conceptual model of the flow behavior of underfill materials, which has four stages, leading to the finding that the contact line jump phenomenon occurs at both entry and exit of the solder bumps. We then propose a more accurate model for describing the flow front based on the principles of mass conservation and force equilibrium. Another contribution of the present paper is the proposal of a computational model for describing the average capillary pressure difference with respect to the area of flow. The verification of these models is presented by comparing the result calculated with our model with the experimental result reported in the literature, which shows the superiority of the models proposed in this paper.