Experimental and numerical study of underfill encapsulation of flip-chips using conductive epoxy polymer bumps

Abstract

The flow of underfill encapsulant for epoxy bonded flip-chips is studied numerically and experimentally and compared to analytical predictions for fully developed 2-D flow. A 2-D VOF (volume of fluid) FLUENT model was used to explore the functional relationships between the gap size, the encapsulant's viscosity and surface tension, and the flow rate. The computational intensiveness of this problem prevented direct comparisons between the numerical and experimental data. However, scaled results indicate that the numerical predictions are in general agreement with analytical predictions-the numerical flow times are proportional to the viscosity and to the square of the distance travelled, and inversely proportional to the gap height and the surface tension. The constant of proportionality, however, differs by up to 60% because the numerical (and experimental) flow is not fully developed near the encapsulant-air interface. Experiments at varying temperatures have allowed the inference of the encapsulant's temperature dependent viscosity and an optimal temperature for the underfill process.