Analysis of the flow of encapsulant during underfill encapsulation of flip-chips

In this paper, the flow of encapsulant during the underfill encapsulation of flip-chips has been studied. Analytical as well as numerical methods have been developed to analyze the flow. For capillary-driven encapsulation (by dispensing), the capillary force at the melt-front has been calculated based on a model for the melt-front shape. A model has also been developed for the analysis of forced-injection encapsulation. The numerical analysis uses a finite-element method based on a generalized Hele-Shaw method for solving the flow field. Experiments have been performed to investigate the flow behavior using actual chips and encapsulants. Short-shot runs have been performed to observe the melt-front advancement at different flow times. In addition, measurements have been made of the material properties of the encapsulant, namely its viscosity, curing kinetics and surface-tension coefficient. The experimental and simulation results have been compared in terms of the flow-front shapes and times at different fill fractions. Such comparisons indicate that the model developed in this study is adequate to approximately simulate the flow during encapsulation of flip chips.