Experimental and analytical study on 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 theoretically and experimentally. Analytical as well as numerical methods have been developed to analyze the flow during the underfill encapsulation process. For capillary-driven encapsulation, 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 to solve for the flow field. Experiments have been performed to investigate the flow behaviour during underfill encapsulation using actual chips and encapsulants. Short-shot experiments have been performed to look at the melt-front at different flow times. Experiments have also been performed to measure the material properties of the encapsulant, namely its viscosity, curing kinetics and surface-tension coefficient. The viscosity has been determined using an oscillatory parallel-plate-type viscometer and the curing kinetics via a differential scanning calorimetry. The experimental and simulation results are compared in terms of the flow-front shape after different flow times. Such comparisons indicate that the approximate model developed in this study is adequate to approximately simulate the flow during encapsulation of flip-chips