Numerical Analysis of Narrow-gap Underfill Flow in the Flip-chip Process Using the Lattice Boltzmann Method

Abstract

Flip-chip packaging is a first-level interconnect technology, and to predict its melt front of capillary driven underfill analytical models have been made. Using the Analytical models, prediction of the melt front can be efficiently done, however it is difficult to reflect the flow resistance resulting from complex solder bump arrangement. Therefore, 3D numerical analysis is required. In this study, Lattice Boltzmann method (LBM) is used for observing the micro void generation that occurs in a underfill process with a height of several tens of microns. LBM statistically expresses the motion of the molecule, to solve the mass transfer of the microscale. In particular, LBM has strengths in multiphase flow analysis of several microscales that cannot be solved by continuum assumption. In this paper, underfill process of several tens of micro scales, which was not covered in previous 3D numerical analysis studies, was simulated using LBM. The improvement of the new LBM model, which more accurately simulates the resistance due to pressure by considering collisions between air and molten mold molecules, is introduced. With the hydraulic diameter greatly reduced due to the narrow gap, it is observed how the solder bump pitch affects the micro void generation. And it is observed whether the air ventilation could change significantly according to the ambient pressure.