Comparative Study of Pressurized and Capillary Underfill Flow Using Lattice Boltzmann Method

This paper studies the three-dimensional simulation of pressurized underfill process for I-type dispensing method on a ten-by-ten, middle empty ball grid array (BGA) using lattice Boltzmann method. Pressurized underfill process is introduced to overcome the drawbacks of capillary underfill process that includes incomplete filling, void formation and extended filling time. The effects of capillary and pressurized underfill methods of BGA encapsulation process on the flow front, filling time and pressure of the fluid are investigated. Four different pressure values are used as the input pressure in the lattice Boltzmann simulation. Experiments are carried out as well to validate the simulation results. In general, pressurized underfill method results in a dramatically shorter filling time compared to capillary underfill. The flow front obtained from both simulation and experimental results is quite similar for the BGA model studied. The pressure inside the flow domain is significantly higher than its inlet pressure and thus, caution needs to be taken by the design engineer such that the printed circuit board (PCB) and flip chip are capable to withstand this pressure to prevent from the occurrence of breakage.