Effects of different air gaps of underfill encapsulant on multi-stack printed circuit board

Abstract

This paper studies the effect of different air gaps on multi-stack printed circuit board (PCB) using finite volume method (FVM). Air gaps of 150 mm² and 450 mm² were introduced to investigate their influence on flow parameters such as filling time, pressure distribution, and void formation during the capillary underfill encapsulation process. It was found that increasing the air gap size improved the filling time by 10 %, though it also doubled the pressure, which helps reduce void formation but requires careful control to prevent encapsulant overflow. Throughout the study, an L-type dispensing method was applied on a perimeter type multi-stack BGA with wall type barrier added on the sides of the multi-stack PCB to prevent encapsulant. This new wall type barrier henceforth requires comprehensive study on the size of the air gaps to ensure crucial flow parameters such as void formation, filling time, pressure distribution and flow front pattern are optimized. Two air gap sizes given as 150 mm² and 450 mm² are used in this study. Moreover, the fluid flow patterns from both studies are investigated to determine the problem of racing effect and void formation. Based on the findings, it was found that 10 % improvement in the filling time can be made by using a bigger air gap of 450 mm². However, with bigger air gap, the pressure increases two-fold which can be good in reducing the void formation though the trade-off can be on the risk of excessive overflow of encapsulant.