Effect of solder bump shapes on underfill flow in flip-chip encapsulation using analytical, numerical and PIV experimental approaches

Consistent with the recent trends in which the flip-chip reliability and performance being prominently prioritized, this paper is devoted to study the effects of solder bump shapes on the capillary underfill flows during the flip-chip encapsulation. A new experimental approach based on the particle image velocimetry (PIV) is set-up to visualize the underfill flow in the flip-chip. A more generalized analytical model for predicting the underfill filling time was developed to be applied for the triple-inlets dispensing (U-shape) underfill process with its application extended to a more generic bump shape. The theoretical analysis based on averaging method has been found viable for prediction of the underfill pressure, velocity and permeability. Findings from both finite volume method (FVM) simulation and PIV experiment achieved great consensus and are in line with the new theoretical estimates. These three distinct methodologies independently concluded the direct impact of different bump shapes design on the underfill flow. Altering the truncated spherical-shaped bump into a narrow-neck bump, for instance hourglass and concave shapes, would respectively reduce the filling times by 8.86% and 11.4% respectively; apart from having the increase in flow pressure and velocity. The mean permeability of the bump array computed is observed to be in an increasing order starting from spherical, cylindrical, hourglass and concave shapes. Generally, this comprehensive study had identified the geometrical parameters of solder bump that necessitated for faster underfill flow, being low sphericity and high pad-to-neck ratio.