Research Of Simulation Methods For Capillary-driven Underfill

In semiconductor industry, Underfill technology is widely used in flip-chip packaging as it can improve the reliability of the interconnect systems effectively. Solder bumps are designed to connect the chip with the substrate in flip-chip packaging. In order to prevent the fatigue cracking of the solder bumps and improve the fatigue life of the chip, underfill encapsulant is filled into the space between the chip and the substrate to minimize thermo-mechanical stresses caused by the mismatch of coefficient of thermal expansion (CTE) between a silicon chip and a organic substrate.Dynamic capillary force is calculated with the adoption of VOF (Volume of Fluid) and CSF (Continue Surface Force) model. A two-dimensional numerical simulation method which both reckons in influences of capillary force and flow resistance induced by the chip, substrate and solder bumps is improved considering the characteristics of constant thickness and laminar flow of the flow field.FLUENT software is employed to simulate underfill process for varying bump pitches. The numerical results are compared with the previous experimental results and found in good conformity. Moreover, there is a significant increase in calculating efficiency versus three-dimensional method. At the same time, simulation of underfill for various encapsulant dispensing types such as I type, L type and U type are carried out, and desired results are gotten.As for non-Newtonian fluid, the3D simulation of the underfill flow is operated as the power-law type equation is applied to reflect the non-Newtonian behavior of the underfill encapsulant. And it shows that the simulation results agree well with the previous experimental results. This simulation provides a reasonable flow front prediction for underfill flow of encapsulant with non-Newtonian feature.