Capillary Flow Analysis of Underfill in Flip-Chip Package

In order to improve the reliability of solder joints, it is generally necessary to add underfill (UF) between chip and substrate to slow down the thermomechanical stress on solder joints. At present, capillary filling is still the main process for adding UF into flip-chip package, which relies on the surface tension of the UF to fill into the gap between the solder balls. However, different properties of UF, process parameters and package structure may have a greater impact on the filling results. In this paper, we systematically study the influences of UF properties, process parameters and package structure on the filling results using mold flow simulation. In terms of material properties, the zero shear viscosity, surface tension coefficient, wetting angle of UF were studied; in terms of process parameters, different dispensing methods and dotting path lengths were studied; in terms of package structures, the number of solder balls and chip edge size was studied. It is found that the filling ability of UF is strongly related to the material properties. Reducing viscosity, increasing surface tension coefficient and reducing wetting angle may improve the filling results of UF, which have been explained from the principles of fluid mechanics. Our research helps to improve the understanding of the factors affecting capillary filling, and may provide guiding suggestions for optimizing the physical properties of UF and the dispensing process in experiments.