Analysis polymer flow due to capillary forces for flip-chip package designs

摘要

The IC chips of the flip-chip packages are mounted directly to substrates using a grid-array of solder bumps. The standoff regions between the chips and substrates are filled with epoxy resins to protect both the chips and the solder joints. It is important to optimize encapsulation conditions and material properties for good quality packages.The underfill materials in the standoff regions flow spontaneously due to capillary forces in the processes of encapsulation. However, the conventional numerical analysis methods, based on the theories of polymer flows due to external forces, consider nocapillary actions.We have developed a numerical analysis technique for free surface flows due to capillary forces. The FEM (Finite Element Method), the SMAC (Simplified Marker And Cell) method, and the VOF (Volume Of Fluid) method are employed for discretization of twodimensional Navier-Stokes equations, time integration, and representation of the free surfaces. The capillary forces, which are governed by surface tensions of resins, contact angles between resins to chips and to substrates, and standoff heights, aretaken into consideration as boundary conditions of pressure at flow front. Numerical calculations were carried out for the underfill material flows in the standoff regions of the flip-chip BGA (Ball Grid Array). The results of filling times and flow front shapes for settings of encapsulation conditions and material propertiesshowed good agreements with experimental ones using flow resistance modification factor.