Stress Analysis of Underfill during Curing Process in Flip Chip Package

In flip-chip package, underfill (UF) was usually applied to enhance the reliability of solder joints. UF needs to be dispensed and filled, and then cured at high temperature for a long time. However, the curing process always results in volume shrinkage for polymers. The volumetric shrinkage of UF in package may causes internal stress and adversely affect solder joints. Moreover, the stress induced by UF cure shrinkage is most likely anisotropic because of the spherical shape of solder joints. Thus, the specific form or direction of cure-induced stress and its influence degree is a problem worthy of study. In this paper, based on finite element analysis (FEA), the UF curing induced internal deformation in flip chip package during isothermal curing process are analyzed in detail. It was found that UF curing shrinkage will cause additional compressive deformation at the corner of package. So, the solder at corner were subjected to compressive stress from outside of the package to inside. While, for the inner region, the deformation is significant anisotropy, which may not only induce tensile stress to solders in the plane, but also cause a slight tensile stress trend to substrate and chip in the height direction. Finally, the influence of UF curing stress on subsequent process are further discussed.