An extensive simulation study of the interfacial delamination in molded underfill flip-chip packages by finite element method based on virtual crack closure technique

In the present work, under the theoretical framework of interface fracture mechanics, the interfacial delamination of molded underfill (MUF) encapsulated flip-chip (FC) packages is studied intensively by means of finite element simulation via virtual crack closure technique (VCCT). The crack propagation driving force, in terms of the strain energy release rate (SERR) is calculated by finite element analyses. Then, the components of Mode I strain energy release rate (G(II)) and Mode II strain energy release rate (G(II)) are used to determine complex stress intensity factors (SIFs) and the associated phase angle at the interface. However, due to the oscillatory nature of the stresses at the crack tip, the results of G(I) and G(II) are usually not converging when the element size decreases by using VCCT. Thus, the mesh independent technology must be used to evaluate the phase angle in interface crack analysis. To this end, tour different methods, which are independent of parameter Delta (i.e., the crack extension size used in VCCT), are adopted to obtain the phase angle. Meanwhile, the values of the phase angle and SERR subjected to different thermal histories are calculated and the differences among these values are compared. It is found that the zero-stress temperature of the molded underfill flip-chip (MUF FC) package is very important for obtaining reasonable and correct simulation results, in particular, below this temperature the crack tip penetration occurs between the upper and lower surfaces of an embedded crack at the interface. A frictionless contact pair should be applied so as to prevent the interpenetration of the crack. Whereas above the zero-stress temperature, no inter-penetration occurs. So, there is no need to apply the contact pair to calculating phase angles and SIFs. Furthermore, the effect of viscoelasticity of MUF on the delamination driving force in packages has also been investigated, which shows that the viscoelasticity can effectively alleviate the delamination driving force in MUF encapsulated FC packages.