Three-dimensional Finite Element Analysis of Interfacial Delamination in Molded Underfill Flip-Chip Packages by Virtual Crack Closure Technique

In the present work, fracture mechanics parameters including strain energy release rate (SERR) and phase angle along the crack front of a three-dimensional interface crack in molded underfill (MUF) encapsulated flip-chip (FC) packages during subsequent reflow process are studied using finite element method via virtual crack closure technique (VCCT). The effect of embedded crack shapes (three different cracks, i.e., straight crack, quarter elliptical crack and quarter circular crack) on the crack propagation behavior is characterized first. A phase angle mesh independent technology called modified virtual crack closure method (MVCCM) is used to evaluate the in-plane mixed mode at the crack tip. It is found that at crack tip of straight crack the Mode II component of the crack driving force is dominant, while both Mode II and Mode III components drive the circular crack to propagate. In addition, there is no significant difference in the in-plane phase angle and SERR among three cracks with different shapes when the Mode III component is not considered. It has also been shown that a straight crack may propagate first from the center of the crack and eventually develop into a circular or elliptical crack. Furthermore, the effect of different crack lengths on the driving force of the quarter circular crack seems to show that there exists a critical crack length, below which the SERR increases rapidly with the crack length. However, SERR tends to decrease when the crack reaches the critical length. This means that once a crack has initiated, it will continue to grow until the critical length is reached.