A numerical study of 3D interfacial delamination in PQFP packages

Abstract

Interfacial delamination of the leadframe pad and encapsulant is the precursor to type I popcorn cracking. Due to high stress concentration at the corner of the interface, delamination usually initiates and propagates from the corner of the interface. The mechanics of delamination propagation from such a corner is three dimensional and can best be studied with a 3D model. When adopting the fracture mechanics approach in the study of delamination, the assumption of the initial shape of the delamination can have significant impact on the fracture mechanics parameters obtained from the numerical analysis. Hence, the main objective of this paper is to investigate the effect of the shape of the initial delamination on energy release rates and the stress intensity based mode mixity. Both the modified virtual crack closure method (MVCCM) and the J-integral method are used in the determination of energy release rate while the stress intensity based mode mixity is obtained from a method based on MVCCM. Three different shapes of initial delamination are studied, namely, convex (bulge towards the corner), concave (bulge towards the center) and straight (initial delamination is triangular in shape). It is found that the maximum energy release rates obtained from the convex case is the highest, follow by the straight case and the concave case. The distribution of the J-integral along the crack front shows that the energy release rate is the highest at the center of the initial delamination for all assumed shapes of initial delamination. Moreover, the mode mixity in all the three cases are relatively constant across the crack front. Comparing the distribution of J-integral among different cases, the convex case exhibits the largest variation of J-integral along the crack front while the distribution of the J-integral for the concave case is relatively flat across the crack front. The results seem to suggest that in an event of crack the center of the delamination will propagate first in all the cases analyzed. They also seem to suggest that the initial crack is likely to propagate with a concave crack front.