Effect of the die edge and multiple delaminations on the mechanics of delamination in a PQFP

Abstract

Interfacial delamination between the lead frame pad and the encapsulant can be a concern as the delamination at this interface is the precursor to Type I popcorn cracking. Previous analyses have shown that when the crack tip of a delamination at the pad/encapsulant interface approaches the edge of the die (albeit separated by the lead frame), the crack driving force increases significantly. Such increase may be attributed to the increase of the crack length or to the nearness of the edge of the die, Based on current literature, it is not clear as to which is the dominant cause. The impact of the edge of the die on the delamination at the pad/encapsulant interface can be significant and this issue is addressed in the current work. The effect of the die attach thickness and Young's Modulus is also investigated. Existing literature often focus on the propagation of one crack which originates from the corner of the interface and this is justified by the presence of high stress concentration at the corner of the interface. However, in some cases, it is possible for more than one crack to exist at the interface. When this happens, it is likely that the cracks will interact with one another, causing the crack-tip driving forces to be affected by the presence of other cracks. Such interactions between multiple cracks is another subject of study in this paper. In the current study, the vehicle of study is a plastic quad flat pack (PQFP) and the interface of interest is the pad/encapsulant interface. Finite element models are constructed and the energy release rate (ERR) of the crack tips calculated using the virtual crack closure method (VCCM) and J-integral. Since the fracture toughness of the interface is dependent on mode mixity, the mode mixities of the crack tips are also calculated. In the study on the effect of the die-attach thickness, it is found that as the thickness of the die attach increases, the peak ERR at the vicinity of the die edge and the ERR of short cracks decreases. The peak ERR observed at the vicinity of the die pad ceases to exist in the presence of a compliant die attach. As for the mode mixities, they remain relatively constant with the change in die attach thickness and Young's modulus. A thick die attach appears to be favorable for mitigation of delamination at the pad/encapsulant interface. In the study on the effect of the die edge, the ERR was computed for a single delamination of length 200 μm located at various locations along the interface to the left and right of the edge of the die. As the location of the delamination is shifted further away from the edge of the pad, the ERR at both (left and right) tips of the crack decreases. No significant increase in ERR is found for crack tips near the vicinity of the die edge. The study on multiple delaminations comprises cases in which, two initial delaminations (which are named crack #1 and crack #2) are assumed to be present in the interface. Crack #1 is fixed at the corner of the pad while The position of crack #2 is varied. The influence of crack #1 on crack #2 is more than the influence of crack #2 on crack #1 as the ERR of crack #1 exhibits a smaller increment when compared to the increase in the ERR of the crack tips of crack #2. Crack#1 exhibits the highest ERR. The results suggests that although the edge of the die does have some effect on delaminations located near it, the delamination located at the edge of the pad will always have the highest ERR.