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Over the glass transition temperature, epoxy molding compound strongly exhibits viscoelastic behavior which causes its Young's modulus to be not only temperature-dependent but also time-dependent. In the present study, the viscoelastic properties of an epoxy molding compound were measured and used in a delamination analysis. To establish a suitable failure criterion for delamination, end-notched flexure tests have been conducted at varying temperatures under different loading rates. The C(t) integral and J-integral were investigated as possible criteria for interfacial delamination involving viscoelastic materials. It was found that while both are suitable, the J-integral is better. A very significant finding from this study is that the fracture toughness of the interface between the molding compound and copper increases with temperature when the viscoelastic behaviour of the molding compound is taken into account. Finally, the delamination of the pad/encapsulant interface in a 160-leaded PQFP undergoing leadfree solder reflow was simulated using the delamination criterion established.