Most semi-conductor devices are encapsulated by epoxy moulding compound (EMC) material. Even after curing at the prescribed temperature and time in accordance with the supplier’s curing specifications often the product is not yet 100% fully cured. As a consequence, the curing process of a product continues much longer, leading to curing effects of the EMC during the lifetime of the package. In this paper, the effect of EMC curing during lifetime on package reliability is investigated. The visco-elastic mechanical properties of two commercial EMC materials are measured as a function of aging time. The resulting data is used to construct material models that are used in FE calculations. Aging effects on critical semi-conductorfailure modes die cracking, compound cracking, wedge break, and delamination are addressed. Die and compound crack risks are predicted by common stress analysis. The risk of wedge break occurrence isinvestigated by detailed 3D modeling of the actual wires in the package using a global–local approach.Conclusions on delamination risks are made based on a parameter sensitivity analysis using a 3D cohesivezones approach to predict transient delamination. The package reliability study shows that the effect of EMC aging affects relevant failure modes in different ways.