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A micromechanics model and an associated computational scheme are proposed to study interface delamination in plastic integrated circuit (IC) packages induced by thermal loading and vapor pressure. The die and die-pad are taken as elastic materials, while the die-attach and molding compound are taken as elasto-visco-plastic materials. The interface between molding compound and the die-pad is characterized by a cohesive law. The key parameters of this law are the interface strength and interface energy. The vapor-induced pressure along the interface is incorporated by way of a micromechanics model. Parametric studies are conducted to understand interface properties and vapor pressure effects on interface delamination. Under purely thermal loading, both weak and strong interfaces are highly resistant to interface failure. However, the combined effects of thermal loading and vapor pressure arising from moisture trapped within the interface can cause total delamination at the interface. Once delamination has initiated at a weak interface, no significant increase in thermal loading and vapor pressure is required for the delaminated zone to grow to a macro-crack and subsequently to catastrophic failure referred to as popcorn cracking. The critical factors controlling the occurrence of popcorn cracking are the interface adhesion strength and interface vapor pressure.