In a package, a heat slug, encapsulated by molding compound, encases an integrated circuit device (IC). In an example embodiment, a semiconductor package structure comprises a substrate having conductive traces and pad landings. The conductive traces have pad landings. An IC is mounted on the substrate. The IC has bonding pads. With conductive wires, the IC bonding pads are connected to the pad landings, which in turn, are connected to the conductive traces. A heat slug, having predetermined height, is disposed on the substrate surface. The heat slug includes a plurality of mounting feet providing mechanical attachment to the substrate. A cavity in the heat slug accommodates the IC. A plurality of first-size openings surrounds the IC. A second-size opening constructed from one of the first size-openings, is larger than the first-size opening. The second size-opening facilitates the introduction of molding compounds into the cavity of the heat slug.