A sheet of material includes a layer of the insulative thermoplastic material such as PET (poly(ethylene terephthalate)). The sheet is placed down over the wirebonds and a semiconductor die of a substrate assembly so that the sheet contacts the wirebonds and/or the semiconductor die. In one example, the sheet is a preform and the bottom of the sheet includes a layer of tacky adhesive that adheres the sheet to the substrate assembly. The sheet is then heated such that the PET softens and becomes conformal to the wirebonds and the semiconductor die of the upper surface of the substrate assembly. The resulting encapsulated substrate assembly is then encapsulated (for example, by overmolding in an injection molding process) to form a packaged semiconductor device. The conformal PET sheet is embedded within the packaged semiconductor device in such a way that it separates the wirebonds and semiconductor die from another encapsulant.