A method for packaging semiconductor dies by overmolding is disclosed. The dies are embedded in a substrate of a mold material, and cavities are produced in the mold substrate by producing 3D structures of a sacrificial material prior to the overmolding step. Afterwards, the sacrificial material is removed to thereby create cavities in the mold substrate. A conformal layer is produced on the 3D structures prior to overmolding, and the mold substrate is thinned to expose an upper surface of the 3D structures. The conformal layer is not removed when the sacrificial structures are removed. In this way, the conformal layer remains on the surfaces of the mold substrate inside the cavity. In one aspect, the conformal layer may have a protective function, useful in the production of packages including dies which come into contact with fluid substances.