A method of using sacrificial structures in a mold substrate for packaging a first die and one or more second dies or stacks thereof is disclosed. The method allows testing of the first die prior to mounting the second dies, without requiring a TSV insert. In one aspect, a block of sacrificial material is embedded together with the first die in a first mold substrate and to one side of the first die. The removal of the block creates an opening. The method is configured so that contacts are exposed at the bottom of the opening, the contacts being electrically connected to corresponding contacts on the first die. This may be realized by bonding both the die and the sacrificial block to a redistribution layer, or by mounting a bridge device between the first die and the block prior to a first overmolding applied for producing the first mold substrate. A second die or a stack of second dies is mounted in the opening and bonded to the exposed contacts, after which a second mold substrate is produced, embedding the second die or dies.