A packaging substrate (310) includes a semiconductor interposer (120) and at least one other intermediate substrate (110), e.g. a BT substrate. The semiconductor interposer has first contact pads (136C) attachable to dies (124) above the interposer, and second contact pads (340) attachable to circuitry below the interposer. Through vias (330) are made in the semiconductor substrate (140) of the interposer (120). Conductive paths going through the through vias connect the first contact pads (136C) to the second contact pads (340). The second contact pads (340) protrude on the bottom surface of the interposer. These protruding contact pads (340) are inserted into vias (920) formed in the top surface of the BT substrate. The vias provide a strong mechanical connection and facilitate the interposer handling, especially if the interposer is thin. In some embodiments, an interposer or a die (124.1) has vias in the top surface. Protruding contact pads (340.1, 340.2) of another die (124.1, 124.2) are inserted into these vias to provide a strong connection.