Embodiments of the present disclosure are directed towards techniques and configurations of interconnect structures having a polymer core in integrated circuit (IC) package assemblies. In one embodiment, an apparatus includes a first die having a plurality of transistor devices disposed on an active side of the first die and a plurality of interconnect structures electrically coupled with the first die, wherein individual interconnect structures of the plurality of interconnect structures have a polymer core, and an electrically conductive material disposed on the polymer core, the electrically conductive material being configured to route electrical signals between the transistor devices of the first die and a second die. Other embodiments may be described and/or claimed.