The hybrid substrate (build-up package substrate + organic interposer + solder joints + underfill) of 2.3D IC integration (or multiple systems and heterogeneous integration) is investigated. The organic interposer of the hybrid substrate is fabricated by a fan-out chip-last on a temporary panel with two different kinds of dielectric materials, namely the PID (photoimageable dielectric) and the ABF (Ajinomoto Build-Up Film). These two hybrid substrates are supporting two chips. The flatness of metal lines of the RDLs (redistribution-layers), the quality of the hybrid substrate after chip-to-hybrid substrate bonding, and the reliability of the solder joints between these two hybrid substrates will be presented.