A semiconductor assembly with built-in stiffener and integrated dual routing circuitries is characterized in that a semiconductor device and a first routing circuitry are positioned within a through opening of a stiffener whereas a second routing circuitry extends to an area outside of the through opening of the stiffener. The mechanical robustness of the stiffener can prevent the assembly from warping. The first routing circuitry can enlarge the pad size and pitch of the semiconductor device, whereas the second routing circuitry not only provides further fan-out wiring structure, but also mechanically binds the first routing circuitry with the stiffener.