An assembly of substrate packages interconnected with flex cables. The assembly allows input/output (I/O) signals to be speedily transmitted between substrate packages via flex cable and without being routed through the motherboard. Embodiments relate to a substrate package providing detachable inter-package flex cable connection. The flex cable comprises a transmission region that includes a plurality of signal traces and a ground plane. A plurality of solder mask strips are disposed on the plurality of signals traces to provide anchoring for the signal traces. The solder mask strips intersect the signals traces. The exposed signal traces and the ground plane are coated with organic solderability preservative material. Hermetically-sealed guiding through holes are provided on the substrate package as a mechanical alignment feature to guide connection between flex cables and high speed I/O contact pads on the substrate package. Embodiments of the method of fabrication relate to simultaneously forming hermetically-sealed guiding through holes and I/O contact pads.