An assembly of substrate packages interconnected with flex cables and a method of fabrication of the substrate package. 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 separable inter-package flex cable connection. 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.