A multichip module comprises a multilayer substrate circuit having conductive patterns on its surface(s) to which microelectronic device(s) are attached. The conductive patterns include a series of electrical contacts adjacent to one edge of the substrate. The substrate is bonded to two rigid frames, one on each opposite surface. Each substrate has a series of castellations on one edge that are aligned and electrically connected to the respective contacts on the substrate, preferably by soldering. The castellations can serve as a self-aligning mechanism when the module is brought into contact with a low-profile pin array, and the module may be held in place on a motherboard by guide rails in a socket that engages the edges perpendicular to the castellated edge of the module. The module may further be provided with protective heat spreading covers.