An electronic module, substrate assembly, and fabrication method, the assembly providing thermal conduction between an electronic device to be cooled and an aqueous coolant, while maintaining physical separation between the coolant and electronic device. The assembly includes a substrate, one or more electronic devices to be cooled, and a multilayer, impermeable barrier. The multilayer barrier includes a first layer, providing mechanical support for a second layer. The second, thinner layer provides an impermeable barrier, and a high effective thermal conductivity path between an electronic device and a cooling fluid in contact with an upper surface of the second barrier layer. Mechanical stresses are minimized by appropriate material selection for the first layer, and a thin second layer. When incorporated into an electronic module assembly including a module cap, the substrate assembly provides physical separation between a cooling fluid introduced into the module cap, and both the substrate and electronic devices.