Abstract: A method of fabricating a multilayer circuit board assembly capable of operating at microwave frequencies. Specifically, multilayer circuits such as complex beam forming networks and antennas having internal via holes may be fabricated in a single bonding cycle. This is accomplished by first preparing individual circuit boards (10) by pre-forming holes (18) through circuitry (14,16) etched on opposite sides of a substrate (12). The holes (18) are next selectively plated with a conductive layer (20) and then with a material (24) capable of forming a melted fusion bond and/or a solid-state fusion bond. Bonding film (56) is placed between each board-to-board interface of a multi-board assembly and the assembly of boards is subjected to a bonding cycle of heat and pressure wherein an integral unit is effected.