Abstract: A hermetically sealed microwave integrated circuit (MIC) includes a motherboard with a ground plane of aluminum-silicon alloy, a plastic/ceramic composite dielectric layer, and a copper-nickel-gold upper layer. The alloy for the ground plane is selected to allow fusion with aluminum containing less than 1% silicon to create a hermetic seal. The ground plane is fashioned to beyond the interior and upper layers, forming a welding flange that circumscribes the perimeters of the interior and upper layers. Recesses are cut into the dielectric to expose the ground plane. Active devices and microwave integrated circuits (MICs) are disposed within the recesses and mounted on the ground plane. The metallized upper layer is etched and patterned to create a microwave integrated circuit. The alloy flange is laser-welded to the annular lower surface of a frame made of aluminum containing less than 1% silicon, so that the interior plastic layer, the metallized upper layer, and all active devices are within the frame.