A hermetically sealed package assembly for microwave circuits in the high microwave frequencies having a dielectric gap defined in the substrate. The package assembly comprises a substrate for carrying the circuit on a top surface, a seal ring wall carried on the top surface, a via structure provided through the substrate for transmitting high-frequency signals into and out of the package, and a conducive lead carried on the bottom surface of the substrate and passes under the seal ring wall. A dielectric gap is provided in the substrate between the seal ring wall and the lead to reduce the capacitive coupling between the lead and the wall. The gap may be a sealed cavity provided inside the substrate, or a cutout area in the substrate so that the lead is suspended in air at the location where it passes under the seal ring wall.