A low temperature cofired monolithic ceramic package (10) for use with microwave and millimeter wave gallium arsenide integrated circuits, or the like. In situ buried passive components (28), including capacitors, resistors, couplers and inductors are incorporated within a cofired substrate structure (12). Fields of interconnected staggered or stacked vias (56, 58) are provided in the substrate below the integrated circuit for efficient heat transfer and electrical connection to a ground plane that eliminates complicated heat sink strucutres. At least one cavity (24) is provided in the top surface to confine integrated circuit chips. A plurality of connectable electrical conductors (34,36,38,40,42) including RF input and output leads and DC bias leads for the chip extend from the edge of the substrate (12) to the edge of the cavity (24). A sidewall 14 is extends around the periphery of the cavity (42) and comprises a low temperature cofired structure which is cofired with the substrate (12). The sidewall 14 has a metalized top surface and interconnected metalization and vias extending from the top surface to the top surface layer of the substrate (12). A cover (16) is provided which is attachable to the metalized top surface (44) of the sidewall (14). The electrical and thermal interconnections provided in the sidewall (14) and substrate (12) are provided by means of the plurality of interconnected staggered or stacked via interconnects. Electrical and thermal integrity is achieved which permits dissipation of heat by way of the ground plane and top cover, and electrial shielding is achieved by interconnecting the conductive components.