Vias in substrate between IC seat and peripheral thermal cage

A substrate assembly comprising:

• A. a planar substrate formed of an electrical and thermal insulating material;
• B. an integrated circuit seat formed on a top surface of the substrate, the seat including contact pads and traces extending from the contact pads;
• C. an isothermal cage formed in the substrate that substantially surrounds the seat, the cage being formed of thermal conductors in the substrate at the periphery of the cage with the thermal conductors being at different levels in the substrate;
• and
• D. vias formed through the substrate between the cage and the seat, the vias being filled with an electrically conductive material, a first via being electrically and thermally connected to one of the traces and to the thermal conductors, and a second via being electrically and thermally isolated from the thermal conductors and being electrically connected to another one of the traces.