• Preferably the heat-sink metal layer 22 is formed of a metal of relatively high thermal conductivity material such as copper or aluminum, copper-clad aluminum, or copper-clad Invar (36% nickel, blance iron) or the like and is secured at a side of the organic electrically insulating layer 20 opposite from the thin high electrical and resistivity layers 26, 24 for withdrawing heat from the high resi