Development and application of copper-graphite composite thermal core materials for high reliability RF system PCBs

Printed circuit board (PCB) design and manufacturing has traditionally used heavy copper layers within the board as a method of dissipating heat loads from high performance, high heat-generating devices mounted on the boards. These copper layers add substantial weight, have a very high coefficient of thermal expansion (CTE) in relation to other board fabrication materials and the semiconductor devices mounted, and cannot be tailored in CTE value to match a specific requirement. Relatively high CTE values can influence temperature-induced stress within such a PCB and impact appropriate joining materials for device mounting, factors in PCB reliability. A search for alternative materials has included development of combinations of carbon fibers, organic materials, alternative metals, and similar concepts. A recent development program has been initiated to examine manufacturing methods to yield relatively lightweight and thin copper-graphite composite materials that can be used as a direct replacement for a copper layer, offer selected CTE values, provide relative stiffness to prevent PCB warpage, and dissipate heat loads adequately via conduction cooling to the electronic system enclosure sidewalls. The intended outcome is manufacturing capability to produce sheet forms of such as copper-graphite composite in very thin formats that match PCB industry standard production equipment and processes, as a “drop-in-place” alternative to copper layers, which can be used with all standardized PCB manufacturing processes and which do not impose limitations on where such a thermal control layer may be used within the in-plane design layout of circuit boards.