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Copper/Graphite Product Details


Thermal Expansion

As materials are heated, they expand.  The coefficient of thermal expansion (CTE) is a measurement of that rate of expansion.  Typical units for CTE are parts per million per degree C (ppm/C).  

Silicon has a coefficient of expansion of approximately 3 ppm/C, while the expansion of copper is 17 and the expansion of aluminum is 23.  Because of this large mismatch, silicon or other low expansion materials cannot be bonded directly to copper or aluminum without undergoing unacceptably high stresses.  Likewise, ceramic packages mounted on FRP boards are likewise subjected to large mismatches in CTE.  The copper/graphite composite can act as a constraining core for FRP boards.

Unlike most materials, graphite fibers actually contract as they are heated.  Depending upon the amount of graphite fiber incorporated into the copper/graphite composite, CTE values can be varied over a wide range, as shown:

The copper/graphite composite typically has a CTE between 4 and 6 ppm/C.

Below is a graph showing expansion versus temperature for a typical 6 ppm/C copper/graphite sample.  The CTE of many materials including Cu/Invar/Cu, Cu/Mo/Cu, Kovar, and epoxy, varies as a function of temperature due to such effects as phase changes or glass transition temperatures.  Likewise, the CTE of the copper/graphite varies as a function of temperature.

For most applications, a linear approximation of the CTE value works well.  Because the expansion of the composite can be varied, thermal mismatches between a die and the composite substrate can be minimized.  Die can be bonded with soft solder or by eutectic gold-silicon or gold-tin alloys.  The reduction in thermal resistance between the die and the substrate can be very substantial.

Please Note: The properties of the copper/graphite composites are a function of the type and amount of graphite fiber incorporated in the composite, as well as the orientation of that fiber.  Thus, the properties discussed are typical for a composite reinforced with a chopped graphite fiber oriented within the plane of the composite.


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