Reporting of Cu/C composites

Composite heat-sink family shows the highest potential for application in heat sinks for electronic applications. The great advantage of this kind of materials is that they can combine metal, polymer and ceramics in a broad range of different products to tailor the final properties sought. Nevertheless, final properties of composites greatly depends on manufacturing processes.

Among the huge amount of different composites, the most efficient and so the most commonly developed for heat dissipation purposes, are metal matrix composites, and most specifically those based on copper and aluminum matrices with a broad range of different reinforcements.

Recently, some investigations in epoxy based composites reinforced with an extremely interesting new kind of carbon fibres (Vapour Grown Carbon Fibres, VGCFs) have shown outstanding thermal conductivities, near 650W/mK. These newly developed kinds of micro/nanofibres (commercially available since the nineties) exhibit the highest thermal conductivity of all materials known. Their special structure based on its manufacturing process (they are produced from gases) gives them a high potential to achieve a highly graphitic structure by a proper heat treatment, not achievable in other carbon fibres, thus leading to outstanding physical and mechanical properties. But the main problem encountered with epoxy based composites is their low highest temperature of use. Nevertheless, in any case, these composites could be applied up to a temperature near 120°C, where polymers can start degrading.

Undoubtedly, the most efficient composites for heat sinks are those based on Cu and Al with a number of different reinforcements. In general, metal matrix composites (MMCs) show several improvements in comparison with currently used materials in electronic packaging, among them the following can be underlined:

– Lower and tailorable CTE (the higher the volume fraction of reinforcement used, the lower the CTE is) to be as similar as possible to ceramic substrates. In the present development, in which a suitable electronic packaging component for GaN semiconductor device will be developed, the coefficient of thermal expansion must be as similar as possible to that of the electronic substrate. This reduction of the CTE mismatch supposes, therefore, a reduction of stresses between the components and their substrates during thermal cycling;

– High heat dissipation capability;

– Lightweight, suitable for space applications;

– High stiffness at high temperatures, being an useful characteristic in order to assure dimensional stability of electronic components working at high temperatures.

Due to their high conductivity, low CTE and lightweight, Al/SiCp (50-70%SiCp) composites represent one of the MMC kinds of materials that have found an easier way to be introduced into real industrial applications [Luedkte, 2004]. Some manufacturing processes for this type of composites have been developed along the last years (infiltration and squeeze casting) for these materials, so they are already available (it depends very much on size, shape, etc…). Unfortunately, they are extremely difficult to machine, therefore, manufacturing processes must be net or near-net shape, in order to get a really profitable manufacturing process. Another interesting kind of MMCs for heat sinks is Cu/diamond composites. But their main problem, again, is that they are expensive because diamond is expensive and difficult to machine.

Looking, at the previous table 1, it can be said that copper based materials are the most promising among MMCs. Most of the composites based on this matrix are reinforced by ceramics (particles, short fibres, and long fibres) in order to reduce their extremely high CTE and match it with those of the semiconductor or other ceramic substrates. Nevertheless, carbon fibres also offer the big advantage of high heat and electrical conductivity. So, MMCs reinforced with long C fibres show excellent thermal conductivity along the fibre direction. Unfortunately, these types of composites are not usable in a broad range of heat sinks, as the excellent thermal conductivity is achieved in surface but not across the thickness. Of major interest are those composites reinforced with short fibres, as although in general terms thermal conductivity is more modest than those reinforced with continuous fibres, they have isotropic properties. In addition, their manufacturing processes are less sophisticated and expensive.