Electronic and electrical devices are the sources of both power and heat. As is well known, in order to provide reliable operation of such devices, it is necessary to maintain stable operating conditions and temperatures. Hence, efficient methods for heat management and dissipation are essential. Typically this is done by providing thermal management devices that are arranged adjacent and in contact with the electronic device or circuit board. Heat generated in the circuit is transferred to and dissipated in the thermal management device. For optimum efficiency, it is desirable that thermal management structures have the highest possible thermal conductivity, efficient external connectivity and appropriate mechanical strength.

To achieve these objectives in thermally demanding applications, some known devices encapsulate high thermal conductivity materials into composite structures. However, these devices often achieve only limited performance, with significant conductivity losses, typically 40%, and increases in mass and bulk.

A further problem is that the mass and volume of known thermal management systems are relatively large. This affects the overall size of electronic systems in which such devices are incorporated. In this day and age when the general drive of the electronics industry is towards miniaturisation, this is highly disadvantageous.

Thermal management systems are often used as substrates for supports for hybrid electronic circuits. In one known arrangement, beryllia is used as a heat sink. This has a thermal conductivity of around 280W/mK at room temperature. On top of this dielectric gold contacts are subsequently formed, thereby to enable connection to other electrical circuits. A disadvantage of this arrangement is that beryllia is a hazardous material; in fact it is carcinogenic, and is generally difficult to process. In addition, the dielectric tends to be thick thereby making the overall structure bulky. Furthermore, partly because of the use of gold as a contact material, the overall structure is expensive to manufacture.

One known solution is that described in International patent application no.

WO00/03567 the contents of which are incorporated herein by reference.

According to the approach described in that document a plate of anisotropic carbon, for example pyrolitic graphite or thermalised pyrolitic graphite is encapsulated in an encapsulating material such as polyimide or epoxy resin or acrylic or polyurethane or polyester or any other suitable polymer. The encapsulating material is applied directly to the anisotropic carbon and improves the rigidity of the carbon. The resulting device has an in-plane thermal conductivity of typically 1, 700W/mK at room temperature whilst it can provide a flatness at typically plus or minus 5um across a plate that is 100mm by 100mm. Yet further the device can provide a board having a tensile strength that is significantly higher than that of the original, unencapsulated, carbon plate with a negligible increase in volume and loss of thermal conductivity.

New and future generations of semi-conductors provide a major challenge for thermal management through having localised areas with excessive power

density (hot-spots) where transient temperatures far exceed those safely accessible to conventional packaging techniques. For example increasing power densities and operation frequencies can arise from specific areas of high performance switching inside a given chip that effectively acts as point-like heat sources. As a result yet further improved thermal management devices are required.

The invention is set out in the accompanying claims. By providing localised enhanced heat transfer regions in a thermal management device of anisotropic carbon encapsulated in an encapsulating material, for example in the form of a cavity defining a heat-pipe or cooling tube, buffering is provided that decreases transient temperatures, for example in a chip and its associated packaging materials, and overall heat flow parameters in surrounding materials are also improved providing conditions for continuous operation that do not compromise either the performance or reliability of a device such as a semi- conductor.

Embodiments of the invention will now be described, by way of example, with reference to the drawings of which: Fig. 1 is a side view of a thermal management device defining part of a cavity; Fig. 2 is a sectional view of two mirror-image thermal management devices of the type shown in Fig. 1 defining a cavity; Fig. 3 is a sectional view of the arrangement of Fig. 2 further having a heat- pipe installed in the cavity; Fig. 4 is a sectional view in a direction A-A of the device shown in Fig. 3; Fig. 5 is a plan view showing alternative cavity configurations in a thermal management device; Fig. 6 is a sectional view of a thermal management device with a drilled cavity;

Fig. 7 is a sectional view of the thermal management of Fig. 6 with a heat-pipe installed; Fig. 8 is a sectional view taken along the line A-A in Fig. 3 showing an embodiment including a heat-sink; and Fig. 9 is a sectional view of a thermal management device with a semi- conductor device mounted thereon.

In overview, a cellular thermal management structure is provided in which a localised enhanced heat transfer region comprises a cavity provided in a thermal management device in the form of a heat-pipe or cooling tube. The thermal management device comprises a plate of anisotropic carbon encapsulated in a suitable polymer in which the cavity is formed by cutting complementary grooves into mirror image half-plate devices which are then bonded together to mate the grooves. Alternatively the cavity is drilled through the thermal management device. The cavity can intrinsically form a heat-pipe or cooling tube or a pre-fabricated heat-pipe or cooling tube can be inserted into the cavity.

As a result the essentially passive planar structure provided by a thermal management device of the type described has enhanced internal heat transfer properties provided by active mono-phase (in the case of cooling fluid in a cooling tube) or bi-phase (in the case of a heat-pipe) systems. Customised designs can be provided benefiting from the extremely high diffusivity of the thermal management device minimising temperature gradients within the structure combined with the directional heat flow of the localised enhanced heat transfer region.

The nature and manner of fabrication of a thermal management device to which the localised enhanced heat transfer regions are added is described fully in

International patent application no. WO00/03567 and will be apparent to the skilled reader so that only a summary is provided here for ease of reference. In one embodiment a plate of thermalised pyrolitic graphite with mosaic or full ordering is coated with polyimide applied directly to the carbon surface for example using a brush. If necessary the coating is cured. Where required holes for electrical contact are formed for example by drilling prior to the coating step, encapsulating the drilled plate and then re-drilling the holes to a smaller diameter such that the carbon remains encapsulated.

The device can be applied to a substrate or used itself as a substrate for example for thin film circuits which can be deposited in any appropriate manner. Both sides of the device can be used and the device can form a base or substrate for a multi-layer circuit.

The thermal management device is thus constructed by direct molecular-level encapsulation of the carbon plate allowing interfacing with other heat transfer materials through micron-level fusing and providing an electronic hybrid technology allowing both single and double-side connectivity. The intrinsic thermal performance of the internal carbon substrate is preserved and thermal transfer characteristics expressed in the relevant parameter k/p (Thermal conductivity/density) are improved with respect to copper by a factor of between 18 to 20 and aluminia by nearly 90. At sub-zero temperatures the improvement factors can be dramatically increased further. The encapsulation layers are typically 20 microns and so for substrates of a thickness of a few hundred microns larger this represents a negligible increase in total volume and hence a negligible decrease in thermal conductivity preserving the fundamental thermal properties of the carbon plate whilst enhancing the mechanical properties such as sheer strength and surface integrity. The device provides robust structures with mechanical stability whilst maintaining low density and

high in-plane thermal conductivity and a range of direct electrical processing to provide a new sector of high thermal conductivity hybrids.

Thermal management structures of the type described above form the basis of the cellular thermal management structures in the present embodiment as shown in more detail in the accompanying drawings which illustrate various approaches to fabricating a thermal management device with enhanced heat transfer properties.

In a first embodiment, referring to Fig. 1, a thermal management device designated generally 10 includes an anisotropic graphite plate 12 and a polyimide encapsulating coat 14. Prior to coating the plate 12 a cavity in the form of a channel or groove 16 is cut or otherwise formed in the plate 12 extending across the length of the surface. Referring to Fig. 2 a corresponding groove is cut into a second plate 20 and the plates are attached together to form a cavity 22.

The fabrication steps will be apparent to the skilled reader but will be described briefly here. The groove 16 can be cut, etched or otherwise formed into the plate 12 and the coating 14 applied as described above. The groove 16 can be of any profile for example semi-circular in cross section (so as to provide a circular cavity 20 as shown in Fig. 2 when the two plates are mated) or a more complex cross section. One or both of the plates 10,20 can be encapsulated anisotropic carbon and neither the plates nor the grooves need be symmetrical as long as the grooves mate to form a common channel. In the embodiment shown, the two plates 10,20 are epoxy-fused to provide an appropriate bonding, but any appropriate bonding technique may be used.

The localised enhanced heat transfer region can be formed from the cavity in various manners. In a first approach, the channel 22 in Fig. 2 forms a conduit for direct flow mono-phase heat transfer fluid hence forming a cooling tube.

Alternatively the channel 22 can form a bi-phase heat-pipe of the type described in more detail below which is made"self-wicking"in an appropriate known manner for example by forming an appropriate profile on the inner surface of the channel 22 in the form of narrow axially extending fins providing the required capillary action. If such an in-situ heat-pipe is formed it is filled with a given mass of an appropriate fluid by evaporation under vacuum and then the cavity is sealed, again as will apparent to the skilled reader.

In either case it is desirable to make the cavity 22 non-porous and so an additional coating step is introduced, for example using continuous vapour deposition, to provide a layer of vitreous carbon providing the required non- porosity or providing any other appropriate non-porous coating. It will be seen that the remainder of the plate 12 can alternatively be coated prior to the cutting step in this case and that an additional coating to the non-porous coating is not required in order to encapsulate the cavity. Indeed the non-porous coating can be used to encapsulate the entire device in a single coating step.

Again, in either case, in the process of epoxy-fusing the half-plates 10 and 20 it is desirable to fill the cavity between the plates with metal to avoid entry of epoxy. After fabrication the metal can be removed by etching to provide an enclosed cavity that can if necessary also be cleaned by chemical action avoiding degradation from ingress of epoxy. Alternatively the heat pipe or cooling tube can be located prior to fusing the half plates to provide a barrier to epoxy ingress. This approach is particularly advantageous for cavities having a tortuous configuration in which it could be difficult to insert the pipe/tube after bonding the half-plates.

An alternative manner of forming the localised enhanced heat transfer region is shown in Fig. 3 in which a pre-fabricated heat-pipe 24 of an appropriate profile to match the cavity 22 (in the present instance of circular cross-section) is inserted into the cavity and epoxy-fused or bonded in any other appropriate manner. It will be appreciated that the heat-pipe 24 can be inserted to one of the half plates 10 prior to mating with the other half plate 20, or can be inserted into the cavity formed when the two half plates 10 and 20 have already been mated and epoxy-fused. The former approach has the advantage that the heat- pipe prevents the ingress of epoxy during the epoxy-fusing step mating the two half plates together. Instead of providing a heat-pipe a cooling tube allowing fluid-flow cooling can be inserted in a similar manner. Once again the encapsulating coat can be applied before or after the cutting step, and the cavity does not require additional coating, as it is effectively encapsulated by the wall of the pipe or tube and epoxy fixing.

Turning to Fig. 4 it will be seen that in one embodiment the heat-pipe or cooling tube 24 extends beyond the perimeter of the device. Of course just one or neither end may extend beyond the device in an alternative configuration. In a further alternative configuration the heat-pipe or cooling tube 24 is entirely enclosed in the cavity in a manner allowing an appropriate level of heat transfer.

More complex cavity configurations and multiple cavities are shown in Fig. 5 and it will be seen that complex shapes can be adopted. The cavities thus formed can extend the full length of the surface between any two faces or the same face, or can be contained within the surface either at one or both ends.

Similarly branched cavities and more complex configurations still can be incorporated. Whatever the configuration, the cavities can either act

intrinsically as cooling tubes or self-wicking heat-pipes or pre-fabricated cooling tubes or heat-pipes of appropriate configuration can be inserted, and the fabrication method is selected dependent on the configuration.

The heat-pipes discussed above can be of any appropriate type, for example they can be the pre-fabricated type available from Noren Products, Inc, 1010 O'Brien Drive, Menlo Park, CA, 94025 USA. The operation of a heat-pipe will be well known to the skilled person and is only summarised here for the purposes of completeness. A heat-pipe comprises a hollow cylinder closed at both ends and with a porous wall providing capillary action forming a"wick".

A heat transfer fluid such as methanol is absorbed by the wick. When heat is applied at a point on the heat-pipe, the methanol at that point vaporises and passes into the hollow core of the tube and away from the heated region. The vapour condenses once again at a cooler region and gives up the latent heat of vaporisation hence transferring the heat from the hotter to the cooler region. In alternative approaches, rather than using a porous or capillary material to provide the wick, "self-wicking"is provided in which a grooved tube having axial grooves provides the capillary action required to transfer the fluid back from the cooler region to the hotter region. Heat-pipes are found to provide significantly higher heat transfer properties than equivalent metallic conductors such as a block of copper.

The cooling tube discussed above effectively provides a conduit for heat transfer fluid and can form part of a conventional cooling loop.

A second approach to fabricating a thermal management device with enhanced heat transfer characteristics is shown in Fig. 6 in which a hole 28 is drilled in the plate 10. In a similar manner to the first approach, the hole may be formed prior to the coating step and coated with the remainder of the device or

subsequently and independently coated. In the embodiment shown in Fig. 6 the cavity 28 can form a cooling tube or self-wicking heat-pipe in the manner described above, again with a suitable non-porous coating if required. The hole may as appropriate be mechanically or laser drilled or alternatively be moulded or otherwise formed and it is found that a self-wicking profile is advantageously obtained using laser cutting. It will be appreciated that more complex configurations may be better accommodated by the approach described with reference to Figs. 1 to 5.

Referring to Fig. 7, alternatively a heat-pipe or cooling tube 30 is inserted into the cavity 28 and interfaced with a thin epoxy film, again as discussed in more detail above.

In either case the drilled hole 28 can be blind (in the case of heat-pipes) or can be open at both ends and can be of any configuration within the constraints of the forming technique and the requirement for inserting the pre-fabricated heat- pipe or cooling tube, or forming a self-wicking inner surface for an in-situ heat- pipe. The encapsulating steps can be performed at any appropriate time in a manner similar to that discussed in relation to Figs. 1 to 5.

The manner in which the cellular thermal management structure is attached to additional components is shown in Figs 8 and 9. Referring to Fig. 8 enhanced heat extraction capabilities are provided by including a heat sink 32. In the embodiment shown the condensation end of a heat-pipe 24 is placed in thermal contact with the heat sink 32 which is provided flush with the heat extraction surface of the device 10 and bonded in any appropriate manner for example epoxy-fusing or high temperature brazing. Similarly, the surface of the heat- pipe 24 may be bonded to the wall of drilled hole 24.

A suitable surface for brazing may be provided by coating with metal layers of thickness ranging from a few microns up to tens of microns using a chemical deposition process, electro-plating, sputtering or a similar process. The coating can be made as a single layer of a metal, multiple sandwiched layers of the same or different metals, a combination of different metals or of an alloy. It can comprise two or more sub-layers, each produced by one or more of the above techniques. After coating the surface of the device with metal, the metal layer can be masked with the desired pattern for the final metal configuration, and metal removed from the unwanted areas region by etching, such that the desired areas of the heat extraction surface of the device 10 and/or the wall of the drilled hole 28 are covered with a metal layer.

The heat sink can be any appropriate device such as a cold-mass, a cooling fin assembly, an externally cooled structure using fluid flow pipes and so forth.

Alternatively the heat sink can be provided in thermal contact with a structure extending beyond the surface of the device 10, providing for example cooling capabilities to cooling fluid flowing through a cooling tube.

Referring to Fig. 9 a device 34 to be cooled comprises a semi-conductor device which is bonded in any appropriate manner, for example epoxy-fusing, to form a surface sealed cellular thermal management structure. The device 34 is positioned relative to the cavity 22 in the thermal management device 10 which can provide localised enhanced heat transfer in any of the manners described above, for example by insertion of a further, suitably profiled pre-fabricated heat-pipe or by intrinsic action of the cavity 22 as a heat-pipe or cooling tube.

The under surface of the device 34 becomes part of the cooling structure forming one face adjacent or closing the cavity.

It will be seen, therefore, that the thermal management device described herein can provide customised solutions to the requirements of devices with very high power density and operation frequencies as well as localised hot-spots including the option of direct contact between the semi-conductor and active cooling elements of the device whilst maintaining the significant thermal transport features of the underlying device, including use as electronic hybrid structures. The properties of the device are particularly advantageous for optimising the in-situ performance of micro heat-pipes. Such pipes can have properties that can be characterised by thermal conductivities in excess of 10, 000W/mK, limited only by the allowed cross-sectional area, length and wicking, as well as by the thermo-dynamic properties of the evaporating/condensing liquid. For the micro heat-pipes discussed in the present document heat transfer of around 0.3W might be achieved given appropriate start-up conditions for each heat-pipe cycle and provision of operating temperatures well below the boiling limit of the evaporating/condensing liquid.

It will be appreciated that aspects from different embodiments can be interchanged or juxtaposed as appropriate. Although application of the thermal management device to semi-conductor packaging is discussed, the device can be equally well used in any appropriate cooling/heat-transfer environment and in combination with any of the optimisations discussed in WO00/03567.

Discussion of heat-pipes extends to derivatives thereof such as loop heat-pipes and any other structures using the basic properties of heat-pipes, and discussion of the cavity comprising the localised enhanced heat transfer region extends to any groove, recess, aperture or hole providing the required properties.