Stock material or miscellaneous articles – Self-sustaining carbon mass or layer with impregnant or...
Reexamination Certificate
2001-03-05
2003-02-04
Jones, Deborah (Department: 1775)
Stock material or miscellaneous articles
Self-sustaining carbon mass or layer with impregnant or...
C408S212000, C408S212000, C361S679090, C361S704000, C361S705000
Reexamination Certificate
active
06514616
ABSTRACT:
The present invention relates to a thermal management device for managing the dissipation of heat in, for example, electronic equipment and a method of making such a device. In particular, the invention relates to a thermal management device that has electrical feed through capability and can act as a direct interface to active elements.
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 Examples of such structures are described in EP 0,147,014, EP 0,428,458, U.S. Pat. No. 5,296,310, U.S. Pat. No. 4,791,248 and EP 0,231.823. The best thermal management systems available at present have conductivities that typically do not exceed 1,000 W/mK.
Current technologies do not provide thermal management that is sufficient in many applications whilst at the same time providing efficient electrical interconnection between layers or sides of circuit boards. 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 280 W/mK at room temperature. On top of this is a layer of dielectric on which 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.
An object of the present invention is to provide a thermal management system that has a high thermal conductivity but a low mass and volume.
According to a first aspect of the present invention there is provided a thermal management device comprising anisotropic carbon encapsulated in an encapsulating material that is applied directly to the carbon and is able to improve the rigidity of the carbon, preferably wherein the encapsulating material is polyimide or epoxy resin or acrylic or polyurethane or polyester or any other suitable polymer.
Preferably, the anisotropic carbon has mosaic or full ordering.
Preferably, the anisotropic carbon is thermalised pyrolytic graphite that has mosaic or full ordering. The thermalised pyrolytic graphite may have an in plane thermal conductivity of 1550-1850 W/mK at around room temperature. Typically, the thermalised pyrolitic graphite has a low value of tensile strength in the orthogonal direction.
The anisotropic carbon may alternatively be pyrolytic graphite. The pyrolytic graphite may be in an “as deposited” or partially ordered form. The conductivity of the pyrolytic graphite may be in the range of 300-420 W/mK in one plane. The tensile strength of the plate may be 1.5 Ksi in the orthogonal plane.
Preferably, the anisotropic carbon is a plate. Preferably the carbon plate has a thickness in the range 100-500 &mgr;m. The carbon plate may have a thickness in the range of 200-250 &mgr;m or 250-300 &mgr;m or 300-350 &mgr;m or 350-400 &mgr;m or 400-450 &mgr;m or 450-500 &mgr;m.
Preferably the material encapsulating the carbon has a low thermal expansion coefficient and high degradation temperature, such as a polyimide, for example PI 2734 provided by DuPont (trade mark), where the thermal expansion coefficient is around 13 ppm/C and the degradation temperature is around 500 C.
The coating layer may have a thickness in the range from a few microns to many tens of microns. Multiple layers of coating may be formed on the carbon in order to build up a desired thickness.
A matrix of fine holes, preferably 200 &mgr;m diameter. may be formed through the carbon plate, prior to encapsulation. These holes are filled during encapsulation of the plate. An advantage of this is that it reduces the possibility of internal delamination.
According to a second aspect. of the present invention. there is provided an electrical system comprising a thermal management device in which the first aspect of the invention is embodied, on a surface of which electrical contacts and/or devices are provided.
The devices may be deposited directly on the surface or may be glued using, for example, a thin layer of liquid glue. Preferably, the devices are encapsulated in polyimide or epoxy resin or acrylic or polyurethane or polyester or any other suitable polymer.
Preferably, a plurality of layers of electrical components are provided, each spaced apart by layers of polyimide. Typically, the electrical contacts are made of thin film metal, for example aluminium.
According to a third aspect of the present invention there is provided a method of fabricating a thermal management device comprising:
applying a coat of encapsulating material, preferably polyimide or epoxy
resin or acrylic or polyurethane or polyester or any other suitable
polymer directly to a clean carbon surface, the encapsulating material
being such as to improve the rigidity of the carbon; and
repeating the foregoing steps until the carbon is encapsulated.
The method may additionally involve curing the encapsulating material.
Preferably, the step of applying involves brushing, rolling, dipping, spraying, spinning, stamping or screen-printing. Preferably, for polyimide, which consists of a single-component, the step of applying the coating involves brushing the polyimide or applying it using a roller. For solid phase application a cast can be used. This requires a pre-polymerised foil of the encapsulating material to be applied directly on to the clean surface. This can be useful when simple thermal management devices are required with no internal holes. Preferably, the carbon and cast are compressed within a vacuum and at high temperature.
Preferably, the step of applying involves applying multiple layers of encapsulating material, such as polyimide or epoxy resin or acrylic or polyurethane or polyester or any other suitable polymer, until a desired thickness is reached.
Preferably, the method includes cleaning a surface of the carbon thereby to produce said clean carbon surface.
Preferably, the step of cleaning involves using pumice powder under water to remove loose materials, followed by drying. Preferably, the step of drying involves drying the carbon by baking the carbon surface to remove moisture, for example, at 100 C for one hour.
Preferably the step of cleaning includes degreasing the carbon by, for example, rinsing it with acetone.
When polyimide is used, it is preferable that the step of curing involves heating the carbon to 150 C for, for example, 1 hour and subsequently temperature cycling the board to 150 C for 30 minutes, 250 C for 30 minutes and finally 300 C for 30 minutes.
In the case of epoxy, this can consist of a single component or else
Carter Anthony Arthur
De Oliveira Rui
Gandi Angelo
Bahta Abraham
Dicke, Billig & Czaja P.A.
Jones Deborah
Queen Mary and Westfield College, University of London
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