HK1078926B - Heat dissipating component using high conducting inserts - Google Patents

Description

Heat dissipating component using high thermal conductivity insert

Technical Field

The present invention relates to a heat dissipating member capable of managing heat from a heat source such as an electronic device. More particularly, the present invention relates to a heat dissipating member that effectively dissipates heat generated by an electronic device, wherein the heat dissipating member is constructed by assembling an anisotropic graphite flat plate member with a core member having high thermal conductivity.

Background

As more and more complex electronic devices are developed, including those that increase processing speed and higher frequencies, have smaller size and more complex power requirements, and exhibit other technological advances, such as microprocessors and integrated circuits in electronics, electrical components and systems, and other devices, such as high power optical devices, may generate relatively high temperatures. However, microprocessors, integrated circuits, and other complex electronic components typically operate effectively only at a certain critical temperature range. The excess heat generated during operation using these components not only compromises their own performance, but also reduces the performance and reliability of the overall system and even causes system failure. The range of environmental conditions in which electronic systems can operate is constantly expanding, including temperature extremes, exacerbating the adverse effects of excess heat.

As the demand for heat dissipation in microelectronic devices has increased, heat management has become an increasingly important factor in the design of electronic products. The performance reliability and lifetime of electronic devices is in turn related to the component temperatures of the devices. For example, a decrease in the operating temperature of a device, such as a typical silicon semiconductor, corresponds to an exponential increase in the reliability and lifetime of the device. Therefore, it is of great importance to control the operating temperature of the device within the limits set by the designer in order to maximize the lifetime and reliability of the components.

Several types of heat dissipation components are used to facilitate heat dissipation from electronic devices. The present invention is applicable to such several heat dissipating components, including those known as heat sinks (heat spreaders), those known as cooling plates, those known as heat sinks (heat sinks), and others.

These heat dissipating components facilitate the dissipation of heat from the surface of a heat source, such as an electronic device that generates heat, to a cooler environment, typically air. In many typical applications, heat transfer between the solid surface of the electronic device and the air is the least efficient within the system, and the solid-air interface generally has the greatest thermal barrier to dissipation. The heat sink component seeks to increase the efficiency of heat transfer between the electronic device and the outside atmosphere by increasing the surface area in direct contact with air or other heat transfer medium. This allows more heat to be dissipated, thereby reducing the operating temperature of the electronic device. The primary purpose of the heat sink member is to help maintain the temperature of the device below the maximum available temperature set by the designer/manufacturer.

Generally, heat dissipation components are made of metals, particularly copper and aluminum, due to the ability of metals, such as copper, to readily absorb and transfer heat to the entire structure. In the case of heat sinks, copper heat sinks are often fabricated with fins or other structures to increase the surface area of the heat sink, with air forced over or through the fins (such as by a fan) influencing the heat dissipation process from the electronic components through the copper heat sink and then to the air.

However, there is still a limitation in using metal heat dissipation members. One limitation relates to the relative isotropy of the metal, i.e., the metal structure tends to distribute heat relatively evenly throughout the structure. The isotropy of the metal means that the heat transferred to the metal heat sink is distributed throughout the structure rather than being preferentially directed to the desired location.

Furthermore, the use of copper or aluminum heat dissipating elements can be problematic due to the weight of the metal, especially when the heat transfer area of the heat dissipating component is significantly larger than the area of the electronic device. For example, pure copper may weigh 8.96 grams per cubic centimeter (g/cm)³) The weight of pure aluminum is 2.70g/cm³(with a weight of less than about 1.8g/cm³Compared to graphite products).

For example, in many applications, it is desirable to arrange multiple heat sinks, for example, on a circuit board, in order to dissipate heat from multiple components on the circuit board. If a metal heat sink is used, the net weight of the metal on the circuit board may increase the chance that a circuit board will break or other equally undesirable effects will occur, and increase the weight of the component itself.

In the case of larger heat dissipating components, such as those known as heat sinks, the weight of a pure copper heat sink requires special mechanical features and designs to hold the heat sink.

Therefore, what is needed is a heat dissipation member for efficiently dissipating heat from a heat source, such as an electronic device. Advantageously, the heat sink component should be relatively anisotropic and exhibit a high thermal conductivity to weight ratio compared to metals such as copper or aluminum. One group of materials suitable for use in heat sinks are the materials commonly referred to as graphite, particularly anisotropic graphite, such as natural graphite and flexible graphite based materials as described below.

Graphites are made up of layers of hexagonal arrays or networks of carbon atoms. These layers of hexagonally arranged carbon atoms are substantially planar and are oriented or ordered substantially parallel to each other and equidistant from each other. The substantially flat, parallel equidistant sheets or layers of carbon atoms, commonly referred to as graphitic layers or basal planes, are linked or bonded together and arranged in groups within the grains. Highly ordered graphites contain crystallites of considerable size; the crystal grains are highly aligned or oriented with each other, and have an aligned carbon layer. In other words, highly ordered graphites have a high degree of preferred grain orientation. It should be noted that graphites possess anisotropic structures and thus exhibit or possess highly directional properties such as thermal and electrical conductivity and fluid diffusion.

Briefly, graphite is characterized by a layered structure formed of carbon, i.e., a structure comprising laminations or laminae of carbon atoms joined together by weak van der waals forces. In view of the structure of graphite, two axes or directions are generally indicated, namely, a "c" axis or direction and an "a" axis or direction. For simplicity, the "c" axis or direction may be considered as the direction perpendicular to the carbon layers. The "a" axis or direction may be considered as the direction parallel to the carbon layers or perpendicular to the "c" direction. Graphite is suitable for making flexible graphite sheets with a high degree of orientation.

As described above, the bonding force that bonds together the parallel carbon atom layers is only a weak van der waals force. Natural graphites can be treated so that the spacing between the superposed carbon layers or sheets can be appreciably opened up so as to provide a marked expansion in the direction perpendicular to the layers, that is, in the "c" direction, and thus form an expanded or intumesced graphite structure in which the laminar character of the carbon layers is substantially retained.

Graphite flakes which have been greatly expanded, particularly those expanded so that the final thickness or "c" direction dimension is about 80 or more times the original "c" direction dimension, can be formed without the use of a binder between bonded or integrated sheets of expanded graphite, e.g., webs, papers, strips, tapes, foils, mats, etc. (commonly referred to as "flexible graphite"). Without the use of any binding material, it is believed that the expanded graphite particles, which have a final thickness or "c" direction dimension that is about 80 or more times the original "c" direction dimension, can be formed into an integral flexible sheet by compression, due to the mechanical interlocking or bonding achieved between the voluminously expanded graphite particles.

In addition to flexibility, as noted above, it has been found that the sheet material has a high degree of anisotropy with respect to thermal and electrical conductivity and fluid diffusion, as compared to natural graphite starting materials, due to the orientation of the expanded graphite particles and graphite layers substantially parallel to the opposed faces of the sheet as a result of very strong compression, e.g., rolling. Thus, the resulting sheet material has good flexibility, high strength, and high orientation.

Briefly, the process for producing a flexible, binderless anisotropic graphite sheet material, e.g. a web, paper sheet, tape, strip, foil, mat, or the like, comprises compressing or compacting under a predetermined load and in the absence of a binder, expanded graphite particles which have a dimension in the "c" direction which is about 80 or more times that of the original dimension in the "c" direction, to form a substantially flat, flexible, unitary graphite sheet. The expanded graphite particles are generally worm-shaped or vermiform in appearance and, once compressed, will remain in the compressed state in alignment with the opposed major surfaces of the sheet. By controlling the degree of compression, the density and thickness of the sheet material can be varied. The sheet material may have a density of about 0.04g/cm³To about 2g/cm³Within the range of (1). Because the graphite particles are parallel to the major opposed planes of the sheet, the flexible graphite sheet material exhibits significant anisotropy, with the degree of anisotropy increasing as the sheet material is rolled to increase orientation. In the rolled anisotropic sheet material, the thickness, i.e., the direction perpendicular to the opposed parallel sheet surfaces comprises the "c" direction and the direction of variation in length and width, i.e., the direction along or parallel to the opposed sheet surfaces comprises the "a" direction, the thermal and electrical conductivity and fluid diffusion properties of the sheet are quantitative in the "c" and "a" directionsThe difference in the stages.

Disclosure of Invention

The present invention provides a thermal management system comprising an anisotropic graphite flat plate element having a high thermal conductivity in the plane of the flat plate element and a low thermal conductivity through the thickness of the flat plate element in a direction perpendicular to the plane of the flat plate element. The planar element has a cavity formed therein and a core or insert is closely received within the cavity. In this embodiment the core is formed from an isotropic core material so that heat from a heat source can be directed through the core into the thickness of the plate member and then out through the plane of the plate member.

In another embodiment of the present invention, a heat treatment system is provided that includes a heat source having a heat transfer surface, an anisotropic graphite flat plate member, and an insert. The planar element has x and y dimensions defining a substantially planar extent of the planar element and has a z dimension defining a thickness of the planar element. The plate element has a high thermal conductivity in the x and y directions and a low thermal conductivity in the z direction. Thus, the x and y directions as used herein correspond to the "a" axis of what is commonly referred to as anisotropic graphite, and the z direction as used herein corresponds to the "c" direction or axis of the anisotropic graphite. The planar member has a cavity formed therein that extends at least partially through a thickness of the planar member. The insert is received within the cavity in thermally conductive engagement with the planar element. The insert has a heat receiving surface that engages the heat transfer surface of the heat source such that heat from the heat source flows through the heat transfer surface and the heat receiving surface, enters the insert in the z-direction, and then exits through the planar element in the x-and y-directions.

In another embodiment of the present invention, a method for dissipating heat from a heat source is provided. The method comprises the following steps:

(a) providing an anisotropic heat dissipating element having a relatively high thermal conductivity in x and y directions and a relatively low thermal conductivity in a z direction perpendicular to the x and y directions, the heat dissipating element having a cavity formed therethrough in the z direction and having an isotropic thermally conductive insert disposed within the cavity;

(b) bringing the insert into thermally conductive engagement with a heat source;

(c) conducting heat from the heat source through the insert and into the anisotropic heat dissipating element; and

(d) heat is conducted in the x and y directions through the heat dissipating element.

It is therefore an object of the present invention to provide an improved design of a heat sink member comprising an anisotropic graphite plate member.

It is another object of the present invention to provide a heat dissipating component that includes a high thermal conductivity core for conducting heat from a heat source to an isotropic graphite flat plate element.

It is another object of the present invention to provide a heat sink that is lightweight, such as made of graphite, but has a high thermal conductivity at the interface of the heat sink and the heat source.

It is another object of the present invention to provide a composite heat sink member that uses an anisotropic graphite material to conduct heat across a major surface area of the member while using an isotropic high thermal conductivity material, such as copper, to conduct heat from a heat source into the anisotropic material.

It is another object of the present invention to provide an economical structure for a heat dissipating member.

In view of the above objects, the present invention provides a thermal management system comprising a flat plate element made of at least one anisotropic graphite sheet comprising compressed particles of exfoliated graphite, said flat plate element having parallel planes and a thickness, said flat plate element having a relatively high thermal conductivity in said parallel planes and a relatively low thermal conductivity across said thickness in a direction perpendicular to said planes, said flat plate element having a cavity formed therein; a core closely received within said cavity, said core being comprised of an isotropic core material and having a high thermal conductivity such that heat from a heat source may be conducted from said heat source through said core to and along one or more of said parallel planes of said planar members.

In other aspects, the flat sheet element is shrink-fitted to the core by thermal expansion and contraction of at least one of the flat sheet element and the core. The heat management system further comprises a lubricant layer between the core and the cavity of the planar element, such that the lubricant layer forms a thermal interface between the core and the planar element. The flat plate element comprises a plurality of anisotropic graphite sheets laminated together, the sheets being oriented parallel to the plane of the flat plate element. The graphite sheet is resin impregnated. The core extends completely through the laminated anisotropic graphite sheets. The core material is a metal. The core has a cylindrical or rectangular shape.

The present invention also provides a thermal management system comprising a heat source having a heat transfer surface; a flat plate element made of at least one anisotropic graphite sheet comprising compressed particles of exfoliated graphite, said flat plate element defining a plane in x and y directions and a thickness in a z direction perpendicular to said x and y directions, said flat plate element having a relatively high thermal conductivity in the x and y directions and a relatively low thermal conductivity in the z direction, said flat plate element having a cavity formed therein extending at least partially through said thickness of said flat plate element; an insert is received within the cavity in thermally conductive engagement with the planar element, the insert having a relatively high thermal conductivity in the z-direction of the planar element, the insert having a heat receiving surface operatively engaged with a heat transfer surface of the heat source such that heat from the heat source flows through the heat transfer surface and the heat receiving surface, flows into the insert in the z-direction, and is then conducted through the planar element in the x-and y-directions.

In other aspects, the insert is constructed of an isotropic material. The system also includes at least one fin made of anisotropic graphite received in a groove disposed on a surface of the flat plate member opposite the heat source.

The present invention also provides a method of dissipating heat from a heat source comprising (a) providing an anisotropic heat dissipating element having a plane, said heat dissipating element comprising compressed particles of exfoliated graphite, said heat dissipating element having a relatively high thermal conductivity in x and y directions of said plane and a relatively low thermal conductivity in a z direction perpendicular to said x and y directions, said heat dissipating element having a cavity formed therethrough in said z direction and having an isotropic thermally conductive insert disposed within said cavity; (b) bringing the insert into a heat-conducting relationship with a heat source; (c) conducting heat from the heat source through the insert and into the anisotropic heat dissipation element; and (d) conducting heat in the x and y directions through the heat dissipating element.

Other objects, features and advantages of the present invention will become more readily apparent to those skilled in the art after reviewing the following disclosure in conjunction with the accompanying drawings.

Drawings

FIG. 1 is a perspective schematic view of a heat dissipating component in the form of a heat sink having a graphite plate member and a cylindrical insert;

FIG. 2 is a cross-sectional view taken along line 2-2 of FIG. 1, showing a heat sink assembled with a heat source, such as an electronic device;

FIG. 3 is a perspective view similar to FIG. 1 showing a heat sink with a plurality of inserts;

FIG. 4 is a perspective view of a heat dissipating component in the form of a heat sink having a cylindrical insert;

FIG. 5 is a cross-sectional view taken along line 5-5 of FIG. 4;

FIG. 6 is a view of a heat sink similar to FIG. 4 with a square insert;

FIG. 7 is a perspective view of a large size rectangular heat sink having rectangular or square inserts;

FIG. 8 is a perspective view of a pin fin heat sink with a cylindrical insert; and

fig. 9 is a perspective view of a pin fin heat sink with a square insert.

Detailed Description

As stated, the present invention relates to a heat sink member comprised of a graphite flat plate member having a high thermal conductivity core or insert disposed within a cavity of the graphite member.

Before describing the construction of the heat-radiating member, a brief description of graphite and the formation of a flexible sheet, which will become a main substrate forming the product of the present invention, are in order.

Preparation of Flexible graphite sheet

Graphite is a form of carbon crystal that includes covalently bonded carbon atoms within planar layers that are weakly bonded to each other. By treating particles of graphite, such as natural graphite flakes, with an intercalation solution, such as a solution of sulfuric and nitric acids, the crystal structure of the graphite reacts to form a composite of graphite and the intercalation solution. The treated graphite particles are hereinafter referred to as "intercalated graphite particles". Upon exposure to high temperatures, the intercalation solution in the graphite decomposes and volatilizes, causing the intercalated graphite particles to expand 80 or more times its original volume in an accordion-like manner in the c-direction, i.e., in the direction perpendicular to the planes of the graphite crystal grains. The exfoliated graphite particles are vermiform in appearance and are therefore commonly referred to as worms. The worms may be compressed together into flexible sheets, unlike the original graphite sheets, which can be formed and cut into various shapes and formed into small transverse openings by deforming mechanical impact.

Graphite starting materials suitable for use in the present invention include highly graphitic carbon materials capable of intercalating organic and inorganic acids as well as halogens and then expanding when heated. It is desirable that these highly graphitized carbon materials have a degree of graphitization of about 1.0. As used herein, the term "degree of graphitization" refers to the value g obtained by the following formula:

where d (002) is the spacing, measured in angstroms, between graphitic layers of carbon in the crystal structure. The spacing d between graphite layers is measured by standard X-ray diffraction techniques. The positions of the diffraction peaks corresponding to the (002), (004) and (006) Miller (Miller) indices are measured and the spacing that minimizes the total deviation of all these peaks is extracted using standard least squares. Examples of highly graphitized carbon materials include natural graphite from a variety of sources, as well as other carbon materials such as graphite prepared by chemical vapor deposition and the like. Natural graphite is most preferred.

The graphite raw material used in the present invention may contain non-carbon components as long as the crystal structure of the raw material maintains a desired degree of graphitization and is capable of delamination (exfoliation). Generally, any carbon-containing material is suitable for use in the present invention, so long as the crystal structure has the desired degree of graphitization and is capable of exfoliation. Preferably, the graphite has an ash content of less than about 20% by weight. Preferably, the graphite used in the present invention has a purity of at least 94%. In the preferred embodiment, the graphite employed has a purity of at least 98%.

One common method of making graphite sheets is described by Shane et al in U.S. patent 3404061, the contents of which are incorporated herein by reference. In the typical practice of the Shane et al method, natural graphite flakes are immersed in a solution containing, for example, a mixture of nitric and sulfuric acids, advantageously at a level of about 20 to about 300 parts intercalation solution (pph) by weight per 100 parts of graphite flakes. The intercalation solution contains oxidizing or other intercalating agents known in the art. Examples include those containing oxidizing agents and oxidizing mixtures such as solutions containing nitric acid, potassium chlorate, chromic acid, potassium permanganate, potassium chromate, potassium dichromate, perchloric acid, and the like, or mixtures such as concentrated nitric acid and chlorate, chromic acid and phosphoric acid, sulfuric acid and nitric acid, or mixtures of strong organic acids, e.g., trifluoroacetic acid, and strong oxidizing agents soluble in the organic acids. Alternatively, an electric potential can be used to oxidize the graphite. Chemical species that may be introduced into the graphite crystal using electrolytic oxidation include sulfuric acid as well as other acids.

In a preferred embodiment, the intercalating agent is a solution of a mixture of sulfuric acid, or sulfuric acid and phosphoric acid, and an oxidizing agent, i.e., nitric acid, perchloric acid, chromic acid, potassium permanganate, hydrogen peroxide, iodic or periodic acids, or the like. Although less preferred, the intercalation solution may contain a metal halide salt, such as ferric chloride, and ferric chloride mixed with sulfuric acid, or a halide, such as bromine, as a solution of bromine and sulfuric acid, or bromine in an organic solvent.

The amount of intercalation solution may range from about 20 to about 150pph, with about 50 to about 120pph being more typical. After the insertion of the wafer, any excess solution is drained from the wafer and the wafer is water-washed. Alternatively, the amount of intercalation solution may be limited to between about 10 and about 50pph, which may eliminate the washing step, as described and suggested in U.S. patent No. 4895713, the disclosure of which is incorporated herein by reference.

The particles of graphite flake treated with intercalation solution may be contacted, for example by blending, with a reducing organic additive selected from alcohols, sugars, aldehydes and esters which react with the surface film formed by the oxidizing intercalation solution at temperatures in the range of 25 c and 125 c. Suitable organic additives include cetyl alcohol, stearyl alcohol, 1-octanol, 2-octanol, decyl alcohol, 1, 10 sebacoyl, decanal, 1-propanol, 1, 3 propylene glycol, ethylene glycol, polypropylene glycol, glucose, fructose, sucrose, potato starch, ethylene glycol monostearate, diethylene glycol dibenzoate, propylene glycol stearate, glycerol monostearate, dimethylglycolate, diethylglycolate, methylformate, ethylformate, ascorbic acid and lignin-derived compounds, such as sodium lignosulfate. The amount of organic reducing agent is suitably from about 0.5 to 4% by weight of the particles of graphite flake.

The use of an expansion aid applied before, during or immediately after intercalation can also provide improvements. These improvements can reduce the stratification temperature and increase the expansion volume (also referred to as "peristaltic volume"). Advantageously, the expansion aid in this context is an organic material sufficiently soluble in the intercalation solution to achieve an improvement in expansion. More narrowly, organic materials of this type containing carbon, hydrogen and oxygen, preferably exclusively, may be employed. Carboxylic acids have been found to be particularly effective. Suitable carboxylic acids for use as expansion aids may be selected from aromatic, aliphatic or cycloaliphatic, straight or branched chain, saturated and unsaturated monocarboxylic acids, dicarboxylic acids and polycarboxylic acids having at least one carbon atom, and desirably up to about 15 carbon atoms, which may be dissolved in the intercalation solution in an effective amount to provide one or more measurable improvements in delamination. Suitable organic solvents may be used to improve the solubility of the organic expansion aid in the intercalation solution.

Representative examples of saturated aliphatic carboxylic acids are e.g. those of formula H (CH)₂)_nThose acids of COOH, wherein n is a number from 0 to about 5, include formic, acetic, propionic, butyric, valeric, caproic, and the like. Instead of carboxylic acids, it is also possible to use anhydrous or activated carboxylic acid derivatives. A representative alkyl ester is methyl formateAnd ethyl formate. Sulfuric acid, nitric acid and other known aqueous intercalation solutions have the ability to decompose formic acid, ultimately to water and carbon dioxide. To this end, it is advantageous that formic acid and other sensitive expansion aids contact the graphite flakes prior to immersion of the flakes in the aqueous intercalation solution. Representative carboxylic acids are aliphatic carboxylic acids having 2 to 12 carbon atoms, especially oxalic acid, fumaric acid, malonic acid, maleic acid, succinic acid, glutaric acid, adipic acid, 1, 5-pentanedicarboxylic acid, 1, 6-octanedicarboxylic acid, 1, 10-decanedicarboxylic acid, cyclohexane-1, 4-carboxylic acid and aromatic carboxylic acids, such as phthalic acid or terephthalic acid. Representative alkyl esters are dimethyl oxalate and diethyl oxalate. Representative cycloaliphatic acids are cyclohexanecarboxylic acid and representative aromatic carboxylic acids are benzoic acid, naphthoic acid, anthranilic acid, p-aminobenzoic acid, salicylic acid, o-, m-, p-toluic acid, methoxy and ethoxybenzoic acid, acetoacetamidobenzoic acid and, acetamidobenzoic acid, phenylacetic acid and naphthoic acid. Representative hydroxyaromatic acids are hydroxybenzoic acid, 3-hydroxy-1-naphthoic acid, 3-hydroxy-2-naphthoic acid, 4-hydroxy-2-naphthoic acid, 5-hydroxy-1-naphthoic acid, 5-hydroxy-2-naphthoic acid, 6-hydroxy-2-naphthoic acid, and 7-hydroxy-2-naphthoic acid. Among the polycarboxylic acids, citric acid is important.

The intercalation solution will be aqueous and will desirably contain an amount of expansion aid of from about 1 to 10%, the amount being effective to enhance exfoliation. In embodiments in which the expansion aid contacts the graphite flake prior to or after immersion in the aqueous intercalation solution, the expansion aid can be admixed with the graphite by suitable means, such as a V-blender, typically in an amount of about 0.2% to about 10% by weight of the graphite flake.

After intercalation of the graphite flake, the intercalated solution coating the intercalated graphite flake is then mixed with an organic reducing agent, and the mixture is exposed to a temperature in the range of 25 ° to 125 ℃ to promote reaction of the reducing agent with the intercalated coating. The heating time is up to about 20 hours, with shorter heating times, e.g., at least about 10 minutes, for higher temperatures in the above range. At higher temperatures, a time of one-half hour or less, such as about 10 to 25 minutes, may be employed.

The graphite particles so treated are sometimes referred to as "intercalated graphite particles". Upon exposure to elevated temperatures, for example temperatures of at least about 160 c, and particularly temperatures of about 700 c to 1000 c or higher, the intercalated graphite particles expand in an accordion-like fashion in the c-direction, i.e., in a direction perpendicular to the crystallographic planes that make up the graphite particles, to about 80 to 1000 times their original volume. The expanded, i.e., exfoliated, graphite particles are vermiform in appearance and are therefore commonly referred to as worms. The worms may be compressed together into flexible sheets that, unlike the raw graphite particles, can be formed and cut into various shapes and formed into small transverse openings by deforming mechanical impact as described below.

Flexible graphite sheets and foils are coherent, have good processing strength, and are suitable for compression, such as by rolling, to a thickness of about 0.075 to 3.75mm, typically a density of about 0.1 to 1.5 grams per cubic centimeter (g/cm)³). As described in US5902762 (incorporated herein by reference), about 1.5-30% by weight of a ceramic additive may be blended with intercalated graphite flakes to provide enhanced resin impregnation in the final flexible graphite product. The additive includes ceramic fiber particles having a length of about 0.15 to 1.5 millimeters. The width of the particles is suitably from about 0.04 to 0.004 mm. The ceramic fiber particles are non-reactive, non-adherent to graphite, and stable at temperatures up to about 1100 ℃, preferably about 1400 ℃ or higher. Suitable ceramic fiber particles are formed from impregnated quartz glass fibers, carbon and graphite fibers, zirconia, boron nitride, silicon carbide and magnesia fibers, naturally occurring mineral fibers such as calcium metasilicate fibers, calcium aluminum silicate fibers, aluminum oxide fibers, and the like.

Preparation of laminated graphite Material

Preferably, the flat plate graphite elements of the heat sink components described below are constructed of a laminated resin impregnated with a graphite material, in the manner disclosed in U.S. patent application No.09/943131, filed 2001, 8, 31, entitled "LAMINATESPREPARED FROM IMPREGNATED FLEXIBLE GRAPHITE SHEETS," assigned to the assignee of the present invention and incorporated herein by reference in its entirety.

According to the Norley et al process, a sheet of flexible graphite prepared as described above and having a thickness of about 4mm to 7mm is impregnated with a thermosetting resin such as an epoxy, acrylic or phenolic resin system. Suitable epoxy resins include diglycidyl ethers of bisphenol a (dgeba) resin systems; other multifunctional epoxy system fluids are suitable for use in the present invention. Suitable phenolic resin systems include those comprising resole and novolac phenolic resins. The sheet is then calendered to a thickness of about 0.35mm to 0.5mm, at which point the calendered, epoxy impregnated flexible mat has a thickness of about 1.4g/cm³To about 1.9g/cm³The density of (c).

The amount of resin in the epoxy impregnated graphite flake should be an amount sufficient to ensure that the final assembled cured layered structure is dense, coherent, yet the thermal conductivity of the anisotropy associated with the densified graphite structure is not adversely affected. Suitable resin content is preferably at least about 3% by weight, more preferably from about 5% to about 35% by weight, depending on the desired characteristics in the final product.

In a typical resin impregnation step, a sheet of flexible graphite is passed through a vessel and impregnated with a resin system, advantageously "pulled through" the mat by a vacuum chamber, from, for example, a nozzle. Typically, but not necessarily, the resin system is a solvent to facilitate application to the flexible graphite sheet. Thereafter, it is desirable to dry the resin to reduce gaps between the resin and the resin impregnated sheet.

An apparatus for continuously forming resin impregnated and calendered flexible graphite sheets is shown in international publication WO00/64808, the contents of which are incorporated herein by reference.

After the calendering step, the impregnated sheets are cut into appropriately sized pieces, stacked together, placed in a press, and cured at elevated temperatures. The temperature should be sufficient to ensure that the layered structure is at the curing pressureThe densification improves the anisotropy of the structure and the thermal properties when used as a heat sink. Typically, this will require a temperature of about 150 ℃ to 200 ℃. The pressure used for curing is a function of the temperature used to some extent, but should be sufficient to ensure that the layered structure is densified without adversely affecting the thermal properties of the structure. Typically, for ease of manufacture, the minimum pressure required to densify the structure to the desired degree will be used. Such pressures are typically 1000 to 3000 pounds per square inch (psi). The curing time may vary depending on the resin system and the temperature and pressure employed, but is typically in the range of 0.5 hours to 2 hours. After curing was complete, the composite was seen to have a density of about 1.8g/cm³To 2.0g/cm³The density of (c).

Advantageously, the resin in the impregnated sheet may act as a binder for the composite material. Alternatively, the calendered, impregnated flexible graphite sheet is coated with a binder prior to stacking and curing of the flexible sheets. Suitable binders include epoxy-, acrylic-and phenolic-based resins. Phenolic resins found to be particularly useful in the present invention include phenolic-based resin systems comprising resole and novolak phenolic resins.

Non-graphite layers may be included in the pre-compressed stack. Such non-graphite layers may comprise metal, plastic or other non-metals, such as fiberglass or ceramic. The epoxy polymer in the impregnated graphite sheet is sufficient to adhesively locate the non-graphite layers of the structure as well as the impregnated graphite layers upon curing.

The following examples further illustrate and explain the construction of suitable laminated structures and are in no way limiting. All parts and percentages are by weight unless otherwise indicated.

Example 1

The weight per unit area was 70mg/cm²Graphite sheets of dimensions of about 30cm by 30cm were impregnated with epoxy resin to produce calendered mats of 12 weight percent epoxy resin. The epoxy resin used is a diglycidyl ether high-temperature curing formula formed by bisphenol A (DGEBA), andthe impregnation procedure consisted of saturation with acetone resin solution followed by drying at about 80 ℃. After impregnation, the sheet was calendered from a thickness of about 7mm to a thickness of about 0.4mm, and 1.63g/cm³The density of (c). The calendered, impregnated sheets were then cut into disks of about 50mm diameter, which were stacked to 46 layers high. This disc stack was then placed in a TMP (Technical machinery products) press and cured at 2600psi, 150 ℃ for 1 hour. The resulting laminate had a thickness of 1.90g/cm³A density of 8000psi, a flexural strength of 8000psi, a young's modulus of 7.5Msi (million pounds per square inch), and a sheet resistance of 6 micro-ohms. The in-plane and through-thickness thermal conductivity values were 396W/m ℃ and 6.9W/m ℃, respectively. The laminate exhibits excellent machinability, has a continuous non-porous surface and finish, and is suitable for use in electrical thermal management devices. The highly anisotropic thermal conductivity results in a structure that is highly suitable for transporting heat from sensitive electronic devices to a heat sink. Furthermore, the density of the material is about 1.94g/cm³Significantly lower than aluminum (2.7 g/cm)³) And even less than copper (8.96 g/cm)³). Thus, the specific thermal conductivity (i.e., the ratio of thermal conductivity to density) of the graphite laminate is about 3 times that of aluminum and 4 to 6 times that of copper.

Laminated graphite materials suitable for use in the present invention are not limited to those specified above and may include, for example, laminates formed from sheets of pyrolytic graphite, such as those manufactured by Matsushita electric Components Co.Ltd.ceramic Division, 1006Kadoma, Osaka, Japan under the trade name Panasonic "PGS"^®A graphite sheet.

The embodiments of fig. 1-9

Referring now to the drawings, and more particularly to FIGS. 1 and 2, a thermal management system is illustrated and designated by reference numeral 10. The system 10 is in the form of a heat sink assembled with an electronic device 14, which may be referred to as a heat source 14. The heat sink 12 comprises an anisotropic graphite plate member 16 having a relatively high thermal conductivity in the plane of the plate member 16 along dimensions x and y and a relatively low thermal conductivity through the thickness 18 of the plate member in a direction z perpendicular to the plane defined by the dimensions x and y.

The planar member 16 has a cylindrical cavity 20 formed therein, preferably through the entire thickness 18. However, it should be understood that the cavity 20 may extend only partially into the thickness 18.

The heat sink 12 includes a core or insert 22 received within the cavity 20.

Preferably, the core 22 is constructed of an isotropic material, preferably a metal such as copper or aluminum. However, in certain embodiments, the core 22 may be constructed of an anisotropic material, such as the noted graphite material, wherein the core is constructed such that the direction of high thermal conductivity (commonly referred to as the "a" axis) of the anisotropic material is generally parallel to the z-axis of the fin shown in FIGS. 1 and 2.

Preferably, the planar member 16 is constructed of a laminated graphite material as described above, wherein a plurality of resin impregnated graphite sheets are stacked and compressed together to form the rigid planar graphite member 16. Typical dimensions for the component 16 of the heat sink device have a length of about 6 to 12 inches along the x-axis, a width of about 6 to 12 inches along the y-axis, and a thickness 18 of about 1/2 inches along the z-axis. A typical size for cylindrical cavity 20 is a diameter of 1 to 1-1/2 inches.

As shown in fig. 2, the electronic device 14 has a top surface 24, which may also be referred to as a thermal conduction surface or a heat transfer surface. The core 22 has a heated surface 26 that effectively engages the surface 24 of the electronic device 14 in a conventional manner, which may include the use of a thin thermal interface 25, or a phase change layer or thermal grease layer therebetween. The thermal interface 25 may be, for example, a thin layer of flexible graphite material.

Although the core 22 and cavity 20 are shown in fig. 1 and 2 as being cylindrical, they may be any shape, including circular, square, rectangular, or other shapes. The cylindrical shape is preferred because of its ease of processing.

Preferably, the heated surface 26 of the core 22 is larger than the heat transfer surface 24 of the device 14, such that the heated surface 26 effectively contacts and covers the entire top surface of the electronic device 14. Otherwise, if a portion of the top surface 24 of the electronic device 14 is covered by the graphite material 16, the graphite material will not effectively transfer heat in the z-direction, which will cause the temperature of the device 14 to rise.

Thus, when the electronic device 14 is operating, which generates heat that must be dissipated, the heat flows through the heat transfer surface 24 and the heat receiving surface 26, into the core 22, which conducts the heat in the direction z through the thickness 18 of the planar graphite component 16. Heat is conducted through the interface between the core 22 and the cavity 20 into the planar graphite component 16 surrounding the cavity 20, which then conducts the heat along the x-y plane of the planar graphite element 16 of the heat sink 12. The thermal energy must then be dissipated by known techniques, such as transferring heat to a heat transfer fluid or the like.

Fig. 3 shows another embodiment of the device 10 having a plurality of inserts, designated 22A and 22B.

Table I below compares the device performance for different material combinations shown in figures 1 and 2. The data was generated by numerical simulation.

TABLE I

In design #1 of table I, an all-copper heat spreader is shown for comparison. In design #2, an all-graphite heat sink is shown for comparison, with the highly thermally conductive "a" axis of the graphite aligned with the x and y axes of the heat sink member. Option #2 is a graphite heat sink without any inserts. Design #3 shows the invention using a graphite plate element with a copper insert.

Designs #4 and #5 show graphite plate elements with graphite inserts having a graphite high conductivity direction determined in a direction perpendicular to the plane of the heat sink. Designs #4 and #5 performed almost as well as the copper insert of design #3 without increasing the weight of the copper insert.

In the data of table I, the units are shown below. T is_maxIs the highest temperature on the electronic device used for condition simulation; a lower temperature is an indication of better performance of the heat dissipating component. R_saRepresenting the thermal resistance of the heat sink member, and a lower number is an indication of better performance of the heat sink member. The numbers for the thermal conductivity of the graphite material indicate the conductivity in each of the x, y and z directions, indicating the orientation of the anisotropic graphite; higher numbers correspond to directions of higher thermal conductivity.

Referring now to fig. 4 through 9, various embodiments of the present invention are shown in the context of a heat sink rather than a heat sink.

In fig. 4, a perspective view of a finned heat sink 30 having a cylindrical insert 32 is shown. Fig. 5 shows a cross-sectional view of the finned heat sink taken along line 5-5 of fig. 4 having a flat graphite substrate 34 with a plurality of graphite fins. The anisotropy of the substrate 34 has a high thermal conductivity direction aligned with the x and y axes. Preferably, the fins 36 are discrete elements having a direction of high thermal conductivity including the z-direction. The fins 36 are received in slots 35 machined in the base plate 34 and are positioned by epoxy or other suitable bonding material. The base plate 34 has a cavity 38 extending partially therethrough in the z-direction. The core 32 is received within the cavity 38.

Table II below shows performance data for the designs of fig. 4 and 5 using different materials. The data was generated by numerical simulation. Design #1 refers to a pure copper heat sink without any inserts. Design #2 is a pure aluminum heat sink without any inserts. Design #3 is a pure graphite heat sink without any inserts. The graphite orientation of design #3 was such that the direction of high thermal conductivity of the substrate was parallel to the plane of the fins. Design #4 is representative of the present invention, where the heat spreader is constructed of graphite with copper insert 32, as shown in fig. 5. The graphitic orientation of the substrate in design #4 has high thermal conductivity in the x and y directions. As shown in table II, the performance of the present invention using a graphite heat spreader with a copper insert is nearly as good as a pure copper heat spreader and far superior to a pure aluminum or pure graphite heat spreader.

TABLE II

Referring now to fig. 6, an embodiment similar to that of fig. 4 is shown and indicated by reference numeral 40. The heat sink 40 differs in that it has a rectangular insert 42 received in a rectangular cavity 44 on a base plate 46.

Fig. 7 shows another embodiment of a heat sink, indicated by reference numeral 50. The heat spreader in fig. 7 is for a large-sized electronic chip. The heat sink 50 has a base plate 52 with fins 54. The square insert 56 is received in a square cavity 58 defined through the base plate 52.

The data shown in table III below compares the performance of a heat sink constructed according to fig. 7 with a prior art aluminum heat sink having a copper substrate (design # 1). Design #1 is currently being sold by Radian corporation, Walsh Avenue, Santa Clara, California 95050; it is an aluminum heat sink with a partially hollow substrate filled with copper plugs. Design #2 is of the present invention having a graphite heat sink 50 with a graphite base plate 52 and graphite fins 54 and a copper insert 56. As shown in table III, the performance of the graphite heat spreader with the copper insert of the present invention is far superior to the performance of the aluminum and copper heat spreader of design # 1.

TABLE III

#	Design scheme	T(℃)	R(℃/W)	Weight (kg)
					1	Aluminum (with copper base)	64.27	0.33 (power 90W)	3.292
2	Graphite (with copper plug-in)	55.21	0.22 (power 90W)	1.647

Finally, fig. 8 and 9 show perspective views of another type of heat sink, known as a "pin fin" type heat sink. Fig. 8 shows a pin fin heat sink 60 having a base plate 62 and a plurality of pin fins 64. A cylindrical cavity 66 in the base plate 62 houses a cylindrical insert 68.

Fig. 9 shows a similar pin-fin heat sink 70, except that a square or rectangular insert 72 is utilized.

The data in table IV below shows the performance of a pin fin heat sink shaped like that of fig. 8, with design #1 and #2 being conventional, existing aluminum or copper heat sinks, and design #3 showing the performance of a graphite heat sink with copper insert 68 utilizing the present invention.

TABLE IV

#	Design scheme	T(℃)	R(℃/W)
				1	Aluminium	82.13	1.89
2	Copper (Cu)	77.08	1.683
				3	Graphite (copper plug-in components)	78.79	1.751

Construction method

The insert can be placed within the cavity of the heat sink in several different ways.

A preferred method is to machine the insert to a diameter or size slightly larger than the cavity machined in the graphite. The insert is then cooled so that it shrinks to a diameter or size less than the cavity and placed in the cavity. Once the insert is heated to room temperature, the insert will expand to fit tightly within the cavity without any adhesive or cement. If desired, a thin layer of thermal grease or phase change material or other lubricant may be applied to the insert or inside the cavity prior to insertion of the insert.

Thus, a thermal management system has been provided in which the heat-dissipating elements provide a thermal conductivity comparable to copper, but only 1/5, the weight of which is the same as copper. This facilitates lighter electronic devices, alleviating the need for special mechanical fastening and design solutions for fastening heavier copper components. The reduction in mass of the heat sink reduces the acceleration stress of the electronic components due to vibration and can dissipate higher power than a conventional aluminum or copper heat sink. Furthermore, design flexibility is provided, and the required cooling can be achieved with a smaller heat sink than with existing designs.

It will thus be seen that the apparatus and method of the present invention are susceptible to being embodied in the form described, and that certain of its objects and advantages are inherent. While certain preferred embodiments of the invention have been shown and described for purposes of disclosure, numerous changes in the arrangement and construction of parts and steps may be made by those skilled in the art, which changes are encompassed within the scope and spirit of the invention as defined by the appended claims.

Claims (12)

1. A thermal management system, comprising:

a flat plate element made of at least one anisotropic graphite sheet comprising compressed particles of exfoliated graphite, said flat plate element having parallel planes and a thickness, said flat plate element having a high thermal conductivity in said parallel planes and a low thermal conductivity through said thickness in a direction perpendicular to said planes, said flat plate element having a cavity formed therein;

a core closely received within said cavity, said core being comprised of an isotropic core material and having a high thermal conductivity such that heat from a heat source may be conducted from said heat source through said core to and along one or more of said parallel planes of said planar members.

2. The thermal management system of claim 1, wherein said planar element is shrink-fitted to said core by thermal expansion and contraction of at least one of said planar element and said core.

3. The thermal management system of claim 1 or 2, further comprising:

a lubricant layer between the core and the cavity of the planar element, whereby the lubricant layer forms a thermal interface between the core and the planar element.

4. The thermal management system of claim 1,

the flat plate element comprises a plurality of anisotropic graphite sheets laminated together, the sheets being oriented parallel to the plane of the flat plate element.

5. The thermal management system of claim 4, wherein said graphite sheets are resin impregnated.

6. The thermal management system of claim 4, wherein said core extends completely through said laminated anisotropic graphite sheets.

7. The thermal management system of claim 1 or 6, wherein said core material is a metal.

8. The thermal management system of claim 1, wherein said core has a cylindrical or rectangular shape.

9. A thermal management system, comprising:

a heat source having a heat transfer surface;

a flat plate element made of at least one anisotropic graphite sheet comprising compressed particles of exfoliated graphite, said flat plate element defining planes in x and y directions and a thickness in a z direction perpendicular to said x and y directions, said flat plate element having a relatively high thermal conductivity in the x and y directions and a relatively low thermal conductivity in the z direction, said flat plate element having a cavity formed therein extending at least partially through said thickness of said flat plate element;

an insert is received within the cavity in thermally conductive engagement with the planar element, the insert having a relatively high thermal conductivity in the z-direction of the planar element, the insert having a heat receiving surface operatively engaged with a heat transfer surface of the heat source such that heat from the heat source flows through the heat transfer surface and the heat receiving surface, flows into the insert in the z-direction, and is then conducted through the planar element in the x-and y-directions.

10. The thermal management system of claim 9, wherein said insert is comprised of an isotropic material.

11. The heat management system of claim 9, further comprising at least one fin made of anisotropic graphite received in a slot disposed on a surface of the flat plate member opposite the heat source.

12. A method of dissipating heat from a heat source, comprising:

(a) providing an anisotropic heat dissipating element with a plane, said heat dissipating element comprising compressed particles of exfoliated graphite, said heat dissipating element having a relatively high thermal conductivity in x and y directions of said plane and a relatively low thermal conductivity in a z direction perpendicular to said x and y directions, said heat dissipating element having a cavity formed therethrough in said z direction and having an isotropic thermally conductive insert disposed within said cavity;

(b) bringing the insert into a heat-conducting relationship with a heat source;

(c) conducting heat from the heat source through the insert and into the anisotropic heat dissipation element; and

(d) conducting heat in the x and y directions through the heat dissipating element.