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  Sandwiched thermal articleUSPTO Application #: 20060225874Title: Sandwiched thermal article Abstract: A thermal material for heat dissipation, which includes at least one sheet of flexible graphite sandwiched about a non-graphite core layer. (end of abstract) Agent: Waddey & Patterson - Nashville, TN, US Inventors: Gary D. Shives, David S. Flaherty USPTO Applicaton #: 20060225874 - Class: 165185000 (USPTO) Related Patent Categories: Heat Exchange, Heat Transmitter The Patent Description & Claims data below is from USPTO Patent Application 20060225874. Brief Patent Description - Full Patent Description - Patent Application Claims
TECHNICAL FIELD [0001] The present invention relates to a sandwiched structure having an isotropic core and being capable of use as a thermal spreader or a finstock in the manufacture of heat sinks and other thermal dissipation devices. By thermal spreader is meant a material or article that functions to spread heat from a heat source over an area greater than one or more of the surfaces of the heat source; by finstock is meant a material or article that can be utilized as, or to form, fins used to dissipate heat. BACKGROUND OF THE INVENTION [0002] With the development of more and more sophisticated electronic devices, including those capable of increasing processing speeds and higher frequencies, having smaller size and more complicated power requirements, and exhibiting other technological advances, such as microprocessors and integrated circuits in electronic and electrical components, high capacity and response memory components such as hard drives, electromagnetic sources such as light bulbs in digital projectors, as well as in other devices such as high power optical devices, relatively extreme temperatures can be generated. However, microprocessors, integrated circuits and other sophisticated electronic components typically operate efficiently only under a certain range of threshold temperatures. The excessive heat generated during operation of these components can not only harm their own performance, but can also degrade the performance and reliability of the overall system and can even cause system failure. The increasingly wide range of environmental conditions, including temperature extremes, in which electronic systems are expected to operate, exacerbates the negative effects of excessive heat. [0003] With the increased need for heat dissipation from microelectronic devices, thermal management becomes an increasingly important element of the design of electronic products. Both performance reliability and life expectancy of electronic equipment are inversely related to the component temperature of the equipment. For instance, a reduction in the operating temperature of a device such as a typical silicon semiconductor can correspond to an increase in the processing speed, reliability and life expectancy of the device. Therefore, to maximize the life-span and reliability of a component, controlling the device operating temperature within the limits set by the designers is of paramount importance. [0004] One group of relatively light weight materials suitable for use in the dissipation of heat from heat sources such as electronic components are those materials generally known as graphites, but in particular graphites such as those based on natural graphites and flexible graphite as described below. These materials are anisotropic and allow thermal dissipation devices to be designed to preferentially transfer heat in selected directions. Graphite materials are much lighter in weight than metals like copper and aluminum and graphite materials, even when used in combination with metallic components, provide many advantages over copper or aluminum when used to dissipate heat by themselves. [0005] For instance, Tzeng, in U.S. Pat. No. 6,482,520 teaches a graphite based thermal management system which includes a heat sink formed of a graphite article formed so as to have a heat collection surface and at least one heat dissipation surface. Krassowski and Chen take the Tzeng concept a step further in International Patent Application No. PCT/US02/38061, where they teach the use of high conducting inserts, formed, for instance, of a metal like copper or aluminum, in a graphite base. Indeed, the use of sheets of compressed particles of exfoliated graphite (i.e., flexible graphite) has been suggested as thermal spreaders, thermal interfaces and as component parts of heat sinks for dissipating the heat generated by a heat source (for example, see, U.S. Pat. Nos. 6,245,400; 6,503,626; and 6,538,892). [0006] Graphites are made up of layer planes of hexagonal arrays or networks of carbon atoms. These layer planes of hexagonally arranged carbon atoms are substantially flat and are oriented or ordered so as to be substantially parallel and equidistant to one another. The substantially flat, parallel equidistant sheets or layers of carbon atoms, usually referred to as graphene layers or basal planes, are linked or bonded together and groups thereof are arranged in crystallites. Highly ordered graphites consist of crystallites of considerable size: the crystallites being highly aligned or oriented with respect to each other and having well ordered carbon layers. In other words, highly ordered graphites have a high degree of preferred crystallite orientation. It should be noted that graphites possess anisotropic structures and thus exhibit or possess many properties that are highly directional e.g. thermal and electrical conductivity and fluid diffusion. [0007] Briefly, graphites may be characterized as laminated structures of carbon, that is, structures consisting of superposed layers or laminae of carbon atoms joined together by weak van der Waals forces. In considering the graphite structure, two axes or directions are usually noted, to wit, the "c" axis or direction and the "a" axes or directions. For simplicity, the "c" axis or direction may be considered as the direction perpendicular to the carbon layers. The "a" axes or directions may be considered as the directions parallel to the carbon layers or the directions perpendicular to the "c" direction. The graphites suitable for manufacturing flexible graphite sheets possess a very high degree of orientation. [0008] As noted above, the bonding forces holding the parallel layers of carbon atoms together are only weak van der Waals forces. Natural graphites can be treated so that the spacing between the superposed carbon layers or laminae 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. [0009] Graphite flake which has been greatly expanded and more particularly expanded so as to have a final thickness or "c" direction dimension which is as much as about 80 or more times the original "c" direction dimension can be formed without the use of a binder into cohesive or integrated sheets of expanded graphite, e.g. webs, papers, strips, tapes, foils, mats or the like (typically referred to as "flexible graphite"). The formation of graphite particles which have been expanded to have a final thickness or "c" dimension which is as much as about 80 times or more the original "c" direction dimension into integrated flexible sheets by compression, without the use of any binding material, is believed to be possible due to the mechanical interlocking, or cohesion, which is achieved between the voluminously expanded graphite particles. [0010] In addition to flexibility, the sheet material, as noted above, has also been found to possess a high degree of anisotropy with respect to thermal and electrical conductivity and fluid diffusion, comparable to the natural graphite starting material due to orientation of the expanded graphite particles and graphite layers substantially parallel to the opposed faces of the sheet resulting from very high compression, e.g. roll pressing. Sheet material thus produced has excellent flexibility, good strength and a very high degree of orientation. [0011] Briefly, the process of producing flexible, binderless anisotropic graphite sheet material, e.g. web, paper, strip, tape, 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 "c" direction dimension which is as much as about 80 or more times that of the original particles so as to form a substantially flat, flexible, integrated graphite sheet. The expanded graphite particles that generally are worm-like or vermiform in appearance, once compressed, will maintain the compression set and alignment with the opposed major surfaces of the sheet. The density and thickness of the sheet material can be varied by controlling the degree of compression. The density of the sheet material can be within the range of from about 0.04 g/cm.sup.3 to about 2.0 g/cm.sup.3. The flexible graphite sheet material exhibits an appreciable degree of anisotropy due to the alignment of graphite particles parallel to the major opposed, parallel surfaces of the sheet, with the degree of anisotropy increasing upon roll pressing of the sheet material to increase orientation. In roll pressed anisotropic sheet material, the thickness, i.e. the direction perpendicular to the opposed, parallel sheet surfaces comprises the "c" direction and the directions ranging along the length and width, i.e. along or parallel to the opposed, major surfaces comprises the "a" directions and the thermal, electrical and fluid diffusion properties of the sheet are very different, by orders of magnitude, for the "c" and "a" directions. [0012] However, the flexible nature of graphite materials makes it difficult to form complex structures or shapes unless the materials are first impregnated with a resin or the like. Such complex shapes are desirable when the materials are to be used, for example, as complex fin shapes or configurations to maximize heat transfer and dissipation. In addition, the attachment of graphite fins to metallic bases is also problematic, since graphite cannot be soldered into place in the same way metallic fins can. Moreover, while the anisotropic nature of graphite articles can be employed advantageously to move or spread heat, their relatively low through-plane conductivity (as compared to in-plane conductivity) can retard heat spreading through the graphite structure. [0013] Accordingly, there is a continuing need for improved designs for graphitic materials for heat dissipation solutions for electronic devices which provide the weight and thermal advantages of graphite elements, with the formability, isotropy and other advantages of metallic elements. SUMMARY OF THE INVENTION [0014] The present invention provides a thermal spreader material and finstock for thermal solutions for dissipating the heat from an electronic component. The inventive article comprises anisotropic sheets of compressed particle of exfoliated graphite (sometimes referred to with the term of art "flexible graphite") sandwiched around non-graphitic materials, especially metallic materials like aluminum or copper, advantageously in the form of a mesh. As used herein, the term "flexible graphite" also refers to sheets of pyrolytic graphite, either singly or as a laminate. The flexible graphite sheets employed in the inventive article have an in-plane thermal conductivity substantially higher than its through-plane thermal conductivity. In other words, the article of the present invention has a relatively high (on the order of 10 or greater) thermal anisotropic ratio. The thermal anisotropic ratio is the ratio of in-plane thermal conductivity to through-plane thermal conductivity. [0015] By sandwiching the non-graphite material between layers of graphite sheets, the thermal properties of graphite are maintained, while providing additional benefits, such as isotropic thermal spreading and moldability or formability, as well as improved attachment to a heat sink base. Most preferably, the non-graphite core layers comprise a metallic material, especially aluminum. Although aluminum is not as thermally conductive as copper, aluminum is preferred due to its lighter weight as compared to copper. Advantageously, when the non-graphite layer comprises a metal material, and the inventive article is employed as finstock, the metal core material can extend beyond the graphite layers and provide a substrate for soldering of the finstock to a metallic base or the like. In addition, the use of a metallic core layer permits the resulting structure to be molded and/or formed into complex shapes that meet specific space demands. The use of a metallic core also makes use of the isotropic nature of the metal to more efficiently spread heat along the graphite outer layers; indeed, it may be advantageous to expose a portion of the metallic core layer through one or both of the graphite outer layers to permit direct operative contact between the core layer and the heat source, in order to facilitate thermal spreading thorugh the core and the graphite layers, when the inventive article is employed as a thermal spreader. [0016] The use of a metal mesh or tanged metal core material is considered most advantageous, especially when the inventive article is employed as finstock. The spaces in the mesh or tanged metal provide a passageway for resin to flow between the graphite outer layers to more securely adhere the finstock sandwich together. Moreover, there can also be some graphite "flow" through the metal core passageways, again to provide more secure sandwich formation and adherence. Additionally, the tangs in a tanged metal core can also assist in securely maintaining the structure of the finstock sandwich of the present invention. When employed as a thermal spreader, a metal mesh or tanged metal core can be employed for the above-noted reasons; the use of a solid metal sheet can also be advantageously used. [0017] The inventive sandwich can be formed by a variety of methods. For instance, the graphite sheet or laminate of sheets can be disposed about the core material layers and the edges of the graphite layers adhered together using an adhesive or impregnated resin. In the alternative, the edges of the outer layers can be folded together to form the sandwich, or, an adhesive material can be applied to the surfaces of the graphite layers and/or the core layer, to adhere the graphite to the core material. In addition, where the core layer is formed of a mesh or tanged material, the passageways can be advantageously employed for adhesion of the sandwich layers, as discussed hereinabove. [0018] The inventive sandwich thermal solution comprises two major surfaces and four edge surfaces between the major surfaces, at least one of the major surfaces of which can be arrayed in operative contact with a heat source (in the case of a thermal spreader); or at least one of the edge surfaces of which can be arrayed in operative contact with a heat collection article or material, such as the base of a heat sink. For example, an edge of finstock can be fit into a slot or the like formed in the heat collection article or material as described hereinbelow. If the core of the sandwich is a metal, the inventive sandwich can be formed such that a portion of the metal extends from the edge fit into the heat collection article or material, allowing it to be soldered into the heat collection article or material (if formed of the appropriate material), for use as a finstock for optimum heat transfer and reduced contact resistance between the finstock and the heat collection article or material. If soldering is not appropriate or desired, the finstock can be pressure fit or attached to the heat collection article or material using a suitable adhesive or resin. [0019] Once fit into the heat collection article or material, the remaining surface area of the finstock functions to dissipate heat transferred to the finstock from the heat collection article or material. For instance, heat is transferred to the inventive finstock article from the heat collection article or material, and the heat is then conducted along the finstock due to the in-plane thermal conductivity of the inventive finstock. Air or another fluid can be passed along or across the surface area of the inventive finstock material to carry heat away from the heat source. [0020] As noted, the inventive finstock can be attached to a heat collection article or material, such as a heat sink base, via welding or soldering (in the case of a metallic core layer that extends beyond an edge of the finstock) or melting thereto (in the case of a plastic core layer that extends beyond an edge of the finstock). In the alternative, the inventive material can be formed into a series of discrete fins, which can be mounted to a heat sink base individual by, for instance, forming channels in the heat sink base and pressure fitting or soldering the individual fins into the channels to maximize thermal contact between the base and the fins. [0021] The formable nature of the inventive sandwich permits the formation of complex fin shapes and structures. For instance, folded or loop structures which optimize contact with the heat sink base while still providing substantial heat dissipation surface area are possible using the sandwich structure of the present invention. Continue reading... Full patent description for Sandwiched thermal article Brief Patent Description - Full Patent Description - Patent Application Claims Click on the above for other options relating to this Sandwiched thermal article patent application. ###  How KEYWORD MONITOR works... a FREE service from FreshPatents1. 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