Patent application title: THERMALLY CONDUCTIVE GEL PACKS
Eoin O'Riordan (Buckinghamshire, GB)
Philip Blazdell (Bicester, GB)
Gary Wood (Windham, NH, US)
Michael H. Bunyan (Chelmsford, MA, US)
Harish Rutti (Slough Berks, GB)
IPC8 Class: AF28F700FI
Class name: Heat exchange heat transmitter
Publication date: 2011-12-22
Patent application number: 20110308781
A conformable, thermally-conductive gel pack is provided having a thermal
gel encapsulated by a compliant packaging material formed from a
dielectric polymer. The gel pack is adapted for emplacement between
opposed heat transfer surfaces in an electronic device. One heat transfer
surface can be part of a heat-generating component of the device, while
the other heat transfer surface can be part of a heat sink or a circuit
1. A conformable thermally-conductive gel pack adapted to be positioned
intermediate a first heat transfer surface and an opposing second heat
transfer surface to provide a thermal pathway there between, the gel pack
comprising a thermally conductive polymeric gel encapsulated in a
compliant package comprising a layer of a polymeric material.
2. The gel pack of claim 1 wherein one of the first or second heat transfer surfaces is located on a heat-generating source.
3. The gel pack of claim 2 wherein: the heat-generating source is an electronic component; and the other one of the first or second heat transfer surface is located on a thermal dissipation member.
4. The gel pack of claim 3 wherein the thermal dissipation member is a heat sink or a circuit board.
5. The gel pack of claim 1 wherein the gel comprises a silicone polymer.
6. The gel pack of claim 1 wherein the gel is filled with a thermally-conductive particulate filler.
7. The gel pack of claim 6 wherein the particulate filler is selected from the group consisting of boron nitride, titanium diboride, aluminum nitride, silicon carbide, graphite, metals, metal oxides, and mixtures thereof.
8. The gel pack of claim 6 wherein the filled gel comprises between about 20-80% by weight of the filler.
9. The gel pack of claim 6 wherein the filler has a thermal conductivity of at least about 20 W/m-K.
10. The gel pack of claim 6 the filled gel has a thermal conductivity of at least about 0.5 W/m-K.
11. The gel pack of claim 1 wherein the interface has a thermal impedance of less than about 1.degree. C.-in2/W (6.degree. C.-cm2/W).
12. The gel pack of claim 1 wherein the polymeric material forming the layer of the package is a dielectric.
13. The gel pack of claim 1 herein the polymeric material forming the layer of the package is selected from the group consisting of polyimides, polyamides, and copolymers and blends thereof.
14. A thermal management assembly comprising: a first heat transfer surface; a second heat transfer surface opposing said first heat transfer surface; and a conformable thermally-conductive gel pack disposed intermediate said first and said second heat transfer surfaces to provide a thermally-conductive pathway there between, the gel pack comprising a thermally-conductive polymeric gel encapsulated in a compliant package comprising at least one layer of a polymeric material.
15. The assembly of claim 14 wherein one of the first or second heat transfer surfaces is located on a heat-generating source.
16. The assembly of claim 15 wherein: the heat-generating source is an electronic component; and the other one of the first or second heat transfer surface is located on a thermal dissipation member.
17. The assembly of claim 16 wherein the thermal dissipation member is a heat sink or a circuit board.
18. The assembly of claim 14 wherein the gel comprises a silicone polymer.
19. The assembly of claim 14 wherein the gel is filled with a thermally-conductive particulate filler.
20. The assembly of claim 19 wherein the particulate filler is selected from the group consisting of boron nitride, titanium diboride, aluminum nitride, silicon carbide, graphite, metals, metal oxides, and mixtures thereof
21. The assembly of claim 19 wherein the filled gel comprises between about 20% to about 80% by weight of the filler.
22. The assembly of claim 19 wherein the filler has a thermal conductivity of at least about 20 W/m-K.
23. The assembly of claim 19 the filled gel has a thermal conductivity of at least about 0.5 W/m-K.
24. The assembly of claim 14 wherein the interface has a thermal impedance of less than about 1.degree. C.-in2/W (6.degree. C.-cm2/W).
25. The assembly of claim 14 wherein the polymeric material forming the layer of the package is a dielectric.
26. The assembly of claim 14 herein the polymeric material forming the layer of the package is selected from the group consisting of polyimides, polyamides and copolymers and blends thereof
CROSS-REFERENCE TO RELATED APPLICATIONS
 This application claims the benefit of U.S. Provisional Application No. 61/100,297, filed on Sep. 26, 2008, the disclosure of which is incorporated herein by reference.
BACKGROUND OF THE INVENTION
 The present invention relates to a new thermal product or form factor which combines the thermal and mechanical performance of a fully dispensable material with the ease of use of traditional gap filler pads. In particular, the invention relates to a thermal gel material encapsulated in a compliant polymeric dielectric package that can be conveniently utilized in an electronic application requiring thermal management.
 Circuit designs for modem electronic devices such as televisions, radios, computers, medical instruments, business machines, communications equipment, and the like have become increasingly complex. For example, integrated circuits have been manufactured for these and other devices which contain the equivalent of hundreds of thousands of transistors. Although the complexity of the designs has increased, the size of the devices has continued to shrink with improvements in the ability to manufacture smaller electronic components and to pack more of these components in an ever smaller area.
 In recent years, electronic devices have become smaller and more densely packed. Designers and manufacturers are now facing the challenge of dissipating the heat generated in these devices using various thermal management systems. Thermal management has evolved to address the increased temperatures created within such electronic devices as a result of the increased processing speed and power of these devices. The new generation of electronic components squeeze more power into a smaller space; and hence the relative importance of thermal management within the overall product design continues to increase.
 An integral part of a thermal design process is the selection of the optimal Thermal Interface Material ("TIM") for a specific product application. New designs have been devised for thermal management to help dissipate the heat from electronic devices for further enhancing their performance. Other thermal management techniques utilize concepts such as a "cold plate", or other heat sinks which can be easily mounted in the vicinity of the electronic components for heat dissipation. The heat sink may be a dedicated, thermally-conductive metal plate, or simply the chassis or circuit board of the device.
 To improve the heat transfer efficiency through the interface, a pad or other layer of a thermally-conductive, electrically-insulating material often is interposed between the heat sink and electronic component to fill in any surface irregularities and eliminate air pockets. Initially employed for this purpose were materials such as silicone grease or wax filled with a thermally-conductive filler such as aluminum oxide. Such materials usually are semi-liquid or solid at normal room temperature, but may liquefy or soften at elevated temperatures to flow and better conform to the irregularities of the interface surfaces.
 The greases and waxes of the aforementioned types generally are not self-supporting or otherwise form-stable at room temperature, and are considered to be messy to apply to the interface surface of the heat sink or electronic component. Consequently, these materials are typically provided in the form of a film, which often is preferred for ease of handling, a substrate, a web, or other carrier which introduces another interface layer in or between the surfaces in which additional air pockets may be formed. Moreover, the use of such materials typically involves hand application or lay-up by the electronics assembler which increases manufacturing costs.
 Alternatively, another approach is to substitute a cured, sheet-like material in place of the silicone grease or wax. Such materials may contain one or more thermally-conductive particulate fillers dispersed within a polymeric binder, and may be provided in the form of cured sheets, tapes, pads, or films. Typical binder materials include silicones, urethanes, thermoplastic rubbers, and other elastomers, with typical fillers including aluminum oxide, magnesium oxide, zinc oxide, boron nitride, and aluminum nitride.
 Exemplary of the aforesaid interface materials are alumina or boron nitride-filled silicone or urethane elastomers. Additionally, U.S. Pat. No. 4,869,954 discloses a cured, form-stable, sheet-like, thermally-conductive material for transferring thermal energy. The material is formed of a urethane binder, a curing agent, and one or more thermally conductive fillers. The fillers, which may include particles of aluminum oxide, aluminum nitride, boron nitride, magnesium oxide, or zinc oxide.
 Sheets, pads, and tapes of the above-described types have garnered general acceptance for use as interface materials in the conductive cooling of electronic component assemblies such as semiconductor chips, as described in more detail in U.S. Pat. No. 5,359,768. In certain applications, however, fastening elements such as springs, clamps, and the like are required to apply enough force to conform these materials to the interface surfaces in order to attain enough surface for efficient thermal transfer. This represents a distinct disadvantage for deploying these materials in practical applications.
 Phase-change materials have recently been introduced which are self-supporting and form-stable at room temperature for ease of handling, but which liquefy or otherwise soften at temperatures within the operating temperature range of the electronic component to form a viscous, thixotropic phase which better conforms to the interface surfaces. These phase-change materials, which may be supplied as free-standing films, or as heated screens printed onto a substrate surface, advantageously function much like greases and waxes in conformably flowing within the operating temperature of the component under relatively low clamping pressures. Such materials are further described in U.S. Pat. No. 6,054,198.
 For typical commercial applications, the thermal interface material may be supplied in the form of a tape or sheet which includes an inner and outer release liner and an interlayer of a thermal compound. Unless the thermal compound is inherently tacky, one side of the compound layer may be coated with a thin layer of a pressure-sensitive adhesive (PSA) for application of the compound to the heat transfer surface of a heat sink. In order to facilitate automated dispensing and application, the outer release liner and compound interlayer of the tape or sheet may be die cut to form a series of individual, pre-sized pads. Each pad thus may be removed from the inner release liner and bonded to the heat sink using the adhesive layer in a conventional "peel and stick" application which may be performed by the heat sink manufacturer.
 U.S. Pat. No. 6,054,198 discloses a thermally-conductive interface for cooling a heat-generating electronic component having an associated thermal dissipation member such as a heat sink. The interface is formed as a self-supporting layer of a thermally-conductive material which is form-stable at normal room temperature in a first phase, and substantially conformable in a second phase to the interface surfaces of the electronic component and thermal dissipation member. The material has a transition temperature from the first phase to the second phase which is within the operating temperature range of the electronic component.
 U.S. Pat. No. 7,208,192 discloses the application of a thermally and/or electrically conductive compound to fill a gap between a first and second surface. A supply of fluent, form-stable compound is provided as an admixture of a cured polymer gel component and a particulate filler component. The compound is dispensed from a nozzle under an applied pressure onto one of the surfaces which is contacted with the opposing surface to fill the gap there between.
 The respective disclosures of each of the patents and patent applications listed above are incorporated by reference herein in their entireties.
 In view of the variety of materials and applications currently used in thermal management, as exemplified by the foregoing, it is to be expected that continued improvements in thermal management materials and applications would be well-received by electronics manufacturers.
 Accordingly, it is an objective of the present invention to provide improved thermal managements materials which provide a high degree of heat transfer efficiency and heat dissipation, are fully conformal to the particular application, and are easy to use and manufacture.
SUMMARY OF THE INVENTION
 The invention is a thermal gel material encapsulated in a dielectric polymer, such as a polyimide, polyamide or other such material, formed into a package, a bag or similar enclosure that confers the benefits of a fully cured, dispensable gap filler material without the need for using or investing in expensive dispensing equipment. This allows the customer to use ultra-compliant materials for sensitive applications, while maintaining the ease of pick-and-place technology and a convenient product form factor.
 In one embodiment, the invention is a conformable, thermally-conductive interface adapted to be positioned between two heat transfer surfaces to provide a thermal pathway there between, the interface comprising a thermally conductive polymeric gel encapsulated in a compliant package comprising a polymeric material. In one aspect, the thermally conductive polymeric gel comprises a silicone polymer containing a thermally conductive particulate filler, such as particles of boron nitride, and the polymeric packaging material is a dielectric polymer such as a polyimide or a polyamide. In another aspect, the package comprises two layers of heat sealable polymeric material encapsulating the thermally conductive gel, with one layer optionally comprising a thermal tape layer.
 In another embodiment, a conformable, thermally-conductive interface material is prepared by dispensing a thermally conductive polymeric gel onto a first layer of a dielectric polymer or a thermal tape. A second layer of dielectric polymer is place over the first layer, and heat sealed (or sealed with an adhesive) to the first layer to encapsulate the thermally conductive gel. The resulting gel packs can be manufactured as discrete items or using automated processing machinery, if desired, to dispense the gel pack in a roll on an assembly line. The amount of polymeric gel in the gel pack can be varied depending on customer requirements and can address a range of thickness requirements. The packaging material can additionally be slit or cut to allow for material displacement under load.
 The gel pack can be used in an electronic device where it can be disposed between a first heat transfer surface and a second heat transfer surface. The first heat transfer surface can be part of a component designed to absorb heat, such as a heat sink or a circuit board. The second heat transfer surface can be part of a heat generating source, such as an electronic component. In use, the gel pack is place between the first and second surfaces and is displaced under low deflection forces allowing the material to conform to the joint surfaces, thereby providing excellent thermal conductivity using only a low closure pressure.
BRIEF DESCRIPTION OF THE DRAWINGS
 FIG. 1 is a cross-sectional view of one embodiment of the invention showing a gel pack comprising a thermally conductive gel sandwiched between two layers of plastic sheet material heat sealed at the edge portions thereof to encapsulate the gel.
 FIG. 2 is a cross-sectional view of another embodiment of the invention showing a gel pack comprising a thermally conductive gel sandwiched between a layer of plastic sheet material and a thermal tape heat sealed at the edge portions thereof to encapsulate the gel.
 Those skilled in the art will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of embodiments of the present invention. Certain terminology may be employed in the description to follow for convenience rather than for any limiting purpose.
DETAILED DESCRIPTION OF THE INVENTION
 The invention provides a thermally conductive gel pack adapted to be positioned between two heat transfer surfaces of components used in electronic devices. The gel pack of the invention has improved heat transfer and handling characteristics for enhanced thermal management as compared to other products currently in use.
 As used herein, the term "thermal management" refers to the capability of keeping temperature-sensitive elements in an electronic device within a prescribed operating temperature in order to avoid system failure or serious system performance degradation.
 The term "EMI shielding" includes, and is interchangeable with, electromagnetic compatibility (EMC), electrical conduction and/or grounding, corona shielding, radio frequency interference (RFI) shielding, and anti-static, i.e., electro-static discharge (ESD) protection.
 A "conformable" product is one which displays sufficient flexibility to conform to the contours of the interface with minimal or low force deflection characteristics.
 As described herein, the thermally and/or electrically-conductive gel packs of the invention are principally described in connection with the usage of such gel packs within a thermal management assembly as a thermal interface material interposed between adjacent heat transfer surfaces. The heat transfer surfaces may be part of heat generating components, such as electronic components, or heat dissipation components, such as heat sinks or electronic circuit boards. However, one skilled in the art will readily appreciate that the present gel packs can have other uses which are fully intended to be within the scope of the present invention.
 In accordance with the present invention, therefore, a gel pack is provided comprising a flexible, conformable plastic package, such as a bag or other container, having an interior compartment for containing a thermal conductive substance, such as a thermally conductive polymeric gel. Preferably, the plastic is a conformable dielectric polymer, such as a polyamide or a polyimide.
 The package can be conveniently formed from two layers of plastic material by, for instance, dispensing the gel onto a first plastic layer, and placing a second plastic layer over the first layer to thereby encapsulate the gel within both layers of plastic. The plastic layers can then be heat sealed or glued at the outer edges where the layers overlap to form the gel pack. The resulting gel pack is fully conformable so as to be capable of filling gaps between adjoining surfaces of the circuitry components, circuit boards, and housings of electronic devices and electrical equipment, or between other adjoining surfaces such as may be found in building structures and the like.
 Gels useful as the polymer gel component of the invention include gels based on silicones, i.e., polysiloxanes, such as polyorganosiloxane, as well as gels based on other polymers, which may be thermoplastic or thermosetting, such as polyurethanes, polyureas, fluoropolymers, chlorosulfonates, polybutadienes, butyls, neoprenes, nitrites, polyisoprenes, and buna-N, copolymers such as ethylene-propylene (EPR), styrene-isoprene-styrene (SIS), styrene-butadiene-styrene (SBS), ethylene-propylene-diene monomer (EPDM), nitrile-butadiene (NBR), styrene-ethylene-butadiene (SEB), and styrene-butadiene (SBR), and blends thereof such as ethylene or propylene-EPDM, EPR, or NBR. Suitable thermal gels include the THERM-A-GAP® gel products, which are highly conformable, pre-cured, single-component compounds requiring only a relatively small compression force.
 As used herein, the terms "polymer gel" or "polymeric gel" generally have their conventional meaning of a fluid-extended polymer system which may include a continuous polymeric phase or network, which may be chemically, e.g., ionically or covalently, or physically cross-linked, and an oil, such as a silicone or other oil, a plasticizer, unreacted monomer, or other fluid extender which swells or otherwise fills the interstices of the network. The cross-linking density of such network and the proportion of the extender can be controlled to tailor the modulus, i.e., softness, and other properties of the gel. The term "polymer gel" or "polymeric gel" should also be understood to encompass materials which alternatively may be classified broadly as pseudogels or gel-like having viscoelastic properties similar to gels, such as by having a "loose" cross-linking network formed by relatively long cross-link chains, but as, for example, lacking a fluid-extender.
 In accordance with one aspect of the present invention, the polymer gel component is rendered thermally-conductive by loading the gel with a filler component which may comprise one or more thermally-conductive particulate fillers. In this regard, the polymer gel component generally forms a binder into which the thermally-conductive filler is dispersed. The filler is included in proportion sufficient to provide the thermal conductivity desired for the intended application, and generally will be loaded in an amount of between about 20% and about 80% by total weight of the compound. The size and shape of the filler is not critical for the purposes of the present invention. In this regard, the filler may be of any general shape, referred to broadly as "particulate," including solid or hollow spherical or microspherical flake, platelet, irregular, or fibrous, such as chopped or milled fibers or whiskers, but preferably will be a powder to assure uniform dispersal and homogeneous mechanical and thermal properties. The particle size or distribution of the filler typically will range from between about 0.01 mil to about 10 mil (0.25 μm-250 μm), which may be a diameter, imputed diameter, length, or other dimension of the particle, but may further vary depending upon the thickness of the gap to be filled. If desired, the filler may be electrically-nonconductive such that compound may be both dielectric or electrically-insulating and thermally-conductive. Alternatively, the filler may be electrically-conductive in applications where electrical isolation is not required.
 Suitable thermally-conductive fillers generally include oxide, nitride, carbide, diboride, graphite, and metal particles, and mixtures thereof, and more particularly boron nitride, titanium diboride, aluminum nitride, silicon carbide, graphite, metals such as silver, aluminum, and copper, metal oxides such as aluminum oxide, magnesium oxide, zinc oxide, beryllium oxide, and antimony oxide, and mixtures thereof. Such fillers characteristically exhibit a thermal conductivity of at least about 20 W/m-K. For reasons of economy, an aluminum oxide, i.e., alumina, may be used, while for reasons of improved thermal conductivity a boron nitride would be preferred. Loaded with the thermally-conductive filler, the compound typically may exhibit a thermal conductivity, per ASTM D5470, of at least about 0.5 W/m-K, which may vary depending upon the thickness of the compound layer.
 In accordance with another aspect of the present invention, the polymer gel component can be rendered electrically-conductive by loading with an electrically-conductive filler, which may be provided in addition to, i.e., a blend, or instead of a thermally-conductive filler. Also, depending upon the filler selected, such filler may function as both a thermally and an electrically-conductive filler. The use of electrically conductive materials provides the gel with EMI shielding characteristics.
 Suitable electrically-conductive fillers include: noble and non-noble metals such as nickel, copper, tin, aluminum, and nickel; noble metal-plated noble or non-noble metals such as silver-plated copper, nickel, aluminum, tin, or gold; non-noble metal-plated noble and non-noble metals such as nickel-plated copper or silver; and noble or non-noble metal plated non-metals such as silver or nickel-plated graphite, glass, ceramics, plastics, elastomers, or mica; and mixtures thereof. The filler again may be broadly classified as "particulate" in form, although the particular shape of such form is not considered critical to the present invention, and may include any shape that is conventionally involved in the manufacture or formulation of conductive materials of the type herein involved including hollow or solid microspheres, elastomeric balloons, flakes, platelets, fibers, rods, irregularly-shaped particles, or a mixture thereof. Similarly, the particle size of the filler is not considered critical, and may be or a narrow or broad distribution or range, but in general will be from about 0.250 μm to about 250 μm.
 The thermal gel is packaged in a plastic film by encapsulating the gel in a dielectric polymer. Exemplary dielectric polymers include various thermoplastic polymers, such as polyimides (e.g. Kapton®), polyamides, and copolymers and blends thereof. These thermoplastic polymers can be formed into films and heat sealed at the edge portions, thereby enclosing the thermal gel in a sealed bag or pouch. In practice, the thermal gel is deposited on a first layer of polymer film, and a second layer of polymer film is placed over and heat sealed to the first film layer. In one embodiment, both the first and second layers are dielectric polymer film layers, preferably formed from the same polymer. In another embodiment, one of the layers, typically the bottom layer, is a thermal tape. Suitable thermal tapes include the THERMATTACH® thermally conductive attachment tapes, which are based on a polyimide carrier and have excellent dielectric strength.
 Multiple gel packs can be advantageously and efficiently manufactured in an automated assembly process on an assembly line, thereby allowing the packs to be produced in rolls and individually cut prior to use.
 The gel packs are adapted to be used with electronic equipment by emplacement intermediate a first heat transfer surface and a second heat transfer surface to provide a thermal pathway there between. One heat transfer surface can be a component designed to absorb heat, such as a heat sink or an electronic circuit board. The other (opposed) heat transfer surface can be a heat generating source, such as a heat generating electronic component. The opposed heat transfer surfaces preferably have a thermal impedance of less than about 1° C.-in2/W (6° C.-cm2/W).
 Typical electronic equipment within the scope of the present invention include, by way of example, automotive electronic components and systems, telecom base stations, and consumer electronics, such as computer monitors and plasma TVs.
 Referring now to the figures, FIGS. 1 and 2 show two embodiments of the thermal gel packs according to the present invention. In FIG. 1, thermal gel 1 is shown encapsulated by an upper layer 2 and a lower layer 3 of a dielectric polymer film. The edges of the upper and lower layers of film are heat sealed to enclose the gel. FIG. 2 is similar to FIG. 1 and shows thermal gel 4 encapsulated by an upper film layer of dielectric polymer 5 and a lower layer of a thermal tape 6. The thermal gel packs of the invention can be prepared individually, or can be part of a number of such packs prepared in an automated manufacturing process.
 It is anticipated that certain changes may be made in the present invention without departing from the concepts herein involved, and it is intended that all matter contained in the foregoing description shall be interpreted as illustrative and not in a limiting sense. All references including any priority documents cited herein are expressly incorporated by reference herein in their entirety.
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Patent applications in class HEAT TRANSMITTER
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