WO2019136151A2

WO2019136151A2 - Thermal interface material

Info

Publication number: WO2019136151A2
Application number: PCT/US2019/012186
Authority: WO
Inventors: Martin D. Smalc; Prashanth SUBRAMANIAN
Original assignee: Neograf Solutions, Llc
Priority date: 2018-01-05
Filing date: 2019-01-03
Publication date: 2019-07-11
Also published as: WO2019136151A3

Abstract

A thermal interface material is disclosed. The thermal interface material comprises a flexible graphite sheet having mechanical alteration; and a heat transfer fluid incorporated in the flexible graphite sheet, where the heat transfer fluid has an operating temperature ranging from -40° C to 300° C and passes the UL94 V-0 flame test when used as a thermal interface material in a thermal interface for a heat source. The thermal interface material has a thermal impedance (Y) at least 10% lower than that defined by Y = 1.02(10^-7)X²-2.8(10^-4)X+0.26 when X is a contact pressure ranging from 400 kPa to 1400 kPa. Also disclosed are a thermal interface assembly comprising the thermal interface material, and a thermal management system comprising the thermal interface assembly for facilitating the management of the heat from a heat source.

Description

THERMAL INTERFACE MATERIAL

CROSS REFERENCE TO RELATED APPLICATIONS

[001] This application claims priority to and the benefit of U. S. Application No.

62/614,138, filed on January 5, 2018, the entire content of which is incorporated herein by reference.

TECHNICAL FIELD

[002] The present invention relates to a thermal interface material, a thermal interface assembly comprising the thermal interface material, and a thermal management system comprising the thermal interface assembly for facilitating the management of the heat from a heat source. The thermal interface material, a thermal interface assembly, and a thermal management system are effective for facilitating the dissipation of the heat generated by a heat source such as an electronic component.

BACKGROUND

[003] The development of more sophisticated electronic components having smaller sizes and higher power densities creates an ever increasing need for improved heat dissipation. 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.

[004] Both performance reliability and life expectancy of electronic equipment are inversely related to the component temperature of the equipment. With the increased need for heat dissipation from microelectronic devices caused by these conditions, thermal management becomes an increasingly important element of the design of electronic products.

[005] One element of a thermal management system is a thermal interface, which includes a thermal interface material. The thermal interface thermally connects a heat source such as an electronic component ( e.g ., a computer chip) to a cooling module, such as a heat sink or chassis body to overcome contact resistance and lack of surface conformity between the heat sink, or the cooling module and the chip or other heat source. See US Patent No. 6,746,768 to Greinke, et al. and US Patent No. US 6245400 to Tzeng et al.

[006] In addition, when the surface area of the thermal interface is greater than the surface area of the heat source it contacts, the thermal interface can sometimes be referred to as a thermal spreader or heat spreader. This is due to the properties of the thermal interface material being able to spread the heat along the surface of the interface. For example, when the thermal interface includes a thermal interface material comprising a flexible graphite sheet, the anisotropic nature of the graphite material spreads the heat from the heat source along the in-plane surface of the graphite material, in addition to through the surface (perpendicular or orthogonal to the in-plane or planar surface of the material), thus reducing so-called hot spots and facilitating the use of heat sinks and other thermal dissipation devices having greater effective surface areas.

[007] It is also desirable to improve the through-plane conductivity of the flexible graphite sheet material to further facilitate heat transfer from a heat source when functioning as a thermal interface.

SUMMARY

[008] In accordance the present invention, a thermal interface material is disclosed. The thermal interface material comprises a flexible graphite sheet having a mechanical alteration and a heat transfer fluid incorporated in the flexible graphite sheet, where the heat transfer fluid has an operating temperature ranging from -40° C to 300° C and passes the UL94 V-0 flame test when used as a thermal interface material in a thermal interface assembly for a heat source. The thermal interface material has a thermal impedance (Y) at least 10% lower than that defined by

Y = l .02(l0 ⁷)X²-2.8(l0 ⁴)X+0.26 when X is a contact pressure ranging from 400 kPa to 1400 kPa.

[009] Further in accordance with the present invention, a thermal interface assembly comprising the thermal interface material of the present disclosure is disclosed. For example, a thermal interface assembly for a heat source comprises a thermal interface material, where the thermal interface material comprises a flexible graphite sheet having a mechanical alteration and a heat transfer fluid incorporated in the flexible graphite sheet, where the heat transfer fluid has an operating temperature ranging from -40° C to 300° C and passes the UL94 V-0 flame test when used as the thermal interface material in the thermal interface assembly for a heat source. The thermal interface material of the thermal interface assembly has a thermal impedance (Y) at least 10% lower than that defined by Y = l .02(l0 ⁷)X²-2.8(l0 ⁴)X+0.26 when X is a contact pressure ranging from 400 kPa to 1400 kPa.

[0010] Yet further in accordance with the present invention, a thermal management system comprising the thermal interface assembly of the present disclosure is disclosed. For example, a thermal management system for moving heat away from a heat source having an external surface comprises: a heat dissipative member having an external surface and a thermal interface assembly having a first surface and a second surface disposed opposite the first surface. The first surface of the thermal interface assembly is disposed adjacent to and in thermal communication with the external surface of the heat source, and the second surface of the thermal interface assembly is disposed adjacent to and in thermal communication with the external surface of the heat dissipative member. The thermal interface assembly comprises a thermal interface material, where the thermal interface material comprises a flexible graphite sheet having a mechanical alteration and a heat transfer fluid incorporated in the flexible graphite sheet, where the heat transfer fluid has an operating temperature ranging from -40° C to 300° C and passes the UL94 V-0 flame test when used as the thermal interface material in the thermal interface assembly for the heat source. The thermal interface material has a thermal impedance (Y) at least 10% lower than that defined by Y = l .02(l0 ⁷)X²-2.8(l0 ⁴)X+0.26 when X is a contact pressure ranging from 400 kPa to 1400 kPa.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011] The present invention will be better understood and its advantages more apparent in view of the following detailed description, especially when read with reference to the appended drawings.

[0012] FIG. 1 is a top perspective view of an exemplary embodiment of a thermal management system of the present invention comprising a thermal interface assembly in association with a heat source.

[0013] FIG. 2 is a bottom perspective view of the thermal management system of FIG. 1.

[0014] FIG. 3 is a side plan view of the thermal management system of FIG. 1.

[0015] FIG. 4 is a top perspective view of an exemplary embodiment of a thermal management system of the present invention comprising a heat spreader utilizing a thermal interface assembly in association with a heat source (in phantom).

[0016] FIG. 5 is a bottom perspective view of the thermal management system of FIG. 4.

[0017] FIG. 6 is a side plan view of the thermal management system of FIG. 4. [0018] FIG. 7 is a graph of thermal impedance versus contact pressure obtained from tests conducted with various samples of thermal interface materials in the Example; and

[0019] FIG. 8 is a table providing numerical results of tests conducted with various samples of thermal interface materials shown in FIG. 7.

DETAILED DESCRIPTION

[0020] As stated above, the present invention includes a thermal interface material, a thermal interface assembly comprising the thermal interface material, and/or a thermal management system comprising the thermal interface assembly for facilitating the management of the heat from a heat source.

[0021] Referring now to FIGS. 1-6, a thermal management system prepared in accordance with the present invention is shown and generally designated by the reference numeral 110. It should be noted that for the sake of clarity not all the components and elements of system 110 may be shown and/or marked in all the drawings. Also, as used in this description, the terms“up,” “down,”“top,”“bottom,” etc. refer to thermal management system 110 when in the orientation shown in FIGS. 3 and 6. However, the skilled artisan will understand that thermal management system 110 can adopt any particular orientation when in use.

[0022] Thermal management system 110 is used to facilitate the dissipation of heat from a heat source 100. In one non-limiting example, the heat source 100 can be an electronic component. Electronic component 100 can comprise any electronic device or component that produces sufficient heat to interfere with the operation of the electronic component or the system of which electronic component is an element, if not dissipated. To that end, efficient heat transfer (i.e., low thermal resistance) is important in the performance and life span of an electric component. Electronic component 100 can comprise a microprocessor or computer chip, an integrated circuit, control electronics for an optical device like a laser or a field-effect transistor (FET), rectifier, inverter, converter, variable speed drive, insulated gate bipolar transistor, thyristor, amplifier, inductors, capacitors or components thereof, or other like electronic elements. In other examples, electronic component 100 can be a wireless charging component, such as for example, an induction coil. Heat source 100 includes at least one external surface lOOa from which heat radiates.

[0023] The thermal management system 110 includes a thermal interface assembly 120 comprising a thermal interface material 130. In addition to the thermal interface material 130, the thermal interface assembly 120 may further comprise optional layers that are conventionally used with thermal interface assemblies (not shown in the FIGS.). Such optional layers include one or more coating layer(s) formed from polymers such as polyethylene terephthalates (PET), polyamides (nylons), temporary coating layers or liners, or other coatings known in the field. The optional layers may also include one or more adhesive layer(s) formed from adhesives suitable for use in such thermal interfaces such as acrylic adhesives, PET adhesives, or like adhesives. In accordance with the present invention, the thermal interface assemblies may comprise a coating layer, an adhesive layer, and combinations thereof. The terms“thermal interface assembly” and “thermal interface” can be used interchangeably herein and refer to the same structure.

[0024] Thermal interface material 130 comprises one or more flexible graphite sheet(s) and a heat transfer fluid incorporated in the flexible graphite sheet. The thermal interface material 130 has a first surface l30a and an oppositely disposed second surface l30b. A principal function of thermal interface assembly 120 is to form sufficient operative thermal communication of the thermal interface assembly with the external surface lOOa of heat source 100 to maximize the removal of heat from the heat source 100 at an acceptable contact pressure. In accordance with the present invention, this is accomplished by contacting first surface l30a of the thermal interface material 130 with the external surface lOOa of the heat source 100. Depending on the nature of the other constituents of thermal management system 110, a second function of thermal interface assembly 120 can be to increase the effective surface area of external surface lOOa of heat source 100 to facilitate heat dissipation from heat source 100 by using a thermal interface assembly 120 having a thermal interface material 130 with a larger surface area than the heat source 100 and its external surface lOOa as shown in FIGS. 4-6. Such embodiments shown in FIGS. 4-6 may be referred to as a thermal spreader or a heat spreader. As discussed above, the anisotropic properties of the flexible graphite sheet facilitate heat transfer along the surface (in-plane) as well as through the surface (perpendicular to the in-plane direction).

[0025] As discussed above, thermal interface material 130 comprises a heat transfer fluid incorporated in the flexible graphite sheet. The heat transfer fluid of the present invention includes any fluid that has an operating temperature ranging from -40° C to 300° C, passes the EIL94 V-0 flame test when used as a thermal interface material in a thermal interface for a heat source, and provides a thermal impedance (Y) at least 10% lower than that defined by Y = l .02(l0 ⁷)X² -2.8(10 ⁴)C+0.26 when X is a contact pressure ranging from 400 kPa to 1400 kPa when used in a thermal interface material in a thermal interface for a heat source. Nonlimiting examples of such heat transfer fluids include fluorinated synthetic oils such as perfluorinated synthetic oils; certain mineral oils, and other fluids such as ethylene glycol. A nonlimiting specific example of a suitable perfluorinated synthetic oil includes a perfluoropolyether lubricant (also known as PFPE). Specific examples of suitable perfluorinated synthetic oils include NOVEC PFPE (available from 3M of Maplewood, MN) and FOMBLIN, particularly FOMBLIN Y, a PFPE (available from Solvay SA of Belgium).

[0026] Preferably, fluorinated synthetic oils used in the present invention have a kinematic viscosity of from about 25 to about 280 cSt (centistokes) @ 20 °C.

[0027] Preferred embodiments of the heat transfer fluid of the present invention further includes any fluid that has an operating temperature ranging from -40° C to 120° C, and/or has a thermal impedance (Y) less than or equal to that defined by Y = 3.31(10 ⁸)C²-1.15(10 ⁴)C+0.136 when X is the contact pressure ranging from 400 kPa to 1400 kPa. It should be understood that certain fluids that do not have operating temperature ranging from -40° C to 300° C or that fail the EIL94 V-0 flame test, such as certain silicone oils, ester type oils, polyalfaolefm oligomers, and alkylated benzenes, are not suitable as the heat transfer fluids of the present invention.

[0028] As discussed above, the thermal interface material 130 includes a heat transfer fluid incorporated into the one or more graphite sheet(s). To that end, the thermal interface material 130 may be prepared by any means known in the art, including but not limited to, contacting the flexible graphite sheet in the heat transfer fluid such as by spraying, dipping, immersion or any other suitable technique so as to at least partially incorporate the heat transfer fluid in the flexible graphite sheet. In one example, this may be accomplished by immersing the graphite sheet in a bath of the heat transfer fluid for a predetermined time, after which is it then removed, to obtain a graphite sheet having the desired amount of incorporated heat transfer fluid after removal from the bath. After removing the graphite sheet from the bath of fluorinated synthetic oil, the flexible graphite sheet can be rinsed with a solvent in one or more rinsing steps. In one example the solvent can be alcohol, such as for example isopropanol including isopropyl alcohol and water. The graphite sheet can be dried after the one or more rinsing steps, if so desired.

[0029] To that end, the thermal interface material 130 can be made by contacting the flexible graphite sheet with the heat transfer fluid until the heat transfer fluid is incorporated into the graphite sheet to obtain about 2 to about 75 parts by weight heat transfer fluid based on 100 parts by weight of the flexible graphite sheet. Further examples include a thermal interface material 130 comprising a flexible graphite sheet and heat transfer fluid with about 2 to about 60 parts by weight heat transfer fluid based on 100 parts by weight of the flexible graphite sheet, including about 2 to about 50 parts by weight heat transfer fluid based on 100 parts by weight of the flexible graphite sheet, including about 2 to about 20 by parts by weight heat transfer fluid based on 100 parts by weight of the flexible graphite sheet, including about 10 to about 60 by parts by weight heat transfer fluid based on 100 parts by weight of the flexible graphite sheet, and including about 10 to about 50 by parts by weight heat transfer fluid based on 100 parts by weight of the flexible graphite sheet. In a preferred embodiment, the heat transfer fluid is added to the extent where any pores of the graphite sheet are substantially filled with the fluid. In other words, the gas present in the pores is replaced with the heat transfer fluid as it is incorporated into the graphite sheet. Therefore, in accordance with the present invention it should be understood that the amount of the heat transfer fluid present in the graphite sheet may change due to different graphite sheets having a different volume of pores.

[0030] In accordance with the present invention, preferably the surface of the one or more graphite sheet(s) of the thermal interface material 130 absorbs the heat transfer fluid such as a fluorinated synthetic oil resulting in a surface that does not evidence an“oily” feel or texture. In one non-limiting specific example, a graphite sheet is immersed in a bath of a heat transfer fluid such as a fluorinated synthetic oil for about 1 hr to about 60 days, including for about 1 hr to about 30 days, and including for about 1 hr to about 24 hours.

[0031] As discussed above, the thermal interface material 130 comprises one or more flexible graphite sheet(s). Flexible graphite sheets of the present invention include synthetic graphite formed from a graphitized polymer sheet (AKA synthetic graphite). Unless indicated herein,“graphite sheet” is interchangeable with“flexible graphite sheet” and refers to flexible graphitized polymer sheets and exclude synthetic graphite made from pitch and coke. Having thermal interface assembly 120 composed of flexible graphite will offer the advantage of conformability with the heat source 100 and also low contact resistance with the heat source when in operative thermal communication therewith. As used herein, two materials are in operative thermal communication when heat can be conducted from one to the other through matter by communication of kinetic energy from particle to particle with no net displacement of the particles. [0032] The precursor polymer for the flexible graphite sheet formed of a graphitized polymer can be a polymer film selected from polyphenyleneoxadiazoles (POD), polybenzothiazole (PBT), polybenzobisthiazole (PBBT), polybenzooxazole (PBO), polybenzobisoxazole (PBBO), poly(pyromellitimide) (PI), poly(phenyleneisophthalamide) (PPA), poly(phenylenebenzoimidazole) (PBI), poly(phenylenebenzobisimidazole) (PPBI), polythi azole (PT), and poly(para-phenylenevinylene) (PPV). The polyphenyleneoxadiazoles include poly-phenylene-l, 3, 4-oxadiazole and isomers thereof. These polymers are capable of conversion into graphite of good quality when thermally treated in an appropriate manner as known in this field of technology. Although the polymer for the starting film is stated as selected from POD, PBT, PBBT, PBO, PBBO, PI, PPA, PBI, PPBI, PT and PPV, other polymers that can yield graphite by thermal treatment, which when used in a thermal interface assembly 120 for a heat source and having a thermal impedance (Y) at least 10% lower than that defined by Y = l .02(l0 ⁷)X²-2.8(l0 ⁴)X+0.26 when X is a contact pressure ranging from 400 kPa to 1400 kPa, may also be used.

[0033] Specific examples of a flexible graphite sheet that may be used in accordance with the present invention include EGRAF HITHERM (available from NEOGRAF Solutions LLC of Lakewood, OH) and Panasonic PGS, a pyrolytic, flexible graphite sheet (available from Matsushita Electric Components Company Ltd., Ceramic Division, Japan).

[0034] The thickness of the flexible graphite sheet of the present invention comprises up to about 500 microns, including from about 5 microns to about 500 microns, including from about 10 microns to about 500 microns, including from about 50 microns to about 500 microns, including from about 100 microns to about 500 microns, including from about 50 microns to about 400 microns, including from about 50 microns to about 300 microns, including from about 100 microns to about 250 microns, including from about 5 microns to about 400 microns, including from about 5 microns to about 300 microns, including from about 5 microns to about 250 microns, including about 5 microns to about 100 microns, including about 5 microns to about 50 microns, including from about 5 microns to about 30 microns, including from about 10 microns to about 50 microns, and including about 10 microns to about 30 microns.

[0035] The density of the flexible graphite sheets according to the present invention is typically less than 1.50 grams/cubic centimeter (g/cc) prior to compression. For any given embodiment of the present invention, the density may range from less than 1.50 g/cc to about 0.2 g/cc, including less than 1.25 g/cc to about 0.2 g/cc. Examples of exemplary flexible graphite sheet densities in accordance with the present invention include less than 1.50 g/cc, less than 1.25 g/cc, less than 1.0 g/cc, less than 0.85 g/cc, less than 0.7 g/cc, less than 0.6 g/cc, less than 0.5 g/cc, less than 0.4 g/cc, and less than 0.3 g/cc.

[0036] In accordance with the present invention, the flexible graphite sheet of the thermal interface material includes a mechanical alteration. Examples of the mechanical alternation include at least one of embossing, altering the porosity, perforating, and combinations thereof. It should be understood that the mechanical alterations may not be exclusive of each other. For example perforating the graphite sheet may also alter the porosity and vice versa. Likewise, embossing the sheet may also alter its porosity, etc. The disclosure herein is not limited by the mechanical alterations disclosed, but instead include any such mechanical alteration that results in a thermal interface material with the thermal impedance disclosed herein when used as a thermal interface for a heat source. The mechanical alterations do not have to be visible to the unaided eye on both of the first and second surfaces of the flexible graphite sheet. Without intending to be limited to any theory, it is believed that the mechanical alteration(s) applied to the flexible graphite sheet of the thermal interface material 130 acts to improve the contact area of the surfaces l30a and 130b of the thermal interface material 130 with the external surface lOOa of heat source 100 and the surface l50b of heat dissipative member 150, thereby improving the operative thermal communication with the heat source. This improved contact area and thus improved operative thermal communication is believed to result from (1) the physical change to the structure of the flexible graphite sheet due to the mechanical alteration and/or (2) due to improved absorption or uptake of the heat transfer fluid in the flexible graphite sheet due to the mechanical alteration. The improved absorption or uptake of the heat transfer fluid may improve the contact area of surface l30a and l30b of the thermal interface material 130 by filling interstices or“air gaps” in the flexible graphite sheet, which act as thermal insulators between the surfaces of the thermal interface and the heat dissipation member and/or the heat source. In other words, the improved absorption or uptake of the heat transfer fluid in the flexible graphite sheet alters (i.e., reduces) porosity of the thermal interface material, and as a result, reduces the thermal impedance of the thermal interface material caused by the porosity of the material.

[0037] In one non-limiting example, an embossing roller can be used to provide a mechanical alteration to the flexible graphite sheet. The flexible graphite sheet can be embossed between a grooved steel roller and rubber roller or rubber mat. The roller is constructed so that there are a series of alternating grooves and lands across the width of the roller. The roller lands press down on the flexible graphite sheet material while the roller grooves do not contact the material. This creates alternating rows of graphite valleys (created by the roller lands) and peaks (created by the roller grooves). The width of these peaks can be varied between 0.016 - 0.039 inches and the width of the valleys from 0.010 - 0.039 inches. Exemplary typical values may be 0.020 inch wide valleys and 0.039 inch wide peaks. The embossing pattern can be applied to one of both sides of the flexible graphite sheet material. If applied to one side, a second pattern can also be applied to the side at a 90° angle to the first pattern. The second pattern can have the same or different spacing of peaks and valleys. If applied to both sides, the second pattern can be applied in the same direction as the first pattern or at 90° to the first pattern. Spacing of peaks and valleys in this second pattern can be the same or different from the first pattern. The rubber roller or mat used with the embossing roller can have a hardness that varies from 20 durometer to 60 durometer.

[0038] In another non-liming example, the flexible graphite sheet can be embossed against a screen to provide a mechanical alteration. In this example, the flexible graphite sheet is embossed between a rubber roller and a woven metal screen. The metal screen can be composed of a metallic material such as steel, stainless steel, copper, and/or brass. Various weave patterns can be used. These can include, for example, a simple over and under woven wire cloth with 30 x 28 wires per inch with a 0.008 inch wire diameter or 60 x 40 wires per inch with a 0.006 inch wire diameter. A rubber roller with a hardness of 40 durometer can be used with these screens. A more complex Dutch weave stainless steel filtering cloth can also be used. This Dutch weave pattern has 24 x 110 wires per inch and consists of 0.014 inch diameter horizontal wires and 0.010 inch diameter vertical wires. Another Dutch weave pattern has 30 x 150 wires per inch and consists of 0.009 inch diameter horizontal wire and 0.007 inch diameter vertical wires. These Dutch weaves generate a highly textured predominately linear pattern in the graphite. A rubber roller with a hardness of 40 durometer can be used with these screens, or a grooved steel roller with a spacing of grooves and lands that matches the linear pattern of the Dutch weave.

[0039] In other non-limiting examples, the flexible graphite sheet can be perforated, e.g., pierced with a plurality of small slits or holes, to provide a mechanical alteration. In one embodiment, the perforations suitable for the present invention penetrate one surface of the sheet, but do not make it through to the other surface of the sheet. Alternatively, in the same or different embodiments, the perforations may extend through the entire graphite sheet. These perforations are believed to improve porosity so that the flexible graphite sheet can soak in a larger amount of heat transfer fluid. In one non-limiting example, the perforation holes can have a diameter from about .0625 inches (1.6 mm), in other examples the holes can have a diameter from about 0.01 inch (0.25 mm) to about 0.1 inch (2.5 mm). The perforations may be applied at less than about 40% of flexible graphite sheet. Unless otherwise indicated herein, the perforation percent represents the reduction in mass of the graphite sheet. Preferably, the perforations are applied at less than 25%, more preferably at less than 20%, and even more preferably at 15% or less.

[0040] Referring to FIGS. 1-6, the thermal management system 110 also includes a heat dissipative member 150 disposed in a proximate relationship to the thermal interface assembly 120 and thermal interface material 130 such that the thermal interface material 130 is in operative thermal communication with the heat source 100 and the heat dissipative member 150. Heat dissipative member 150 may be any suitable active or passive heat exchange article, such as a heat sink, that transfers heat from a heat source 100 and dissipates it into an adjacent or proximate medium such as a fluid or air. The heat dissipative member 150 has an external surface l50b that provides a compressive force against the thermal interface material 130 in such a manner to press the first surface 130a of the thermal interface material 130 against the external surface lOOa of the heat source 100 at a predetermined contact pressure. The compressive force can be provided by any suitable means known in the field, such as, for example, mechanical fasteners, springs, clips, etc. The contact pressure ranges from about 50 kPa to about 1500 kPa, including from about 200 kPa to about 1500 kPa, and including from about 400 kPa to 1400 kPa.

[0041] An advantage of the use of thermal interface 110 of the present invention is in its conformability. Since external surface lOOa of heat source 100 is generally formed of a metallic, ceramic material, or other like material, the surface of external surface lOOa is not perfectly smooth (even though it may appear so to the naked eye, or to the touch), but is rather covered by surface deformations, irregularities, “peaks and valleys,” etc. This causes air gaps (which, as discussed above, act as thermal insulators between the surfaces of the thermal interface 120 and the external heat dissipative member surface l50b and/or the heat source 100. As discussed above, the mechanical alterations (i.e., physical changes) to the flexible graphite sheet and/or the heat transfer fluid within the thermal interface material fills the air gaps to improve, i.e., lower, the through thermal impedance between the thermal interface material 130 (and consequently thermal interface 120) and the heat source 100. As used herein, the through thermal impedance, measured in cm²K/W, is defined as a measure of a material’s resistance to transfer heat through the body of the material. When referring to sheet materials, such as a flexible graphite sheet, the through thermal impedance is determined by multiplying the through plane thermal conductivity times the thickness of the flexible graphite sheet. The term“through plane” can be defined as an orthogonal direction (i.e., a direction forming a 90° angle) with the in-plane direction for sheet materials. In other words, the“through plane” is understood to be the C-direction in a 3 -dimensional space relative to an A-B Cartesian plane, where the C-direction is perpendicular or orthogonal to the A- B plane).

[0042] Since thermal interface 120 of the present invention is more conformable to the surface topography of external surface lOOa of heat source 100 as well as to an external surface l50b of heat dissipative member 150, a better thermal connection (that is, a better thermal communication or a better thermal transfer) between heat source 100 and heat dissipative member 150 having surface deformations can be achieved.

[0043] Heat dissipative member surface l50a as used herein refers to an area of heat dissipative member 150 from which the heat transmitted from external surface lOOa through thermal interface assembly 120 is dissipated into the environment. Most commonly, the at least one heat dissipation surface l50a of heat dissipative member 150 are those surfaces where air or another coolant fluid is passed across heat dissipative member 150 such as by the action of a fan (not shown). To maximize heat transfer from heat dissipative member 150 to a coolant medium (e.g., a coolant fluid), the at least one heat dissipation surface l50a of heat dissipative member 150 should be designed and/or shaped so as to have as great a surface area as feasible.

[0044] The following example is presented to further illustrate the present invention, and are not intended to limit the present invention in any way.

EXAMPLE

[0045] Several thermal interface materials were tested to compare their impedance over a range of contact pressures ranging from 400 kPa to 1400 kPa. The thermal impedance was measured using a modified version of the test method described in ASTM D5470. In the standard test, a pressure of 2760 kPa is applied to the test specimen. This test pressure is well in excess of pressures typically encountered in electronics applications. In the modified test, the pressure is reduced to between 400 and 1400 kPa, a more appropriate range for a thermal interface material (“TIM”). In the test, the TIM sample is sandwiched between a pair of identical aluminum meter bars, and a heater and cooling plate are used to generate a one-dimensional heat flow through the specimen. Three thermocouples located in each meter bars are used to measure the thermal gradient in the bar.

[0046] Knowing the thermal conductivity of the meter bars, the heat flow through the TIM sample can also be determined, and thus the thermal impedance of the TIM can be determined where,

YJ=ATJ/(Q/AA)

AA = contact area (cm²),

Q = joint heat transfer rate (W), and

Y_j = thermal impedance (cm^{2 o}C/W, and can also be expressed as cm² K/W).

[0047] The meter bars were made from 6063 aluminum alloy and had a diameter of 5.08 cm. Bottom and top surfaces of the meter bars were lapped to a finish of 0.08 microns and a flatness of less than 13 microns. Three type T, 32 gauge, bare junction thermocouples were installed in each meter bar. Data analysis was performed using thermocouple calibrations obtained from tests of the meter bars in an oven at 40-60 °C, the observed operating temperature range of the meter bars. Thermal interface tests were conducted in air at a nominal specimen temperature of 50 °C and at a heat flow rate of 45-50 Watts. Test pressure was varied from 400-1400 kPa in these tests. Pressures were generated using a pneumatic cylinder to apply a compression load to the heater. A load cell placed beneath the cooling plate was used to measure the actual load. A closed loop control system was used to maintain the desired pressure on the test specimen.

[0048] Test samples had a diameter of 5.2 cm. Heat flow was from an electric heater on top of the meter bars to a cooling plate below them. The nominal heater temperature was between 80-100 °C while the cooling plate was held at 1-2 °C below room temperature. This arrangement ensured one-dimensional heat flow through the test sample. Temperature measurements were made after the system reached thermal equilibrium, which took 20-30 minutes. Measurements were then made at 2 second intervals over a 5 minute period and averaged.

[0049] The relevant results of these test are shown in the graph of FIG. 7 and corresponding numerical results shown in the table of FIG. 8. The tested materials include: [0050] Sample 1A: a flexible graphite sheet formed of a graphitized polymer having a thickness of 184 microns, was embossed in accordance with the present disclosure and did not contain fluorinated synthetic oil shown at Sample 1A (control A with mechanical alternation only);

[0051] Sample 1B: a flexible graphite sheet formed of a graphitized polymer having a thickness of 152 microns, was embossed in accordance with the present disclosure and did not contain fluorinated synthetic oil shown at Sample 1B (control A with mechanical alternation only);

[0052] Sample 2: a flexible graphite sheet formed of a graphitized polymer having a thickness of 206 microns, which was not embossed and did not contain fluorinated synthetic oil shown at Sample 2 (control A);

[0053] Sample 3 : a flexible graphite sheet formed of a graphitized polymer having a thickness of 139 microns, which was embossed in accordance with the present disclosure and had a porosity of 7% and which did contain 2-75 parts by weight fluorinated synthetic oil (PFPE) based on 100 parts by weight of the flexible graphite sheet shown at Sample 3 (porosity was created by punching 1.6 mm holes uniformly distributed on the surface in a manner sufficient to obtain the 7% porosity);

[0054] Sample 4: an indium material having a thickness of 156 microns is shown at Sample

4 (best in class commercially available control with no mechanical alteration or heat transfer fluid);

[0055] Sample 5: a flexible graphite sheet formed of a graphitized polymer having a thickness of 154 microns, which was embossed in accordance with the present disclosure and which did contain 2-75 parts by weight fluorinated synthetic oil (PFPE) based on 100 parts by weight of the flexible graphite sheet shown at Sample 5;

[0056] Sample 6: a flexible graphite sheet formed of a graphitized polymer having a thickness of 147 microns, which was embossed in accordance with the present disclosure and which did contain 2-75 parts by weight fluorinated synthetic oil (PFPE) based on 100 parts by weight of the flexible graphite sheet shown at Sample 6;

[0057] Sample 7: a flexible graphite sheet formed of a graphitized polymer having a thickness of 170 microns, which was embossed in accordance with the present disclosure and which did contain 2-75 parts by weight fluorinated synthetic oil (PFPE) based on 100 parts by weight of the flexible graphite sheet shown at Sample 7; [0058] Sample 8: a comparable flexible graphite sheet formed of a graphitized polymer having a thickness of 225 microns commercially available as PGS from Panasonic, which was not embossed and which did not contain fluorinated synthetic oil shown at Sample 8;

[0059] Sample 9: a comparable flexible graphite sheet formed of compressed particles of exfoliated (AKA expanded) graphite sample having a thickness of 122 microns, which was not embossed and did contain 2-75 parts by weight fluorinated synthetic oil (PFPE) based on 100 parts by weight of the flexible graphite sheet shown at Sample 9 (this sample is the natural graphite equivalent of the polymerized graphite sheets of Samples 3 and 5-7);

[0060] Sample 10: a flexible graphite sheet formed of a graphitized polymer having a thickness of 143 microns, which was not embossed and had a porosity of 15% and which did not contain fluorinated synthetic oil shown at Sample 10 (control A with porosity that was created by punching 1.6 mm holes uniformly distributed on the surface in a manner sufficient to obtain the 15% porosity); and

[0061] Sample 11 : a flexible graphite sheet formed of a graphitized polymer having a thickness of 125 microns, which was not embossed and had a porosity of 6% and which did not contain fluorinated synthetic oil shown at Sample 11 (control A with porosity that was created by punching 1.6 mm holes uniformly distributed on the surface in a manner sufficient to obtain the 6% porosity).

[0062] Unless otherwise indicated, the same embossing was applied to all Samples in this

Example that were embossed.

[0063] The indium material shown at 4 in FIG. 7, which as discussed above is currently the best in class TIM, provides good thermal impedance at relatively high contact pressures, however it has an undesirable effect of cold soldering to materials at higher pressures such as those used in the tests described herein. As shown in FIG. 7, the thermal interface materials (and consequently the thermal interface assemblies and thermal management systems) in accordance with the present invention shown at 3 and 5-6 have substantially the same thermal impedance or lower than the thermal impedance as the best in class material indium shown at 4, which is a thermal impedance (Y) less than or equal to that defined by Y = 3.31(10 ⁸)C²-1.15(10 ⁴)C+0.136 when X is the contact pressure ranging from 400 kPa to 1400 kPa. Thermal interface material shown at 7 of the present appears to have a slightly higher thermal impedance over this contact pressure range than indium at 4, but as noted below, is still lower than a Control 2. These results are supported by the numerical results of these tests shown in the table of FIG.8.

[0064] Control A shown at 2 in FIG. 7, which is a flexible graphite sheet without mechanical alteration and the thermal transfer fluid (e.g., fluorinated synthetic oil), has a thermal impedance (Y) defined by Y = 1 02(l0 ⁷)X²-2.8(l0 ⁴)X+0.26 when X is a contact pressure ranging from 400 kPa to 1400 kPa. As shown in FIG. 7, the thermal interface materials (and consequently the thermal interface assemblies and thermal management systems) in accordance with the present invention shown as curves 3 and 5-7 have thermal impedance (Y) at least 10% lower than that defined by Y = l .02(l0 ⁷)X²-2.8(l0 ⁴)X+0.26 when X is a contact pressure ranging from 400 kPa to 1400 kPa (i.e., the thermal impedance of Sample 2). This is supported by the numerical results of these tests shown in the table of FIG.8.

[0065] Likewise, as shown in FIGS. 7-8, the thermal interface materials (and consequently the thermal interface assemblies and thermal management systems) in accordance with the present invention shown as curves 3 and 5-7 have respective thermal impedances much lower than those of control Samples 1A, 1B, and 8-11. Notably, control samples 1A, 1B, and 8-11 either have some form of mechanical alteration such as embossing or porosity but without the thermal transfer fluid (fluorinated synthetic oil) (Samples 1A, 1B, 8, and 10-11) while Sample 9 formed from compressed particles of expanded (exfoliated) graphite (not the graphitized polymer sheet of the present invention) and includes the thermal transfer fluid (fluorinated synthetic oil) but no mechanical alteration.

[0066] All cited patents and publications referred to in this application are incorporated by reference.

[0067] All such weights as they pertain to listed ingredients are based on the active level and, therefore, do not include solvents or by-products that may be included in commercially available materials, unless otherwise specified.

[0068] All references to singular characteristics or limitations of the present disclosure shall include the corresponding plural characteristic or limitation, and vice versa, unless otherwise specified or clearly implied to the contrary by the context in which the reference is made. Thus, in the present disclosure, the words“a” or“an” are to be taken to include both the singular and the plural. Conversely, any reference to plural items shall, where appropriate, include the singular. [0069] All combinations of method or process steps as used herein can be performed in any order, unless otherwise specified or clearly implied to the contrary by the context in which the referenced combination is made.

[0070] All ranges and parameters, including but not limited to percentages, parts, and ratios, disclosed herein are understood to encompass any and all sub-ranges assumed and subsumed therein, and every number between the endpoints. For example, a stated range of“1 to 10” should be considered to include any and all subranges between (and inclusive of) the minimum value of 1 and the maximum value of 10; that is, all subranges beginning with a minimum value of 1 or more ( e.g ., 1 to 6.1), and ending with a maximum value of 10 or less ( e.g ., 2.3 to 9.4, 3 to 8, 4 to 7), and finally to each number 1, 2, 3, 4, 5, 6, 7, 8, 9, and 10 contained within the range.

[0071] The system and method of the present disclosure can comprise, consist of, or consist essentially of the essential elements and limitations of the disclosure as described herein, as well as any additional or optional ingredients, components, or limitations described herein or otherwise useful in thermal interface materials, thermal interface assemblies, and/or thermal management systems.

[0072] To the extent that the terms“include,”“includes,” or“including” are used in the specification or the claims, they are intended to be inclusive in a manner similar to the term “comprising” as that term is interpreted when employed as a transitional word in a claim. Furthermore, to the extent that the term "or" is employed (e.g., A or B), it is intended to mean“A or B or both A and B.” When the Applicant intends to indicate“only A or B but not both,” then the term“only A or B but not both” will be employed. Thus, use of the term“or” herein is the inclusive, and not the exclusive use.

[0073] In some embodiments, it may be possible to utilize the various inventive concepts in combination with one another. Additionally, any particular element recited as relating to a particularly disclosed embodiment should be interpreted as available for use with all disclosed embodiments, unless incorporation of the particular element would be contradictory to the express terms of the embodiment. Additional advantages and modifications will be readily apparent to those skilled in the art. Therefore, the disclosure, in its broader aspects, is not limited to the specific details presented therein, the representative apparatus, or the illustrative examples shown and described. Accordingly, departures may be made from such details without departing from the spirit or scope of the general inventive concepts.

Claims

What is claimed is:

1. A thermal interface material comprising:

a flexible graphite sheet having a mechanical alteration; and

a heat transfer fluid incorporated in the flexible graphite sheet, wherein the heat transfer fluid has an operating temperature ranging from -40° C to 300° C and passes the UL94 V-0 flame test when used as a thermal interface material in a thermal interface assembly for a heat source,

wherein the thermal interface material has a thermal impedance (Y) at least 10% lower than that defined by Y = l.02(l0 ⁷)X²-2.8(l0 ⁴)X+0.26 when X is a contact pressure ranging from 400 kPa to 1400 kPa.

2. The thermal interface material of claim 1, wherein the thermal interface material has a thermal impedance (Y) less than or equal to that defined by Y = 3.31(10 ⁸)C² - 1.15(10 ⁴)C+0.136 when X is the contact pressure ranging from 400 kPa to 1400 kPa.

3. The thermal interface material of claim 1 or 2, wherein the heat transfer fluid is a perfluorinated synthetic oil.

4. The thermal interface material of claim 3, wherein the perfluorinated synthetic oil comprises perfluoropolyether.

5. The thermal interface material of any of claims 1-4, wherein the heat transfer fluid has an operating temperature ranging from -40° C to 120° C

6. The thermal interface material of any of claims 1-5, wherein the flexible graphite sheet has a thickness of up to about 500 microns.

7. The thermal interface material of any of claims 1-6, wherein the flexible graphite sheet has a thickness of from about 5 microns to about 300 microns.

8. The thermal interface material any of claims 1-7, wherein the thermal interface material comprises about 2 to about 75 parts by weight heat transfer fluid based on 100 parts by weight of flexible graphite sheet.

9. The thermal interface material any of claims 1-8, wherein the thermal interface material comprises about 2 to about 60 parts by weight heat transfer fluid based on 100 parts by weight of flexible graphite sheet.

10. The thermal interface material any of claims 1-9, wherein the mechanical alternation includes at least one of embossing, altering the porosity, perforating, and

combinations thereof.

11. A thermal interface assembly for a heat source comprising a thermal interface material, the thermal interface material comprising:

a flexible graphite sheet having a mechanical alteration; and

a heat transfer fluid incorporated in the flexible graphite sheet, wherein the heat transfer fluid has an operating temperature ranging from -40° C to 300° C and passes the UL94 V-0 flame test when used as the thermal interface material in the thermal interface assembly for a heat source,

12. The thermal interface assembly of claim 11, wherein the thermal interface material has a thermal impedance (Y) less than or equal to that defined by Y = 3.31(10 ⁸)C²- 1.15(10 ⁴)C+0.136 when X is a contact pressure ranging from 400 kPa to 1400 kPa.

13. The thermal interface assembly of claim 11 or 12, wherein the heat transfer fluid is a perfluorinated synthetic oil.

14. The thermal interface assembly of claim 13, wherein the perfluorinated synthetic oil comprises perfluoropolyether.

15. The thermal interface assembly of any of claims 11-14, wherein the heat transfer fluid has an operating temperature ranging from -40° C to 120° C

16. The thermal interface assembly of any of claims 11-15, wherein the flexible graphite sheet has a thickness of up to about 500 microns.

17. The thermal interface assembly of any of claims 11-16, wherein the thermal interface material comprises about 2 to about 75 parts by weight heat transfer fluid based on 100 parts by weight of flexible graphite sheet.

18. The thermal interface assembly of any of claims 11-17 further comprising at least one of a coating layer, an adhesive layer, and combinations thereof.

19. The thermal interface assembly any of claims 11-18, wherein the mechanical alternation includes at least one of embossing, altering the porosity, perforating, and

combinations thereof.

20. A thermal management system for moving heat away from a heat source having an external surface, the thermal management system comprising:

a heat dissipative member having an external surface; and

a thermal interface assembly having a first surface and a second surface disposed opposite the first surface, the thermal interface assembly comprising a thermal interface material, the thermal interface material comprising:

a flexible graphite sheet having a mechanical alteration, and

a heat transfer fluid incorporated in the flexible graphite sheet, wherein the heat transfer fluid has an operating temperature ranging from -40° C to 300° C and passes the UL94 V-0 flame test when used as a thermal interface material in a thermal interface for a heat source, wherein the thermal interface material has a thermal impedance (Y) at least 10% lower than that defined by Y = l .02(l0 ⁷)X²-2.8(l0 ⁴)X+0.26 when X is a contact pressure ranging from 400 kPa to 1400 kPa,

wherein the first surface of the thermal interface assembly is disposed adjacent to and in thermal communication with the external surface of the heat source, and

wherein the second surface of the thermal interface assembly is disposed adjacent to and in thermal communication with the external surface of the heat dissipative member.

21. The thermal management system of claim 20, wherein the planar surface area of the first surface of the thermal interface is substantially the same as the surface area of the external surface of the heat source.

22. The thermal management system of claim 20, wherein the planar surface area of the first surface of the thermal interface is greater than the surface area of the external surface of the heat source.

23. The thermal management system of any of claims 20-22, wherein the mechanical alternation includes at least one of embossing, altering the porosity, perforating, and

combinations thereof.

24. The thermal management system of any of claims 20-23, wherein the contact pressure of the external surface of the heat dissipative member against the second surface of the thermal interface assembly comprises about 50 kPa to about 1500 kPa.

25. The thermal management system of any of claims 20-24, wherein the thermal interface material has a thermal impedance (Y) less than or equal to that defined by

Y = 3.31(10 ⁸)C²-1.15(10 ⁴)C+0.136 when X is a contact pressure ranging from 400 kPa to 1400 kPa.