Heat sink with expanded graphite wick

Abstract

A heat sink includes a baseplate of thermally-conductive material defining a lower surface for conducting heat from a heat source, and an upper surface opposite of the lower surface. The heat sink also includes a radiator and a heat spreader. The radiator is disposed upon the baseplate away from the lower surface and includes a housing enclosing a chamber. The heat spreader includes expanded graphite disposed within the chamber and coating the upper surface of the baseplate. The heat spreader also includes a binder of Styrene-Butadiene Rubber (SBR) for holding particles of the expanded graphite into a solid structure. A phase-change material (PCM) is disposed within the chamber and flowable through the heat spreader. The PCM is changeable between a liquid and a vapor to convey heat from the baseplate to the housing of the radiator.

Description

HEAT SINK WITH EXPANDED GRAPHITE WICKCROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This PCT International Patent Application claims the benefit of and priority to U.S. Provisional Patent Application Serial No. 63 / 615,114, filed December 27, 2023, titled “Heat Sink With Expanded Graphite Wick,” and U.S. Provisional Patent Application Serial No. 63 / 647.641, filed May 15, 2024. titled “Heat Sink With Expanded Graphite Wick.” the entire disclosures of which are hereby incorporated by reference in their entirety.FIELD

[0002] The present disclosure relates generally to a heat sink for conveying heat from a baseplate to a cover. More specifically, it relates to a two-phase heat sink having a wick that includes expanded graphite material.BACKGROUND

[0003] Heat sinks are used to convey heat away from a heat source, such as an electronic device, to prevent the heat source and / or other components from being damaged due to excessive temperatures. One type of heat sink that is conventionally known is a heat pipe, which uses a refrigerant fluid that changes between two different phases, from a liquid to a gas at an evaporator to transmit heat from the heat source to a condenser, where heat exits as the refrigerant fluid condenses back to a liquid. Some heat pipes use a wick to transfer the condensed refrigerant from the condenser back to the evaporator.SUMMARY

[0004] The present disclosure provides a heat sink that includes: a baseplate defining an upper surface; a housing enclosing a chamber extending from the upper surface of the baseplate; and a heat spreader including expanded graphite disposed within the chamber and coating at least one of: the upper surface of the baseplate, or a heat source within the chamber.

[0005] The present disclosure also provides a heat sink that includes: a housing defining a chamber; a heat source located within the chamber; and a heat spreader including expanded graphite disposed within the chamber and coating the heat source within the chamber.

[0006] A method of forming a heat sink is also provided. The method includes: forming a slum of expanded graphite (EG) particles and a binder; spreading the slum- onto a substrate; and curing the binder by heating the slurry in an oven, thereby forming a heat spreader.BRIEF DESCRIPTION OF THE DRAWINGS

[0007] Further details, features and advantages of designs of the invention result from the following description of embodiment examples in reference to the associated drawings.

[0008] FIG. 1 shows a cut-away side view of a first heat sink according to some embodiments of the present disclosure;

[0009] FIG. 2 shows a cut-away side view of a second heat sink according to some embodiments of the present disclosure; and

[0010] FIG. 3 shows a flow chart listing steps in a method of forming a heat sink, in accordance with an aspect of the present disclosure.DETAILED DESCRIPTION

[0011] Recurring features are marked with identical reference numerals in the figures, in which example embodiments of a heat sink are disclosed. The heat sink of the present disclosure includes expanded graphite (EG), which can provide several advantages over alternative materials. EG has excellent wicking properties, providing enhanced fluid flow therethrough. It is relatively lightweight, having less density than metals. Thus, use of an EG material can reduce the overall module weight. EG particles possess a porous architecture. EG foam exhibits plenty of micro & macro pores, resulting in capillary action. Porous EGallows the PCM to directly touch the backing-plate, not indirectly contact via the intervening metallic wick. EG provides enhanced Corrosion Resistance. Unlike non-corrosive EG, metals are much more susceptible to corrosion. EG is economical and can be sourced using relatively inexpensive materials and a simple fabrication process. In some embodiments, the EG heat spreader can be manufactured without 3D printing, which can add cost and complexity for manufacturing.

[0012] FIG. 1 shows a cut-away side view of the first heat sink 20 that includes a first baseplate 22 of thermally-conductive material for conducting heat from a first heat source 10. The first baseplate 22 is shaped as a flat plate extending between a first lower surface 24 and a first upper surface 25. The first lower surface 24 of the first baseplate 22 is configured to be in thermal communication with the first heat source 10, such as an integrated circuit or a power electronic device. The first heat sink 20 also includes a first radiator 26 disposed upon the first upper surface 25 of the first baseplate 22, away from the first lower surface 24 for transferring heat to atmosphere, such as air or liquid that surrounds the first radiator 26. The first radiator 26 may transfer heat to the atmosphere by any means such as radiation, conduction, and / or convection. The first radiator 26 includes a first housing 32 of solid material, such as cast metal, which encloses a first chamber 36. The first housing 32 includes a first cover 30 disposed parallel to and spaced apart from the first baseplate 22, with the first chamber 36 extending therebetween.

[0013] The first heat sink 20 also includes a first heat spreader 38 that includes expanded graphite (EG) and which is disposed within the first chamber 36 and coating the first upper surface 25 of the first baseplate 22. The first heat spreader 38 is permeable to liquid, allowing liquid and / or gases to flow therethrough with relatively low restrictions to flow.

[0014] In some embodiments, the first heat spreader 38 also includes a binder for holding particles of the expanded graphite into a solid structure. The binder may include Styrene-Butadiene Rubber (SBR). However, other materials may be used. The first heat spreader 38 is permeable to liquid flow, allowing a liquid or a gas to pass therethrough. The first heat spreader 38 may function as a porous wick, promoting capillary7action to convey liquid therethrough.

[0015] In some embodiments, and as shown in FIG. 1, a phase-change material (PCM) 50, such as a refrigerant, is disposed within the first chamber 36. In some embodiments, the PCM may include a polar fluid. In some embodiments, the EG may be made hydrophilic, for example by treatment using a wetting agent, such as Triton X-100, to allow or enhance usage of polar fluids as the PCM.

[0016] The PCM 50 may be free to flow7through the first heat spreader 38. The first heat spreader 38 may hold the PCM 50 near the first housing 32, thereby improving the ability of the first heat sink 20 to dissipate heat. The PCM 50 may boil, or change between a liquid 52 and a vapor 54 to convey heat from the first baseplate 22 to the first cover 30. For example, the PCM 50 may boil from a first region 56 adjacent to the first baseplate 22 and proximate to the first heat source 10, and travel in the form of the vapor 54 to a second region 58 proximate to the first cover 30. At the second region 58, the PCM 50 may condense back to the liquid 52. The PCM 50, in the form of the liquid 52, may be conveyed through the first heat spreader 38 and back to the first region 56 proximate to the first heat source 10 by capillary7action.

[0017] In some embodiments, and as shown in FIG. 1. the first radiator 26 includes a plurality of first fins 60 disposed on the first housing 32. extending away from the first baseplate 22. More specifically, the first cover 30 may extend in a generally flat plane, with the plurality of first fins 60 extending generally transversely to the generally flat plane. Thefirst cover 30 could define one or more curved surfaces, which may or may not include the first fins 60 extending therefrom. The first fins 60 may be formed as pillars or posts. Alternatively or additionally, the first fins 60 may be formed as ribs that extend for a substantial length along the first cover 30. The first fins 60 may function to increase the surface area of the first housing 32 to promote heat transfer to a fluid, such as a gas or a liquid, contacting an outer surface of the first housing 32 opposite the first chamber 36.

[0018] In some embodiments, and as shown in FIG. 1, the first fins 60 are solid. Alternatively or additionally, some or all of the first fins 60 may be hollow and / or filled with a permeable material, which may be in fluid communication with the first chamber 36. In this way, the PCM 50, in the form of the vapor 54, can travel into the first fins 60 to reach the second region 58, which is sufficiently cold to cause the vapor 54 to condense back to the liquid 52.

[0019] In some embodiments, one or more parameters, such as EG to binder ratio, EG particle size, and / or compaction-induced packing density can be tuned to optimize the synergistic-relation of thermal conductivity and capillary action for maximum heat spreading effects.

[0020] FIG. 2 shows a cut-away side view of a second heat sink 120 according to some embodiments of the present disclosure. The second heat sink 120 may be similar or identical to the first heat sink 20 except for differences described herein. The second heat sink 120 includes a second baseplate 122 and a second housing 132 of solid material, such as cast metal, which encloses a second chamber 136. The second housing 132 includes a second cover 130 disposed parallel to and spaced apart from the second baseplate 122, with the second chamber 136 extending therebetween.

[0021] The second heat sink 120 includes a second heat source 110, such as an integrated circuit or a power electronic device, located within the second chamber 136 andsubmerged in the PCM 50. In some embodiments, a conformal coating 112, such as a dielectric material, coats the second heat source 110. The conformal coating 112 may prevent the PCM 50 and / or other contaminants from contacting the second heat source 110. The second heat source 110 may be located on or within a printed circuit board (PCB) 114 that is mounted to the second baseplate 122 and within the second chamber 136. In some embodiments, the conformal coating 112 may coat the entire PCB 114.

[0022] The second heat sink 120 also includes a second heat spreader 138 that includes expanded graphite (EG) and which is disposed within the second chamber 136 and coating the second heat source 110. In some embodiments, and as shown on FIG. 2, the second heat spreader 138 coats the entire PCB 114. The second heat spreader 138 is permeable to liquid, allowing liquid and / or gases to flow therethrough with relatively low restrictions to flow. Thus, the second heat spreader 138 may function as a wick for conveying fluid such as the PCM 50.

[0023] The second heat spreader 138 including a blanket of EG foam over the PCB 114 can also serve the dual functionality of protecting delicate electronic components and stabilizing them from shocks and vibrations, providing efficient performance and longer lifetimes.

[0024] In some embodiments, the second heat spreader 138 also includes a binder for holding particles of the expanded graphite into a solid structure. The binder may include Styrene-Butadiene Rubber (SBR). However, other materials may be used. The second heat spreader 138 is permeable to liquid flow, allowing a liquid or a gas to pass therethrough. The second heat spreader 138 may function as a porous wick, promoting capillary action to convey liquid therethrough.

[0025] In some embodiments, the EG may be made hydrophilic, for example by treatment using a wetting agent, such as Triton X-100, to allow' or enhance usage of polar fluids as the PCM.

[0026] The PCM 50 may be free to flow through the second heat spreader 138. The second heat spreader 138 may hold the PCM 50 near the PCB 114 and / or the second housing 132, thereby improving the ability of the second heat sink 120 to dissipate heat. The PCM 50 may boil, or change between a liquid 52 and a vapor 54 to convey heat from the second baseplate 122 to the second cover 130. For example, the PCM 50 may boil from a third region 156 adjacent to the second heat source 110, and travel in the form of the vapor 54 to a fourth region 158 proximate to the second cover 130. At the fourth region 158, the PCM 50 may condense back to the liquid 52. The PCM 50, in the form of the liquid 52, may be conveyed through the second heat spreader 138 and back toward the second heat source 110 by capillary action.

[0027] In some embodiments, and as shown in FIG. 2, the second radiator 126 includes a plurality of second fins 160 extending away from the second baseplate 122. More specifically, the second cover 130 may extend in a generally flat plane, with the plurality’ of second fins 160 extending generally transversely to the generally flat plane. The second cover 130 could define one or more curved surfaces, which may or may not include the second fins 160 extending therefrom. The second fins 160 may be formed as pillars or posts. Alternatively or additionally, the second fins 160 may be formed as ribs that extend for a substantial length along the second cover 130. The second fins 160 may function to increase the surface area of the second housing 132 to promote heat transfer to a fluid, such as a gas or a liquid, contacting an outer surface of the second housing 132 opposite the second chamber 136.

[0028] In some embodiments, and as shown in FIG. 2. the second fins 160 are solid. Alternatively or additionally, some or all of the second fins 160 may be hollow and / or filledwith a permeable material, which may be in fluid communication with the second chamber 136. In this way, the PCM 50, in the form of the vapor 54, can travel into the second fins 160 to reach the second region 58, which is sufficiently cold to cause the vapor 54 to condense back to the liquid 52.

[0029] The second heat sink 120 may include Expanded Graphite coating directly deposited oved the PCB 114 covering hot-spots like LEDs, integrated circuits, and power regulating and control devices. The conformal coating 112 may include a dielectric protective coating capable of withstanding extreme temperatures and chemical environments. The second heat sink 120 may include a non-polar, dielectric, 2-phase heat transfer fluid susceptible to wicking action of the porous EG structure. The second heat sink 120 may provide heat regulation via vaporization of the PCM 50 over one or more hot-spots, dumping heat via condensation of PCM over the second fins 160 and process automation via recirculation of the PCM 50 through the second heat spreader 138.

[0030] A predetermined mass of slurry may be deposited directly over the PCB 114, hot-chips or any electronic component in need of thermal regulation. Aided by agitation, allow the DI water to uniformly spread out the EG particles in the slurry. Due to the non- uniform nature of the substrate, it may be necessary to bolster the non-uniform regions like the step features or other irregular geometries with additional slurry.

[0031] In some embodiments, the conformal coating 112 may include a pinhole and defect-free Parylene-based dielectric coating over PCB / electronics with excellent moisture barrier and chemical resistance, high dielectric strength and capacity to uniformly complex geometries without affecting the functionality of delicate electronic components. The conformal coating 112 may include, for example, a commercially available coating material, such as the guardian series coating by HZO, Inc.

[0032] An additional advantage of this design can come from increased contact areas between electronics and cooling media. In standard designs the thermally relevant contact area occurs where the electronic component is linked to the baseplate / PCB. The complete free surface of the electronic component is thermally relevant in the second heat sink 120 of the present disclosure.

[0033] Highly porous architecture of EG coating combined with the inherent affinity of graphite to non-polar fluids may provide exceptional capillary forces and wicking action on organic phase change materials (PCMs) like silicone oils, mineral oils, alkanes and synthetic dielectrics. Suitable choice of PCMs based on boiling point, enthalpy of vaporization and compatibility' with electronics. The PCM 50 may include one or more commercially available products, such as Polydimethylsiloxane Fluid / PDMS Silicone Oil, which is sold by Clearco Products as “Low Temperature Silicone Heat Transfer Fluids”, or 72052 EDM-30 synthetic dielectric by Rustlick.

[0034] The heat sinks 20, 120 of the present disclosure provide 2-phase heat transfer due to the liquid-to-gas transition of the PCM 50, along with the automatic capillary-based recirculation of the condensed vapor working in tandem for passive thermal management of hot-spots and electronics.

[0035] A method 300 of forming a heat sink 20, 120 is shown in the flow7chart of FIG. 3. As can be appreciated in light of the disclosure, the order of operation within the method is not limited to the sequential execution as illustrated in FIG. 3, but may be performed in one or more varying orders as applicable and in accordance with the present disclosure.

[0036] The method 300 includes forming, at step 302. a slurry of expanded graphite (EG) particles and a binder. The EG particles may be EG- 1520 material. However, other grades and / or types of EG particles may be used. The binder may include, for example,styrene-butadiene rubber (SBR). However, the binder may include other materials. The EG and binder may be combined in a 1:1 weight ratio. However, other ratios may be used. A dilutant, such as de-ionized (DI) water may also be used to form the slurry.

[0037] In some embodiments, the slurry may be heated in an oven at 70 C for 12 hours to evaporate off the DI water and cure the polymeric SBR binder without affecting the electronics. However, a different temperature and / or a different heating time may be used. The SBR may function to both join the EG particles firmly with each other but also to a substrate underneath. This may allow for direct contact of the thermal management material with the heated source without an intervening laminate (like circuit board, baseplate, etc.) inbetween or requiring any thermal paste. Irregular geometries may also be coated.

[0038] The method 300 also includes spreading, at step 304, the slurry onto a substrate. Step 304 may include spreading the slurry directly onto the substrate at a predefined thickness. The substrate may include a baseplate of the heatsink. Alternatively or additionally, the substrate may include a PCB and / or one or more heat sources located within a chamber of the heat sink. High capillary' forces for non-polar PCMs and low contact resistance with the heat source make the resulting structure especially well-suited for 2-phase heat spreading and exchange.

[0039] The method 300 also includes curing, at step 306, the binder by heating the slurry in an oven, and thereby forming a heat spreader. Step 306 may include operating the oven at a temperature of at least about 80 degrees Celsius. In some embodiments, Step 306 may include operating the oven at a temperature of at least about 90 degrees Celsius. However, other temperatures may be used, which may depend on a length of time and / or a particular material used for the binder. The slurry may be cured in the oven for a period of 3 hours. However, step 306 may be performed over a shorter or a longer period of time. Curing the binder may simultaneously form the heat spreader while causing the heat spreader to befirmly atached to the baseplate in thermal contact therewith. Curing the binder to adhere the head spreader directly to the baseplate may also be achieved without requiring any additional interface material, such as adhesive or solder between the heat spreader and the baseplate.

[0040] In some embodiments, the method 300 further includes calendering, at step 308, the heat spreader between a plurality of rollers and to a uniform thickness. The calendering may include forming the heat spreader to the uniform thickness of about 1.0 mm or a thickness of at least about 2 mm.

[0041] In some embodiments, the method 300 further includes treating, at step 310, the EG with a weting agent. The weting agent may include a non-toxic weting agent, such as Triton X-100. Treating the EG with the weting agent may make the EG hydrophilic, and may allow or enhance usage of polar fluids as the PCM.

[0042] The foregoing description of the embodiments has been provided for purposes of illustration and description. It is not intended to be exhaustive or to limit the disclosure. Individual elements or features of a particular embodiment are generally not limited to that particular embodiment, but, where applicable, are interchangeable and can be used in a selected embodiment, even if not specifically shown or described. The same may also be varied in many ways. Such variations are not to be regarded as a departure from the disclosure, and all such modifications are intended to be included within the scope of the disclosure.

Claims

CLAIMSWhat is claimed is:

1. A heat sink, comprising: a baseplate defining an upper surface; a housing enclosing a chamber extending from the upper surface of the baseplate; and a heat spreader including expanded graphite disposed within the chamber and coating at least one of: the upper surface of the baseplate, or a heat source within the chamber.

2. The heat sink of Claim 1, wherein the baseplate is made of a thermally - conductive material defining a lower surface opposite the upper surface for conducting heat from a heat source therebelow, and wherein the heat spreader coats the upper surface of the baseplate.

3. The heat sink of Claim 1 , wherein the heat spreader coats a heat source within the chamber.

4. The heat sink of Claim 3, wherein the heat source within the chamber includes at least one of an integrated circuit or a power electronic device.

5. The heat sink of Claim 1 , wherein the heat spreader further comprises a binder for holding particles of the expanded graphite into a solid structure.

6. The heat sink of Claim 5, wherein the binder includes Styrene-Butadiene Rubber (SBR).

7. The heat sink of Claim 5. wherein the heat spreader is directly bonded onto the upper surface of the baseplate by the binder.

8. The heat sink of Claim 1, wherein the heat spreader is attached by a thermally - conductive material to the at least one of the upper surface of the baseplate or the heat source within the chamber.

9. The heat sink of Claim 1, further comprising a phase-change material (PCM) disposed within the chamber and flowable through the heat spreader, wherein the PCM is changeable between a liquid and a vapor to convey heat from the at least one of the upper surface of the baseplate or the heat source within the chamber and to the housing.

10. The heat sink of Claim 1, further including a plurality of fins disposed on the housing and extending away from the baseplate.11 . The heat sink of Claim 10, wherein the chamber extends into the plurality of fins.

12. A method of forming a heat sink, comprising: forming a slurry of expanded graphite (EG) particles and a binder; spreading the slurry onto a substrate; and curing the binder by heating the slurry in an oven, and thereby forming a heat spreader.

13. The method of Claim 12, wherein the substrate includes a baseplate of the heat sink, and wherein curing the binder further bonds the EG particles to the baseplate.

14. The method of Claim 12, further including calendering the heat spreader between a plurality of rollers and to a uniform thickness.

15. The method of Claim 12, further including treating the EG with a wetting agent.

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