Thermal diffusion of clamp-annealed pyrolytic graphite.

Abstract

A heat spreader is provided that includes a core of an orthotropic material having a first preferred direction of heat conduction and a second preferred direction of heat conduction, a top plate, a bottom plate, and a pyrolytic graphite sheet (PGS) interposed in compression between the core and the top plate and between the core and the bottom plate as a thermal interface material (TIM) with a compression direction aligned with at least one of the first preferred direction of heat conduction and the second preferred direction of heat conduction.

Description

[Technical Field]

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS This application claims the benefit of U.S. Patent Application No. 17 / 991,138, filed November 21, 2022, the entire contents of which are incorporated herein by reference. [Background technology]

[0002] FIELD OF THE DISCLOSURE The present disclosure relates to heat spreading, and more particularly to a clamp-on annealed pyrolytic graphite heat spreader.

[0003] A heat spreader is an assembly used to transfer heat generated by an electronic device or module in a first direction away from the electronic device or module and spread the heat in a second direction. In some cases, the heat spreader includes a module, a cold plate or heat exchanger, and a heat spreading element interposed between the module and the cold plate. The heat spreading element transfers heat generated by the module to the cold plate, spreading the heat in the process.

[0004] In conventional heat spreaders, the heat spreading element is provided by a monolithic copper spreader. These monolithic copper spreaders tend to be heavy. Therefore, conventional heat spreaders that include a monolithic copper spreader also tend to be heavy. Summary of the Invention

[0005] According to an aspect of the present disclosure, a heat spreader is provided that includes a core of an orthotropic material having a first preferred direction of thermal conduction and a second preferred direction of thermal conduction, a top plate, a bottom plate, and a pyrolytic graphite sheet (PGS) interposed in compression between the core and the top plate and between the core and the bottom plate as a thermal interface material (TIM) with a compressive orientation aligned with at least one of the first preferred direction of thermal conduction and the second preferred direction of thermal conduction.

[0006] According to additional or alternative embodiments, the orthotropic material comprises annealed pyrolytic graphite (APG).

[0007] According to additional or alternative embodiments, the first preferred direction of heat conduction and the second preferred direction of heat conduction are in-plane directions of the core.

[0008] According to additional or alternative embodiments, the top plate and the bottom plate are aligned in a through-thickness direction and the core is rotated such that at least one of the first preferred direction of heat conduction and the second preferred direction of heat conduction is aligned with the through-thickness direction.

[0009] According to additional or alternative embodiments, the PGS comprises compressible PGS that, in a compressed state, provides in-plane thermal conductivity and coefficient of thermal expansion (CTE) matching of the core and the top and bottom plates.

[0010] According to additional or alternative embodiments, the compressed state of the PGS is characterized in that the PGS is compressed to a predetermined degree of compression, and the heat spreader further includes a stopper configured to stop further compression of the PGS in the compression direction beyond the predetermined degree of compression.

[0011] According to additional or alternative embodiments, the stopper includes a pedestal extending upwardly from the bottom plate and toward the top plate, and the heat spreader further comprises a threaded screw extending through the top plate to threadably engage the pedestal to retract the top plate toward the bottom plate until the top plate impacts the pedestal.

[0012] According to additional or alternative embodiments, the threaded screws include countersunk screws and the top plate is formed to define countersunk holes through which the countersunk screws extend.

[0013] According to additional or alternative embodiments, the heat spreader further includes an additional PGS lining the stopper.

[0014] According to an aspect of the present disclosure, a heat spreader is provided, comprising a core of an orthotropic material having a first preferred direction of thermal conduction and a second preferred direction of thermal conduction, an encapsulant, and a pyrolytic graphite sheet (PGS), the PGS being compressed and interposed between the core and the encapsulant as a thermal interface material (TIM) with at least a compressive direction aligned with at least one of the first preferred direction of thermal conduction and the second preferred direction of thermal conduction.

[0015] According to additional or alternative embodiments, the orthotropic material comprises annealed pyrolytic graphite (APG).

[0016] According to additional or alternative embodiments, the first preferred direction of heat conduction and the second preferred direction of heat conduction are in-plane directions of the core.

[0017] According to additional or alternative embodiments, the encapsulant has an overall through-thickness direction, and the core is rotated such that at least one of the first preferred direction of heat conduction and the second preferred direction of heat conduction is aligned with the through-thickness direction.

[0018] According to additional or alternative embodiments, the PGS comprises a compressible PGS that, in a compressed state, provides in-plane thermal conductivity and coefficient of thermal expansion (CTE) matching of the core and encapsulant.

[0019] According to additional or alternative embodiments, the compressed state of the PGS is characterized in that the PGS is compressed to a predetermined degree of compression, and the heat spreader further includes a stopper configured to stop further compression of the PGS in the compression direction beyond the predetermined degree of compression.

[0020] According to additional or alternative embodiments, the stopper includes a pedestal extending upwardly from the bottom plate of the encapsulant and extending toward the top plate of the encapsulant, and the heat spreader further includes a threaded screw extending through the top plate to threadably engage the pedestal to retract the top plate toward the bottom plate until the top plate impacts the pedestal.

[0021] According to additional or alternative embodiments, the threaded screws include countersunk screws and the top plate is formed to define countersunk holes through which the countersunk screws extend.

[0022] According to additional or alternative embodiments, the heat spreader further includes an additional PGS lining the stopper.

[0023] According to an aspect of the present disclosure, there is provided a method for assembling a heat spreader, the method including providing a core of orthotropic material having a first preferred direction of heat conduction and a second preferred direction of heat conduction, disposing the core between a top plate and a bottom plate in a compression orientation aligned with at least one of the first preferred direction of heat conduction and the second preferred direction of heat conduction, interposing a pyrolytic graphite sheet (PGS) between the core and the top plate and between the core and the bottom plate in the compression orientation, and compressing the PGS into a compressed state such that the PGS provides a thermal interface material (TIM) between the core and the top plate and between the core and the bottom plate.

[0024] According to further or alternative embodiments, the PGS comprises a compressible PGS that, in a compressed state, provides in-plane thermal conductivity and coefficient of thermal expansion (CTE) matching between the core and the top and bottom plates, the compressed state of the PGS being characterized by the PGS being compressed to a predetermined degree of compression, and the method further comprising stopping further compression of the PGS in the compression direction beyond the predetermined degree of compression.

[0025] Further features and advantages are realized through the techniques of the present disclosure. Other embodiments and aspects of the present disclosure are described in detail herein and are considered part of the claimed technical concept. For a better understanding of the present disclosure, together with its advantages and features, please refer to the description and drawings.

[0026] For a more complete understanding of the present disclosure, reference is now made to the following brief description taken in conjunction with the accompanying drawings and detailed description, wherein like reference characters represent like parts, and wherein: [Brief explanation of the drawings]

[0027] [Figure 1] 1 is a schematic side view of a heat spreading element, according to an embodiment. [Figure 2] 1 is a schematic side view of a heat spreader with a heat spreading element, according to an embodiment. [Figure 3] 3 is a graphical representation of an assembly of the heat spreader and heat spreading element of FIG. 2, according to an embodiment. [Figure 4] 1 is a schematic side view of a heat spreader with a heat spreading element and a cross section, according to an embodiment. [Figure 5] 1 is a schematic side view of a heat spreader with a heat spreading element, according to an embodiment. [Figure 6] 1 is a schematic side view of a heat spreader with a heat spreading element, according to an embodiment. [Figure 7] 1 is a schematic side view of a heat spreader with a heat spreading element, according to an embodiment. [Figure 8] 1 is a flow chart illustrating a method for assembling heat spreading, according to an embodiment. [Figure 9] FIG. 1 is a perspective view of annealed pyrolytic graphite (APG) according to an embodiment of the present invention. [Figure 10] 10 is a side view of an unassembled heat spreader including the APG of FIG. 9, according to an embodiment. [Figure 11] 10 is a side view of an assembled heat spreader including the APG of FIG. 9, according to an embodiment. [Figure 12] 10 is a side view of a heat spreader including the APG and PGS lining stopper of FIG. 9 in an assembled state, according to an embodiment. [Figure 13] 1 is a flow chart illustrating a method for assembling a heat spreader including an APG, according to an embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0028] As described below, pyrolytic graphite sheet (PGS) material is used as a thermal interface material (TIM) between a bulk annealed pyrolytic graphite (APG) spreader and the encapsulant. This allows for an optimized APG preferred orientation that balances compression and CTE mismatch while using a bolted joint design. Using PGS material as a TIM reduces manufacturing complexity by avoiding hot isostatic pressing (HIP) operations or active solder welding processes, enabling the expanded use of high thermal conductivity APG in product lines. The resulting structure uses one or more continuous or discontinuous PGS to join the bulk pyrolytic graphite (TPG) of the heat spreader with aluminum top and bottom clamp plates that provide encapsulation. The APG orientation is optimized for heat transfer between the source and sink and diffusion in the bulk material through two preferred directions of the spreader: through the thickness and in one plane. The preferred orientation of the PGS is in the plane of the spreader, a design goal that provides some spreading relief in the unpreferred orientation of the TPG. Tight tolerances on the wall and pedestal geometry control compression of the PGS, and the encapsulated APG is sealed from the environment to protect it from corrosion.

[0029] Referring to FIG. 1 , a heat spreading element 101 is provided, including a compressible PGS 110 and a PGS 120. The rigid PGS 120 are alternately arranged with the compressible PGS 110. At least one of the compressible PGS 110 and the rigid PGS 120 exhibits an in-plane thermal conductivity of greater than approximately 1000 W / mK. The compressible PGS 110 and the rigid PGS 120 have a much lower density than a metallic material, such as copper. For example, the density of the rigid PGS 120 may be less than approximately 10-15% of the density of the metallic material. The density of the compressible PGS 110 may be less than approximately 10-15%, or in some cases, less than approximately 5%, of the density of the metallic material. The compressible PGS 110 and the rigid PGS 120 may be compressed together in an interleaving direction A, for example, by a clamp 130. This compression effectively activates the in-plane thermal conductivity of the compressible PGS 110 and the rigid PGS 120, allowing the heat spreading element 101 to transfer heat in a first direction (i.e., interleaving direction A) and spread heat in a second direction (i.e., in-plane direction B).

[0030] According to an embodiment, the rigid PGS 120 may have a density of about 1200-1300 kg / m 3 , and the compressible PGS 110 may have a sponge-like nature with a density of about 400-500 kg / m 3 .

[0031] Referring to FIG. 2 , a module 210 including an electronic device 211 that generates heat during operation and includes a heat spreader 201, a heat exchanger or cold plate (hereinafter referred to as a “cold plate”) 220 configured to absorb and dissipate the heat generated by the electronic device 211, a heat spreading element 230, and a clamp 240. The heat spreading element 230 includes a compressible PGS 231 and a rigid PGS 232 interleaved with the compressible PGS 231. At least one of the compressible PGS 231 and the rigid PGS 232 exhibits an in-plane thermal conductivity greater than approximately 1000 W / mK. The compressible PGS 231 and the rigid PGS 232 have much lower densities than metallic materials such as copper. For example, the density of the rigid PGS 120 can be approximately 10% to 15% or less of the density of the metallic material. The density of the compressible PGS 110 can be approximately 10% to 15% or less, and in some cases, less than approximately 5% of the density of the metallic material.

[0032] As mentioned above, according to an embodiment, the rigid PGS 232 may have a density of about 1200-1300 kg / m 3 , and the compressible PGS 231 may have a sponge-like property with a density of about 400-500 kg / m 3 .

[0033] The heat spreading element 230 is interposed between the module 210 and the cold plate 220 and transfers heat generated by the electronic device 211 from the module 210 to the cold plate 220 in a first direction (i.e., an interleaving direction A relative to the interleaving direction of the compressible PGS 231 and the rigid PGS 232). The heat spreading element 230 also spreads heat in a second direction transverse to the first direction (i.e., an in-plane direction B of the compressible PGS 231 and the rigid PGS 232). The clamp 240 can include a screw 241 that can engage with at least the cold plate 220 to draw the cold plate 220 toward the module 210. The clamp 240 can alternatively include or be provided with various other configurations that serve to draw the cold plate 220 toward the module 210. In either case, the clamp 240 serves to sandwich the heat spreading element 230 between the module 210 and the cold plate 220 and compress the compressible PGS 231 and the rigid PGS 232 in a first or interleaving direction A. (Although FIG. 3 illustratively shows only the compressible PGS 231 as being compressed, this is for clarity and is not necessarily the case.) This compression of the compressible PGS 231 and the rigid PGS 232 effectively activates the in-plane thermal conductivity of the compressible PGS 231 and the rigid PGS 232.

[0034] According to an embodiment, the clamp 240 can be used to compress the compressible PGS 231 and the rigid PGS 232 to adjust the contact resistance between the compressible PGS 231 and the rigid PGS 232, thereby optimizing the thermal capacity of the heat spreading element 230 as a whole and optimizing the thermal capacity and in-plane thermal conductivity of the compressible PGS 231 and the rigid PGS 232.

[0035] It should be understood that, according to further or alternative embodiments, the compressible PGS 231 and the stiff PGS 232 need not be interleaved with one another in a 1:1 order, and other configurations are possible, including configurations where only a compressible PGS 231 is provided, and / or configurations where multiple compressible PGSs 232 are interleaved with a single stiff PGS 232.

[0036] 2, the heat spreader 201 can also include a monolithic metal element 250 interposed between the module 210 with the heat spreading element 230 and the cold plate 220. The monolithic metal element 250 can be formed of a metallic material such as copper or other suitable metal or metal alloy. The monolithic metal element 250 can be provided in a variety of configurations, but is typically provided as a single, integral element formed to define a pocket 251 in which the heat spreading element 230 can be positioned. In this sense, the monolithic metal element 250 can also engage with and be engaged by the clamp 240.

[0037] 2, the heat spreader 201 may also include a seal 245. The seal 245 may be of any size, shape, and dimension for use in the heat spreader 201. In some, but not all, cases, the seal 245 may be positioned and configured to prevent moisture from entering the heat spreading element 230. In some other cases, the seal 245 may also be positioned and configured to prevent moisture from flowing around the clamp 240 and then entering the heat spreading element 230.

[0038] 3 , according to an embodiment, the height H1 of the pocket 251 before the compressible PGS 231 and the rigid PGS 232 are compressed can be lower than the height H2 of the heat spreading element 230. Therefore, as described above, the height H1 of the pocket 251 can be adjusted together with the clamp 240 to compress the compressible PGS 231 and the rigid PGS 232 and optimize the thermal capacity of the heat spreading element 230. That is, when the compressible PGS 231 and the rigid PGS 232 are compressed from the height H2 of the pocket 251 to the height H1, the degree of compression optimizes the thermal capacity of the heat spreading element 230.

[0039] According to an embodiment, the degree of compression can be about 5-60% of the height H2.

[0040] The compressibility of the compressible PGS 231 allows for compliance with the coefficient of thermal expansion (CTE) mismatch between at least two or more of the module 210 , the monolithic metal element 250 , the rigid PGS 232 , and the cold plate 220 .

[0041] Referring to FIG. 4 , according to yet further embodiments, the heat spreading element 230 of FIGS. 2 and 3 can be provided in a hybrid configuration, arrangement, and formation. For example, as shown in FIG. 4 , the heat spreading element 230 can include a compressible PGS 231 and a rigid PGS 232, as well as a cross section 401. This cross section 401 can be, but is not required to be, provided within the heat spreading element 230 and can include the compressible PGS 231 and the rigid PGS 232. The compressible PGS 231 and the rigid PGS 232 of the cross section 401 can be oriented transversely or perpendicularly or otherwise rotated relative to the rest of the heat spreading element 230. In this manner, the cross section 401 can function like a via for through-thickness heat spreading (in addition to the in-plane heat spreading of the compressible PGS 231 and the rigid PGS 232). The compressible PGS 231 and rigid PGS 232 of the cross section 401 are compressible (i.e., by the rest of the heat spreading element 230), and therefore can provide a high degree of in-plane heat transfer between the module 210 and the cold plate 220. This cross section 401 can be placed at or near a hot spot of the module 210.

[0042] 4, the cross section 401 is shown as being sandwiched between the compressible PGS 231 and the rigid PGS 232, but it should be understood that this is not required and other embodiments are possible. For example, the cross section 401 could be in direct contact with one or both of the cold plate 220 and the monolithic metal element 250.

[0043] 5-7, according to further embodiments, the monolithic metal element 250 can be formed such that the pocket 251 can have a number of different configurations. For example, the pocket 251 of FIGS. 2 and 3 can be adjacent to the cold plate 220, the pocket 251 can be adjacent to the module 210 (see FIG. 5), the pocket 251 can be sandwiched on either side by portions 601 of the monolithic metal element 250 (FIG. 6), or the pocket 251 can span the entire distance between the module 210 and the cold plate 220 (see FIG. 7).

[0044] Referring to FIG. 8 , a method for assembling a heat spreader, such as the heat spreader 201 described above, is presented. As shown in FIG. 8 , the method includes interleaving a rigid PGS and a compressible PGS to form a heat spreading element for heat transfer and heat spreading (block 801) and compressing the compressible PGS and the rigid PGS in a heat transfer direction (block 802). According to an embodiment, the method may further include interposing a heat spreading element between the module and a cold plate to transfer heat in a first direction from the module to the cold plate and spread heat in a second direction transverse to the first direction (block 803). Furthermore, the compression may include clamping the heat spreading element between the module and the cold plate to compress the compressible PGS and the rigid PGS in the first direction (block 804). The method may further include interposing a monolithic metal element between the module with the heat spreading element and the cold plate (block 805).

[0045] 9-11, a heat spreader 901 (see FIG. 10) is presented and includes a core 910. The core 910 is made of an orthotropic material, which by definition has a first preferred direction of heat conduction D1 and a second preferred direction of heat conduction D2, such as annealed pyrolytic graphite (APG). The heat spreader 901 further includes a top plate 920 and a bottom plate 930, which may be provided as part of an encapsulant 940 (see FIG. 11), which may include additional external structures and sidewalls. The top plate 920 and bottom plate 930 (and the entire encapsulant 940) may be made from a variety of materials, including, but not limited to, metallic materials or alloys such as aluminum or copper. The heat spreader 901 also includes a first PGS 951 and a second PGS 952. The first PGS 951 can be interposed in compression as a TIM between the core 910 and the top plate 920 with a compressive orientation aligned with at least one of the first preferred direction of heat conduction D1 and the second preferred direction of heat conduction D2 (e.g., D2). The second PGS 952 can be interposed in compression as a TIM between the core 910 and the bottom plate 930 with a compressive orientation aligned with at least one of the first preferred direction of heat conduction D1 and the second preferred direction of heat conduction D2 (again, e.g., D2).

[0046] For clarity and brevity, the following description will be for the case where the compression direction is aligned with the first preferred direction of heat conduction D1.

[0047] As shown in Figure 9, core 910 is provided as a volume of orthotropic material, with a first preferred direction of heat conduction D1 and a second preferred direction of heat conduction D2 in the plane of core 910. As shown in Figure 10, top plate 920 and bottom plate 930 are aligned in a through-thickness direction D3, and core 910 is rotated so that first preferred direction of heat conduction D1 is aligned with through-thickness direction D3.

[0048] The first PGS 951 and the second PGS 952 comprise or are provided as compressible PGS that, in a compressed state, have a relatively high in-plane thermal conductivity and coefficient of thermal expansion (CTE) that matches the compliance of the core 910 and the top and bottom plates 920 and 930, respectively. As used herein, the compressed state of the first PGS 951 and the second PGS 952 is characterized by the first PGS 951 and the second PGS 952 being compressed to a predetermined degree of compression.

[0049] According to this description, the heat spreader 901 also includes a stopper 960 configured to stop further compression of the first PGS 951 and the second PGS 952 in the compression direction (i.e., the first heat conduction preferred direction D1 and the through-thickness direction D3) beyond a predetermined degree of compression. The stopper 960 may include or be provided as a pedestal 961 extending upward from the bottom plate 930 toward the top plate 920. The heat spreader 901 further includes a threaded screw 962 extending through the top plate 920 to threadably engage with the pedestal 961 to retract the top plate 920 toward the bottom plate 930 until the top plate 920 impacts the pedestal 961. According to an embodiment, the threaded screw 961 may include or be provided as a countersunk head screw, and the top plate 920 is formed to define a countersunk head screw hole 962 through which the countersunk head screw passes.

[0050] During assembly of the heat spreader 901, as shown in FIG. 10 , the initial thickness T1 of the first PGS 951 and the second PGS 952 is such that the combined thickness T2 of the core 910, the first PGS 951, and the second PGS 952 exceeds the height H1 of the top plate 920 above the bottom plate 930 (i.e., the height of the stopper 960). However, as shown in FIG. 11 , when the top plate 920 is assembled onto the stopper 960, the first PGS 951 and the second PGS 952 are compressed to a secondary thickness T3 such that the combined compressed thickness of the core 910, the first PGS 951, and the second PGS 952 in the compressed state is equal to the height H1 (compression of the core 910 in the first preferred direction of heat conduction D1 occurs, but is minimal relative to the degree of compression of the first PGS 951 and the second PGS 952). The dimensions of the stopper 960 are set according to this desired effect.

[0051] Referring to FIG. 12, the heat spreader 901 may include an additional PGS 970 positioned and configured to line the stopper 960 .

[0052] As described above, the core 910, the first and second PGSs 951 and 952, and the additional PGS 970 exhibit in-plane thermal conductivities greater than about 1,000 W / mK, or between about 400 and 1,500 W / mK, and have densities between about 428 and 1,200 kg / m. Therefore, the heat spreader 901 can spread heat relatively quickly in a first preferred direction of thermal conduction D1 and in a thickness direction D3 (due to the rotation of the core 910 relative to the global coordinate system of the top plate 920 and the bottom plate 930 (and the encapsulant 940). The heat spreader 901 can also spread heat relatively quickly in a second preferred direction of thermal conduction D2 of the core 910 and in the in-plane thermal conduction of the first and second PGSs 951 and 952 and the additional PGS 960.

[0053] Referring to FIG. 13, a method for assembling a heat spreader such as heat spreader 901 of FIGS. 9-12 is presented. The method includes providing a core of an orthotropic material having a first preferred direction of thermal conduction and a second preferred direction of thermal conduction (block 1301), positioning the core between a top plate and a bottom plate in a compressive orientation aligned with at least one of the first preferred direction of thermal conduction and the second preferred direction of thermal conduction (block 1302), interposing a PGS between the core and the top plate and between the core and the bottom plate in the compressive orientation (block 1303), and compressing the PGS into a compressible state such that the PGS forms a thermal interface material between the core and the top plate and between the core and the bottom plate (block 1304). The PGS may include a compressible PGS or may be provided as a compressible PGS that, in a compressed state, provides in-plane thermal conduction and CTE matching between the core and the top plate and the bottom plate. The compressed state of the PGS is characterized by the PGS being compressed to a predetermined degree. The method further includes stopping further compression of the PGS in the compression direction beyond a predetermined degree of compression (block 1305).

[0054] A technical effect and advantage of the present disclosure is the provision of a compressible PGS that can be used as a TIM between an APG core and an encapsulant, providing a low thermal resistance contact and a flexible connection that limits distortion due to thermal expansion.

[0055] Corresponding structure, materials, acts, and equivalents of all means or step-plus-function elements in the following claims are intended to include any structure, material, or acts for performing the function as specifically claimed in combination with other claimed elements. The description of the present disclosure has been presented for purposes of illustration and description, but is not intended to be exhaustive or limiting to the technical concepts in the disclosed form. Many modifications and variations will be apparent to those skilled in the art without departing from the scope and spirit of the present disclosure. The selection and description of the embodiments was made to best explain the principles and practical applications of the present disclosure and to enable others skilled in the art to understand the present disclosure in various embodiments with various modifications suitable for the particular use intended.

[0056] While preferred embodiments of the present disclosure have been described, it should be understood that those skilled in the art, both now and in the future, may make various improvements and enhancements that fall within the scope of the following claims, which should be interpreted to maintain the appropriate protection for the disclosure as originally described.

Claims

1. A heat spreader, a core of orthotropic material having a first preferred direction of heat conduction and a second preferred direction of heat conduction; Top plate, a bottom plate, and the heat spreader including a pyrolytic graphite sheet (PGS) interposed in a compressed state as a thermal interface material (TIM) between the core and the top plate and between the core and the bottom plate with a compression direction aligned with at least one of the first preferred direction of heat conduction and the second preferred direction of heat conduction.

2. The heat spreader of claim 1 , wherein the orthotropic material comprises annealed pyrolytic graphite (APG).

3. The heat spreader of claim 1 , wherein the first preferred direction of heat conduction and the second preferred direction of heat conduction are in-plane directions of the core.

4. the top plate and the bottom plate are aligned in a through-thickness direction; The heat spreader of claim 1 , wherein the core is rotated so that at least one of the first preferred direction of heat conduction and the second preferred direction of heat conduction is aligned with the through-thickness direction.

5. 2. The heat spreader of claim 1, wherein the PGS comprises compressible PGS that, in the compressed state, provides in-plane thermal conductance and coefficient of thermal expansion (CTE) matching of the core and the top and bottom plates.

6. the compressed state of the PGS is characterized in that the PGS is compressed to a predetermined degree of compression; The heat spreader of claim 1 , further comprising a stopper configured to stop further compression of the PGS in the compression direction beyond the predetermined degree of compression.

7. the stopper includes a base extending upward from the bottom plate toward the top plate; 7. The heat spreader of claim 6, further comprising a threaded screw extending through the top plate to threadably engage the base to draw the top plate toward the bottom plate until the top plate impacts the base.

8. the threaded screw comprises a countersunk head screw; The heat spreader of claim 7 , wherein the top plate is formed to define countersunk holes through which the countersunk screws extend.

9. The heat spreader of claim 6 further comprising an additional PGS lining the stopper.

10. A heat spreader, a core of orthotropic material having a first preferred direction of heat conduction and a second preferred direction of heat conduction; an encapsulant; a pyrolytic graphite sheet (PGS) interposed in a compressed state between the core and the encapsulant as a thermal interface material (TIM) in at least a compressive direction aligned with at least one of the first preferred direction of thermal conduction and the second preferred direction of thermal conduction.

11. The heat spreader of claim 10 , wherein the orthotropic material comprises annealed pyrolytic graphite (APG).

12. The heat spreader of claim 10 , wherein the first preferred direction of heat conduction and the second preferred direction of heat conduction are in-plane directions of the core.

13. the encapsulant has an overall through-thickness direction; The heat spreader of claim 10 , wherein the core is rotated so that at least one of the first preferred direction of heat conduction and the second preferred direction of heat conduction is aligned with the through-thickness direction.

14. 11. The heat spreader of claim 10, wherein the PGS comprises compressible PGS that, in the compressed state, provides in-plane thermal conductivity and coefficient of thermal expansion (CTE) matching of the core and the encapsulant.

15. the compressed state of the PGS is characterized in that the PGS is compressed to a predetermined degree of compression; The heat spreader of claim 10 , further comprising a stopper configured to stop further compression of the PGS in the compression direction beyond the predetermined degree of compression.

16. the stopper includes a pedestal extending upward from a bottom plate of the encapsulant toward a top plate of the encapsulant; 16. The heat spreader of claim 15, further comprising a threaded screw extending through the top plate to threadably engage the base to draw the top plate toward the bottom plate until the top plate impacts the base.

17. the threaded screw comprises a countersunk head screw; 17. The heat spreader of claim 16, wherein the top plate is formed to define countersunk holes through which the countersunk screws extend.

18. The heat spreader of claim 15 further comprising additional PGS lining the stopper.

19. 1. A method of assembling a heat spreader, comprising: providing a core of orthotropic material having a first preferred direction of heat conduction and a second preferred direction of heat conduction; disposing the core between a top plate and a bottom plate in a compression orientation aligned with at least one of the first preferred direction of heat conduction and the second preferred direction of heat conduction; Interposing pyrolytic graphite sheets (PGS) between the core and the top plate and between the core and the bottom plate in the compression direction; and compressing the PGS into a compressed state such that the PGS provides a thermal interface material (TIM) between the core and the top plate and between the core and the bottom plate.

20. the PGS comprises a compressible PGS that, in the compressed state, provides in-plane thermal conductivity and coefficient of thermal expansion (CTE) matching between the core and the top and bottom plates; the compressed state of the PGS is characterized in that the PGS is compressed to a predetermined degree of compression; 20. The method of claim 19, further comprising: ceasing further compression of the PGS in the compression direction once the predetermined degree of compression is exceeded.

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