Patent Publication Number: US-2022228820-A1

Title: Graphene and carbon nanotube based thermal management device

Description:
TECHNICAL FIELD 
     The present disclosure relates generally to thermal management, and more particularly, to a graphene and carbon nanotube based thermal management device. 
     BACKGROUND 
     Over the past several years, there has been a tremendous increase in the need for higher performance communications networks. Increased performance requirements have led to an increase in energy use resulting in greater heat dissipation from components. High power components such as ASICs (Application Specific Integrated Circuits) require high performance thermal management. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic side view of a graphene and carbon nanotube based heat sink, in accordance with one embodiment. 
         FIG. 2A  is a schematic perspective of a graphene layer and carbon nanotube array, in accordance with one embodiment. 
         FIG. 2B  is a schematic side view of a plurality of graphene layers with the carbon nanotubes interposed therebetween. 
         FIG. 2C  is a simplified side view illustrating offset carbon nanotube arrays between the graphene layers. 
         FIG. 3  is an exploded view of a graphene and carbon nanotube heat sink, a heat sink protector, and an integrated circuit package on a printed circuit board, in accordance with one embodiment. 
         FIG. 4  is a bottom perspective of the exploded view shown in  FIG. 3  with the printed circuit board removed. 
         FIG. 5  is a cross-sectional view of the assembled heat sink and fin protector mounted on the integrated circuit package of  FIG. 3 . 
         FIG. 6A  is a flowchart illustrating an overview of a process for making the graphene and carbon nanotube based heat sink, in accordance with one embodiment. 
         FIG. 6B  is a flowchart illustrating an overview of a process for heat dissipation with the graphene and carbon nanotube heat sink, in accordance with one embodiment. 
     
    
    
     Corresponding reference characters indicate corresponding parts throughout the several views of the drawings. 
     DESCRIPTION OF EXAMPLE EMBODIMENTS 
     Overview 
     In one embodiment, a thermal management device generally comprises a heat sink base and heat sink fins comprising a single element formed from a plurality of graphene layers with carbon nanotubes interposed between the graphene layers. 
     In one embodiment, the heat sink base comprises a pedestal for direct contact with an integrated circuit package, which may comprise a lidless integrated circuit package. 
     In one embodiment, the thermal management device further comprises a frame configured for mounting on a printed circuit board and supporting the heat sink base. The frame may comprise a shock absorbing material and at least two side walls extending upward from the frame and positioned adjacent to the heat sink fins. 
     In one embodiment, the thermal management device is configured for direct contact with an integrated circuit package without a thermal interface material interposed therebetween. 
     In one embodiment, an apparatus generally comprises a printed circuit board, an integrated circuit package mounted on the printed circuit board, a heat sink formed from a plurality of graphene layers with carbon nanotubes interposed between the graphene layers, and a frame mounted on the printed circuit board, wherein the heat sink is positioned on the frame and in direct contact with the integrated circuit package through an opening in the frame. 
     In yet another embodiment, a method generally comprises creating an element comprising a plurality of graphene layers and carbon nanotubes interposed between the graphene layers and removing material from the element to define heat sink fins integrated with a heat sink base. The graphene layers provide in-plane thermal conductivity and the carbon nanotubes provide through-plane thermal conductivity. 
     Further understanding of the features and advantages of the embodiments described herein may be realized by reference to the remaining portions of the specification and the attached drawings. 
     Example Embodiments 
     The following description is presented to enable one of ordinary skill in the art to make and use the embodiments. Descriptions of specific embodiments and applications are provided only as examples, and various modifications will be readily apparent to those skilled in the art. The general principles described herein may be applied to other applications without departing from the scope of the embodiments. Thus, the embodiments are not to be limited to those shown, but are to be accorded the widest scope consistent with the principles and features described herein. For purpose of clarity, details relating to technical material that is known in the technical fields related to the embodiments have not been described in detail. 
     High power components such as ASICs (Application Specific Integrated Circuits) often need high performance heat sinks to quickly dissipate and spread excessive heat to fins, which may be cooled via forced airflow or immersion cooling in a liquid, for example. Increased die size may result in multiple localized hot spots on the die, and cooling ASICs with such die profiles is a challenge. Mitigation of hot spots and heat removal may result in a need for both heat spreading and heat conduction in single and multiple dimensions, which may be followed by natural, mixed, or forced convection. Heat spreading may be achieved with heat conductors that possess high thermal conductivity in at least two directions (i.e., in a plane). Graphene provides high thermal conductivity in a plane and is therefore a good material for use as a heat spreader. Even though layers of graphene demonstrate very high in-plane thermal conductivity, stacking layers of graphene significantly decreases the through-plane thermal conductivity. Thus, the through-plane heat conductivity of graphene is less than desirable for a heat conductor that may be used as a heat sink or an ASIC lid. 
     The embodiments described herein provide a graphene and carbon nanotube (CNT) based thermal management device. As described below, the superior in-plane thermal conductivity of graphene is combined with the high thermal conductivity of CNTs in the through-plane direction. CNT structures provide directional high thermal conductivity when the CNTs are properly arranged in a specific direction in the structure. The assembly of graphene layers and carbon nanotubes may be used as a passive thermal management element such as a heat sink or cold plate base that improves thermal performance considerably by utilizing superior heat spreading capabilities of graphene and directional heat conduction capabilities of carbon nanotubes. In addition to providing a lightweight solution, the thermal management device may eliminate the need for low thermal conductivity elements such as at least one layer of thermal interface material. As described in detail below, the thermal management device may be implemented to cool hot spots on a chip by vertical and horizontal heat transfer. 
     Referring now to the drawings, and first to  FIG. 1 , a schematic side view of a thermal management device  10  is shown, in accordance with one embodiment. The thermal management device  10  comprises a heat sink base  12  and heat sink fins  14  defining a single element (unit, piece) formed from a plurality of graphene layers with carbon nanotube arrays interposed between the graphene layers. For example, each of the heat sink fins  14  may include any number of graphene layers with any number of CNTs interposed between the graphene layers or extending therethrough as shown in  FIGS. 2B and 2C , depending on the height, width, and thickness of the fins. 
     In one or more embodiments, the heat sink base  12  may comprise a pedestal  16  extending downward therefrom for direct contact with an integrated circuit package. The pedestal  16  of the base  12  may, for example, replace an integrated circuit package lid (cover) and thermal interface material (TIM) (also referred to as TIM 2 ) for direct contact with a lidless integrated circuit package, as described below with respect to  FIGS. 3-5 . The heat sink  10  transfers heat generated by the integrated circuit package, which may one or more components in which heat dissipation capability of the component is insufficient to moderate its temperature. The heat sink base  12  and fins  14  allow excess thermal energy to dissipate into the environment by conduction. The heat sink  10  is configured to maximize the surface area in contact with a cooling medium (e.g., air, liquid) surrounding the heat sink and may be used to dissipate heat from an integrated circuit package comprising one or more ASIC, NPU (Network Processing Unit), chip, die, integrated circuit, photonic chip, electronic component, optical component, or other heat generating component or combination of heat generating components. 
     It is to be understood that the thermal management device shown in  FIG. 1  is only an example, and other shapes, sizes, or types of heat sinks may be used without departing from the scope of the embodiments. The heat sink  10  may have any shape (e.g., height, width, length, ratio of width to length, base footprint, base thickness, number of fins, size of fins, shape or thickness of pedestal, or number of pedestals). In one example, the fins are 0.4 mm thick (e.g., between 0.2 mm and 0.6 mm thick). Also, the thermal management device may comprise a base plate of a cold plate, vapor chamber, or other thermal management device. 
     Referring now to  FIGS. 2A, 2B, and 2C , examples of graphene layers  20  and carbon nanotubes  22  are schematically shown, in accordance with one embodiment. Perforated, thin, stacked graphene layers (sheets)  20  are filled in between with CNTs  22  to provide a composite structure with very good thermal transfer characteristics. As shown in  FIG. 2B , a plurality of carbon nanotubes  22  are arranged with a plurality of graphene layers  20  (e.g., comprising one graphene sheet). This structure may include any number of graphene layers  20  with any number of carbon nanotubes  22  to build an element corresponding to the thermal management device (e.g., height of base plus fins). In one example, each CNT  22  may form a chain in a height (through-plane) direction with multiple layers of graphene  20  around it (i.e., the CNT structure does not have to terminate at individual graphene layers). 
       FIG. 2C  schematically illustrates another example structure comprising a plurality of graphene layers  20  and CNTs  22  with some of the CNTs offset between layers. One or more CNTs  22  may extend inline through one or more graphene layers  20 , one or more CNTs may be offset between one or more graphene layers, or any combination thereof. It is to be understood that the arrangement of graphene layers  20  and CNTs  22  shown in  FIG. 2C  is only an example and the CNTs may be arranged in many other configurations, without departing from the scope of the embodiments. For example, as previously noted, the CNT  22  may extend through two or more graphene layers  20 , or the CNTs may be offset between each layer. Also, one or more of the CNT arrays between the graphene layers  20  may comprise a different number or spacing of CNTs  22 . 
     In one example, each CNT  22  comprises a first end in thermal contact with a first graphene layer  20  and a second end in thermal contact with a second graphene layer. Each end of the CNT  22  may be covalently bonded to the respective graphene layer  20 . For example, the graphene layer  20  may undergo an atomic substitution reaction in which the carbon nanotubes  22  are directly infused to create a three-dimensional structure from the graphene layer and the carbon nanotubes. In one example, the CNTs  22  are single-walled carbon nanotubes. The covalent bonds are defined by carbon-carbon bonds at one or more intersections between each CNT  22  and the adjacent graphene layers  20 . As previously noted, there may also be multiple layers of graphene around the CNT  22 , in addition to the graphene layers  20  at each end of the CNT. Thus, the CNT  22  may be covalently bonded to one or more of the graphene layers or extend through or connect with one or more of the graphene layers. 
     The first graphene layer  20  provides a large contact area to transfer heat from the surface of the integrated circuit package in direct contact with the thermal management device. While graphene has an outstanding in-plane thermal conductivity, its through-plane (cross-plane) heat transfer (i.e., perpendicular to the graphene plane) is hindered by a large inter-layer thermal resistance. The hybrid graphene and carbon nanotube structure described herein significantly reduces the interlayer thermal resistance in the normal direction between the graphene layers. High thermal conductivity is provided in-plane from the graphene layers (e.g., k in-plane  up to 2000 W/mK) and in the normal (through-plane) direction from the CNTs (e.g., k normal  up to 200 W/mK) to provide thermal conductivity in all directions. Improvement in the through-plane (CNT) thermal conductivity and in-plane (graphene) thermal conductivity provides a reduction in thermal resistance, resulting in high thermal conductivity of the thermal management device, thereby enabling sufficient cooling of the integrated circuit package (e.g., ASIC, NPU (Network Processing Unit) or other chip, die, integrated circuit, photonic chip, electronic component, optical component, or electrical/optical component). Graphene also provides advantages in strength and the composite structure is lightweight (e.g., at least 20-25% lower density than that of aluminum and about 80% lower density than that of copper). 
     As previously noted with respect to  FIG. 1 , the thermal management device (heat sink element) may include any number of graphene layers and any number of CNTs interposed between the layers depending on the size of the heat sink element. The stacked tiers of graphene layers and CNT arrays extend from a bottom surface of the heat sink element to a top of the heat sink element (e.g., top edge of heat sink fins) and are included in each integrated portion of the heat sink element. For example, the heat sink base, pedestal, and fins each comprise a plurality of graphene layers and CNT arrays interposed therebetween 
     It is to be understood that the schematic depiction shown in  FIGS. 2A-2C  are only examples, and the relative size of the carbon nanotubes  22  and thickness of the graphene layers  20  or spacing and position of the nanotubes relative to each other in an array or between arrays may be different than shown, without departing from the scope of the embodiments. For example, the arrangement of the carbon nanotubes  22  may be based on a desired density of the hybrid structure, which may be defined by the percentage of an area of the graphene layer  20  bonded to the carbon nanotubes  22 . The thermal conductivity of the carbon nanotube array may also depend on the length of the carbon nanotubes, which may be optimized based on implementation and environment of the thermal management device. Any suitable length or diameter of carbon nanotubes or process to grow the heat sink element may be used. In one or more embodiments, the structure shown in  FIG. 2B  may be positioned offset at an angle within the thermal management device (e.g., graphene layer offset from surface of heat sink base) to provide a different thermal conductivity profile in the heat sink. Also, fin pitch and thickness may be optimized for thermal management. 
       FIG. 3  is an exploded view of a thermal management device and printed circuit board, in accordance with one embodiment. In one embodiment, an apparatus comprises the printed circuit board  31 , an integrated circuit package  35  (substrate  37   a,  die  37   b ) mounted on the printed circuit board, a heat sink  30  formed from a plurality of graphene layers with carbon nanotubes interposed between the graphene layers, and a frame  42  mounted on the printed circuit board. The heat sink  30  is positioned on the frame  42  and in direct contact with the integrated circuit package  35  through an opening  45  in the frame  42 . 
     In the example shown in  FIGS. 3-5 , the thermal management device includes an integrated graphene and carbon nanotube heat sink  30  comprising a heat sink base  32  and heat sink fins  34  extending upward from the base. As previously described, the heat sink  30  is formed as a single unit from multiple layers of graphene with carbon nanotubes interposed between the layers. The heat sink base  32  thermally couples the integrated circuit package  35  to the heat sink and fluid (e.g., airflow provided by one or more fans or a cooling liquid) flows across the fins  34 . The heat sink base  32  transfers heat to the heat sink fins  34  and the fluid carries heat away from the heat sink  30  as it travels past the fins, thereby cooling the integrated circuit package  35 . 
     A heat sink protector  38  is provided to protect the heat sink  30  (e.g., protect thin fins  34  of the heat sink and protect heat sink from shock or vibration). The heat sink protector  38  comprises the frame  42  for mounting on the printed circuit board  31  and supporting the heat sink base  32 . In the example shown in  FIGS. 3-5 , the heat sink protector  38  further comprises side walls  44  (e.g., two or more side walls extending upward from the frame and positioned adjacent to outer fins  34 ). The side walls  44  may be formed, for example, from an aluminum material and comprise a 1 mm thick wall (or formed from any other suitable material with any wall thickness). The base  32  of the heat sink  30  sits within the frame  42  and is in contact with the integrated circuit package  35  through opening  45  in the frame. The frame  42  may be attached to printed circuit board  31  with one or more fasteners  40  (e.g., screws, spring loaded screws, or any other suitable fastener). The frame  42  preferably comprises a shock absorbing material  47  (e.g., gasket extending around inner periphery of frame) to reduce amplitude or vibration quickly and effectively and protect the heat sink  30 . The frame  42  is positioned over the integrated circuit package  35  mounted on PCB  31 . In one or more embodiments, the package  35  is a lidless integrated circuit package and the thermal management device is configured for direct contact with the integrated circuit package without a thermal interface material (TIM 2 ) interposed therebetween. 
     The printed circuit board  31  may be any type of conductive platform or board for installing and electrically connecting electrical and mechanical components to create an electrical circuit. The circuit board  31  may be securely positioned within a housing and have one or more integrated circuit packages installed thereon. One or more thermal management devices may be attached to the circuit board and positioned over one or more heat generating components. The printed circuit board  31  may include one or more active devices (e.g., transistor, chip, processor, circuit, application specific integrated circuit, field programmable gate array, memory, etc.) and one or more passive devices (e.g., capacitor, resistor, inductor, connector, via, pad, etc.). The traces, pads, and electronic components may be arranged in any configuration to perform any number of functions (e.g., network server card, graphics card, motherboard, device card (line card, fabric card, controller card), and the like) for operation on any type of network device (e.g., computer, router, switch, server, gateway, controller, edge device, access device, aggregation device, core node, intermediate node, or other network device). The circuit board  31  may be located within a line card, fabric card, controller card, or other modular device, including, for example, 1RU (rack unit), 2RU, or other size modular device. 
       FIG. 4  is a bottom perspective of the heat sink  30 , heat sink protector  38  (frame  42 , fin protector  44 ) and lidless package  35  of  FIG. 3 . A pedestal  36  is integrally formed with the base  32  of the heat sink for direct contact with the die of the integrated circuit package  35 . As previously described, the pedestal  36  is part of the one piece element formed from the graphene layers and carbon nanotube arrays, and may replace a lid and thermal interface material. 
       FIG. 5  is a cross-sectional view of the assembled thermal management device of  FIG. 3 . As previously described, the pedestal  36  is integrally formed with the heat sink base  32  and fins  34 . The heat sink base  32  is supported by the frame  42  comprising the shock absorbing material  47  and outer heat sink fins  34  are positioned adjacent to the side walls  44  ( FIGS. 3 and 5 ). 
     It is to be understood that the thermal management device shown in  FIGS. 3-5  is only an example. Also, it should be noted that the terms, downward, upward, bottom, top, lower, upper, below, above, and the like as used herein are relative terms dependent upon orientation of the printed circuit board and should not be interpreted in a limiting manner. These terms describe points of reference and do not limit the embodiments to any particular orientation or configuration. 
       FIG. 6A  is a flowchart illustrating an overview of a process for manufacturing the thermal management device, in accordance with one embodiment. At step  60 , an element is created comprising a plurality of graphene layers and carbon nanotubes interposed between the graphene layers. Material is removed from the element to define heat sink fins integrated with a heat sink base (step  62 ). As previously described, the graphene layers provide in-plane thermal conductivity and the carbon nanotubes provide through-plane thermal conductivity. In one or more embodiments, the graphene layers may be offset from a surface of the heat sink base to optimize thermal conductivity. As previously described, the heat sink base with the integrated heat sink fins may be mounted on a frame attached to a printed circuit board with the heat sink base (e.g., pedestal of the heat sink base) in direct contact with an integrated circuit package through an opening in the frame. The frame may comprise a shock absorbing material and at least two walls extending upward therefrom to protect the heat sink fins. 
       FIG. 6B  is a flowchart illustrating an overview of a process for implementing the thermal management device, in accordance with one embodiment. At step  64 , the heat sink is positioned in the heat sink protector. The heat sink and heat sink protector are mounted on the printed circuit board with a pedestal of the base in direct contact with the integrated circuit package (step  66 ). Heat is dissipated from the integrated circuit package through in-plane thermal conductivity in the graphene layers and through-plane thermal conductivity in the carbon nanotubes to surrounding fluid (air, liquid) (step  68 ). 
     It is to be understood that the processes shown in  FIGS. 6A and 6B  are only examples, and steps may be added, modified, combined, removed, or reordered without departing from the scope of the embodiments. For example, the frame may first be attached to the PCB and the heat sink inserted into the mounted frame. 
     As can be observed from the foregoing, the thermal management device described herein provides many advantages. For example, the integrated graphene and carbon nanotube based heat sink may eliminate drawbacks with conventional heat sinks such as flatness, high contact resistance, vapor chamber dry-out, or the need for a thermal management interface. In one example, an improvement of thermal performance of 45% may be provided while reducing the weight by at least 30% compared to a traditional vapor chamber/parallel plate fin assembly. 
     Although the method and apparatus have been described in accordance with the embodiments shown, one of ordinary skill in the art will readily recognize that there could be variations made to the embodiments without departing from the scope of the invention. Accordingly, it is intended that all matter contained in the above description and shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.