Patent Publication Number: US-2009225515-A1

Title: Thermal bus or junction for the removal of heat from electronic components

Description:
RELATED APPLICATIONS 
     This application claims priority of U.S. provisional application, Ser. No. 61/068,891, filed Mar. 10, 2008, and entitled “Fan Tray for Supplemental Air Flow”, by these same inventors. This application incorporates U.S. provisional application, Ser. No. 61/068,891 in its entirety by reference. 
    
    
     FIELD OF THE INVENTION 
     The invention relates to a method of and apparatus for cooling a heat producing device in general, and specifically, to a method of and apparatus for cooling electronic components using fluid-based cooling systems. 
     BACKGROUND OF THE INVENTION 
     Cooling of high performance integrated circuits with high heat dissipation is presenting significant challenge in the electronics cooling arena. Electronics servers, such as blade servers and rack servers, are being used in increasing numbers due to the higher processor performance per unit volume one can achieve. However, the high density of integrated circuits also leads to high thermal density, which is beyond the capability of conventional air-cooling methods. 
     A particular problem with cooling integrated circuits on electronics servers is that multiple electronics servers are typically mounted in close quarters within a server chassis. In such configurations, electronics servers are separated by a limited amount of space, thereby reducing the dimensions within which to provide an adequate cooling solution. Often electronics server stacks within a single server chassis are cooled with one or more fans, one or more heat sinks, or a combination of both. Using this configuration, the integrated circuits on each electronics server are cooled using the heat sink and the large fan that blows air over the heat sink, or simply by blowing air directly over the electronics servers. However, considering the limited free space surrounding the stacked electronics servers within the server chassis, the amount of air available for cooling the integrated circuits is limited. 
     Many conventional electronic cooling solutions use heat pipe assemblies to remove and reject heat from an electronic component and to the surrounding air flow or rejected to another heat sink, such as a liquid cold plate. Due to the nature of heat pipes, the thermal performance is highly sensitive to orientation and length between the electronic component and the heat sink. Heat pipes are generally configured to rely on wicking structures and are aided by gravitational effects. A condenser portion is positioned at a top of the heat pipe assembly and an evaporation portion is positioned at a bottom, which is thermally coupled to a heat source such as an electronic component. Heat from the electronics component evaporates liquid in the evaporation portion of the heat pipe assembly. This vapor rises from the bottom to the top of the heat pipe assembly, where the vapor condenses to the liquid. The liquid drains from the top to the bottom of the heat pipe assembly, thereby forming a loop. The greater the distance between the evaporation end and the condensation end of the heat pipe assembly, the more sensitive the heat pipe performance is to the heat pipe orientation. For blade server applications, this is a greater issue because of the high aspect ratio of the blade itself. These thin blades are normally place on one edge on the bottom of the rack chassis. 
       FIG. 1  illustrates an exemplary block diagram of a electronics board turned on edge, such as in the case of a mounted blade server. The electronics board  10  includes multiple electronic components, such as central processing unit (CPU)  12 , CPU  14 , memory device  16 , and memory device  18 . Each of the electronic components generates heat to be removed. In one manifestation of a heat removal design, air flow is generated across the electronic components. Heat from the electronic components is transferred to the crossing air flow. In another manifestation of a heat removal design, the heat is transferred from the electronic components to a liquid cold plate. The cold plate, which can be attached to an edge of the electronics board, serves as a heat sink for the heat generated by the electronic components. Heat pipe assemblies couple the electronic components to the liquid cold plate so that heat is transferred from the electronic components to the edge(s) of the electronics board. Unfortunately, the reduced performance of the heat pipe assembly due to the orientation problem associated with heat pipe assemblies, allows only the top edge of the electronics board to be used as a viable heat sink. For the electronic components nearer the bottom edge of the electronics board, the heat must be transferred to the top edge of the electronics board. Increasing the length of the heat pipe assemblies drastically reduces the thermal performance of the heat pipe assemblies. 
     The physical layout of an electronics board may require bends and turns in the heat pipe assemblies, as well as increasing the length, to work around physical obstacles on the electronics board. In some cases, the physical layout may prohibit positioning the cold plate at the closest position to the electronic components, further lengthening the heat pipe assemblies. Bends, turns, and increased length all decrease the thermal performance of a heat pipe assembly. 
     SUMMARY OF THE INVENTION 
     Embodiments of the present invention are directed to a thermal bus, or junction, that enables the use of multiple separate heat pipe assemblies instead of using a single heat pipe assembly spanning the distance from heat source to cold plate. The use of a thermal bus, which may be a heat pipe assembly or a liquid loop, can decrease the orientation effects as well as decrease the travel length of any single heat pipe assembly. In addition, the use of multiple heat pipe assemblies enables each individual heat pipe assembly to be optimized to meet localized heat transfer characteristics between each heat source, the thermal bus, and the cold plate. Such optimization can include, but is not limited to, the use of differently sized heat pipes, wick structures within the heat pipe, and working fluid used within the heat pipe. The thermal bus provides an intermediate thermal transfer from one heat pipe assembly serially coupled to another heat pipe assembly, thereby enabling multiple serially coupled heat pipe assemblies to transfer heat from a given heat source to the cold plate at the edge of the electronics board. 
     Another advantage of using a thermal bus is the elimination of sharp bending angles of the heat pipe assemblies. The heat removal capacity of a heat pipe assembly can be adversely affected by bends in the heat pipe structure. The use of thermal bus eliminates or minimizes the number and severity of the bends. This technique can be extended to the use of multiple thermal buses to eliminate bends in a single heat pipe configuration from heat source to cold plate. 
     Besides taking heat directed from the electronic components, an air-fin heat pipe assembly can be used to remove heat from the air flow as well. This air-fin heat pipe assembly can also be attached to the thermal bus. Subsequently, the heat taken from the air flow via the air-fin heat pipe assembly can then be transferred to the cold plate. 
     The air-fin heat pipe assemblies can be placed at the upstream side and/or the downstream side of the electronics board. When the air-fin heat pipe assemblies are placed at the downstream side, the heat generated from the electronic components and expelled into the air flow can be absorbed by the air-fin heat pipe assemblies and rejected to the liquid cold plate via the thermal bus. Alternatively, the air-fin heat pipe can be placed at the upstream side, the air initially crossing the electronics board is cooled. This becomes important should the inlet air temperature rise above the optimal value. In conventional cooling systems, a higher air temperature necessitates an increase in air flow to increase the cooling capacity. This increase in air flow requires increased power to the cooling fans that generate the air flow. Therefore, with increased inlet air temperature, the overall energy efficiency of such a cooling system is decreased. However, with the air-fin heat pipe assemblies placed at the upstream entrance of the electronics board, the air-fin heat pipe assemblies serve to pre-cool the air temperature. In this case, the increase in air flow is not needed. 
     The air-fin heat pipe assembly design can be used in conjunction with the heat pipe assemblies coupled to the heat sources to form a hybrid design, which incorporates both the heat removal directly off electronic components as well as from the air flow. This hybrid configuration allows for the direct removal of heat from larger heat emitting components as well as the indirect heat removal from the aggregate of the smaller heat emitting components. 
     Other features and advantages of the present invention will become apparent after reviewing the detailed description of the embodiments set forth below. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates an exemplary block diagram of a electronics board turned on edge. 
         FIG. 2  illustrates an exemplary block diagram of a cooling system including a thermal bus according to an embodiment of the present invention. 
         FIG. 3A  illustrates a top down view of a first embodiment of the thermal bus. 
         FIG. 3B  illustrates a side view of the first embodiment of  FIG. 3A . 
         FIG. 3C  illustrates a side view of a second embodiment of the thermal bus. 
         FIG. 3D  illustrates a side view of a third embodiment of the thermal bus. 
         FIG. 4  illustrates an exemplary block diagram of a cooling system including multiple thermal buses according to an embodiment of the present invention. 
         FIG. 5A  illustrates an exemplary block diagram of a cooling system including an air-fin heat pipe assembly and a thermal bus according to an embodiment of the present invention. 
         FIG. 5B  illustrates an exemplary block diagram of a cooling system including an air-fin heat pipe assembly and a thermal bus according to another embodiment of the present invention. 
         FIG. 6  illustrates an exemplary block diagram of a cooling system including a hybrid configuration according to an embodiment of the present invention. 
     
    
    
     The present invention is described relative to the several views of the drawings. Where appropriate and only where identical elements are disclosed and shown in more than one drawing, the same reference numeral will be used to represent such identical elements. 
     DETAILED DESCRIPTION OF THE PRESENT INVENTION 
     Reference will now be made in detail to the embodiments of the cooling system of the invention, examples of which are illustrated in the accompanying drawings. While the invention will be described in conjunction with the embodiments below, it will be understood that they are not intended to limit the invention to these embodiments and examples. On the contrary, the invention is intended to cover alternatives, modifications and equivalents, which may be included within the spirit and scope of the invention as defined by the appended claims. Furthermore, in the following detailed description of the present invention, numerous specific details are set forth in order to more fully illustrate the present invention. However, it will be apparent to one of ordinary skill in the prior art that the present invention may be practiced without these specific details. In other instances, well-known methods and procedures, components and processes haven not been described in detail so as not to unnecessarily obscure aspects of the present invention. 
     Embodiments of the present invention are directed to a cooling system that transfers heat generated by one or more heat generating devices on an electronics board. The cooling system described herein can be applied to any electronics board including, but not limited to, a motherboard, a graphics card, or any electronics sub-system that is mounted to a backplane, including, but not limited to, a blade server and a rack server, herein referred to collectively as an electronics board. An electronics board can be coupled to a backplane or mid-plane within a server or computer chassis. Embodiments of the cooling system are described below in the context of one or more heat generating devices coupled to an electronics server. It is understood that these concepts can be expanded to include the cooling of one or more heat generating devices coupled to any type of electronics board. As used herein, references to a heat source, a heat generating source, a heat generating device, and the like, as well as specific references to exemplary heat generating devices such as an integrated circuit, an integrated microprocessor circuit, and a semiconductor heat source, are used interchangeably and refer in general to any apparatus or source capable of generating heat. 
     Embodiments of the cooling system includes a thermal bus, or junction, that thermally couples separate heat pipe assemblies. As used herein, a heat pipe assembly is a device that is made of a thermally conductive material with a fluid sealed within the heat pipe assembly interior. The heat pipe assembly includes an evaporation end and a condensation end. Heat is transferred to the heat pipe assembly at the evaporation end, where the liquid evaporates. The vapor moves from the evaporation end to the condensation end, where the vapor releases heat thereby condensing the vapor into liquid. The liquid moves from the condensation end back to the evaporation end. The thermal bus provides a thermal interface between the condensation end of a first heat pipe assembly and the evaporation end of a second heat pipe assembly. As a first fluid in the first heat pipe assembly condenses from vapor to liquid at the condensation end, heat is released. This released heat is thermally transferred to the evaporation end of the second heat pipe assembly such that a second fluid in the second heat pipe assembly evaporates from a liquid to a vapor. The evaporated second fluid then moves to the condensation end of the second heat pipe assembly. 
       FIG. 2  illustrates an exemplary block diagram of a cooling system including a thermal bus according to an embodiment of the present invention. An electronics board  100  includes multiple electronic components, such as a CPU  102  and a CPU  104 . Each of the electronic components generates heat to be removed. Although only two electronic components are shown in the exemplary configuration of  FIG. 2 , it is understood that more, or less, than two electronic components can be coupled to the electronics board  100 . As also shown in  FIG. 2 , the electronics board  100  is positioned on edge, as in a blade server. In this configuration, an electronics board edge  101  is a top edge and an electronics board  103  is a bottom edge. This configuration is for exemplary purposes only. In an alternative embodiment, the electronics board  100  is positioned horizontally, such as component-side up or component-side down. 
     A heat pipe assembly  106  includes an evaporation end coupled to an electronics component  102 , such as a processor, and a condensation end coupled to a thermal bus  10 . In some embodiments, the evaporation end of the heat pipe assembly includes a flat surface to be coupled directly or indirectly via a thermal interface material to the electronic component  102 . In other embodiments, the evaporation end of the heat pipe assembly is fitted within a thermally conductive block, which in turn is thermally coupled to the electronic component  102 . A heat pipe assembly  104  includes an evaporation end coupled to an electronics component  104 , such as a processor, and a condensation end coupled to the thermal bus  110 . The evaporation end of the heat pipe assembly  104  can be thermally coupled to the electronic component  104  in one of the manners described above in relation to the evaporation end of the heat pipe assembly  106 . A heat pipe assembly  112  includes an evaporation end coupled to the thermal bus  110  and a condensation end coupled to a cold plate  120 . The condensation end of the heat pipe assembly  112  can be thermally coupled to the cold plate  120  in one of the manners described above in relation to the evaporation end of the heat pipe assembly  106 . In this exemplary configuration, the cold plate  120  is coupled to the top edge  101  of the electronics board  100  to allow gravity to improve the efficiency of the heat pipe assemblies. However, this is not a requirement, and the cold plate can be positioned on any of the electronic board edges, with the heat pipe assemblies and thermal bus appropriately positioned to provide a condensation end of a heat pipe assembly at a location of the cold plate. 
     In some embodiments, the cold plate  120  is a fluid-based cold plate. The cold plate is made of a thermally conductive material configured with fluid channels to allow fluid to pass through. Heat is transferred from the heat pipe assembly  112  to the cold plate and to the fluid flowing through the channels of the cold plate. In some embodiments, the fluid-based cold plate is coupled to a cooling loop.  FIG. 7  illustrates the electronics board  100  coupled to an external fluid-based cooling loop. The cooling loop includes the fluid-based cold plate  120 , a heat rejector  140 , and a pump  142 . The fluid cold plate  120  is coupled to the heat rejector  140  and the pump  142  via fluid lines  144 . In other embodiments, the cold plate  120  can be any heat exchanging device that transfers heat from the heat pipe assembly  112 . 
     The thermal bus  110  is configured as an intermediate heat exchanger between two heat pipe assemblies. In some embodiments, the thermal bus  110  is formed by stacking the evaporating end of one heat pipe assembly on top of the condensation end of another heat pipe assembly, or stacking the condensation end of one heat pipe assembly on top of the evaporating end of another heat pipe assembly. In some embodiments, the ends of each heat pipe assembly that form a thermal bus include a flat surface to thermally couple with each other, either directly or indirectly via a thermal interface material. Although the ends of each heat pipe assembly  106 ,  108 , and  112  are described above as being cylindrical or flattened, the shape of the ends can be alternatively configured to mate with each other to form a larger contact surface area for thermal interface. Similarly, the ends of the heat pipe assemblies  106 ,  108 , and  112  can be configured to mate with the corresponding cold plate  120  ( FIG. 2 ), electronic component  102  ( FIG. 2 ), or electronic component  104  ( FIG. 2 ). 
     In some embodiments, each end of each heat pipe assembly that forms the thermal bus is fitted within a thermally conductive block, and the blocks are thermally coupled to each other.  FIG. 3A  illustrates a top down view of a first embodiment of the thermal bus.  FIG. 3B  illustrates a side view of the first embodiment of  FIG. 3A . The evaporation end of the heat pipe assembly  112  is positioned in a hole of a thermally conductive block  111 . The condensation end of the heat pipe assembly  106  is positioned in a hole of a thermally conductive block  107 . The condensation end of the heat pipe assembly  108  is positioned in a hole of a thermally conductive block  109 . The block  109  and the block  108  are each thermally coupled to the block  111 . In some embodiments, each of the blocks  107 ,  109 , and  111  have a flat surface, which are mated to each other. In some embodiments, a thermal interface material is positioned between the block  109  and the block  111 , and a thermal interface material is positioned between the block  107  and the block  111 . The block  111  is coupled to the blocks  107  and  109  by any conventional securing means including, but not limited to, adhesive, bonding material, solder, mechanical clamp, screw, or bolt, that enables and forms a thermal interface between the evaporation end of the heat pipe assembly  112  and the condensation ends of each of the heat pipe assemblies  106  and  108 . In some embodiments, the block and ends of the heat pipe assemblies are press fit together to ensure thermal conductivity between the heat pipe assemblies and the block. In some embodiments, an additional thermal interface material is positioned between the outer surface of the ends of the heat pipe assemblies and the surface in the holes of the block. 
     In some embodiments, the evaporating end of each of the heat pipe assemblies  106  and  108  are fitted within a common thermally conductive block. This common block is then thermally coupled to the block  111 . 
     As shown in  FIGS. 3A and 3B , the appropriate ends of the heat pipe assemblies  106 ,  108 ,  112  are fully embedded in the corresponding blocks  108 ,  109 ,  111 , respectively. Alternatively, the ends and the blocks can be configured such that an end is only partially embedded in a block such that a portion of the end is exposed. This exposed portion can then be thermally coupled to an exposed portion of a partially embedded end of another heat pipe assembly.  FIG. 3C  illustrates a side view of a second embodiment of the thermal bus. The thermal bus of  FIG. 3C  is similar to the thermal bus of  FIG. 3B  except that the ends of the thermal bus assemblies are not completely embedded in the blocks. Specifically, the evaporation end of a heat pipe assembly  112 ′ is partially embedded in a thermally conductive block  111 ′. A condensation end of a heat pipe assembly  106 ′ is partially embedded in a thermally conductive block  107 ′, and a condensation end of a heat pipe assembly  108 ′ is partially embedded in a thermally conductive block  109 ′. An exposed portion of the condensation end of the heat pipe assembly  106 ′ is thermally coupled to an exposed portion of the evaporation end of the heat pipe assembly  112 ′, either directly or indirectly via a thermal interface material. An exposed portion of the condensation end of the heat pipe assembly  108 ′ is thermally coupled to an exposed portion of the evaporation end of the heat pipe assembly  112 ′, either directly or indirectly via a thermal interface material. In some embodiments, the evaporation end of the heat pipe assembly  112 ′ and the condensation end of the heat pipe assemblies  106 ′ and  108 ′ are not cylindrical. In some cases, as is shown in  FIG. 3C , these ends are compressed together to form two opposing flat surfaces connected by two radial surfaces. One of the flat surfaces is mated to a flat surface of another end. 
     In some embodiments, the thermal bus  110  includes a single thermally conductive block including holes into which the ends of the heat pipe assemblies are positioned.  FIG. 3D  illustrates a side view of a third embodiment of the thermal bus. A thermally conductive block  130  includes a hole  136 , a hole  132 , and a hole  134 . The evaporation end of the heat pipe assembly  112  is positioned in the hole  136 . The condensation end of the heat pipe assembly  108  is positioned in the hole  132 . The condensation end of the heat pipe assembly is positioned in the hole  106 . The block  130  functions as a thermal interface material between the heat pipe assemblies  106 ,  108 , and  112 . In some embodiments, the block and ends of the heat pipe assemblies are press fit together to ensure thermal conductivity between the heat pipe assemblies and the block. In some embodiments, an additional thermal interface material is positioned between the outer surface of the ends of the heat pipe assemblies and the surface in the holes of the block. 
     In some embodiments, the heat pipe assemblies are coupled to the thermally conductive block such that the evaporating end(s) is positioned above the condensation end(s), as shown in  FIG. 3A-3D . In other embodiments, the heat pipe assemblies are coupled to the thermally conductive block such that the condensation end(s) is positioned above the evaporating end(s). In still other embodiments, there is no preference as to the position of the evaporation end or the condensation end in the block as long as there is sufficient thermal coupling between the evaporation end the conduction end. 
     Referring back to  FIG. 2 , a single heat pipe assembly is shown between each electronic component and the thermal bus, or between the thermal bus and the cold plate. In some embodiments, multiple heat pipe assemblies can be coupled between an electronic component and the thermal bus and/or between the thermal bus and the cold plate. The configuration in  FIG. 2  also shows two electronic components coupled to the thermal bus. It is understood that more or less than two electronic components can be coupled to the thermal bus. It is also understood that not every electronic component on the electronics board  100  need be coupled to the thermal bus. In some embodiments, only select electronic components are coupled to the thermal bus. The configuration of the thermal bus is application-specific to accommodate the application-specific number of heat pipe assemblies that are thermally coupled. Accordingly, where the thermal bus is configured using a thermally conductive block, as in  FIG. 3C , the number of holes in the block is also application-specific. 
     The use of a thermal bus can decrease the orientation effects as well as decrease the travel length of any single heat pipe assembly. For example, a single heat pipe assembly that couples the electronic component  104  to the cold plate  120  has a greater length than either the heat pipe assembly  108  or the heat pipe assembly  112 . In addition, each individual heat pipe assembly coupled to the thermal bus can be designed to optimize the local heat transfer characteristics corresponding to that particular heat pipe assembly. For example, the heat pipe assembly  108  can be optimized to the heat transfer characteristics in the electronic component  104 /thermal bus  110  domain, and the heat pipe assembly  112  can be optimized to the heat transfer characteristics in the thermal bus  110 /cold plate  120  domain. Such heat pipe assembly optimization can include, but is not limited to, the use of differently sized heat pipes, wick structures, and working fluid. 
     If the heat transfer path between a specific electronic component and the cold plate is considered as a series of thermally coupled heat pipe assemblies and thermal bus(es), then each heat pipe assembly in the series can be independently configured. For example, the heat pipe assembly  106  can have a different configuration than the heat pipe assembly  112 . Different heat pipe assemblies can have different interior wicking structures, different wicking mechanisms such as groove or foam, different types of fluids, and different physical dimensions. An advantage of using different heat pipe configurations is to optimize heat pipe performance for each heat pipe assembly. Individual heat pipe assemblies are optimized to work in specific temperature ranges. When the temperature extends beyond the designed temperature range, performance decreases. So one heat pipe assembly can be optimized differently than another heat pipe assembly to match the respective operating temperature ranges of each. As each individual heat pipe assembly has a limited operating temperature range, use of multiple heat pipe assemblies, as enabled by the thermal bus, extends the overall temperature range to which the cooling system can be applied. As opposed to using a single heat pipe assembly with a single fluid type and structure, the single heat pipe assembly is segmented into multiple heat pipe assemblies, each segment corresponding to a different temperature range and each segmented heat pipe assembly optimized to the corresponding temperature range. 
     By way of example, consider the heat transfer path between the electronic component  102  and the cold plate  120 , which includes the heat pipe assembly  106 , the thermal bus  110 , and the heat pipe assembly  112 . For hotter temperature ranges, such as at the electronics component  102 , water is a good fluid choice for use in a heat pipe assembly. However, water does not behave as well in extreme cold, such as at the interface at the cold plate  120 . If a single heat pipe assembly is used between the electronic component  102  and the cold plate  120 , this is an undesirable temperature range for water due to the low temperature at the cold plate  120 . However, alcohol does behave well at lower temperatures and is a good design choice for a heat pipe assembly coupled to the cold plate  120 . But alcohol does not behave well at hotter temperatures, such as at the electronics component  102 . If a single heat pipe assembly is used between the electronic component  102  and the cold plate  120 , this is an undesirable temperature range for alcohol due to the high temperature at the electronics component  102 . By using the two heat pipe assemblies  106  and  112 , the heat pipe assembly  106  can be designed using water, and the heat pipe assembly  112  can be designed using alcohol. This improves thermal performance of the overall heat removal process. It is understood that this is a simplified example, and that other factors other than the fluid type can be taken into consideration in determining the respective configurations of the heat pipe assemblies. 
     Selecting the proper fluid type is not the only challenge for a conventional single heat pipe assembly configuration. An electronic component is often conventionally cooled using a single heat pipe assembly that includes an evaporation portion coupled to the heat generating electronic component and a condensation portion having an air-fin structure. Air moving across the air-fin structure removes heat causing condensation of the fluid within the heat pipe assembly. In an exemplary application, an integrated circuit operates at an approximate temperature of 60 degrees Celsius. Ambient air temperature before heating is approximately 25-30 degrees Celsius, which results in an operating temperature range of approximately 30-35 degrees Celsius. However, if the heat removal process is improved using a cold plate instead of air cooling, the operating temperature range increases to approximately 45 degrees Celsius for a cold plate operating at a temperature of approximately 15 degrees Celsius. This increase in operating temperature range is enough to negatively impact the overall thermal performance of the single heat pipe assembly. 
     In an exemplary application of the multiple heat pipe assemblies and thermal bus of the present invention, an operating temperature of the thermal bus is approximately 30-35 degree Celsius. With the approximate operating temperature of the electronics component again being 60 degrees Celsius, the operating temperature range of the heat pipe assembly  106  is approximately 30-35 degrees Celsius, which is an effective temperature range and an effective absolute temperature for water. Within the operating temperature range of the cold plate  120  again being approximately 15 degrees Celsius, the operating temperature range of the heat pipe assembly  112  is approximately 15-20 degrees Celsius, which is also an effective temperature range and an effective absolute temperature for alcohol. The operating temperature of the thermal bus  110  is sufficiently low to condense the water in the heat pipe assembly  106 , and sufficiently high to evaporate the alcohol in the heat pipe assembly  112 . It is again understood that this is a simplified example, and that other factors other than the fluid type can be taken into consideration in determining the respective configurations of the heat pipe assemblies. 
     In general, the portion of the heat pipe assembly  106  coupled at the thermal bus  110  must provide condensation of the fluid used in the heat pipe assembly  106 , and the portion of the heat pipe assembly  112  coupled at the thermal bus  110  must provide evaporation of the fluid used in the heat pipe assembly  112 . These condition can be met by appropriate configuration of the heat pipe assemblies using different fluid types, different conditions such as pressure within the heat pipe, and different heat pipe physical structures. 
     The heat removal capacity of the heat pipe assembly can be adversely affected by bends in the heat pipe assembly structure. The use of a thermal bus eliminates or minimizes the number and severity of the bends. The cooling system of  FIG. 2  is described in terms of a single thermal bus coupling two heat pipe assemblies. These concepts can be extended to couple additional thermal buses and heat pipe assemblies in series. The use of multiple thermal buses can be used to eliminate multiple bends, to accommodate restricted space due to other components on or near the electronics board  100 , and to accommodate different locations of the cold plate  120  relative to the electronics board  100 . The use of multiple thermal buses can also be used to further optimize the local heat transfer characteristics, as described above. 
       FIG. 4  illustrates an exemplary block diagram of a cooling system including multiple thermal buses according to an embodiment of the present invention. The cooling system of  FIG. 4  is similar to the cooling system of  FIG. 2  with the addition of a second thermal bus  130 . Heat pipe assembly  114  functions similarly as the heat pipe assembly  112  except that the condensation end of the heat pipe assembly  114  is coupled to the thermal bus  130 , instead of to the cold plate  120 . An evaporation end of a heat pipe assembly  116  is thermally coupled to the condensation end of the heat pipe assembly  114  via the thermal bus  130 . A condensation end of the heat pipe assembly is thermally coupled to the cold plate  120 . The thermal bus  130  functions in a similar manner as the thermal bus  110 . The physical configuration of the thermal bus  130  can be the same as the configuration of the thermal bus  110 , such as both thermal buses include a thermally conductive block as in  FIG. 3C . Alternatively, the physical configuration of the thermal bus  120  can be different than the thermal bus  110 , such as one thermal bus having a thermally conductive block and the other thermal bus using the stacking configuration as in  FIGS. 3A-3B . 
     The cooling system of  FIG. 4  shows two thermal buses. In some embodiments, more than two thermal buses can be used, thermally coupled by one or more additional heat pipe assemblies. In general, the number of thermal buses and heat pipe assemblies used in the cooling system is limited only by the thermal performance requirements of the overall system. Each thermal bus can be independently configured to couple one or more condensation ends of heat pipe assemblies to one or more evaporation ends of other heat pipe assemblies. Thermal buses and heat pipe assemblies can be configured in series or in parallel. Heat can be transferred from one heat pipe assembly to multiple other heat pipe assemblies via the thermal bus, or heat can be transferred from multiple heat pipe assemblies to one or more other heat pipe assemblies via the thermal bus. In some embodiments, the heat generated by a specific electronic component can be transferred to the cold plate using dedicated heat pipe assemblies and thermal bus(es). 
     In addition to or alternatively to taking heat directly from the electronic components, an air-fin heat pipe assembly can be used to remove heat from the air flow crossing the electronics board. This air-fin heat pipe assembly can also be attached to a thermal bus. Subsequently, the heat taken from the air flow can then be transferred to the cold plate. In some embodiments, a heat sink is thermally coupled to one or more electronic components. Air flowing over the electronics board absorbs heat as it moves across the heat sink(s). This transfers heat from the electronic component to the heat sink to the air. 
       FIG. 5A  illustrates an exemplary block diagram of a cooling system including an air-fin heat pipe assembly and a thermal bus according to an embodiment of the present invention. The cooling system of  FIG. 5A  functions similarly as the cooling system of  FIG. 2  except that no heat pipe assembly is positioned on-device, that is there is no heat pipe assembly coupled directly to the electronic components  102  and  104 . Relative to the air flow direction across the electronics board  100 , an evaporation end of a air-fin heat pipe assembly  206  and an evaporation end of an air-fin heat pipe assembly  208  are positioned downstream of the electronic components  102  and  104 . In the exemplary configuration of  FIG. 5A , the evaporation ends of the air-fin heat pipe assemblies  206  and  208  are positioned at a back end, or air flow exit, of the electronics board  100 . The evaporation end of the air-fin heat pipe assembly  206  and the evaporation end of the air-fin heat pipe assembly  208  are each fitted with fins. A condensation end of the air-fin heat pipe assembly  206  is thermally coupled to the thermal bus  210 , and a condensation end of the air-fin heat pipe-assembly  208  is thermally coupled to the thermal bus  210 . Heat is transferred to air flowing over the electronics components  102  and  104  (and any other heat generating devices on the electronics board  100 ), or over heat sinks coupled to the electronics components  102  and  104 . As the heated air moves across the fins on the evaporation ends of the air-fin heat pipe assemblies  206  and  208 , heat is transferred to the evaporation ends, thereby evaporating the fluid within. In this manner, the temperature of the air exiting the electronics board  100  is decreased. 
     In an alternative configuration, air-fin heat pipe assemblies are positioned upstream of the electronics components.  FIG. 5B  illustrates an exemplary block diagram of a cooling system including an air-fin heat pipe assembly and a thermal bus according to another embodiment of the present invention. The cooling system of  FIG. 5B  functions similarly as the cooling system of  FIG. 5A  except that the evaporation ends of the air-fin heat pipe assembles  306  and  308  are positioned upstream of the electronic components  102  and  104 . In the exemplary configuration of  FIG. 5A , the evaporation ends of the air-fin heat pipe assemblies  306  and  308  are positioned at a front end, or air flow entrance, of the electronics board  100 . The evaporating end of the air-fin heat pipe assembly  306  and the evaporating end of the air-fin heat pipe assembly  308  are each fitted with fins. A condensation end of the air-fin heat pipe assembly  306  is thermally coupled to the thermal bus  310 , and a condensation end of the air-fin heat pipe-assembly  308  is thermally coupled to the thermal bus  310 . Air entering the electronics board is cooled by flowing over the air-fin heat pipe assemblies  306  and  308 . Heat is transferred to the cooled air flowing over the electronics components  102  and  104  (and any other heat generating devices on the electronics board  100 ), or over heat sinks coupled to the electronics components  102  and  104 . 
     The air-fin heat pipe assemblies can be placed at the entrance, exit, or both the entrance and exit of the of electronics board. When the air-fin heat pipe assemblies are placed at the exit, as shown in  FIG. 5A , the heat generated from the electronic components and transferred into the air flow can be absorbed by the air-fin heat pipe assemblies and rejected to the cold plate via the thermal bus. However, if the air-fin heat pipe assemblies are placed at the entrance, as shown in  FIG. 5B , the entering air is cooled before reaching the electronic components. This configuration is useful should the inlet air temperature rise above the optimal value. In most cooling systems, the higher air temperature necessitates the increase in air flow to increase the cooling capacity. This increase in air flow requires increase power to the cooling fans that generate the air flow. In this event, the overall energy efficiency of the cooling system is decreased. However, placed at the entrance the air-fin heat pipe assemblies serve as pre-coolers or air temperature controllers. The increase in air flow therefore, is reduced if not eliminated. 
       FIGS. 4A-4B  show the air-fin heat pipe assemblies attached to the thermal bus. Alternatively, the air-fm heat pipe assemblies can be directly attached to the cold plate  120 . If the limitations of the air-fin heat pipe assembly and the physical layout of the electronics board prevent the direct connection, a thermal bus can be used. 
     The air-fin heat pipe assembly design can be combined with the on-device heat pipe assembly design as a hybrid design which incorporates both heat removal directly off electronic components as well as from the air flow. This hybrid configuration allows for the direct removal of heat from the larger heat emitting components as well as the indirect heat removal from the aggregate of the smaller heat emitting components. 
       FIG. 6  illustrates an exemplary block diagram of a cooling system including a hybrid configuration according to an embodiment of the present invention. The exemplary hybrid configuration of  FIG. 6  includes the on-device heat pipe assembly design as shown in  FIG. 2  and the air cooling air-fin heat pipe assembly design as shown in  FIG. 5A . Air is heated by flowing over electronic components  116  and  118  (and any other heat generating devices on the electronics board  100 ), or over heat sinks coupled to the electronics components  116  and  118 . Heat is transferred from the heated air to an air-fin heat pipe assembly  406  and an air-fin heat pipe assembly  408 , which are each thermally coupled to a thermal bus  410 . A heat pipe assembly  402  thermally couples the electronic component  102  to the thermal bus  410 , and a heat pipe assembly  404  thermally couples the electronic component  104  to the thermal bus  410 . A heat pipe assembly  412  thermally couples the thermal bus  410  to the cold plate  120 . Although the hybrid configuration of  FIG. 6  is shown and described in terms of a downstream configuration, it is understood that the upstream configuration of the air cooling air-fin heat pipe assembly design of  FIG. 5B  can be alternatively used or added to the hybrid configuration. 
     An attribute of the hybrid design is its ability to self-regulate the cooling capacity of the heat pipe assembly/thermal bus cooling system. Under the ideal conditions, the hybrid heat pipe assembly is designed such that the heat directly removed from the electronic components, as well as the heat indirectly removed via the air flow, is rejected to the cold plate. However, if the heat generated from the electronic components that are directly attached to the heat pipe assemblies exceed that of the cold plate, or other secondary loop capacity, the air-fin heat pipe assemblies that are attached to the thermal bus can act as a rejecter of heat to the air flow. The switch from absorbing to rejecting heat is passive in nature and the switching point can be designed into the original assembly. This scenario can occur if there is a reduction in the cold plate cooling or secondary cooling loop capacity. Therefore, the air-fin heat pipe assemblies attached to the thermal bus can offer some relief or redundancy if the main cooling path is compromised. 
     In another scenario, the hybrid heat pipe assembly design can be used as an air conditioner for the air exiting the electronics board. If a certain exiting air temperature is desired, the air-fin heat pipe assemblies can either reject or absorb heat to or from the air flow as needed. If the exiting air is above a desired temperature, the air-fin heat pipe assemblies can absorb heat from the air. If the exiting air is below the desired temperature, perhaps due to low inlet air temperature or electronic components operating at low powers, the air-fin heat pipe assemblies can reject heat into the air flow. Again, the control point (desired temperature) can be designed into the original heat pipe assembly/thermal bus cooling system. The need for air temperature control both at the entrance and exit of the electronics board is desired for the efficient operation of any rack level, or room level cooling system design. 
     The various configurations of the cooling system on the electronics board  100  provide adaptable solutions for application-specific thermal requirements. Such thermal requirements may be dictated by the configuration and components on the electronics board itself and/or the configuration and components positioned before or after the electronics board  100  along the air flow direction. The configuration of the cooling system can be determined to account for various input air flow temperatures and to provide various output air flow temperatures. 
     It is typically more efficient to transfer heat directly off the electronic component, such as in  FIGS. 2 and 4 , than to cool heated air, as in  FIGS. 5A-5B . However, there may be limitations such as board layout and relative electronic component positions that necessitates one implementation versus another. As such, any of the heat transfer concepts and configurations described above can be combined and manipulated. 
     The thermal bus can be modular in design. The heat pipe assembly for each electronic component can be readily attached and detached from the thermal bus. This allows for easy access to the electronic component as well as easy optimization of the heat pipe solution for each electronic component. 
     Although heat pipe assemblies typically utilize wicking structures in addition to gravitational effects, alternative structures can be used that rely solely on gravity, for example vapor chambers or thermal siphons. In blade server applications where the blade server is positioned on edge, certain configurations may require that the position of the evaporation end relative to the condensation end works against gravity. This situation necessitates the use of a heat pipe assembly that includes wicking structure. In other applications, however, such as when the electronics board is horizontally positioned instead of vertically positioned as in the blade server, the alternative structures can be used. In an exemplary configuration, a vapor chamber can be positioned on an electronics component. An evaporating end of the vapor chamber is at the bottom end proximate the electronics component, and a condensing end is at the vertical top end of the vapor chamber. An evaporating end of a heat pipe assembly is thermally coupled to the top end of the vapor chamber. A thermal bus is formed between the top end of the vapor chamber and the evaporation end of the heat pipe assembly. This configuration initially removes heat from the electronic component in a vertical direction using the vapor chamber. This type of thermal bus provides an additional axis that provides further alternatives for designing the overall configuration of the cooling system. 
     The present invention has been described in terms of specific embodiments incorporating details to facilitate the understanding of the principles of construction and operation of the invention. Such reference herein to specific embodiments and details thereof is not intended to limit the scope of the claims appended hereto. It will be apparent to those skilled in the art that modifications may be made in the embodiment chosen for illustration without departing from the spirit and scope of the invention.