Patent Publication Number: US-2007119199-A1

Title: System and method for electronic chassis and rack mounted electronics with an integrated subambient cooling system

Description:
TECHNICAL FIELD OF THE INVENTION  
      This invention relates generally to the field of cooling systems and, more particularly, to a system and method for electronic chassis and rack mounted electronics with an integrated subambient cooling system.  
     BACKGROUND OF THE INVENTION  
      Chassis mounted electronics continue to generate higher and higher levels of heat or thermal energy. With the generation of such thermal energy, such chassis mounted electronics need to be cooled to prevent overheating. However, conventional cooling systems do not always meet the current or future needs for such chassis based systems.  
     SUMMARY OF THE INVENTION  
      According to one embodiment of the invention, a cooling system for a heat-generating structure that is disposed in an environment having an ambient pressure comprises a fluid coolant and three structures. The first structure allows the heat generating structure to removably couple to the cooling system. The second structure reduces a pressure of the fluid coolant to a subambient pressure at which the fluid coolant has a boiling temperature less than a temperature of the heat-generating structure. The third structure directs a flow of the fluid coolant in the form of a liquid at the subambient pressure in a manner causing the fluid coolant to be brought into thermal communication with the heat-generating structure where heat from the heat-generating structure causes the fluid coolant in the form of the liquid to boil and vaporize so that the fluid coolant absorbs heat from the heat-generating structure as the fluid coolant changes state.  
      Certain embodiments of the invention may provide numerous technical advantages. For example, a technical advantage of one embodiment may include the capability to enhance cooling capability for chassis based systems. Other technical advantages of other embodiments may include the capability to enable flexibility in the integration of a cooling system with a heat generating structure or the capability to compensate for adverse orientations, including tilting, that may be experienced by a cooling system.  
      Although specific advantages have been enumerated above, various embodiments may include all, some, or none of the enumerated advantages. Additionally, other technical advantages may become readily apparent to one of ordinary skill in the art after review of the following figures and description.  
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      For a more complete understanding of example embodiments of the present invention and its advantages, reference is now made to the following description, taken in conjunction with the accompanying drawings, in which:  
       FIG. 1  is a block diagram of a cooling system, according to an embodiment of the invention; and  
       FIG. 2  show an integration of a cooling system with a chassis of a circuit card assembly, according to another embodiment of the invention;  
       FIG. 3  show an integration of a cooling system with a chassis of a circuit card assembly, according to another embodiment of the invention;  
       FIGS. 4A and 4B  show an integration of a cooling system with a rack, according to another embodiment of the invention;  
       FIG. 5  is a block diagram of a cooling system, according to another embodiment of the invention;  
       FIGS. 6A and 6B  illustrate a problem that can develop in a channel;  
       FIGS. 7A, 7B , and  7 C illustrate a channel, according to an embodiment of the invention;  
       FIG. 8  is an illustration of an interior wall of cut-away channel, according to another embodiment of the invention; and  
       FIG. 9  illustrates a channel, according to another embodiment of the invention.  
    
    
     DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS OF THE INVENTION  
      It should be understood at the outset that although example embodiments of the present invention are illustrated below, the present invention may be implemented using any number of techniques, whether currently known or in existence. The present invention should in no way be limited to the example embodiments, drawings, and techniques illustrated below, including the embodiments and implementation illustrated and described herein. Additionally, the drawings are not necessarily drawn to scale.  
      As briefly referenced in the Background, current cooling systems do not always meet the current or future needs for chassis based systems. The teachings of certain embodiments of the invention recognize that systems that rely purely on convection or conduction cooling provide limited heat removal capability. Some air flow systems require 4° C. inlet air to maintain the component temperatures limits, thereby causing undesirably large temperature gradients. Other air flow systems operating at higher temperatures (e.g., 49° C.) require undesirably high air mass flow rates. Additionally, with conventional systems, there is little, if any, flexibility in the integration of the cooling system with the structure in which they are cooling. Accordingly, teachings of some embodiments of the invention recognize a cooling system that enhances cooling capability for chassis based systems. Additionally, teachings of some embodiments of the invention recognize a cooling system that enables flexibility in the integration of the cooling system with a heat generating structure.  
      Cooling systems may additionally be subjected to adverse orientations in adverse environments that tilt and rock the cooling systems. Accordingly, teachings of some embodiments of the invention recognize components that may be utilized in a cooling system to compensate for such adverse orientations.  
       FIG. 1  is a block diagram of a cooling system  10 , according to an embodiment of the invention. In this embodiment, the cooling system  10  is shown cooling a circuit card assembly  12 . Electronic or circuit components within the circuit card assembly  12  may take on a variety of configurations. Accordingly, the details of the circuit card assembly are not illustrated and described. The cooling system  10  of  FIG. 1  includes channels  23  and  24 , pump  46 , inlet orifices  47  and  48 , a condenser heat exchanger  41 , an expansion reservoir  42 , and a pressure controller  51 .  
      The circuit card assembly  12  may be arranged and designed to conduct heat or thermal energy from the electronic or circuit components on the circuit card assembly  12  to the channels  23 ,  24 . To receive this thermal energy or heat, the channels  23 ,  24  may be disposed on an edge of the circuit card assembly or may extend through portions of the circuit card assembly  12 , for example, through a thermal plane of circuit card assembly  12 . In particular embodiments, the channels  23 ,  24  may extend up to the circuit components, directly receiving thermal energy from the circuit components. Although two channels  23 ,  24  are shown in the embodiment of  FIG. 1 , one channel or more than two channels may be used to cool a circuit card assembly  12 , according to other embodiments of the invention.  
      In operation, a fluid coolant flows through each of the channels  23 ,  24 . As discussed later, this fluid coolant may be a two-phase fluid coolant, which enters inlet conduits  25  of channels  23 ,  24  in liquid form. Absorption of heat from the circuit card assembly  12  causes part or all of the liquid coolant to boil and vaporize such that some or all of the fluid coolant leaves the exit conduits  27  of channels  23 ,  24  in a vapor phase. To facilitate such absorption or transfer of thermal energy, the channels  23 ,  24  may be lined with pin fins or other similar devices which increase surface contact between the fluid coolant and walls of the channels  23 ,  24 . Additionally, in particular embodiments, the fluid coolant may be forced or sprayed into the channels  23 ,  24  to ensure fluid contact between the fluid coolant and the walls of the channels  23 ,  24 .  
      The fluid coolant departs the exit conduits  27  and flows through the condenser heat exchanger  41 , the expansion reservoir  42 , a pump  46 , and a respective one of two orifices  47  and  48 , in order to again to reach the inlet conduits  25  of the channels  23 ,  24 . The pump  46  may cause the fluid coolant to circulate around the loop shown in  FIG. 1 . In particular embodiments, the pump  46  may use magnetic drives so there are no shaft seals that can wear or leak with time  
      The orifices  47  and  48  in particular embodiments may facilitate proper partitioning of the fluid coolant among the respective channels  23 ,  24  , and may also help to create a large pressure drop between the output of the pump  46  and the channels  23 ,  24  in which the fluid coolant vaporizes. The orifices  47  and  48  may have the same size, or may have different sizes in order to partition the coolant in a proportional manner which facilitates a desired cooling profile.  
      A flow  56  of fluid (either gas or liquid) may be forced to flow through the condenser heat exchanger  41 , for example by a fan (not shown) or other suitable device. In particular embodiments, the flow  56  of fluid may be ambient fluid. The condenser heat exchanger  41  transfers heat from the fluid coolant to the flow  56  of ambient fluid, thereby causing any portion of the fluid coolant which is in the vapor phase to condense back into a liquid phase. In particular embodiments, a liquid bypass  49  may be provided for liquid fluid coolant that either may have exited the channels  23 ,  24  or that may have condensed from vapor fluid coolant during travel to the condenser heat exchanger  41 .  
      The liquid fluid coolant exiting the condenser heat exchanger  41  may be supplied to the expansion reservoir  42 . Since fluids typically take up more volume in their vapor phase than in their liquid phase, the expansion reservoir  42  may be provided in order to take up the volume of liquid fluid coolant that is displaced when some or all of the coolant in the system changes from its liquid phase to its vapor phase. The amount of the fluid coolant which is in its vapor phase can vary over time, due in part to the fact that the amount of heat or thermal energy being produced by the circuit card assembly  12  will vary over time, as the circuit card assembly  12  system operates in various operational modes.  
      Turning now in more detail to the fluid coolant, one highly efficient technique for removing heat from a surface is to boil and vaporize a liquid which is in contact with a surface. As the liquid vaporizes in this process, it inherently absorbs heat to effectuate such vaporization. The amount of heat that can be absorbed per unit volume of a liquid is commonly known as the latent heat of vaporization of the liquid. The higher the latent heat of vaporization, the larger the amount of heat that can be absorbed per unit volume of liquid being vaporized.  
      The fluid coolant used in the embodiment of  FIG. 1  may include, but is not limited to mixtures of antifreeze and water. In particular embodiments, the antifreeze may be ethylene glycol, propylene glycol, methanol, or other suitable antifreeze. In other embodiments, the mixture may also include fluoroinert. In particular embodiments, the fluid coolant may absorb a substantial amount of heat as it vaporizes, and thus may have a very high latent heat of vaporization.  
      Water boils at a temperature of approximately 100° C. at an atmospheric pressure of 14.7 pounds per square inch absolute (psia). In particular embodiments, the fluid coolant&#39;s boiling temperature may be reduced to between 55-65° C. by subjecting the fluid coolant to a subambient pressure of about 2-3 psia. Thus, in the embodiment of  FIG. 1 , the orifices  47  and  48  may permit the pressure of the fluid coolant downstream from them to be substantially less than the fluid coolant pressure between the pump  46  and the orifices  47  and  48 , which in this embodiment is shown as approximately 12 psia. The pressure controller  51  maintains the coolant at a pressure of approximately 2-3 psia along the portion of the loop which extends from the orifices  47  and  48  to the pump  46 , in particular through the channels  23  and  24 , the condenser heat exchanger  41 , and the expansion reservoir  42 . In particular embodiments, a metal bellows may be used in the expansion reservoir  42 , connected to the loop using brazed joints. In particular embodiments, the pressure controller  51  may control loop pressure by using a motor driven linear actuator that is part of the metal bellows of the expansion reservoir  42  or by using small gear pump to evacuate the loop to the desired pressure level. The fluid coolant removed may be stored in the metal bellows whose fluid connects are brazed. In other embodiments, the pressure controller  51  may utilize other suitable devices capable of controlling pressure.  
      In particular embodiments, the fluid coolant flowing from the pump  46  to the orifices  47  and  48  may have a temperature of approximately 55° C. to 65° C. and a pressure of approximately 12 psia as referenced above. After passing through the orifices  47  and  48 , the fluid coolant may still have a temperature of approximately 55° C. to 65° C., but may also have a lower pressure in the range about 2 psia to 3 psia. Due to this reduced pressure, some or all of the fluid coolant will boil or vaporize as it passes through and absorbs heat from the channels  23  and  24 .  
      After exiting the exits ports  27  of the channels  23 ,  24 , the subambient coolant vapor travels to the condenser heat exchanger  41  where heat or thermal energy can be transferred from the subambient fluid coolant to the flow  56  of fluid. The flow  56  of fluid in particular embodiments may have a temperature of less than 50° C. In other embodiments, the flow  56  may have a temperature of less than 40° C. As heat is removed from the fluid coolant, any portion of the fluid which is in its vapor phase will condense such that substantially all of the fluid coolant will be in liquid form when it exits the condenser heat exchanger  41 . At this point, the fluid coolant may have a temperature of approximately 55° C. to 65° C. and a subambient pressure of approximately 2 psia to The fluid coolant may then flow to pump  46 , which in particular embodiments  46  may increase the pressure of the fluid coolant to a value in the range of approximately 12 psia, as mentioned earlier. Prior to the pump  46 , there may be a fluid connection to an expansion reservoir  42  which, when used in conjunction with the pressure controller  51 , can control the pressure within the cooling loop. 3 psia.  
      It will be noted that the embodiment of  FIG. 1  may operate without a refrigeration system. In the context of electronic circuitry, such as may be utilized in the circuit card assembly  12 , the absence of a refrigeration system can result in a significant reduction in the size, weight, and power consumption of the structure provided to cool the circuit components of the circuit card assembly  12 .  
      Although components of one embodiment of a cooling system  10  have been shown in  FIG. 1 , it should be understood that other embodiments of the cooling system  10  can include more, less, or different component parts. For example, although specific temperatures and pressures have been described for one embodiment of the cooling system, other embodiments of the cooling system  10  may operate at different pressures and temperatures. Additionally, in some embodiments a coolant fill port and/or a coolant bleed port may be utilized with metal-to-metal caps to seal them. Further, in some embodiments, all or a portion of the joints between various components may be brazed, soldered or welded using metal-to-metal seal caps.  
       FIG. 2  show an integration of a cooling system  100  with a chassis  162  of a circuit card assembly  112 , according to another embodiment of the invention. The cooling system  100  of  FIG. 2  may utilize similar or different component parts than those outlined in the block diagram of the cooling system  10  of  FIG. 1 . In this integration, components of the cooling system  100  such as, but not limited to, the condenser heat exchanger, the pump, the pressure controller, and the expansion reservoir may be disposed in an end piece  160  on one end of the chassis  162 . In a manner similar to that described above with reference to  FIG. 1 , the components of the end piece  160  may circulate fluid coolant through channels  123 ,  124  that are disposed in coldwalls  163  of the chassis  162 . For example, subambient pressurized fluid coolant may enter the channels  123 ,  124  disposed within the coldwalls  163  in a substantially liquid state.  
      Thermal energy from the circuit card assembly  112  boils or vaporizes at least a portion of the subambient fluid coolant, allowing in particular embodiments a high heat flux or high heat load. The fluid coolant exits the channels  124  disposed in the coldwalls  163  in a substantially vapor state. The heat inherent within the vapor fluid coolant is then removed as the vapor fluid coolant travels through the condensing heat exchanger in the end piece  160 . To facilitate this removal, the end piece  160  may include vents  157  (only one shown in  FIG. 2 ) for the heat exchanger (not explicitly shown. The vents  157  are operable to interact with a flow of ambient fluid for transfer of thermal energy. Upon condensing, the fluid coolant may be circulated back to the channels  123 ,  124 .  
       FIG. 3  show an integration of a cooling system  200  with a chassis  262  of a circuit card assembly  212 , according to another embodiment of the invention. The integration of the cooling system  200  with the chassis  262  may be similar to that described above with reference to the embodiment of  FIG. 2 , except that the chassis  262  of  FIG. 3  is removable from the cooling system  200 . In this embodiment, the cooling system  200  may have similar or different features than the cooling system  100  of  FIG. 2  and the cooling system  10  of  FIG. 1 . For example, the cooling system  200  of  FIG. 3  may includes channels  223 ,  224  and an end piece  260  with vents  257 . The end piece  260 , similar to end piece  160  of  FIG. 2  may include, but is not limited to, a heat exchanger, a pump, a pressure controller, and an expansion reservoir. The inlet conduit  225  and exit conduit  227  for the cooling system  200  can also be seen.  
      The chassis  262  includes an inner chassis wall  265 , which can be coupled to the channels  223 ,  224  in a variety of manners, including, but not limited to, bolting, clamping, or use of a variety of actuating devices. Thermal energy or heat may be conducted from components of the circuit card assembly  212  to the inner chassis wall  265  to the channels  223 ,  224 . From the channels  223 ,  224 , the thermal energy or heat may be transferred through the remaining portion of the cooling system  200  in a similar manner to that described above with reference to  FIG. 2 , namely through vaporization in the channels  223 ,  224  and then offloading to a flow of ambient fluid in the condensing heat exchanger in the end piece  260 , for example, through use of the vent  257 .  
      With the embodiment of  FIG. 3 , the circuit card assembly  212  and chassis  262  can be can be removed and replaced without disrupting or disassembling the sealed cooling system  200 . Although the channels  223 ,  224  have been described as coupling to an inner chassis wall  265  in this embodiment, in other embodiments the cooling system may be used to circulate flow directly to the circuit card assembly  212 . In such an embodiment, components of the circuit card assembly  212  may be attached to a hollow thermal plane, which may include any of a variety of features to direct flow or improve heat transfer. In this embodiment, the fluid coolant may enter the hollow thermal plane in a liquid state and vaporize within the thermal plane. Such an embodiment may provide high heat transfer at substantially low fluid flow rates.  
       FIGS. 4A and 4B  show an integration of a cooling system  300  with a rack  380 , according to another embodiment of the invention. The rack  380  may be designed to hold a plurality of circuit card assemblies  312  and their associated chassis using shelves  382  or other suitable components.  
      Similar to the embodiments of  FIGS. 2 and 3 , portions of the cooling system  300  may be may be disposed in an end piece  360  on an end of the rack  380 . The end piece  360  may include, but is not limited to, a heat exchanger, a pump, a pressure controller, and a expansion reservoir. Further details of other components that may be used with a cooling system  300  are described below in  FIG. 5  with reference to another cooling system  400 .  
      The cooling system  300  of  FIGS. 4A and 4B  includes a coolant manifold  308 , which may deliver liquid coolant (e.g., received from the end piece  360 ) and receive vapor coolant (e.g, for delivery to the end piece  360 ). To deliver such liquid coolant and receive vapor coolant, the coolant manifold  308  may be arranged in a variety of configurations. In particular embodiments, the coolant manifold  308  may be vertically disposed in a rear portion of the rack  380 .  
      One or more electronic chassis  362  may respectively be plugged into the manifold  308  to obtain cooling functionality. The chassis  362  may have a fluid channel  324  in its wall, which contain an inlet port  325  (e.g., for substantial liquid fluid coolant) and an exit port  327  (e.g., for substantially vapor fluid coolant). The inlet port  325  and the exit port  327  of the electronic chassis  362  may respectively be fluidly coupled to the manifold  308  using a variety of fluid coupling techniques, including but not limited to techniques which utilize seals, O-rings, and other devices. In this embodiment, each chassis  362  may utilize the centralized cooling system  300  without having its own separate cooling system.  
      Although the chassis  362  has been described as fluidly coupling to a coolant manifold  308  in the rack  380  in this embodiment, in other embodiments, the rack  380  may provide a series of coolant channels plumbed into the walls of the rack  380 . Accordingly, each chassis  362  would simply slide into its allocated slot where it may be coupled or clamped to the coolant channels in a manner similar to that described above with reference to  FIG. 3 . An advantage of such an embodiment is that the cooling system may be sealed. Accordingly, minimized perturbances to such a sealed system would occur during insertion or removal of a chassis  362 .  
       FIG. 5  is a block diagram of a cooling system  400 , according to another embodiment of the invention. The cooling system  400  of  FIG. 5  may operate in a similar to the cooling system  10  of  FIG. 1 ; however, the cooling system  400  of  FIG. 5  also incorporates an air removal system  490 . For a variety of reasons, unintended air or other fluids may be introduced into the cooling system  400 . For example, with reference to  FIG. 4 , the insertion of the inlet port  325  and the exit port  327  into the coolant manifold  308  may undesirably insert air or air may undesirably leak into the system, for example, through O-ring connections used in a fluid coupling between the inlet port  325  and the manifold  380  or the exit port  327 . Accordingly the cooling system  400  utilize the air removal system  490  to remove air from the cooling system  400 . The air removal system  490  in the embodiment of  FIG. 5  includes an air pump  492 , a reclamation heat exchanger  494 , an air trap  496 , and a reclamation fill valve  498 .  
      With reference to  FIG. 5 , the cooling loop for the cooling system  400  is similar to cooling loop for the cooling system  10  of  FIG. 1  for example, including a pump  460 , an expansion reservoir  442 , a pressure controller  451 , and a condenser heat exchanger  441 . However, air leaks  402  may enter the system at a rack  480  and travel to the condenser heat exchanger  441 . At the condenser heat exchanger  441 , condensed coolant liquid may pass though while air (and any associated coolant vapor that may be present therein) may be pumped using air pump  492  to a reclamation heat exchanger  494 . The reclamation heat exchanger  494  may cool the air/coolant vapor combination, which condenses the vapor from the air stream being removed from the bottom of the condenser heat exchanger  441 . Coolant separates from the air in a trap  496  while the air exits through a vent  495 . A level switch  497  may be in communication with a reclamation fill valve  498  to allow the reclamation fill valve  498  to open when recovered coolant is present. The recovered coolant may be reintroduced to the loop through the reclamation fill valve  498  and a conduit in communication with the pump  446 . Although one example of an air removal system  490  has been shown with reference to  FIG. 5 , other air removal systems may be used in other embodiments of the invention with more, less, or alternative component parts.  
       FIGS. 6A and 6B  illustrate a problem that can develop in a channel  524 . In particular embodiments, the channel  524  may be positioned adjacent a chassis that is subjected to movement. For example, the chassis and channel  524  may be subjected to as much as plus or minus 60° rolling and pitching in a combat vehicle. In particular embodiments, it is desirable for the interior of the wall of the channel  524  to be wet, for example, to ensure appropriate heat transfer. Accordingly, a chamber  502  of the channel  524  may be substantially filled with liquid fluid coolant. Vapor that develops during boiling heat transfer needs to be allowed to exit the channel  524 , for example, through a an exit port  527 . However, during such rolling and pitching, an undesirable vapor pocket  590  can arise as shown in  FIG. 6B . When such a vapor pocket  590  develops, vapor does not exit through the exit port  527 .  
       FIGS. 7A, 7B , and  7 C illustrate a channel  724 , according to an embodiment of the invention. Given the problem with the development of vapor pockets, the configuration of the channel  724  of  FIGS. 7A, 7B , and  7 C minimizes development of vapor pockets. The channel  724  includes a coolant chamber  702 , a vapor passage  704 , a rib  706 , a check ball  708 , a ball stop  710 , a ball seat  712 , an inlet port  725 , and an exit port  727 .  
      When the channel  724  is level as shown in  FIG. 7A , vapor can exit from either of the passageways  713 ,  714  in the upper corners of the chamber  702 . When the channel  724  is tilted with the exit port  727  upward, the check ball  708  rests on the ball stop  710  as shown in  FIG. 7B . In  FIG. 7B , the vapor will exit from the passageway  713  adjacent the ball seat  712  as the check ball  708  does not block passageway  713 . When the channel  724  is tilted with the exit port  727  downward, the check ball  708  rests on the ball seat  712  as shown in  FIG. 7C . In this case, the check ball  708  closes the passageway  713 , which forces the vapor to exit through passageway  714 .  
      In the embodiment of  FIGS. 7A, 7B , and  7 C, the fluid coolant remains in contact with the wall of the chamber  702  which, for example, may be lined with pin fins. Accordingly, proper heat transfer may occur while still allowing vapor to exit the exit port  727 .  
       FIG. 8  is an illustration of an interior wall  894  of cut-away channel  824 , according to another embodiment of the invention. The embodiment of  FIG. 8  minimizes development of vapor pockets by using internal jet impingement. The interior wall  894  of the channel  824  includes a plurality of pin fins  896  extending therefrom. With internal jet impingement, coolant jets with small nozzles (not expressly shown) direct coolant on to the plurality of pin fins  896 . The liquid fluid vaporizes upon contact with the pins fins  896 . Accordingly, liquid fluid coolant is not free standing and vapor pockets do not develop. In operation, the plurality of pin fins  896  may linearly be aligned in clusters or strips that coincide with a location where heat transfer is expected to occur, for example, an edge of the circuit card assembly.  
       FIG. 9  illustrates a channel  924 , according to another embodiment of the invention. For purposes of illustration, portions of the channel  924  are shown in a cut-way view. The channel  924  of  FIG. 9  includes a plurality of walled areas or passages  902  connected by a common vapor passage  904 . Each of the passages  902  may be fed liquid coolant through a coolant feed hole  905 . In particular embodiments, each of the passages  902  may be lined with pin fins  906 . The pin fins  906  may couple to a plane of thermal transfer and extend outwards across each passage  902  to increase surface area in the thermal transfer. Each of the coolant feed holes  905  may be connected through a common feed passage  906 , which receives the liquid coolant from an inlet conduit  925 .  
      In particular embodiments, the feed passage  906  may be disposed in a separate sheet of material within the channel  924 . For example, the passages  902  may be directly adjacent the plane of thermal transfer while the feed passage  906  is at least one layer removed from the plane of thermal transfer. Controlled liquid coolant may be forced into the bottom of each respective feed hole  905  up through passages  902  transversing the plurality of pin fins  906 . As the liquid fluid coolant boils to a vapor state, the vapor in each passage  902  will move up to the common vapor passage  904 . If the channel  924  is tilted, liquid coolant may run out of one of the passages  902 , cascading over another passage  902 . However, vapor may still escape to the common vapor passage  904  and out an exit port  927 . Each one of the passages  902  may correspond to a board for a circuit card assembly or another location where heat may be expected.  
      Although the present invention has been described with several embodiments, a myriad of changes, variations, alterations, transformations, and modifications may be suggested to one skilled in the art, and it is intended that the present invention encompass such changes, variations, alterations, transformation, and modifications as they fall within the scope of the appended claims.