Abstract:
A variable gap thermal interface is coupled with a cold or hot plate, forming a low thermal resistance connection between an electronic device module containing at least one heat generating electronic device and a rack or other structure. The variable gap thermal interface and the cold or hot plate are provided in a configuration to allow quick-disconnect of the electronic device module from the rack, allowing for a wide dimensional tolerance between the module and the rack while maintaining a reliable thermal connection. An embodiment including a plurality of server modules within a server rack in conformance with the present invention, allows the replacement of server modules while powered without any disconnection or reconnection of hoses to cold plates used in cooling the server modules, thus greatly reducing the probability of leaks and resulting damage to the system.

Description:
FIELD OF THE INVENTION 
     The present invention is related generally to the field of heat transfer and more specifically to the field of thermal contact resistance during heat transfer. 
     BACKGROUND OF THE INVENTION 
     As modern electronic devices decrease in size and increase in speed, power density rises dramatically. As power density rises, so does the heat produced by these devices. Traditionally, designers have relied on airflow over heat generating parts for cooling. In vacuum tube systems, large enclosures with numerous ventilation openings allowed the heated air to escape the enclosure. Later systems, such as many personal computers, included fans within an enclosure to draw cool air from outside the enclosure force it over the heat generating devices, and push the warmed air out through ventilation openings. 
     Many large modern electronic systems require more cooling than simple airflow, whether forced or not, is capable of providing. Some large computers use a network of cold plates plumbed together with water lines to use water as a thermal liquid carrying the heat away from the heat generating devices. Such systems are very expensive and since each cold plate must be plumbed, repair of these systems becomes very complicated. For example, in replacing a circuit board that has one or more heat generating devices cooled by cold plates, each individual cold plate must either be removed from the device or the plumbing must be disconnected from the overall system before the circuit board can be removed. Use of liquid cooling may make hot swapping boards impossible. The connectors for the liquid would have to be opened in areas of the system including live voltages situated such that even a small spill would be very likely to result in a short circuit with resulting damage to the system, and possibly the user. This makes maintenance of such systems more time-consuming and therefore more costly, particularly when including the risk of leaks. 
     Modern computer servers often comprise a number of individual server modules plugged into racks that supply power to the modules and interconnect the modules to memory, storage, and each other. Such racks present difficult thermal problems, since a large number of heat generating devices are often placed within relatively small server modules that are then placed together tightly in the rack. Ventilation may be constrained by the rack and it&#39;s necessary components, and by the fact that often users will want to place servers close together to reduce floor space required in their computer rooms. Liquid cooling becomes very attractive in situations like this, since liquids are capable of carrying a much greater thermal load than air. However, liquid cooling each server module would require plumbing each server module which, while possible, would eliminate much of the benefit of having readily replaceable server modules. 
     Another problem encountered in systems allowing easy replacement of boards is the issue of tolerance between the board and the rack. Often even boards of identical design will have small manufacturing tolerance differences in their dimensions. Thermal transfer systems relying on contact between a board and an external heat sink may require a greater dimensional tolerance from board to board than may be available with standard thermal grease or elastomeric conductors. Thermal grease only provides a few mils of tolerance. For many applications this is insufficient. This problem only gets worse when systems do not have dedicated slots for each board design, but allow differing boards to be placed in any given location. These different boards may include different thermal transfer needs. They may include heat sinks at different locations on the board and may generate different amounts of power. These problems make it difficult to design a simple heat transfer system for a large system that still allows for flexibility of system configurations. 
     SUMMARY OF THE INVENTION 
     A variable gap thermal interface is coupled with a cold or hot plate, forming a low thermal resistance connection between an electronic device module containing at least one heat generating electronic device and a rack or other structure. The variable gap thermal interface and the cold or hot plate are provided in a configuration to allow quick-disconnect of the electronic device module from the rack, allowing for a wide dimensional tolerance between the module and the rack while maintaining a reliable thermal connection. An embodiment including a plurality of server modules within a server rack in conformance with the present invention, allows the replacement of server modules while powered without any disconnection or reconnection of hoses to cold plates used in cooling the server modules, thus greatly reducing the probability of leaks and resulting damage to the system. 
     Other aspects and advantages of the present invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1A is a top view of an example embodiment of a variable gap thermal interface. 
     FIG. 1B is a cross-sectional view through section line A—A through the example embodiment of a variable gap thermal interface from FIG.  1 A. 
     FIG. 2 is a top view of an example embodiment of a docking thermal interface according to the present invention, before docking of the module. 
     FIG. 3 is a top view of an example embodiment of a docking thermal interface according to the present invention, after docking of the module is completed. 
     FIG. 4 is a flowchart of an example method of cooling an electronic device module according to the present invention. 
     FIG. 5 is a top view of an example embodiment of a docking thermal interface according to the present invention, before docking of the module. 
     FIG. 6 is a flowchart of an example method of cooling an electronic device module according to the present invention. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1A is a top view of an example embodiment of a variable gap thermal interface. A variable gap thermal interface may be constructed with a body  100  and an array of pins  102 . Further detail of the construction of the array of pins  102  is shown in a cross-sectional view along section line A—A in FIG.  1 B. Note that this array of pins  102  may be of any size and dimension within the scope of the present invention. The array of pins  102  may be optimized for a particular purpose by varying such properties as the diameter of the pins, the shape of the pins, the length of the pins, the size of the spring below the pins, and the strength of the spring below the pins, for a given implementation of the present invention. 
     FIG. 1B is a cross-sectional view through section line A—A through the example embodiment of a variable gap thermal interface from FIG.  1 A. An array of pins  102  is placed within cavities in a thermal interface body  100 . Spring elements  104  may reside beneath each pin, applying a vertical force on each pin  102 . These spring elements  104  may be constructed in a wide variety of configurations within the scope of the present invention. Some embodiments of the present invention may use small springs or a quantity of deformable material as spring elements. In some embodiments of the present invention the pins  102  may be surrounded by thermal grease to facilitate movement within the cavities in the body  100  and to improve heat transfer between the pins  102  and the body  100 . A vent hole  106  may be added to the body if desired. The vent hole  106  may be necessary to allow thermal grease to escape from the cavity when the pin is depressed. 
     FIG. 2 is a top view of an example embodiment of a docking thermal interface according to the present invention, before docking of a module. A variable gap thermal interface  202  including an array of spring-loaded pins  204  is attached to an electronic device module  200  including heat generating parts. A variety of mechanisms such as a liquid loop, a heat pipe, spray cooling, refrigeration, and other cooling mechanisms may be used to transfer heat from the heat generating parts to the variable gap thermal interface  202  within the scope of the present invention. A cold plate  208  is attached to a rack  206  or other structure that includes the plumbing  214  necessary for the cold plate  208 . The rack  206  may include rails  210  for aligning the electronic device module  200  within the rack such that a sufficient portion of the variable gap thermal interface  202  comes in contact with the cold plate  208  as is required to remove heat from the electronic device module  200 . The rack  206  may also include latches  212  or other devices to hold the electronic device module  200  in position such that the pins  204  remain in contact with the cold plate  208 . Some example embodiments of the present invention may not require all of the variable gap thermal interface  202  to contact the cold plate  208 . Note that the surface of the cold plate  208  does not need to be perfectly flat or parallel to the variable gap thermal interface  202  to create a low thermal resistance connection. Since the pins are individually compressible, the variable gap thermal interface  202  will form a low thermal resistance contact with non-planar cold plates  208 . Other embodiments of the present invention may be configured to allow a variety of different electronic device modules  200  to be used in a single position within the rack  206  by providing a large cold plate  208  configured to mate with a wide variety of sizes and positions of variable gap thermal interfaces  202 . Note that typical designs including the present invention will also include electrical connections such as power lines between the electronic device module  200  and the rack  206 . Note that the alignment of the electronic device module  200  to the rack  206  is not critical, since the presence of spring-loaded pins  204  in the present invention still creates a robust thermal interface with the cold plate  208  even with a misalignment. The variable gap thermal interface  202  does not need to be perfectly parallel to the cold plate  208  when in use, since it is possible for some of the pins to be compressed further than other pins. Also, in some embodiments of the present invention, thermal grease may be applied to the surface of the cold plate  208  or the pins further reducing the thermal resistance of the thermal contact formed when the module is docked. 
     Note that the cold plate  208  on the rack  206  does not necessarily need to be liquid cooled within the scope of the present invention. Some embodiments of the present invention may include an array of heat sinks thermally coupled to the cold plate  208  and configured to utilize airflow for heat dissipation. Other embodiments of the present invention may include heat pipes thermally connected to the cold plate  208  to transfer heat away from the cold plate  208 . Still other embodiments of the present invention may use techniques such as spray cooling, standard refrigeration techniques, a thermosyphon, or thermoelectrics to transfer heat away from the cold plate  208 . 
     In some embodiments of the present invention the rack  206  may contain a single large cold plate  208  configured to couple with a plurality of variable gap thermal interfaces  202 , while other embodiments of the present invention may contain a single variable gap thermal interface  202  configured to couple with a plurality of cold plates  208 . This allows a single large cold plate or variable gap thermal interface to be built into a rack in a configuration allowing a plurality of electronic device modules to transfer heat to the single large cold plate or variable gap thermal interface. 
     FIG. 3 is a top view of an example embodiment of a docking thermal interface according to the present invention, after docking of a module is completed. In this example embodiment of the present invention, the electronic device module  200  including a variable gap thermal interface  202  from FIG. 2 is docked with the rack  206  resulting in at least some of the array of spring-loaded pins  204  to come in contact with the cold plate  208 . Latches  212 , or other mechanical devices, may be used to hold the electronic device module  200  in place with respect to the rack  208  to keep the pins  204  in contact with the cold plate  208 . In this example embodiment of the present invention spring-loaded latches  212  are used to hold the electronic device module  200  in position, however, those of skill in the art will recognize that may other equivalent devices and configurations may be used to hold the electronic device module  200  in position. 
     FIG. 4 is a flowchart of an example method of cooling an electronic device module according to the present invention. In a step  400  a rack is provided. In a step  402  at least one cold plate is attached to the rack. In an optional step  404  plumbing is attached to the cold plates within the rack configured to liquid cool the cold plates. In an optional step  406  at least one rail is attached to the rack configured to position an electronic device module within the rack. In an optional step  408  at least one latch is attached to the rack configured to secure an electronic device module within the rack. In a step  410  at least one electronic device module is provided. In a step  412  at least one variable gap thermal interface is attached to at least one of the provided electronic device modules. In a step  414  the electronic device module is removeably positioned within the rack such that the variable gap thermal interface contacts at least one of the cold plates within the rack. 
     FIG. 5 is a top view of an example embodiment of a docking thermal interface according to the present invention, before docking of the module. This embodiment of the present invention is similar to that shown in FIG. 2, with the exception that the variable gap thermal interface is now on the rack side of the thermal interface. A variable gap thermal interface  502  including an array of spring-loaded pins  404  is attached to a rack  506  including heat dissipation means. A hot plate  508  is attached to an electronic device module  500  including at least one heat generating part. A variety of mechanisms such as a liquid loop or a heat pipe may be used to transfer heat from the heat generating parts to the hot plate  508  within the scope of the present invention. The rack  506  may include rails  510  for aligning the electronic device module  500  within the rack  506  such that a sufficient portion of the hot plate  508  comes in contact with the variable gap thermal interface  502  as is required to remove heat from the electronic device module  500 . The rack  506  may also include latches  512  or other devices to hold the electronic device module  500  in position such that the pins  504  remain in contact with the hot plate  508 . The rack  506  may include plumbing to liquid cool the variable gap thermal interface  502 . 
     FIG. 6 is a flowchart of an example method of cooling an electronic device module according to the present invention. In a step  600  a rack is provided. In a step  602  at least one variable gap thermal interface is attached to the rack. In an optional step  604  plumbing is attached to the cold plates within the rack configured to liquid cool the variable gap thermal interfaces. In an optional step  606  at least on rail is attached to the rack configured to position an electronic device module within the rack. In an optional step  608  at least one latch is attached to the rack configured to secure an electronic device module within the rack. In a step  610  at least one electronic device module is provided. In a step  612  at least one hot plate is attached to at least one of the provided electronic device modules. In a step  614  the electronic device module is removeably positioned within the rack such that the variable gap thermal interface contacts at least one of the cold plates within the rack. 
     The foregoing description of the present invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed, and other modifications and variations may be possible in light of the above teachings. The embodiments were chosen and described in order to best explain the principles of the invention and its practical application to thereby enable others skilled in the art to best utilize the invention in various embodiments and various modifications as are suited to the particular use contemplated. It is intended that the appended claims be construed to include other alternative embodiments of the invention except insofar as limited by the prior art.