Abstract:
A plate is thermally coupled to a heat generating device and thermally coupled to two heat pipes. Each heat pipe is configured to have a predetermined boiling point temperature selected according to design criteria. One or more additional heat pipes can be coupled to the plate. A heat spreader can be in thermal contact with the heat generating device and with at least one of the heat pipes. The heat pipes can differ in outer cross-sectional dimensions depending on thermal distance position relative to the heat generating device, such that the heat pipes located a farther thermal distance from the heat generating device have smaller outer cross-sectional dimensions than the heat pipes located a shorter thermal distance from the heat generating device.

Description:
This application claims priority under 35 U.S.C. § 119(e) of the co-pending U.S. provisional patent application Ser. No. 60/444,269, filed on Jan. 31, 2003, and titled “REMEDIES FOR FREEZING IN CLOSED-LOOP LIQUID COOLING FOR ELECTRONIC DEVICES.” The provisional patent application Ser. No. 60/444,269, filed on Jan. 31, 2003, and titled “REMEDIES FOR FREEZING IN CLOSED-LOOP LIQUID COOLING FOR ELECTRONIC DEVICES” is hereby incorporated by reference. 

   FIELD OF THE INVENTION 
   The present invention generally relates to a method of and apparatus for removing heat from a heat source, in which boiling temperatures of heat pipes are optimized to ensure that the heat pipes start to boil at approximately the same heat load condition from the chip, thereby utilizing the full heat removal capacity of the heat pipes. 
   BACKGROUND OF THE INVENTION 
   The increasing power of electronic devices is motivating the semiconductor industry to seek effective thermal management solutions. A number of techniques for cooling electronic devices and packages have become widespread. Currently, fin array heat sinks with fans are the most common cooling technique. These devices are typically attached to a package lid or directly to a die. A primary purpose of heat sinks is to increase the area for heat rejection to air. 
   Another common technique that has emerged in the past decade is the use of heat pipes. The heat pipe has become a widely used thermal management tool in the notebook industry. Most current notebooks utilize heat pipes in their thermal management solution. Their primary purpose is to remove heat from a heat source to a heat sink where the heat is dissipated. Heat pipes are vacuum-tight vessels that are evacuated and partially filled with a small amount of water or other working fluid with a wicking structure.  FIG. 1  shows an illustration of how a heat pipe works. As heat is directed into a heat pipe  110  from a heat source  120 , fluid evaporates creating a pressure gradient in the heat pipe  110 . This forces vapor  130  to flow along the heat pipe  110  to a cooler section or condenser  160  where it condenses. The condensed fluid wicks  140  back to the evaporator  150  near the heat source  120 . 
   Heat removal capacity of the heat pipe is controlled by wicking media and heat pipe geometry, among which important parameters are heat pipe cross-sectional dimensions and heat pipe length. Currently, available heat pipes with 6 cm outer diameter (OD) can only dissipate about 30 to 50 W of heat. High performance electronic devices often dissipate more than 100 W of heat. Therefore, multiple heat pipes are used for thermal management. These heat pipes, usually 3 or more, are often embedded in a cooper enclosure or block to form a cooling module. 
   The current designs have an important limitation. Due to their OD sizes, the heat pipes are much comparable with a typical die size. Spreading thermal resistance from the die to each individual heat pipe becomes significant.  FIG. 2  shows an illustration of multiple heat pipes embedded in a copper block  200  and coupled to a heat spreader  210 . The heat spreader  210  sits on top of a heat source  220 , such as a chip. A center heat pipe  230  is positioned closest to the heat source  220 , with outer heat pipes  240  located farther away from the heat source  220 . The center heat pipe  230  and the outer heat pipes  240  have identical boiling points of 50 degrees Celsius. The spreading thermal resistance causes the outer heat pipes  240  to remove much less heat than the center heat pipe  230 . The center heat pipe  230  could reach its boiling limit and exceed its heat removal capacity, causing it to “burn out” before the outer heat pipes  240  have approached their full heat removal potential. Therefore, heat load carried by the outer heat pipes  240  will be far less that the amount carried by heat pipes closer to the heat source, such as the center heat pipe  230 . Thus, the center heat pipe  230  will approach and exceed its heat load capacity before the outer heat pipes  240  reach their heat load capacity, resulting in a system poorly adapted for handling high heat load. 
   What is needed is an apparatus for and method of optimizing boiling points of heat pipes to achieve simultaneous onset of boiling. 
   SUMMARY OF THE INVENTION 
   According to a first aspect of the present invention, an apparatus for removing heat from a heat generating device is provided. The apparatus comprises a plate thermally coupled to the heat generating device and thermally coupled to two heat pipes wherein each heat pipe is configured to have a predetermined boiling point temperature selected according to design criteria. 
   The apparatus can further include a heat spreader. The apparatus can also include one or more additional heat pipes. Preferably, the heat spreader is in thermal contact with the heat generating device and with at least one of the heat pipes. The heat spreader can be made of copper. The heat pipes can differ in boiling point temperature by at least 1 degree Celsius. The heat pipes can differ in outer cross-sectional dimensions depending on thermal distance relative to the heat generating device, such that the heat pipes located a farther thermal distance from the heat generating device have smaller outer cross-sectional dimensions than the heat pipes located a shorter thermal distance from the heat generating device. 
   According to an alternative embodiment of the present invention, an apparatus for removing heat from a heat generating device by optimizing boiling points of heat pipes in an enclosure is disclosed, wherein the heat pipes have an outer cross-sectional dimension greater than the outer cross-sectional dimension of the heat generating device. The enclosure is coupled to a heat spreader. The apparatus includes a first heat pipe positioned at a first thermal distance from the heat generating device and a second heat pipe positioned at a second thermal distance from the heat generating device, wherein the first heat pipe has a boiling point corresponding to the first thermal distance and the second heat pipe has a boiling point corresponding to the second thermal distance to achieve substantially simultaneous onset of boiling of the heat pipes. 
   The second heat pipe can be separated by a predetermined from the first heat pipe. The boiling points of heat pipes can be predetermined. 
   According to another embodiment of the present invention, an apparatus for removing heat from a heat generating device by optimizing boiling points of heat pipes in an enclosure is disclosed. The apparatus includes a first heat pipe positioned at a first thermal distance from the heat generating device; a second heat pipe positioned a second thermal distance from the heat generating device; and means for achieving simultaneous onset of boiling of the heat pipes. 
   According to another embodiment of the present invention, a method of removing heat from a heat generating device by optimizing boiling of heat pipes in an enclosure is disclosed. The method comprises the steps of: positioning a first heat pipe at a first thermal distance from the heat generating device; providing a second heat pipe positioned at a second thermal distance from the heat generating device; and providing means for achieving simultaneous onset of boiling of the heat pipes. 
   According to another embodiment of the present invention, an apparatus for removing heat from a heat generating device by optimizing boiling points of heat pipes in an enclosure is disclosed. The apparatus comprises a plurality of heat pipes each having a predetermined thermal distance from the heat generating device wherein a boiling point for each of the plurality of heat pipes is selected in accordance with the thermal distance. 
   According to another embodiment of the present invention, an apparatus for removing heat from a heat generating device by optimizing boiling points of heat pipes in an enclosure is disclosed. The apparatus comprises a plurality of heat pipes each having a predetermined thermal distance from the heat generating device wherein a boiling point for each of the plurality of heat pipes is selected in accordance with the thermal distance, such that each of the plurality of heat pipes boils at substantially the same time in response to heat generated by the heat generating device. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic diagram which illustrates how a prior art heat pipe works. 
       FIG. 2  is a schematic side view prior art illustration of three pipes embedded in a plate having similar boiling points. 
       FIG. 3  is a schematic side view of three heat pipes embedded in a plate, with a center heat pipe having a higher boiling point than adjacent heat pipes, in accordance with the present invention. 
       FIG. 4  is a schematic side view of three pipes embedded in a plate, with a center heat pipe having a greater outer cross-sectional dimension than adjacent heat pipes, in accordance with the present invention. 
       FIG. 5  is a schematic side view of stacked heat pipes embedded in a plate, with a center heat pipe having a higher boiling point than heat pipes a farther thermal distance away from a heat generating device, in accordance with the present invention. 
       FIG. 6  is a schematic flow chart illustrating steps of a preferred method of the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Reference will now be made in detail to the preferred and alternative embodiments of the invention, examples of which are illustrated in the accompanying drawings. While the invention will be described in conjunction with the preferred embodiments, it will be understood that they are not intended to limit the invention to these embodiments. 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 provide a thorough understanding of the present invention. However, it should be noted that the present invention may be practiced without these specific details. In other instances, well known methods, procedures and components have not been described in detail as not to unnecessarily obscure aspects of the present invention. 
   In accordance with a preferred embodiment of the present invention, an apparatus for removing heat from a heat generating device is shown in a side view of  FIG. 3 . In particular, the preferred embodiment of this invention includes a first heat pipe  310  and a second or additional heat pipes  320  which are maintained in a fixed spaced-apart position and embedded in a plate  300  constructed of, e.g., copper or aluminum. Cylindrical heat pipes can be placed in holes within the plate  300 . The heat pipes  310  and  320  are thermally coupled to a heat generating device  340 , such as a chip. A heat spreader  330  is provided to couple the chip to the heat pipes  310  and  320 . The heat spreader  330  can be made of solid copper or other heat conductive material. Other designs include heat spreaders within the chip or chip package. The heat spreader  330  helps spread heat generated by the chip at hot-spots on the chip to a much larger area, such as heat pipes. The heat pipes  310  and  320  carry heat away from the heat generating device  340 . 
   The first heat pipe  310  is preferably positioned a first thermal distance from the heat generating device  340 . The additional heat pipes  320 , adjacent to the first heat pipe  310 , are preferably positioned a second thermal distance from the heat generating device  340 . The additional heat pipes  320  can be a plurality of heat pipes located a thermal distance from the heat generating device  340 . 
   It will be appreciated that when a heat source in steady state is positioned to provide heat energy to a solid body, eventually the system will reach equilibrium. Because heat can be removed from boundaries of the solid body by convection or radiation or, if it is in contact with other structures heat can be removed by conduction, there will be a temperature gradient across the solid body. The shape, construction and materials of the solid body will determine the difficulty of calculating the equilibrium temperature at any given point in the solid body. For purposes of this document, each point within the solid body that has a same temperature is said to be the same thermal distance from the heat source. By way of example, consider an idealized point heat source at the center of a uniform solid spherical body in space. All points on each concentric sphere having a center at the heat source will be the same temperature and thus the same thermal distance from the heat source. 
   Still referring to  FIG. 3 , the first heat pipe  310  has a boiling point corresponding to a first thermal distance from the heat generating device  340 . The additional heat pipes  320  have a boiling point corresponding to a second thermal distance from the heat generating device  340  to achieve simultaneous onset of boiling of the heat pipes  310  and  320 . The fluid in the additional heat pipes  320  boils at a slightly lower temperature than the center heat pipe  310  to compensate for spreading thermal resistance in the plate  300 . Setting the boiling points of the additional heat pipes  320  at a lower temperature compensates for the spreading thermal resistance by inducing an additional temperature drop to force the heat to spread even further. Boiling temperature of the heat pipes  310  and  320  can be fixed by adjusting the pressure inside the heat pipes  310  and  320 . 
   One reason to optimize boiling points of the heat pipes is to ensure that the heat pipes start to boil at approximately same heat load conditions from heat generating devices. Spreading thermal resistances cause heat pipes located a thermal distance away from the heat generating device to remove less heat than heat pipes located a closer thermal distance to the device. A goal of the present invention is to ensure that all of heat pipes start to boil at approximately similar heat load conditions from the heat generating device, resulting in an optimal and efficient system for handling high heat loads. 
   Another goal of the present invention is optimization of heat load capabilities, which correspond to cross-sectional dimensions, of the heat pipes. In  FIG. 4 , the heat pipes farther from the heat generating device  440  may or may not receive less heat owing to thermal resistances in the plate  400 . An optimal heat removal plate may include heat pipes of varying cross-sectional dimensions, such that the heat pipes a farther thermal distance from a heat generating source have smaller cross-sectional dimensions and lower heat removal capability than the heat pipes a closer thermal distance from the heat generating device. 
   Referring again to  FIG. 3 , the heat spreader  330  is in thermal contact with the heat generating device  340  and with at least one of the heat pipes  310  and  320 . In this example, the first heat pipe  310  can have a boiling point temperature difference of at least 4 degrees, such as a boiling point temperature between 48 degrees Celsius and 52 degrees Celsius. The additional heat pipes  320  can be characterized as having a boiling point temperature difference of at least 10 degrees, such as a boiling point temperature between 37 degrees Celsius and 47 degrees Celsius. To tailor heat toad capabilities, the additional heat pipes  320  can have a lower boiling temperature than the first heat pipe  310 . Preferably, the heat pipes  310  and  320  differ in boiling point temperature by at least 1 degree Celsius. Moreover, the additional heat pipes  320  can have a lower heat removal capacity than the heat removal capacity of the first heat pipe  310 . As shown in  FIG. 3 , the additional heat pipes  320  can be separated by a predetermined distance from the first heat pipe  310 . Alternatively, heat pipes can be stacked in a plate as shown in  FIG. 5 . In this example, four outer heat pipes  520  and one center pipe  510  are enclosed in a copper plate. The four outer heat pipes  520  are separated by a predetermined distance and have boiling points lower than the boiling point of the center heat pipe  510 . The boiling points of the outer heat pipes  520  can be same or different. The boiling points of all the heat pipes can be predetermined. 
   In an alternative embodiment, a method of removing heat from a heat generating device by optimizing boiling points of heat pipes in an enclosure is disclosed, as shown in the flow diagram of  FIG. 6 . In the Step  600 , a first heat pipe is positioned at a first thermal distance from a heat generating device. In the Step  610 , a second heat pipe is positioned a second thermal distance from the heat generating device. In the Step  620 , means for achieving simultaneous onset of boiling of the heat pipes is provided. The first heat pipe can be a center heat pipe and the second heat pipe can be many additional heat pipes or a plurality of heat pipes. 
   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.