Patent Abstract:
A cooling system and method for cooling electronic components. The cooling system employs a cooling device that includes a composite structure having first and second plates arranged substantially in parallel and bonded together to define a sealed cavity therebetween. The first plate has a surface that defines an outer surface of the composite structure and is adapted for thermal contact with at least one electronic component. A mesh of interwoven strands is disposed within the cavity and lies in a plane substantially parallel to the first and second plates. A fluid is contained and sealed within the cavity of the composite structure, and is pumped through interstices defined by and between the strands of the mesh. Flow dividers can define interconnected channels within the cavity.

Full Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application No. 60/826,939, filed Sep. 26, 2006, the contents of which are incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention generally relates to cooling systems for electronic components. More particularly, this invention relates to a sealed cooling device that contains a fluid and a mesh material through which the fluid flows to promote heat transfer through the device. 
     Cooling of electronic devices has become increasingly challenging as electronics have evolved. As manufacturing processes are constantly refined, the migration to smaller design processes and the incumbent reduction in operating voltage has not kept pace with the increased complexity of faster integrated circuits (ICs). Increasing number of transistors in combination with increasing operating frequencies has resulted in higher numbers of switching events over time per device. As a result, within the same market space and price range, ICs are becoming more and more sophisticated and power-hungry with every generation. 
     Compared to earlier generations, the implementation of smaller design processes has allowed the integration of more electronic building blocks such as transistors and capacitors on the same footprint. Consequently, area power densities have increased, resulting in smaller dies dissipating higher thermal load. As a result, formerly sufficient, passive heat spreaders and coolers often do not provide adequate cooling. While sophisticated fin designs and powerful fans increase the active surface area useable for offloading thermal energy to the environment, even extremely well designed coolers are hitting inherent limitations. In particular, significant limitations stem from the bottleneck of limited heat conductivity of the materials used, and specifically the fact that passive heat transfer throughout a solid structure is limited by the thermal conductance coefficient of the material and the cross sectional area of the structure. 
     In a two-dimensional heat spreader of uniform thickness, the amount of heat energy decreases as a square function of the distance from the source, where the thermal conductance coefficient of the material and the cross sectional area define the slope of the decrease. Therefore, even the most highly conductive material will not be able to maintain an even temperature distribution across the entire surface of the cooling device. Any gradient, on the other hand, will cause a decrease in cooling efficiency since the temperature difference (ΔT) between the cooler&#39;s surface and the environment is the primary limiting factor for thermal dissipation to the surrounding. 
     In view of the above, it is desired that coolers transfer heat from a heat source as quickly and efficiently as possible to provide a uniform temperature distribution or isothermicity at the cooler&#39;s surface. In combustion engines, liquid cooling has become the method of choice, using the fact that a liquid (e.g., water) is taking up thermal energy and subsequently being pumped to a remote radiator where it releases the absorbed heat. In electronic devices, liquid cooling is still only marginally accepted for reasons that include the inherent risk of spills, cost overhead, and complexity of the installation, which involves routing of tubing and installation of radiators. Alternatively, some self-contained liquid cooling devices have been proposed and marketed. 
     The four primary factors defining the efficacy of a liquid cooling device are the uptake of heat by the cooling fluid at the heat source, the transport rate of the fluid away from the heat source, the offloading of heat to the solid components of the cooler, and finally the dissipation rate of heat into the environment. The exchange of heat between the fluid and the cooling device largely depends on the routing of the flow of the coolant within the device. If the channels are too wide, laminar flow can cause a decrease in efficacy of heat exchange between the fluid and the device. Therefore, it is desirable to have a capillary system to achieve an optimal surface to volume ratio. Such capillary systems have been referred to as microchannel systems. 
     Different technologies have been employed to create microchannels, including etching and crosshatching of small grooves into a cooling device and even into the die of an electronic device to be cooled, such technologies have required relatively elaborate steps in their design and manufacturing process. Commonly-assigned U.S. Pat. No. 7,219,715 to Popovich, the contents of which are incorporated herein by reference, describes an alternative approach using a mesh or woven screen that is between and bonded to two foils that define a flow cavity. A generic representation of this type of approach is depicted by a cooling device  10  shown in  FIG. 1 , in which a pair of foils  12  and  14  are bonded together, and a mesh  16  is contained within a cavity  18  defined by and between the foils  12  and  14 . The interstices between the warp and weft strands  20  of the mesh  16 , as well as the gaps between the strands  20  and the bordering foils  12  and  14 , allow the passage of a cooling fluid, providing direct contact with the fluid for heat absorption and transfer heat through the bonding contacts with the foils  12  and  14 . As further represented in  FIG. 2 , Popovich also provides an opening  22  in one of the foils  14  that provides for direct contact of the cooling fluid within the device  10  with the die  24  of an IC device, which is represented in  FIG. 2  as projecting into the cavity  18  through the opening  22 .  FIGS. 3 through 4  represent variations of the cooling device  10  of  FIG. 1 . In  FIG. 3 , the device  10  is modified to include cooling fins  28  that promote heat dissipation to the surrounding environment. In  FIG. 4 , the device  10  is modified to include an integrated pump  30  for forcing the flow of the cooling fluid within the device  10 . As would be expected, the fins  28  of  FIG. 3  and the pump  30  of  FIG. 4  can also be combined in the same cooling device  10 . 
     An advantage of the cooling devices represented in  FIGS. 1 through 4  is attributable to the tortuous course of the microchannels through the mesh  16 , meaning that in addition to an X-Y labyrinth of interstices, a Z-plane of tortuousness is created that further increases the internal surface area for heat exchange between the fluid and the solid components of the cooling device  10 . 
     Though the approach represented in  FIGS. 1 through 4  is thermally efficient, exposure of the IC die  24  to the cooling fluid requires a sealing feature  32  (such as an adhesive seal, O-ring, etc.) around the opening  22  in the cavity  18  to allow direct fluid passage over the die  24 . Aside from any potential reliability problems, the requirement for a sealing feature  32  can complicate the serviceability of the cooling device  10 , and may render the device  10  ill-suited for aftermarket retrofitting by certain end users with limited technical skills. 
     In addition to Popovich, the use of microchannels for coolant fluids has been known for some time, as evidenced by U.S. Pat. No. 4,450,472 to Tuckerman et al. The preferred embodiment featured in this patent integrated microchannels into the die of the microchip to be cooled and coolant chambers. U.S. Pat. No. 5,801,442 also describes a similar approach. Still other approaches have focused on the combined use of coolant phase change (condensation) and microchannels, an example of which is U.S. Pat. No. 6,812,563. U.S. Pat. No. 6,934,154 describes a similar two-phase approach including an enhanced interface between an IC die and a heatspreader based on a flip-chip design and the use of a thermal interface material. U.S. Pat. Nos. 6,991,024, 6,942,018, and 6,785,134 describe electroosmotic pump mechanisms and vertical channels for increased heat transfer efficiencies. Variations of microchannel designs include vertical stacking of different orientational channel blocks as described in U.S. Pat. No. 6,675,875, flexible microchannel designs using patterned polyimide sheets as described in U.S. Pat. No. 6,904,966, and integrated heating/cooling pads for thermal regulation as described in U.S. Pat. No. 6,692,700. 
     Additional efforts have been directed to the manufacturing of microchannels. U.S. Pat. Nos. 7,000,684, 6,793,831, 6,672,502, and 6,989,134 are representative examples, and disclose forming microchannels by sawing, stamping, crosscutting, laser drilling, soft lithography, injection molding, electrodeposition, microetching, photoablation chemical micromachining, electrochemical micromachining, through-mask electrochemical micromachining, plasma etching, water jet, abrasive water jet, electrodischarge machining (EDM), pressing, folding, twisting, stretching, shrinking, deforming, and combinations thereof. All of these methods, however, share the drawback of requiring a more or less elaborate and expensive manufacturing process. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention is a cooling system and method for cooling electronic components, including IC dies. The cooling system employs a cooling device that includes a composite structure comprising first and second plates arranged substantially in parallel and bonded together to define a sealed cavity therebetween. At least one of the first and second plates has a surface that defines an outer surface of the composite structure and is adapted for thermal contact with at least one electronic component. At least one and preferably multiple separate meshes, each of interwoven strands, are disposed within the cavity and lie in a plane substantially parallel to the first and second plates, with their strands bonded to the first and second plates. A fluid is contained and sealed within the cavity of the composite structure, and flows through interstices defined by and between the strands of the meshes. 
     The cooling method entails absorbing heat dissipated by an electronic component with a first plate arranged substantially in parallel and bonded to a second plate so as to define a composite structure and a sealed cavity between the first and second plates. The first plate has a surface that defines an outer surface of the composite structure and is adapted for thermal contact with the electronic component. The absorbed heat is transferred through the cavity and into the second plate via a fluid and at least one mesh contained in the cavity. The mesh lies in a plane substantially parallel to the first and second plates, and comprises interwoven strands that are bonded to the first and second plates and define interstices through which the fluid is able to flow. The fluid acts as a secondary heat absorbent and a thermal transport media that transports thermal energy to the mesh at a distance from the first plate. After traveling through the cavity, the absorbed heat is dissipated to the environment with the second plate. 
     In a preferred embodiment, the cooling device has a plate-mesh-plate laminate construction, in which portions of the plates, preferably including their edges, are raised so that by laminating the plates together a channel system is defined between the plates. At least one of the plates is preferably configured to define first order channels or macrochannels within the cooling device in order to direct the general flow of a cooling fluid through the channel system between the plates. Fluid movement through the channel system can be augmented by a pump. 
     Within the channel system, a tortuous three-dimensional labyrinth of microchannels is established by interstices between strands of the one or more meshes. The meshes are preferably bonded to each of the plates at substantially every bump of each strand resulting from the strands passing over and under transverse strands of each mesh. Difficulties associated with directly contacting an electronic component with a cooling fluid are overcome by hermetically sealing the cooling device to prevent contact between the cooling fluid and an electronic component cooled by the cooling device, and then thermally contacting the electronic device with one of the plates or a heat-slug formed as part of the plate or as a separate component attached to the plate. 
     According to a preferred aspect of the invention, the hermetical seal establishes a self-contained, spill-proof, and leak-proof cooling system that can easily be adapted to fit any heat source, while maintaining the advantages of a sealed system. The potential drawback of reduced efficacy of heat uptake compared to a fully immersed IC die can be considered relatively minor compared to the limitations posed by the overall rate of thermal dissipation to the environment that can result in a thermal saturation of the entire cooling apparatus. Additional advantages of the invention include rapid heat distribution throughout the entire cooling device, uncomplicated installation and maintenance, cost-effectiveness, and good scalability that allows for large-scale cooling devices. 
     Other objects and advantages of this invention will be better appreciated from the following detailed description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  represents an open cooling device for fluid immersion of an electronic device in accordance with the prior art. 
         FIG. 2  shows the cooling device of  FIG. 1  mounted and sealed with an electronic device to be cooled. 
         FIGS. 3 and 4  show prior art cooling devices mounted and sealed with electronic devices in a manner similar to that shown in  FIG. 2 , but further equipped with, respectively, cooling fins to provide increased surface area for promoting heat dissipation and an integrated pump for fluid displacement. 
         FIG. 5  represents a first embodiment of a cooling device of the present invention, in which the device defines a completely sealed enclosure between a pair of bonded plates, and has a mesh material within the enclosure and bonded to the plates. 
         FIG. 6  shows the device of  FIG. 5  mounted to an electronic device to be cooled. 
         FIG. 7  shows a cooling device equipped with cooling fins that provide increased surface area for promoting heat dissipation in accordance with a second embodiment of the invention. 
         FIG. 8  shows the cooling device of  FIG. 7  mounted to an electronic device in a manner similar to that shown in  FIG. 6 . 
         FIG. 9  shows a cooling device with an integrated pump for fluid displacement in accordance with a third embodiment of the invention. 
         FIG. 10  shows the cooling device of  FIG. 9  mounted to an electronic device in a manner similar to that shown in  FIG. 6 . 
         FIG. 11  shows a cooling device that includes an integrated pump as shown in  FIG. 9  and cooling fins as shown in  FIG. 7  in accordance with a fourth embodiment of the invention. 
         FIG. 12  shows the cooling device of  FIG. 11  mounted to an electronic device in a manner similar to that shown in  FIG. 6 . 
         FIG. 13  shows a top view of a cooling device equipped with a pump, and embossed regions that define partitions within the cooling device to direct fluid flow in macrochannels within the device in accordance with a fifth embodiment of the invention. 
         FIG. 14  is a cross-sectional view of  FIG. 13  along section line A-A. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention provides a self-contained, closed-loop fluid cooling device suitable for cooling a wide variety of electronic components, including those with high power densities such as microprocessors and power conversion devices used in computers.  FIG. 5  represents a cooling device  110  of this invention comprising an isothermal plate  111  having a composite construction, in which a relatively pliant mesh  116  is sandwiched between two foils or plates  112  and  114  that are substantially parallel to each other. The mesh  116  is represented as being composed of individual strands  120  that are woven together, generally transverse to each other and conventionally referred to as warp and weft strands  120 . The mesh  116  and plates  112  and  114  are preferably formed of materials having physically and chemically compatible properties, including materials having the same composition, though various material combinations are possible. For example, individual strands  120  of the mesh  116  can be formed by an individual wire, braided wires, bundled wires, etc., of copper, silver, aluminum, carbon, or alloys thereof, and the plates  112  and  114  can be formed of the same or similar materials. As discussed below, heat transfer occurs by conduction through the plates  112  and  114  and mesh  116 , such that preferred materials for these components are thermally conductive, though the use of other materials including polymeric and nonmetallic materials is also foreseeable. Suitable thicknesses for the plates  112  and  114  and mesh  116 , suitable cross-sectional shapes and dimensions for the mesh strands  120 , and suitable weaves (including strands per inch) for the mesh  116  may depend on the particular application and the materials from which these components are formed. 
     As evident from  FIG. 5 , the peripheral edges  134  of both plates  112  and  114  are preferably raised relative to the remainder of the plates  112  and  114 , such as by embossing, to form a relief in each plate  112  and  114  that promotes the rigidity of the plates  112  and  114  and further defines a continuous peripheral surface at which the plates  112  and  114  can be bonded to each other, such as with a solder alloy, braze alloy, adhesive, etc. With the plates  112  and  114  laminated together, the reliefs define a cavity  118  between the plates  112  and  114 . As will be discussed in reference to  FIGS. 13 and 14 , additional embossing can be performed on one or both plates  112  and  114  to define within the cavity  118  a channel system between the plates  112  and  114 , by which particular flow routes can be established within the device  110 . Three-dimensional structures formed by such additional embossing have the further advantage of increasing the mechanical stability of the cooling device  110 . 
     As evident from  FIG. 5 , the mesh  116  within the cavity  118  of the composite plate  111  may have approximately the same thickness as the height of the cavity  118  (as measured in the direction normal to the plane of the plate  111 ). The peaks  136  projecting from both sides of the mesh  116  are preferably bonded, such as by soldering or brazing, to the plates  112  and  114  to establish a highly-conductive thermal contact between the mesh  116  and both plates  112  and  114 . Bonding also serves to cross-link the plates  112  and  114 , which resists any shearing forces to which the plates  112  and  114  are subjected and contributes additional mechanical stability and rigidity to the plate  111 . The warp and weft strands  120  of the mesh  116  form interstices that are more or less freely penetrable by any fluid, yet define tortuous paths that avoid laminar flow conditions within the cavity  118  that would reduce the heat transfer rate between the cooling fluid, the plates  112  and  114 , and the mesh  116 . Assuming the plate  114  is in thermal contact with a heat source, e.g., an electronic component  124  shown in  FIG. 6 , heat transfer from the component  124  is through the plate  114 , through the cavity  118  containing the mesh  116  and fluid, and then through the plate  114 , from whose outer surface heat is dissipated by convection. More particularly, heat transfer through the cooling device  110  is by thermal conduction through the plate  114 , the mesh  116 , and then the plate  112 , and by convention between the plate  114  and the cooling fluid and between the cooling fluid and the plate  112 , as well as convection through the cooling fluid from the plate  114  to the mesh  116  and convection through the cooling fluid from the mesh  116  to the plate  112 . Accordingly, heat transfer is generally in a single direction through the thickness of the composite plate  111 , and the fluid acts as a secondary heat absorbent and a thermal transport media capable of transporting thermal energy to the mesh  116  at a distance from the plate  114 . 
     As generally known in the art, suitable coolant fluids include liquids such as water, mineral spirits/oils, alcohols, and fluorocarbonate derivatives, though various other fluids could also be used, including air, vapor, etc., depending on the required temperature range of operation. For example, in extremely cold environments, a fluid with lower viscosity is a better choice than in extremely hot environments. Various other parameters for choosing a cooling fluid exist and are well known, and therefore will not be discussed in any further detail here. 
     As evident from  FIG. 5 , the composite plate  111  lacks an opening in which the cooling fluid within the cooling device  110  is able to directly contact an electronic component intended to be cooled with the device  110 . Instead, the device  110  is self-contained with the cooling fluid being hermetically sealed within the cavity  118 , such that cooling of an electronic component  124  ( FIG. 6 ) is achieved by thermally contacting the component  124  with one of the plates  112 / 114 . This approach greatly simplifies the installation and maintenance of the device  110 . 
     Thermal contact between the component  124  and plate  114  is shown in  FIG. 6  as being promoted with the use of a heat-slug  138 , which is preferably formed of a high thermally-conductive material that also has high thermal capacitance, a notable but nonlimiting example of which is copper and its alloys. The heat slug  138  can be defined by a portion of the plate  114 , or can be separately formed and then attached to the plate  114 . While a small loss in heat transfer is associated with the interfaces  140  and  142  between the heat slug  138  and the plate  114  and component  124 , the resistance to heat transfer caused by these interfaces  140  and  142  is relatively minor compared to the resistance encountered when dissipating heat from the cooling device  110  to the environment (typically atmospheric air) surrounding the device  110 . Moreover, the heat slug  138  is preferably able to offer sufficient thermal capacitance to buffer transient temperature spikes of the component  124 . The thermal capacitance of the slug  138  also overcomes other problems, such as the potential for localized boiling of the cooling fluid in proximity to hot-spots of the component  124 , the occurrence of which could greatly reduce the cooling efficacy of the device  110 . 
     Because the cooling fluid assists the plates  112  and  114  in conducting heat from the component  124 , the coefficient of thermal conductance of the material(s) used to form the plates  112  and  114  is less important than in structures that rely on passive heat transfer. As such, a wider variety of materials could be used to form the composite plate  111  and its individual components. Moreover, because the plate  111  is hollow, the total amount of material used is substantially lower than in a comparable solid structure, resulting in reduced material costs for manufacturing the cooling device  110 . A related issue is the mechanical stability of the cooling device  110 . Hollow structures generally exhibit only a minor reduction in rigidity as compared to a solid body of the same dimensions. The rigidity of the device  10  is promoted as a result of the peripheral edges  134  of the plates  112  and  114  being bonded together, as well as bonding of the mesh  116  to both plates  112  and  114 . Consequently, the cooling device  110  can be much lighter but yet nearly as strong and rigid as a solid heat spreader of comparable size. 
     As evident from  FIG. 7 , the cooling device  110  may include fins  128  to promote heat transfer to the surrounding environment. While fins  128  are shown on only the upper plate  112 , the lower plate  114  or both plates  112  and  114  could be so equipped. As known in the art, the fins  128  effectively increase the surface area of the cooling device  110  and, thus, facilitate offloading of the heat to the surrounding environment. 
     The cooling device  110  may further include a pump  130  as shown in  FIG. 9  by which the cooling fluid is recirculated through the cavity  118 , generally in a direction or directions parallel to the mesh  116 . A wide variety of pumps are possible and suitable for use in the device  110 , and the choice of which will be primarily dependent on the specific application since pressure and noise requirements need to be taken into consideration. Notable but nonlimiting examples of suitable pump types include centrifugal, positive displacement, rotary, and osmotic pumps that are commercially available and have been used in prior cooling systems for electronic components. As seen in  FIG. 11 , the cooling device  110  may also include a combination of fins  128  and pump  130 . 
       FIGS. 9 and 11  depict another aspect of the invention, in which mesh segments  116 A and  116 B are employed in place of the single unitary mesh  116  of  FIGS. 5 through 8 . The mesh segment  116 A is a primary mesh juxtaposed and preferably directly aligned and over the electronic component  124  for heat uptake, and one or more mesh segments  116 B located near the periphery of the device  110  and preferably surround the mesh segment  116 A for faster offloading of heat. Heat transfer between the mesh segments  116 A and  116 B is generally via the cooling fluid and via the plates  112  and  114 . 
       FIGS. 8 ,  10 , and  12  represent, respectively, the use of the devices  110  shown in  FIGS. 7 ,  9 , and  11  for dissipating heat from an electronic component  124 . In each of  FIGS. 8 and 12 , a fan  148  is also shown for promoting heat transfer from the fins  128  through forced convection. Otherwise, the device  110  may rely on natural convection to dissipate heat. 
     Notably, with the inclusion of the pump  130 , heat transfer through the cooling fluid is enhanced as a result of the fluid flow becoming turbulent as a result of the fluid being forced to flow through the interstices between the strands  120  of the mesh  116 . More particularly, assuming the plate  114  is in thermal contact with the electronic component  124  as shown in  FIGS. 6 ,  8 ,  10 , and  12 , heat transfer through the cooling device  110  is by thermal conduction through the plate  114 , the mesh  116 , and then the plate  112 , and by turbulent forced convention between the plate  114  and the cooling fluid and between the cooling fluid and the plate  112 , as well as turbulent forced convection through the cooling fluid from the plate  114  to the mesh  116  and turbulent forced convection through the cooling fluid from the mesh  116  to the plate  112 . Even so, because the cooling fluid is recirculated through the cooling device  110 , heat transfer is through the device  110  is generally through the thickness of the composite plate  111 , in other words, from the electronic component  124 , through the plate  114 , through the cavity  118  containing the mesh  116  and fluid, and then through the plate  112 , from whose outer surface heat is dissipated by convection. 
       FIGS. 13 and 14  depict a variation of the cooling device  110  of  FIGS. 5 through 12 , modified to include embossed regions  144  within the interior of the cavity  118 . The embossed regions  144  are represented as forming walls or dividers within the cavity  118  to define a system of channels  146  in fluidic series. As shown in  FIG. 13 , the channels  146  define a circuitous route (identified by arrows) for the cooling fluid through the cooling device  110 , with flow through the device  110  being maintained at a desired rate with a pump  130 . Alternatively, flow may occur opposite the direction indicated in  FIG. 13 . Another alternative is to use the channel  146  immediately downstream from the pump  130  as a macrochannel or manifold to direct the flow of the cooling fluid in series through the remaining channels  146 , or simultaneously in parallel through two or more of the channels  146 . The mesh  116  can be a single unit having portions clamped between the embossed regions  144  and the opposing interior surface regions of the plate  114 , or can be made up of mesh segments that are each sized to individually fit within one of the channels  146 . 
     While the invention has been described in terms of specific embodiments, it is apparent that other forms could be adopted by one skilled in the art. For example, the functions of the components of the cooling device  110  could be performed by components of different construction but capable of a similar (though not necessarily equivalent) function, the cooling device  110  and its components could differ in appearance and construction from the embodiments shown in the Figures, and appropriate materials could be substituted for those noted. Therefore, the scope of the invention is to be limited only by the following claims.

Technology Classification (CPC): 5