Patent Publication Number: US-6903929-B2

Title: Two-phase cooling utilizing microchannel heat exchangers and channeled heat sink

Description:
FIELD OF THE INVENTION 
   The field of invention relates generally to cooling electronic apparatus&#39; and systems and, more specifically but not exclusively relates to microchannel cooling technology. 
   BACKGROUND INFORMATION 
   Components in computer systems are operating at higher and higher frequencies, using smaller die sizes and more densely packed circuitry. As a result, these components, especially microprocessors, generate large amounts of heat, which must be removed from the system&#39;s chassis so that the components do not overheat. In conventional computer systems, this is accomplished via forced air convection, which transfers heat from the circuit components by using one or more fans that are disposed within or coupled to the chassis to draw air over the components through the chassis. To further aid the heat removal process, heat sinks are often mounted to various high-power circuit components to enhance natural and forced convection heat transfer processes. Heat sinks comprising of an array of fins having a height of approximately 1-2 inches are commonly used to cool microprocessors in desktop systems, workstations, and pedestal-mounted servers. The heat sinks provide significantly greater surface areas than the components upon which they are mounted. 
   For example, a typical processor cooling solution that employs a heatsink is shown in FIG.  1 . The cooling solution is designed to cool a processor die  100 , which is flip-bonded to a substrate  102  via a plurality of solder bumps  104 . Typically, an epoxy underfill  106  is employed to strengthen the interface between die  100  and substrate  102 . Substrate  102 , in turn, is mounted to a chip carrier  108  via a plurality of solder balls  110 . The upper side of the die is thermally coupled to a copper heat spreader  112  via a first layer of thermal interface material (TIM)  114 . Similarly, a heat sink  118  is thermally coupled to the copper heat spreader via a second layer of TIM  118 . 
   During operation, the processor die generates heat due to resistive losses in its circuitry. This heats up the processor. Since heat flows high temperature sources to lower temperature sinks, heat is caused to flow through TIM layer  114  to copper spreader  112 . In turn, heat from the spreader flows through TIM layer  118  to heat sink  116 . The heat sink, in turn, is cooled by air that flows over the heat sink&#39;s fins  120 , either via natural convection or forced convection. Generally, the rate of cooling is a function of the fin area and the velocity of the air convection. 
   Thermal solutions are even more difficult for smaller processor-based devices, such as laptop computers and the like. In this instance, the amount of space available for heat sinks and heat spreaders is minimal, thereby causing the heat transfer capacity to be significantly reduced. The power available to drive fans is also significantly reduced. Even with the use of lower-power dies, the reduced heat transfer capacity often leads to the processors running derated speeds via self-regulation in response to over temp conditions. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing aspects and many of the attendant advantages of this invention will become more readily appreciated as the same becomes better understood by reference to the following detailed description, when taken in conjunction with the accompanying drawings, wherein like reference numerals refer to like parts throughout the various views unless otherwise specified: 
       FIG. 1  is a cross-section view of a conventional cooling assembly employing a metallic spreader and heat sink; 
       FIG. 2   a  is a schematic diagram of a closed loop cooling system employing a microchannel heat exchanger; 
       FIG. 2   b  is a cross-section view of a conventional microchannel heat exchanger that may be employed in the closed loop cooling system of  FIG. 2   a;    
       FIG. 3   a  is a cross-section view of a microchannel heat exchanger that is integrated with an integrated circuit (IC) die in accordance with an embodiment of the invention, wherein a thermal mass including a plurality of open microchannels is coupled to the IC die using a solder and the bottom surfaces of the microchannels comprise the solder material; 
       FIG. 3   b  is a cross-section view of a microchannel heat exchanger that is integrated with an IC die in accordance with an embodiment of the invention, wherein a thermal mass including a plurality of open microchannels is coupled to the IC die using a solder and the bottom surfaces of the microchannels comprise a solderable material; 
       FIG. 3   c  is a cross-section view of a microchannel heat exchanger that is integrated with an IC die in accordance with an embodiment of the invention, wherein a thermal mass including a plurality of open microchannels is coupled to the IC die using a thermal adhesive; 
       FIG. 3   d  is a cross-section view of a exemplary IC package in which the components of  FIG. 3   a  are coupled to a substrate and a chip carrier; 
       FIG. 4   a  is cross-section view of a microchannel heat exchanger that is coupled to an IC die via a thermal interface material layer in accordance with an embodiment of the invention, wherein the microchannel heat exchanger includes a thermal mass having a plurality of open microchannel covered by a plate; 
       FIG. 4   b  is a cross-section view of a microchannel heat exchanger that is coupled to an IC die via a thermal interface material layer in accordance with an embodiment of the invention, wherein the microchannel heat exchanger comprises a single-piece thermal mass in which a plurality of microchannels are formed; 
       FIG. 4   c  is a cross-section view of a exemplary IC package in which the components of  FIG. 4   a  are coupled to a substrate and a chip carrier; 
       FIG. 5   a  is a cross-section view of an IC package in which dual microchannel heat exchangers are employed, wherein the upper portion of the package includes a first integrated heat exchanger corresponding to one of the configurations shown in  FIGS. 3   a-c , and wherein microchannels corresponding to a second microchannel heat exchanger are formed in a flip-chip substrate; 
       FIG. 5   b  is a cross-section view of a variation of the embodiment of  FIG. 5   a  in which a chip carrier is not employed; 
       FIG. 6   a  is a cross-section view of an IC package in which dual microchannel heat exchangers are employed, wherein the upper portion of the package includes a first heat exchanger corresponding to one of the configurations shown in  FIGS. 4   a-b , and wherein microchannels corresponding to a second microchannel heat exchanger are formed in a flip-chip substrate; 
       FIG. 6   b  is a cross-section view of a variation of the embodiment of  FIG. 6   a  in which a chip carrier is not employed; 
       FIG. 7   a  is a cross-section view of an IC package in which dual microchannel heat exchangers are employed, wherein the upper portion of the package includes a first heat exchanger corresponding to one of the configurations shown in  FIGS. 3   a-c , and wherein microchannels corresponding to a second microchannel heat exchanger are formed in a chip carrier; 
       FIG. 7   b  is a cross-section view of an IC package in which dual microchannel heat exchangers are employed, wherein the upper portion of the package includes a first heat exchanger corresponding to one of the configurations shown in  FIGS. 4   a-b , and wherein microchannels corresponding to a second microchannel heat exchanger are formed in a chip carrier; 
       FIG. 8   a  is a plan view of a microchannel heat exchanger including parameters that define the configuration of the heat exchanger; 
       FIG. 8   b  is a cross section view illustrating further details of the channel configuration parameters of  FIG. 5   a;    
       FIGS. 9   a  and  9   b  are external and partial cut-away isometric views of a channeled heat sink in accordance with one embodiment of the invention; 
       FIGS. 9   c ,  9   d , and  9   e  show respective heat sink channel configurations corresponding to the channeled heat sink of  FIGS. 9   a  and  9   b ; and 
       FIG. 10  is a schematic diagram showing an exemplary cooling system in which embodiments of the invention may be employed. 
   

   DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
   Embodiments of microchannel heat exchanger apparatus, channeled heat sinks and cooling systems employing the same are described herein. In the following description, numerous specific details are set forth to provide a thorough understanding of embodiments of the invention. One skilled in the relevant art will recognize, however, that the invention can be practiced without one or more of the specific details, or with other methods, components, materials, etc. In other instances, well-known structures, materials, or operations are not shown or described in detail to avoid obscuring aspects of the invention. 
   Reference throughout this specification to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, the appearances of the phrases “in one embodiment” or “in an embodiment” in various places throughout this specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures, or characteristics may be combined in any suitable manner in one or more embodiments. 
   Recently, research efforts have been focused on providing thermal solutions for densely-packaged high-power electronics. A leading candidate emerging from this research is the use of two-phase convection in micromachined silicon heat sinks, commonly referred to as microchannels. A typical configuration for a microchannel-based cooling system is shown in  FIGS. 2   a  and  2   b . The system includes a microchannel heat exchanger  200 , a heat rejecter  202 , and a pump  204 . The basic premise is to take advantage of the fact that changing a phase of a fluid from a liquid to a vapor requires a significant amount of energy, known as latent heat, or heat of vaporization. Conversely, a large amount of heat can be removed from the fluid by returning the vapor phase of back to liquid. The microchannels, which typically have hydraulic diameters on the order of hundred-micrometers, are very effective for facilitating the phase transfer from liquid to vapor. 
   In accordance with typical configurations, microchannel heat exchanger  200  will comprise a plurality of microchannels  206  formed in a block of silicon  208 , as shown in  FIG. 2   b . A cover plate  210  is then placed over the top of the channel walls to formed enclosed channels. Generally the microchannel heat exchanger performs the function of a heat sink or heat spreader/heat sink combination. Accordingly, in  FIG. 2   b  the microchannel heat exchanger is shown as thermally coupled to a die  100  via a TIM layer  212 . In an optional configuration, a processor die with an increased thickness may include channels formed in the processor die silicon itself. 
   As the die circuitry generates heat, the heat is transferred outward to the microchannel heat exchanger via conduction. The heat increases the temperature of the silicon, thereby heating the temperature of the walls in the microchannels. Liquid is pushed by pump  204  into an inlet port  214 , where it enters the inlet ends of microchannels  206 . As the liquid passes through the microchannels, further heat transfer takes place between the microchannel walls and the liquid. Under a properly configured heat exchanger, a portion of the fluid exits the microchannels as vapor at outlet port  216 . The vapor then enters heat rejecter  202 . The heat rejecter comprises a second heat exchanger that performs the reverse phase transformation as microchannel heat exchanger  200 —that is, it converts the phase of the vapor entering at an inlet end back to a liquid at the outlet of the heat rejecter. The liquid is then received at an inlet side of pump  204 , thus completing the cooling cycle. 
   A significant advantage of the foregoing scheme is that is moves the heat rejection from the processor/die, which is typically somewhat centrally located within the chassis, to the location of the heat rejecter heat exchanger, which can be located anywhere within the chassis, or even externally. Thus, excellent heat transfer rates can be obtained without the need for large heatsinks/spreaders and high airflow rates. 
   However, there are a number of practical problems in the fabrication of microchannels in the silicon block or the silicon die itself. These problems include: bulk machining of brittle silicon might break the silicon; post silicon micromachining may impact IC yields; conventional etching can be used to make the channels, but the etching time required would be extensive; microchannels formed in silicon may reduce the mechanical strength of the silicon or die; IC fabrication processes are well-defined, with corresponding manufacturing infrastructure—introducing one or more extra fabrication processes could disrupt process flows and productivity; and formation of microchannels may lead to contamination or breaking of the die circuitry. 
   In accordance with aspects of the present invention, embodiments of microchannel heat exchangers and corresponding cooling solutions are disclosed herein that employ components that are do not suffer from the problems discussed above with respect to conventional approaches. Furthermore, these microchannel heat exchangers and cooling loops provide enhanced cooling rates when compared with those employed in research studies. 
   An integrated microchannel heat exchanger  300 A in accordance with one embodiment of the invention is shown in  FIG. 3   a . The microchannel heat exchanger includes a metallic thermal mass  302  in which a plurality of microchannels  304  are formed. Metallic thermal mass  302  may be configured in various shapes, including the block shape shown in  FIGS. 3   a-d . For point of illustration, the size and configuration of the microchannels formed in the metallic thermal mass are exaggerated for clarity; details of exemplary channel configurations are discussed below with reference to  FIGS. 5A and 5B . In accordance with principles of the embodiment, the thermal mass is mounted over an integrated circuit (IC) die  100  such that a hermetic seal is formed between the bases of internal channel walls  306  and external channel walls  308  and the top of the die. Thus, each of channels  304  comprises a closed volume configured to facilitate two-phase heat transfer in the manner discussed above. This and other microchannel heat exchangers are termed “integrated” because the IC die surface (or a layer coupled to the IC die surface) forms an integral part (i.e., the base) of the microchannels. 
   In the embodiment illustrated in  FIG. 3   a , the hermetic seal is formed by soldering metallic thermal mass  302  to die  100 . In particular, the bases of internal channel walls  306  and external channel walls  308  are soldered to a layer of solderable material  310  affixed to the top side of the die using a solder  312 . Generally, solderable material  310  may comprise any material to which the selected solder will bond. Such materials include but are not limited to metals such as copper (Cu), gold (Au), nickel (Ni), aluminum (Al), titanium (Ti), tantalum (Ta), silver (Ag) and Platinum (Pt). In one embodiment, the layer of solderable material comprises a base metal over which another metal is formed as a top layer. In another embodiment, the solderable material comprises a noble metal; such materials resist oxidation at solder reflow temperatures, thereby improving the quality of the soldered joints. 
   Generally, the layer (or layers) of solderable material may be formed over the top surface of the die  100  using one of many well-known techniques common to industry practices. For example, such techniques include but are not limited to sputtering, vapor deposition (chemical and physical), and plating. The formation of the solderable material layer may occur prior to die fabrication (i.e., at the wafer level) or after die fabrication processes are performed. 
   In one embodiment solder  312  may initially comprise a solder preform having a pre-formed shape conducive to the particular configuration of the bonding surfaces. The solder preform is placed between the die and the metallic thermal mass during a pre-assembly operation and then heated to a reflow temperature at which point the solder melts. The temperature of the solder and joined components are then lowered until the solder solidifies, thus forming a bond between the joined components. Furthermore, the solidified solder forms a hermetic seals between the bottom of the internal and external walls and the top of the die. 
   As shown in  FIG. 3   b , a solder preform  312 A that is configured such that there is a reduced amount of solder (preferably none) between the base of the microchannels and solderable material layer  310  is employed in an integrated microchannel heat exchanger  300 B. This embodiment provides improved heat transfer when compared with the embodiment of  FIG. 3A  because the thermal resistance represented by the solder material between the microchannels and the solderable material layer is removed. 
   In another embodiment corresponding to an integrated microchannel heat exchanger  300 C shown in  FIG. 3   c , a thermal adhesive  314  is used in place of a solder for securing the bases of internal channel walls  306  and external channel walls  308  to IC die  100 . Thermal adhesives, sometimes called thermal epoxies, are a class of adhesives that provide good to excellent conductive heat transfer rates. Typically, a thermal adhesive will employ fine portions (e.g., granules, slivers, flakes, micronized, etc.) of a metal or ceramic, such as silver or alumina, distributed within in a carrier (the adhesive), such as epoxy. As shown in  FIG. 3   c , in this embodiment the silicon die forms one of the microchannel walls, providing excellent heat transfer to the working fluid, particular when the ratio of the channel width to the wall width is large. Another advantage obtained when using some types of thermal adhesives, such as alumina products, concerns the fact that these thermal adhesives are also good insulators, thereby providing an electric isolation between the die circuitry and the metallic microchannel thermal mass. 
   A further consideration related to the embodiment of  FIG. 3   c  is that the thermal mass need not comprise a metal. In general, the thermal mass may be made of any material that provides good conduction heat transfer properties. For example, a ceramic carrier material embedded with metallic pieces in a manner to the thermal adhesives discussed above may be employed for the thermal mass. It is additionally noted that a thermal mass of similar properties may be employed in the embodiments of  FIGS. 3   a  and  3   c  if a layer of solderable material is formed over surface areas that are soldered to the IC die (i.e., the wall bases). 
   An IC package  320  corresponding to an exemplary use of microchannel heat exchanger  300 A (illustrated),  300 B, and  300 C is shown in  FIG. 3   d . The lower portion of the package is similar to the assembly shown in FIG.  1 . Accordingly, IC die  100  is flip-bonded to substrate  102  via solder bumps  104 , while substrate  102  is secured to chip carrier  108  via a plurality of solder balls  110 . It is noted that a similar package configuration may be employed for each of microchannel heat exchangers  300 A and  300 B. In general, chip carrier  108  represents various types of base components used in IC packaging, including leaded and non-leaded chip carriers, ball grid arrays (BGA&#39;s), pin grid arrays (PGA&#39;s) and the like. For clarity, many of the chip carriers illustrated herein due not show any connection features, although it will be understood that such features exist in an actual package. 
   In an alternative scheme, depicted in  FIGS. 4   a-c , a separate microchannel heat exchanger is thermally coupled to an IC die via a TIM layer, while the heat exchanger is operatively coupled to the die via a physical coupling to a substrate on which the die is mounted. For example, in the microchannel heat exchanger/die subassembly  400 A illustrated in  FIG. 4   a , an IC die  100  is mounted to a substrate  402 . For illustrative purposes, the die is shown to be flipped-bounded to the substrate; however, this is merely an exemplary mounting scheme, and is not meant to be limiting. A microchannel heat exchanger  404 A is then operatively coupled to the die via a physical coupling to substrate  402 . In general, this physically coupling can be provided by one of many well-known assembly techniques, such as via appropriate fasteners and/or adhesives. In the illustrated embodiment, a plurality of standoffs  406  are coupled to substrate  402 , while the microchannel heat exchanger is coupled to the standoffs via threaded fasteners  408 . For simplicity, the configuration for attaching the standoffs to the substrate is not shown—any of many well-known physical coupling techniques may be employed for this purpose. 
   As shown in  FIGS. 4   a-c , the bases of microchannel heat exchangers  404 A and  404 B are not directly coupled to the dies, but rather are thermally coupled via TIM layers  410 . The TIM layer performs several functions. Foremost, it provides a conductive heat transfer path between the microchannel heat exchanger and the top of the die. It also enables the various assembly components to contract and expand in response to temperature changes without inducing any stress on the assembled component while maintaining a good thermal conduction path. For instance, in response to an increase in temperature, most materials expand, while those same materials contract when their temperature is lowered. This rate of expansion/contraction is generally a fixed rate (at least locally) corresponding to the material&#39;s coefficient of thermal expansion (CTE). When the CTE for joined materials differs, one material expands or contracts relative to the other, inducing a stress at the joint between the materials. The CTE mismatch can lead to failure at the joint, especially when thermal cycling is occurs. 
   In most configurations, the material used for the standoffs will be a metal, such as aluminum, steel, or copper. These metals have higher CTE&#39;s than typical die materials (semiconductors, such as silicon). As a result, when the temperature increases, the thickness of the TIM layer will increase due to the higher expansion rate of the metal standoff than the die. Since the TIM layer is very compliant and adheres to the two material faces, it easily accommodates this expansion. At the same time, the metal in the microchannel heat exchanger expands horizontally at a different rate than the die does. The relative expansion between the two components is also easily handled by the TIM layer. 
   Microchannel heat exchanger/die subssemblies  400 A and  400 B are substantially similar except for their respective microchannel heat exchangers  404 A and  404 B. Microchannel heat exchanger  404 A comprises a metallic thermal mass  412  having a plurality of open channels formed therein. A plate  414  is employed to close the channels, thereby forming closed microchannels  416 . Ideally, the plate should be coupled to the top of the channel walls in a manner that forms a hermitic seal. If necessary, one of several well-known sealants may be disposed between the plate and the tops of the channel walls to facilitate this condition. In one embodiment, plate  414  is soldered to thermal mass  412 , in a manner similar to that discussed above with reference to  FIGS. 3   a  and  3   b.    
   In contrast, a plurality of closed microchannels  418  are formed in a metallic thermal mass  420  for microchannel heat exchanger  400 B, thus eliminating the need for a hermetic seal. In general, microchannels  418  may be formed using one of many metal forming techniques, such as casting, forging, and machining (e.g., electrical discharge machining). 
   An exemplary package  450  made from sub-assembly  400 A is shown in  FIG. 4   c . In the illustrated embodiment, substrate  402  is mounted to a chip carrier  452  via a plurality of solder balls  454 . It will be understood that various other types of packaging may also be employed, including BGA and PGA packages and the like. 
   In accordance with further aspects of the invention, IC packaging configurations employing multiple microchannel heat exchangers are now disclosed. In general, the following IC packages employ a pair of microchannel heat exchangers disposed on opposite sides of the IC die. Although the illustrated embodiments show particular configurations, it shall be recognized that any of the microchannel heat exchanger configurations discussed above may be employed for the IC packages. 
   In a first type of configuration shown in  FIGS. 5   a-b  and  6   a-b , a second microchannel heat exchanger is formed in the flip-chip substrate to which the IC die is flip-bonded. For example, in an IC package  500 A, IC die  100  forms part of a subassembly having a configuration similar to microchannel heat exchangers  300 A (illustrated),  300 B, and  300 C discussed above. IC die  100  is flip bonded to a microchannel heat exchanger substrate  502 A, in which a plurality of microchannels  504  are formed via a plurality of solder bumps  506 . An optional underfill  508  may be added to strengthen the coupling between the die and substrate. Microchannel heat exchanger substrate  502  is then mounted on a chip carrier  510  via a plurality of solder balls  512 . In an optional IC package  500 B configuration shown in  FIG. 5   b , a microchannel heat exchanger substrate  502 B comprises the base of the package (e.g., ball grid array package); this type of configuration is typically employed in low-profile electronics, such as laptop computers, PDA&#39;s, pocket PC&#39;s, and cell phones, wherein the package is wave-soldered to a circuit board. In the illustrated embodiment, the IC package may be coupled to a circuit board (not shown) via a plurality of solder balls  514  (i.e., corresponding to a BGA package). 
   IC packages  600 A and  600 B shown in  FIGS. 6A and 6B  employ analogous configurations to IC packages  500 A and  500 B, except for the substitution of microchannel heat exchanger/die subassemblies  400 A (illustrated) or  400 B in place of integrated microchannel heat exchangers  300 A-C, and the replacement of substrates  402  with microchannel heat exchanger substrates  602 . 
   In a second type of configuration shown in  FIGS. 7   a  and  7   b , the second microchannel heat exchanger is formed in the chip carrier. In an IC package  700 A, the upper portion of the package comprises an integrated microchannel heat exchanger  300 A (illustrated),  300 B, or  300 C, while in an IC package  700 B the upper portion comprises a microchannel heat exchanger/die assembly  400 A (illustrated) or  400 B. IC die  100  is mounted to a substrate  702 , e.g., flip bonded to the substrate via a plurality of solder bumps  704  and optional underfill  706 . In turn, substrate  702  is mounted to a microchannel heat exchanger/chip carrier  708 . In the illustrated embodiments of  FIGS. 7A and 7B , microchannel heat exchanger/chip carrier  708  comprises a two-part subassembly including a carrier  710  in which a plurality of open microchannels are formed, mounted to a plate  712 , wherein a hermetic seal is formed between the base of the microchannel walls and the plate to form closed microchannels  714 . In other embodiments (not illustrated), the microchannel heat exchanger/chip carrier may comprise a single component. 
   It is noted that various internal connection paths provided by the substrates and chip carriers are not shown in the Figures for clarity. In actual implementations, these components will need to be configured with appropriate internal connection routing paths that avoid the built-in microchannels. Thus, in most instances corresponding to configurations in which connection paths between pads on the top to pads on the bottom of to be provided, the substrate or carrier chip should comprise a single piece, for ease of manufacture, although it is possible to use a two-piece assembly (which would require electrical connections between the two pieces. In situations in which pads on top of a component are coupled to the side of the components, as is common in leaded and non-leaded chip carrier packages, a two-piece assembly may be employed, such as that shown in  FIGS. 7   a  and  7   b.    
   Plan and cross-section views illustrating typical channel configurations are shown in  FIGS. 8   a  and  8   b , respectively. In general, the channel configuration for a particular implementation will be a function of the heat transfer parameters (thermal coefficients, material thickness, heat dissipation requirements, thermal characteristics of working fluid), working fluid pumping characteristics (temperature, pressure, viscosity), and die and/or heat exchanger area. Although depicted as rectangular in configuration in the figures herein, the actual shape of the channels may include radiused profiles, or may even have substantially circular or oval profiles. The goal is to achieve a two-phase working condition in conjunction with a low and uniform junction temperature and a relatively low pressure drop across the heat exchanger. 
   Channel configuration parameters for rectangular channel shapes are shown in  FIGS. 8   a  and  8   b . The parameters include a width W, a depth D, and a length L. In parallel channel configurations, such as shown in  FIG. 8   a , respective reservoirs  802  and  804  fluidly coupled to an inlet  806  and outlet  808 . In essence, the reservoirs function as manifolds in coupling the microchannels to incoming and outgoing fluid lines. The plurality of microchannels will be formed in a thermal mass having a shape the generally corresponds to the die to which the heat exchanger is thermally coupled. For a rectangular configuration, which is likely to be most common but not limiting, the overall length of the heat exchanger is L HE  and the overall width is W HE . 
   Typically, the microchannels will have a hydraulic diameter (e.g., channel width W) in the hundreds of micrometers (μm), although sub-channels may be employed having hydraulic diameters of 100 μm or less. Similarly, the depth D of the channels will be of the same order of magnitude. It is believed that the pressure drop is key to achieving low and uniform junction temperature, which leads to increasing the channel widths. However, channels with high aspect rations (W/D) may induce flow instability due to the lateral variation of the flow velocity and the relatively low value of viscous forces per unit volume. 
   In one embodiment target for cooling a 20 mm×20 mm chip, 25 channels having a width W of 700 um, a depth D of 300 um and a pitch P of 800 um are formed in a thermal mass  810  having an overall length L HE  of 30 mm and an overall width W HE  of 22 mm, with a channel length of 20 mm. The working fluid is water, and the liquid water flow rate for the entire channel array is 20 ml/min. 
   In addition to heat transfer from the IC die, which converts a portion of the working fluid to vapor, a closed loop cooling cycle removes heat from the system via a second heat exchanger (a.k.a., the heat rejecter). In general, the heat rejecter will comprise a volume or plurality of volumes having walls on which the vapor condenses. If the walls are kept at a temperature lower than the saturation temperature (for a given pressure condition), the vapor will condense, converting it back to the liquid phase. 
   In one embodiment, a “channeled” or hollowed finned heat sink is employed for the heat rejecter. Exemplary configurations for a channels heat sink  900  of such a configuration are shown in  FIGS. 9   a-e . The channeled heat sink includes a plurality of hollow fins  902  having respective channels  904  formed therein. Incoming working fluid in the vapor state is received at an inlet  906  and enters a reservoir  908 . The vapor expands and condenses on the walls of channels  904 , falling down the walls as a liquid that is collected at the bottom of reservoir  908 . The liquid exits the channeled heat sink at an outlet  910  (hidden from view in  FIGS. 9   a-b ). 
   Three exemplary channel configurations corresponding to channeled heat sink embodiments  900 A,  900 B, and  900 C are shown in  FIGS. 9   c ,  9   d , and  9   e , respectively. In general, the configuration of the channeled heat sink is defined by the interior width of the channels W C , the width of the air gap between fins W A , and the depth of the fins D. A wide range of values may be used for each of these parameters, depending on the available space, required cooling rate, and convection considerations (such as whether forced air convection is available). Generally, unlike with the microchannel heat exchangers discussed above, there is more space in which to place the channeled heat sink. Thus, the size of the channels and corresponding fins may be significantly larger than the size of the microchannels discussed above. However, in cases in which a limited amount of space is available, the size of the channels in the channeled heat sink may be similar to those employed for the microchannel heat exchangers. 
   Generally, the channeled heat sink may be formed using well-known manufacturing techniques targeted towards thin-walled components, and may be made from a variety of materials, including various metals and plastics. The manufacturing techniques include but are not limited to casting (e.g., investment casting) and molding (e.g., injection molding, rotational molding for plastic components), and stamping (for metal components). Operations such as brazing may also be employed for assembling multi-piece channeled heat sinks. 
   An exemplary cooling system  1000  that is illustrative of cooling loop configurations employing various embodiments of the cooling system components discussed herein is shown in FIG.  10 . In general, the cooling system may be designed for cooling one or more components, such as IC dies, which produce significant levels of heat in a system, such as a laptop computer, PDA, pocket PC, etc. Typically, dual microchannel heat exchanger packaging will be targeted for higher power components, such as microprocessors, while single microchannel heat exchanger packaging will be targeted for (relatively) lower power components, such as chip sets, video chips, co-processors, and the like. 
   Cooling system  1000  employs a two-phase working fluid, such as but not limited to water. The working fluid is pumped through the system in liquid its liquid phase via a pump  1002 . Generally, the pumps used in the closed loop cooling system employing microchannel heat exchangers in accordance with the embodiments described herein may comprise electromechanical (e.g., MEMS-based) or electro-osmotic pumps (also referred to as “electric kinetic” or “E-K” pumps). In one respect, electro-osmotic pumps are advantageous over electromechanical pumps since they do not have any moving parts, which typically leads to improved reliability. Since both of these pump technologies are known in the microfluidic arts, further details are not provided herein. 
   Pump  1002  provides working fluid in liquid form to the inlets of the various microchannel heat exchangers. In the illustrated embodiments, these correspond to the dual microchannel heat exchangers employed in an IC package  500 A and the single integrated microchannel heat exchanger in an IC package  320 A. As denoted by the “ . . . ” continuation marks, there may be a plurality of each two of IC package configuration employed in an actual system. 
   Upon passing through the various microchannel heat exchangers, a portion of the working fluid is converted from its liquid phase to a vapor phase. This vapor (along with non-converted liquid) exits each microchannel heat exchanger and is routed via appropriate ducting to a heat rejecter. In one embodiment, the heat rejecter comprises a channeled heat sink  900 . Optionally, other types of heat rejecters may also be employed in other embodiments. The vapor is converted back to a liquid in the heat rejecter, which the exits the heat rejecter and is routed to the inlet of pump  1002 , thus completing the cooling loop. In an optional configuration, a reservoir  1104  may be provided to enable additional working fluid to be added to the cooling loop in the event of working fluid losses, such as through evaporation at duct couplings. 
   Heat transfer rates have been simulated and/or calculated to verify the effectiveness of the various microchannel heat exchanger/spreader embodiments discussed above. Based on empirically measured heat transfer rates observed during two-phase water vapor convection heat transfer research studies, the simulations/calculations have indicated favorable cooling rates when compared with the prior art configurations discussed above. Furthermore, the simulations indicate that the microchannel heat exchangers are especially advantageous for cooling hot-spots in IC dies that do not produce an even heat distribution. 
   The above description of illustrated embodiments of the invention, including what is described in the Abstract, is not intended to be exhaustive or to limit the invention to the precise forms disclosed. While specific embodiments of, and examples for, the invention are described herein for illustrative purposes, various equivalent modifications are possible within the scope of the invention, as those skilled in the relevant art will recognize. 
   These modifications can be made to the invention in light of the above detailed description. The terms used in the following claims should not be construed to limit the invention to the specific embodiments disclosed in the specification and the claims. Rather, the scope of the invention is to be determined entirely by the following claims, which are to be construed in accordance with established doctrines of claim interpretation.