Patent Publication Number: US-2005141195-A1

Title: Folded fin microchannel heat exchanger

Description:
BACKGROUND  
      Integrated circuits such as microprocessors generate heat when they operate and frequently this heat must be dissipated or removed from the integrated circuit die to prevent overheating. This is particularly true when the microprocessor is used in a notebook computer or other compact device where space is tightly constrained and more traditional die cooling techniques such as direct forced air cooling are impractical to implement.  
      One technique for cooling an integrated circuit die is to attach a fluid-filled microchannel heat exchanger to the die. A typical microchannel heat exchanger consists of a silicon substrate in which microchannels have been formed using a subtractive microfabrication process such as deep reactive ion etching or electro-discharge machining. Typical microchannels are rectangular in cross-section with widths of about 100 μm and depths of between 100-300 μm. Fundamentally the microchannels improve a heat exchanger&#39;s coefficient of heat transfer by increasing the conductive surface area in the heat exchanger. Heat conducted into the fluid filling the channels can be removed simply by withdrawing the heated fluid.  
      Typically, the microchannel heat exchanger is part of a closed loop cooling system that uses a pump to cycle a fluid such as water between the microchannel heat exchanger where the fluid absorbs heat from a microprocessor or other integrated circuit die and a remote heat sink where the fluid is cooled. Heat transfer between the microchannel walls and the fluid is greatly improved if sufficient heat is conducted into the fluid to cause it to vaporize. Such “two-phase” cooling enhances the efficiency of the microchannel heat exchanger because significant thermal energy above and beyond that which can be simply conducted into the fluid is consumed in overcoming the fluid&#39;s latent heat of vaporization. For example, conductively heating 50 grams of liquid water from 0° C. to 100° C. consumes 21 kJ of heat energy while then vaporizing the same quantity of water at 100° C. consumes a further 113 kJ of energy. This latent heat is then expelled from the system when the fluid vapor condenses back to liquid form in the remote heat sink. Water is a particularly useful fluid to use in two-phase systems because it is cheap, has a high heat (or enthalpy) of vaporization and boils at a temperature that is well suited to cooling integrated circuits.  
      While a conventional microchannel heat exchanger as described above can effectively cool an integrated circuit die, conventional microchannel heat exchangers are expensive to manufacture because the microfabrication techniques used to create the microchannels such as deep reactive ion etching or electro-discharge machining are expensive to implement and have low processing throughput. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The foregoing aspects of this invention will become more readily appreciated 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. In the drawings:  
       FIG. 1  is a cross-section view of a folded fin microchannel heat exchanger in accordance with an embodiment of the invention;  
       FIG. 2  is a cross-section view of an integrated thermal management assembly including a folded fin microchannel heat exchanger coupled to an integrated circuit (IC) die using a thermal interface material and fasteners in accordance with an embodiment of the invention;  
       FIG. 3  is a cross-section view of an integrated thermal management assembly including a folded fin microchannel heat exchanger coupled to an IC die using solder in accordance with an embodiment of the invention;  
       FIG. 4  is a cross-section view of an integrated thermal management assembly including a folded fin microchannel heat exchanger coupled to an IC die using a thermal adhesive in accordance with an embodiment of the invention;  
       FIG. 5  is a block diagram of a mobile computer system employing a closed loop two-phase cooling system including a folded fin microchannel heat exchanger in accordance with an embodiment of the invention;  
       FIG. 6  is a schematic diagram of a closed loop cooling system employing a folded fin microchannel heat exchanger in accordance with an embodiment of the invention;  
       FIG. 7   a  is a plan view of a folded fin microchannel heat exchanger in accordance with an embodiment of the invention including parameters that define the configuration of the heat exchanger;  
       FIG. 7   b  is a cross section view illustrating further details of the channel configuration parameters of  FIG. 7   a  in accordance with an embodiment of the invention; and  
       FIG. 8  is a flow diagram representing implementation of a method for cooling ICs using a folded fin microchannel heat exchanger in accordance with an embodiment of the invention  
    
    
     DETAILED DESCRIPTION  
      Embodiments of folded fin microchannel heat exchanger apparatus, cooling systems employing the same and methods for cooling electronic components using the same are described. In the following description, numerous specific details such as cooling apparatus and system implementations, types and interrelationships of cooling apparatus and system components, and particular embodiments of folded fin microchannel heat exchangers are set forth in order to provide a more thorough understanding of the present invention. It will be appreciated, however, by one skilled in the art that embodiments of the invention may be practiced without such specific details or by utilizing, for example, different embodiments of folded fin microchannel heat exchangers. In other instances, methods for manufacturing folded fin heat exchangers or specific mechanical details for implementing cooling apparatus or systems, for example, have not been shown in detail in order not to obscure the embodiments of the invention. Those of ordinary skill in the art, with the included descriptions will be able to implement appropriate functionality without undue experimentation.  
      References in the specification to “one embodiment”, “an embodiment”, “an example embodiment”, etc., indicate that the embodiment described may include a particular feature, structure, or characteristic, but every embodiment may not necessarily include the particular feature, structure, or characteristic. Moreover, such phrases are not necessarily referring to the same embodiment. Further, when a particular feature, structure, or characteristic is described in connection with an embodiment, it is submitted that it is within the knowledge of one skilled in the art to affect such feature, structure, or characteristic in connection with other embodiments whether or not explicitly described. Moreover, the particular features, structures, or characteristics may be combined in any suitable manner in one or more embodiments.  
      A number of figures show block diagrams of apparatus and systems comprising folded fin microchannel heat exchangers, in accordance with embodiments of the invention. One or more figures show flow diagrams illustrating operations for making or using folded fin microchannel heat exchangers likewise in accordance with embodiments of the invention. The operations of the flow diagrams will be described with references to the systems/apparatus shown in the block diagrams. However, it should be understood that the operations of the flow diagrams could be performed by embodiments of systems and apparatus other than those discussed with reference to the block diagrams, and embodiments discussed with reference to the systems/apparatus could perform operations different than those discussed with reference to the flow diagrams.  
     Folded Fin Microchannel Heat Exchanger  
       FIG. 1  illustrates in cross-sectional view one embodiment of a folded fin microchannel heat exchanger  100  in accordance with the invention. Heat exchanger  100  includes a metal folded fin  102  housed within a metal base  104  to define channels  106  between folded fin  102  and base  104 . A cover plate  108  encloses folded fin  102  within base  104  such that a hermetic seal is formed between base  104  and plate  108  and such that channels  110  are defined between folded fin  102  and plate  108 . For illustration purposes the size and form of folded fin  102  and the dimensions of channels  106  and  110  are exaggerated for clarity. In operation, heat exchanger  100  acts as a thermal mass to absorb heat conducted from integrated circuits. Details of exemplary configurations for channels  106  and  110  are discussed below with reference to  FIGS. 6   a  and  6   b.  Folded fin  102  and base  104  are formed using well-known techniques. For example, folded fin  102  can be formed by folding metal sheet stock and base  104  can be formed by stamping it out of metal sheet stock.  
      Channels  106  and  110  together comprise the microchannels within heat exchanger  100  through which a fluid such as water can be pumped from an inlet manifold and an outlet manifold (not shown in  FIG. 1  but discussed below with reference to  FIGS. 5 and 6 ). In one embodiment, folded fin  102 , base  104  and plate  108  are formed from copper allowing folded fin  102  and plate  108  to be brazed to base  104  using standard copper brazing techniques. The invention is, however, not limited in this respect and any technique of containing folded fin  102  within base  104  and plate  108  may be utilized such that channels  106  and  110  are defined and such that a hermetic seal is formed between base  104  and plate  108 . For example, plate  108  could be soldered or brazed to base  104  such that folded fin  102  is contained within the space formed between base  104  and plate  108  without directly attaching folded fin  102  to base  104 . Alternatively, plate  108  could be affixed to base  104  using any means capable of forming a hermetic seal between plate  108  and base  104  such as adhesives or o-ring seals in combination with clips or other fasteners.  
       FIG. 2  illustrates, in accordance with an embodiment of the invention, an integrated thermal management assembly  200  comprising folded fin microchannel heat exchanger  100  coupled thermally to an integrated circuit (IC) die  202  via a Thermal Interface Material (TIM)  204  and coupled operatively to a substrate  206  to which the IC die  202  is flip-bonded by a plurality of solder bumps  208 . TIM layer  204  serves several purposes; first, it provides a conductive heat transfer path from die  202  to heat exchanger  100  and, second, because TIM layer  204  is very compliant and adheres well to both the die  202  and heat exchanger  100 , it acts as a flexible buffer to accommodate physical stress resulting from differences in the coefficients of thermal expansion (CTE) between die  202  and heat exchanger  100 .  
      Heat exchanger  100  is physically coupled to substrate  206  through a plurality of fasteners  212  each one of the plurality of fasteners  212  coupled to a respective one of a plurality of standoffs  214  mounted on substrate  206 . In addition, an epoxy underfill  210  is typically employed to strengthen the interface between die  202  and substrate  206 . The illustrated fasteners  212  and standoffs  214  are just one example of a number of well known assembly techniques that can be used to physically couple heat exchanger  100  to die  202 . In another embodiment, for example, heat exchanger  100  is coupled to die  202  using clips mounted on substrate  206  and extending over heat exchanger  100  in order to press heat exchanger  100  against TIM layer  204  and die  202 .  
       FIG. 3  illustrates, in accordance with an embodiment of the invention, an integrated thermal management assembly  300  comprising a metallic folded fin microchannel heat exchanger  100  coupled thermally and operatively to an IC die  302  by solder  304  and solderable material  306 . Soldering heat exchanger  100  to die  302  eliminates the need for the fasteners and standoffs of assembly  200  of  FIG. 2 . As above, an epoxy underfill  210  is typically employed to strengthen the interface between die  302  and the substrate  206  that die  302  is flip-bonded to by a plurality of solder bumps  208 .  
      Generally, solderable material  306  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. In one embodiment, both heat exchanger  100  and solderable material  306  are copper.  
      Generally, the layer (or layers) of solderable material may be formed over the top surface of the die  302  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  304  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 heat exchanger 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.  
       FIG. 4  illustrates, in accordance with an embodiment of the invention, an integrated thermal management assembly  400  comprising a folded fin microchannel heat exchanger  100  coupled thermally and operatively to an IC die  402  by a thermal adhesive  404 . 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. One advantage obtained when using some types of thermal adhesives, such as alumina products, concerns the fact that these thermal adhesives are also good electrical insulators, thereby electrically isolating the die circuitry from the metallic folded fin microchannel heat exchanger.  
      A further consideration related to the embodiment of  FIG. 4  is that the heat exchanger need not comprise a metal. In general, the heat exchanger may be made of any material that provides good conductive 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 heat exchanger. It is additionally noted that a heat exchanger of similar properties may be employed in the embodiments of  FIGS. 2 and 3  if, in the case of the embodiment of  FIG. 3 , a layer of solderable material is formed over surface areas that are soldered to the IC die (i.e., the base of folded fin microchannel heat exchanger  100 ).  
      While FIGS.  2  thru  4  illustrate folded fin microchannel heat exchanger  100  thermally and operatively coupled to IC die  202 ,  302  and  402  respectively, the invention is not limited in this respect and one of ordinary skill in the art will appreciate that folded fin heat exchangers  100  can be thermally and operatively coupled to an IC package containing one or more IC die while remaining within the scope and spirit of the invention.  
     Cooling Systems  
       FIG. 5  illustrates one embodiment in accordance with the invention of a mobile computer system  500  having a closed loop two-phase cooling system  502  including a folded fin microchannel heat exchanger (not shown) coupled thermally and operatively to an IC die or package  504 . System  500  includes a bus  506 , which in an embodiment, may be a Peripheral Component Interface (PCI) bus, linking die  504  to a network interface  508  and an antenna  510 . Network interface  508  provides an interface between IC die or package  504  and communications entering or leaving system  500  via antenna  510 . The folded fin microchannel heat exchanger within cooling system  502  acts as a thermal mass to absorb thermal energy from, and thereby cool, IC die or package  504 . Cooling system  502  is described in more detail below with respect to  FIGS. 6, 7   a  and  7   b.  While the embodiment of system  500  is a mobile computer system, the invention is not limited in this respect and other embodiments of systems incorporating cooling systems utilizing folded fin microchannel heat exchangers in accordance with the invention include, for example, desktop computer systems, server computer systems and computer gaming consoles to name only a few possibilities.  
       FIG. 6  illustrates one embodiment in accordance with the invention of closed loop two-phase cooling system  500  having a folded fin microchannel heat exchanger coupled thermally and operatively to an IC die or package (not shown). System  500  includes a folded fin microchannel heat exchanger  100 , a heat rejecter  600 , and a pump  602 . System  500  takes advantage of the fact, as discussed earlier, that a fluid undergoing a phase transition from a liquid state to a vapor state absorbs a significant amount of energy, known as latent heat, or heat of vaporization. This absorbed heat having been converted into potential energy in the form of the fluid&#39;s vapor state can be subsequently removed from the fluid by returning the vapor phase back to liquid. The folded fin microchannels, which typically have hydraulic diameters on the order of hundred-micrometers, are very effective for facilitating the phase transfer from liquid to vapor.  
      System  500  functions as follows. As the die circuitry generates heat, the heat is conducted into the folded fin microchannel heat exchanger  100 . The heat increases the temperature of the heat exchanger  100  thermal mass, thereby heating the temperature of the walls in the folded fin microchannels. Liquid is pushed by pump  602  into an inlet port  604 , where it enters the inlet ends of the folded fin microchannels. 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 a vapor at outlet port  606 . The vapor then enters heat rejecter  600 . The heat rejecter comprises a second heat exchanger that performs the reverse phase transformation as folded fin microchannel heat exchanger  100 —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  602 , thus completing the cooling cycle.  
      In this manner system  500  acts to transfer the heat rejection process from the processor/die, which is typically somewhat centrally located within the chassis of a notebook computer, for example, to the location of the heat rejecter heat exchanger, which can be located anywhere within the chassis, or even externally.  
     Folded Fin Microchannel Configurations  
      Plan and cross-section views illustrating folded fin microchannel heat exchanger configurations in accordance with the invention are shown in  FIGS. 7   a  and  7   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. 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.  
      Example folded fin microchannel configuration parameters are shown in  FIG. 7   a  and  7   b.  The parameters include a width W, a depth D, and a length l. Respective reservoirs  702  and  704  are fluidly coupled to an inlet  706  and outlet  708 . In essence, the reservoirs function as manifolds in coupling the microchannels of folded fin  102  to incoming and outgoing fluid lines. If formed from copper, for example, folded fin  102 , defining rectangular channels  106  and  110 , can be brazed to copper base  104  formed, itself, by stamping from copper sheet stock. Brazing copper plate  108  to base  104  and adding inlet  706  and outlet  708  completes heat exchanger  100 . Incorporating space for reservoirs  702  and  704  yields an overall length of the heat exchanger of L HE  and an overall width of W HE .  
      Typically, the folded fin microchannels  106  and  110  will have a hydraulic diameter (e.g., channel width W) in the hundreds of micrometers (μm), although smaller microchannels may be employed having hydraulic diameters of 100 μm or less. Similarly, the depth D of the microchannels 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 ratios (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 an example of representative dimensions for a folded fin microchannel heat exchanger for cooling a 20 mm×20 mm die, 25 channels having a width w of 700 μm, a depth d of 300 μm and a pitch p of 800 μm are defined by a folded fin contained with a heat exchanger (thermal mass)  100  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. While these dimensions are representative of one embodiment of the invention, the invention is not limited in this respect and other folded fin microchannel heat exchanger dimensions may be utilized while remaining within the scope and spirit of the invention.  
      Generally, the pump  602  used in the closed loop cooling system  500  employing folded fin microchannel heat exchangers  100  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). Electro-osmotic pumps are advantageous over electromechanical pumps because they do not have any moving parts and hence are more reliable than electromechanical pumps. Since both of these pump technologies are known in the microfluidic arts, further details are not provided herein.  
       FIG. 8  illustrates a flow diagram representing implementation of a method for cooling ICs using a folded fin microchannel heat exchanger in accordance with an embodiment of the invention. In the embodiment of  FIG. 8  the ICs being cooled include a processor IC and can include additional components such as platform chipset ICs, memory ICs, video ICs, co-processors or other ICs. Some or all of the additional ICs can be spatially separated from the processor IC or can be included in an IC package along with processor IC. In block  802 , at least one folded fin microchannel heat exchanger is thermally coupled to a least one IC. In block  804 , a working fluid such as water is passed through the folded fin microchannel heat exchanger. At block  806 , heat is transferred from the IC into working fluid within the folded fin microchannel heat exchanger thereby converting a portion of the working fluid from liquid to vapor phase. Finally, at block  808 , the working fluid exiting the folded fin microchannel heat exchanger is passed through a heat rejector where heat is removed from the working fluid converting the working fluid back to a liquid phase.  
      Thus, methods, apparatuses and systems of a folded fin microchannel heat exchanger have been described. Although the invention has been described with reference to specific exemplary embodiments, it will be evident that various modifications and changes may be made to these embodiments without departing from the broader spirit and scope of the invention. For example, while the method, apparatuses and systems for utilizing a folded fin microchannel heat exchanger are described in reference to the invention&#39;s use in a two-phase liquid cooling system, in other embodiments, such method and systems are applicable to use in a single-phase cooling system. Therefore, the specification and drawings are to be regarded in an illustrative rather than a restrictive sense.