Abstract:
A scalable method of edge cooling and making a cold chassis for electronic modules. The steps include fabrication of modular cooling ribs, each including microchannels along the cooling edge and a peripheral flange. A set of adjacent ribs are secured together and assembled onto at least one face of a chassis frame member. The rib flanges are sealed (e.g. friction stir welded) with respect to the frame member. The Application includes new and retrofit environments with increased cooling needs and multi-scaled manufacturing.

Description:
FIELD OF THE INVENTION 
     The subject invention relates primarily to heat sinks and, in particular, edge cooling of electronic modules. 
     BACKGROUND OF THE INVENTION 
     Electronic modules residing in a chassis or cabinet or other higher level assembly or system are often “edge cooled.” The typical electronic module includes a printed circuit board populated with electronic components. U.S. Pat. No. 4,962,444, for example, discloses a cold chassis including a frame with spaced ribs. Electronic modules are placed between the ribs. Coolant flows in the frame and ribs to cool the electronic modules. U.S. Published Patent Application No. 2005/0199372 discloses a cooling rib with machined cooling fins therein. Both of these references are incorporated herein by this reference. 
     In some cases, a radar system is one example, the equipment chassis is already designed and/or in place but higher power electronic modules (e.g., transmit/receive integrated microwave modules or TRIMMs) are to be installed. Changing the overall system architecture to include increased cooling for such modules may be expensive and thus undesirable. 
     Microchannel heat sinks are also known and, in one example, folded fins are inserted into a body and then brazing methods are used to secure the fins in place. During brazing, however, the microchannels formed by the fins can become clogged resulting in low yields. In addition, brazing autoclaves are limited in size. Brazing larger size heat sinks is expensive due to the limited availability of suppliers who have large brazing autoclaves and associated equipment. 
     Prior attempts to implement high-performance cooling schemes for edge-cooled electronic assemblies have had limited impact due to fundamental thermal physical and mechanical constraints preventing implementation on the scale necessary for edge cooling high power electronic systems. One thermal physical constraint includes the ability to fit the necessary volumetric flow of coolant into a constrained geometry and/or excessive temperature rises and pressure drops. An example of a mechanical constraint includes designs which cannot be easily scaled and/or manufactured. Many edge-cooled electronic systems are large and constitute many square meters which must be cooled. Practical large-scale solutions involving high performance cooling techniques such as the use of microchannels have proven elusive. 
     BRIEF SUMMARY OF THE INVENTION 
     The invention, in one aspect, features the use of cooling ribs with microchannels therein in combination with a phase change coolant to provide an increased heat transfer coefficient and also isothermal cooling. One preferred version of the cooling ribs enables them to be individually tested after brazing. Thereafter, welding, (e.g., friction stir welding) techniques are used to secure the ribs to a frame member constructed to supply the phase change coolant to the individual cooling ribs. Large scale manufacturability is possible. 
     Featured are methods of making a cold chassis for electronic modules. One preferred method includes fabricating individual cooling ribs, typically brazed ribs each including microchannels along the length thereof. Each rib also preferably includes a peripheral flange. A plurality of adjacent ribs may first be secured together. The plurality of joined ribs are assembled onto at least one face of a frame member. The rib flanges are then sealed with respect to the frame member typically by friction stir welding. 
     The method may further include the step of forming, in at least one face of the frame member, a supply header spaced from a return header. The supply header and the return header are typically depressions machined in the face of the frame member. The preferred method further includes the step of forming (e.g., machining) patterned recesses in the face of the frame member. The recesses correspond to the ribs. The pattern recesses may include a ledge portion proximate the supply header and the return header. 
     Typically, adjacent flanges of the ribs are conjoined at locations corresponding to the supply and return headers in the frame member. A supply port can be added to the conjoined ribs corresponding to the location of the supply header and a return port can be added to the conjoined ribs corresponding to the location of the return header. 
     Fabricating each rib may include forming a proximal inlet port in fluid communication with a distal outlet port. Typically, a phase change coolant is introduced to the supply port to flow into the supply header and then into the inlet ports of the plurality of ribs and through the rib microchannels. The coolant exits out the outlet port of the ribs, into the return header, and out the return port. 
     The ribs may be machined if necessary, after they are sealed to the frame member and/or tested after fabrication but before they are assembled onto the frame member. 
     One cold chassis according to the invention features a frame member including patterned adjacent recesses, and a plurality of cooling ribs on the frame member. A seal secures peripheral flanges of each rib in recesses in the frame member. The preferred seal is attained via a friction stir weld. 
     Typically, each rib includes microchannels along the length thereof. One example of a frame member includes a proximal supply header and a distal return header and each rib then includes a proximal bottom side inlet port in fluid communication with the supply header and a distal bottom side outlet port in fluid communication with the return header. There may also be at least one supply port in fluid communication with the supply header and at least one return port in fluid communication with the return header. A phase change coolant flows through the supply port into the supply header and into the inlet ports of the ribs, through the ribs to the outlet ports thereof, into the return header, and then exiting via the return port. 
     The subject invention, however, in other embodiments, need not achieve all these objectives and the claims hereof should not be limited to structures or methods capable of achieving these objectives. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       Other objects, features and advantages will occur to those skilled in the art from the following description of a preferred embodiment and the accompanying drawings, in which: 
         FIG. 1A  is a schematic three-dimensional top view of an example of an individual cooling rib in accordance with the invention; 
         FIG. 1B  is a bottom view of the cooling rib shown in  FIG. 1 ; 
         FIG. 2  is a schematic cross-sectional view of the rib shown in  FIG. 1A  taken along line  2 - 2 ; 
         FIG. 3A  is a schematic three-dimensional top view showing a plurality of individual cooling ribs joined together in accordance with an example of the method of the invention; 
         FIG. 3B  is a schematic three-dimensional top view showing an example where coolant supply and return ports are added to the conjoined ribs; 
         FIG. 4  is a schematic three-dimensional view showing one face of a frame member for the cooling ribs in accordance with an example of the invention; 
         FIG. 5  is a schematic three-dimensional top view showing how the conjoined ribs are assembled on to the frame member and sealed with respect thereto; 
         FIG. 6  is a schematic cross-sectional view of a portion of the assembly shown in  FIG. 5 ; 
         FIG. 7  is a schematic three-dimensional top view similar to  FIG. 5  showing the flow of a phase change coolant through the assembly; and 
         FIG. 8  is a schematic three-dimensional front view showing an electronic module disposed between adjacent cooling ribs in accordance with the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Aside from the preferred embodiment or embodiments disclosed below, this invention is capable of other embodiments and of being practiced or being carried out in various ways. Thus, it is to be understood that the invention is not limited in its application to the details of construction and the arrangements of components set forth in the following description or illustrated in the drawings. If only one embodiment is described herein, the claims hereof are not to be limited to that embodiment. Moreover, the claims hereof are not to be read restrictively unless there is clear and convincing evidence manifesting a certain exclusion, restriction, or disclaimer. 
       FIGS. 1A-1B  show an example of a brazed cooling rib  10  including a lengthy member  12  made of, for example, aluminum. The rib preferably includes microchannels along its length. In this particular example, the microchannels are formed via microfins  14 ,  FIG. 2  within the rib extending along the length of member  12  between inlet port  16   a ,  FIG. 1B  and outlet port  16   b  on the bottom of the rib. The microfins may also be made of aluminum. In one preferred design, rib  10  includes peripheral flange  18 . Typically, a plurality of these ribs are fabricated using brazing methods to secure microfins  14  in member  12 . Brazing methods may result in the clogging of the microfins in some cases but fabricating the ribs in this way allows them to be individually pressure and/or flow tested before the next step in the method where the ribs are incorporated into a higher level assembly. Only ribs which pass the test(s) are then used. 
       FIG. 3A  shows a plurality of these ribs  10   a ,  10   b , and  10   c  secured together so that they lie adjacent to each other typically in a parallel arrangement. The lengthy slot between adjacent ribs will accommodate an edge, typically one side, of an electronic module. The number of ribs will vary depending upon the design of the chassis, the number of modules, and the like. In one preferred method, the flanges of ribs  10   a - 10   c  are friction stir welded at locations A and B which typically correspond to the locations of supply header  20 ,  FIG. 4  and return header  22 , respectively, machined in one or both faces of frame member  24 . A friction stir weld is shown, for example, at  30  in  FIG. 3A  between the flanges of ribs  10   a  and  10   b  at location A. A fixture, not shown, may be used to align and support the ribs during the friction stir welding process. The ribs are secured or conjoined together in this example at locations A and B because the force used in the friction stir welding process might bend the ribs at locations A and B if they were first placed over supply header  20  and return header  22 ,  FIG. 4 .  FIG. 3B  shows a design where supply port member  30 , end cap member  32 , and return port members  34   a  and  34   b  are also added to rib assembly typically also by employing friction stir welding techniques. 
     At some point in the process, patterned recesses  40   a ,  40   b  and  40   c ,  FIG. 4  are formed (e.g., machined) in one or both faces of frame member  24 . There is typically one recess for each rib and the recesses, like the ribs, run in a parallel relationship along the extent of frame member  24 . A spaced pair of such frame members are typically used to support a number of edge cooled electronic modules. As shown for recess  40   c , each recess includes a peripheral ledge  42  in which sits the peripheral flange of rib  10   c ,  FIGS. 5-6  at the supply and return header areas of the frame member. Frame member  24  may also be made of aluminum. 
     After the conjoined ribs have been assembled onto the frame member  24  as shown in  FIG. 5 , the rib flanges are sealed to the frame member again preferably using friction stir welding techniques (except at locations A and B,  FIG. 3A  above the supply and return headers,  FIG. 4 , where the ribs are already conjoined together).  FIGS. 5 and 6  also show rib members on the opposite face of frame member  24  which is common. Thus, there is a friction stir weld at locations  44  and  46  in  FIG. 6 , i.e., about the entire periphery of the rib flange. If needed, the entire assembly can then be machined especially if the application demands very tight tolerances of the pitch from rib to rib (distance X in  FIG. 6 ), rib height (Y), or co-planarity (Z). 
     In this specific design, a phase change coolant is introduced via supply port  30 ,  FIG. 7  and flows into supply header  20 ,  FIG. 4  and then into the inlets (see inlet port  16   a ,  FIG. 1B ) of ribs  10   a - 10   c . As the phase change coolant flows through the microchannels of the ribs, it edge cools electronic modules supported between the ribs.  FIG. 8  shows electronic module  50  (e.g., a TRIMM) held in place between ribs  10   a  and  10   b  via one or more wedge locks. The wedge lock applies force to the edge of the heat dissipating electronic assembly to enhance thermal contact with the ribs. A similar cooling manifold is usually fabricated for the other edge of electronic assembly  50 ,  FIG. 8 . The coolant then exits the outlet ports of the ribs (see outlet port  16   b ,  FIG. 1B ) and flows into return header  22 ,  FIG. 4  and then out return ports  34   a  and  34   b , FIGS.  5  and  7 - 8 . The conduits, plenums, valving, and the like at the chassis level for coolant supply and return is not shown. 
     The typical manufacturing process involves fabricating brazed rib assemblies with microfin density and locations appropriate for the thermal load to be addressed. The size of the rib flange can be adjusted to achieve the proper friction stir weld at the next higher assembly. A fixture may be used to properly align a plurality of the rib assemblies and to physically connect the ribs together at the edge flanges again using friction stir welding as discussed above with reference to  FIGS. 3A and 3B . Traditional welding, epoxy, and other techniques, however, may be used. The frame member is machined to create a coolant supply and coolant return headers that will feed the rib assembly in parallel. One or both sides of the frame member may be machined. The entire periphery of the conjoined rib assembly is then friction stir welded to the frame member. This creates a water tight seal and a structural joint around the perimeter of each rib flange. 
     This scalable method for parallel cooling of edge cooled electronic assemblies using modular microchannel rib members can be configured to fit an existing mechanical envelope while at the same time providing significantly improved cooling capabilities. 
     The high density brazed folded fin configuration is preferably used to obtain the heat transfer and pressure drop characteristics required for thermal isolation over large distances (typically, many meters). The configuration of brazed folded fin  14 ,  FIG. 2  can be selected to obtain isothermal rib performance at a specified flow rate. Indeed, varying fin pitches can be used to achieve varying cooling performance along the length of the rib. For example, if certain areas of electronic module  50 ,  FIG. 8  generate more heat than other areas, the configuration of the fin pitches in the cooling ribs can be tailored accordingly. The size of the microchannels along the length of the ribs may thus vary. 
     In one preferred embodiment, the inventive method takes advantage of novel combinations of multi-scaled manufacturing processes to achieve increased heat transfer performance, thermal isolation, and a flow distribution control desired to realize cost-effective high-performance edge cooling over many square meters. The parallel distribution of a phase change coolant in the modular microchannel ribs provides an increase in performance relative to current known state of the art approaches. The result is an increase in the power density for edge-cooled electronic assemblies without a corresponding increase in cold plate temperature within the same form factor. The use of a phase change coolant also increases the heat transfer rate to the modular microchannel rib assemblies and results in a constant rib temperature. This allows the heat to preferentially flow into the ribs instead of the supply and return headers to achieve the required thermal isolation. Existing electronic assemblies can be backfit with the edge cooling subassembly shown in  FIG. 8 . Finally, each rib can be independently flow and pressure tested before it is permanently attached to the next higher assembly. The smaller scale brazement also permits many more suppliers to produce the individual ribs further reducing costs and improving yield at final assembly. 
     Thus, although specific features of the invention are shown in some drawings and not in others, this is for convenience only as each feature may be combined with any or all of the other features in accordance with the invention. The words “including”, “comprising”, “having”, and “with” as used herein are to be interpreted broadly and comprehensively and are not limited to any physical interconnection. Moreover, any embodiments disclosed in the subject application are not to be taken as the only possible embodiments. 
     In addition, any amendment presented during the prosecution of the patent application for this patent is not a disclaimer of any claim element presented in the application as filed: those skilled in the art cannot reasonably be expected to draft a claim that would literally encompass all possible equivalents, many equivalents will be unforeseeable at the time of the amendment and are beyond a fair interpretation of what is to be surrendered (if anything), the rationale underlying the amendment may bear no more than a tangential relation to many equivalents, and/or there are many other reasons the applicant can not be expected to describe certain insubstantial substitutes for any claim element amended. 
     Other embodiments will occur to those skilled in the art and are within the following claims.