Patent Document

FIELD OF THE INVENTION 
   This invention is directed to an apparatus for mounting and cooling circuit card assemblies, and, more particularly, to a chassis including a number of self-sealing, straight-pass heat exchangers each directly mounted to a circuit card assembly forming a number of modules which are individually secured by wedge locks to an inlet card guide and an outlet card guide formed on opposed end walls of the chassis. 
   BACKGROUND OF THE INVENTION 
   Many communications systems, especially those intended for use in mobile platforms, must be environmentally robust both in terms of their hardware and signaling format. Airborne communication systems used with a plurality of UHF line-of-sight and satellite links, for example, may incorporate a transceiver mounting chassis or enclosure containing diverse communication equipment such as RF transmitter modules, RF receiver modules and various digital signal processing modules which control operation of the RF components and interface digital communications signals with attendant encryption and decryption circuits. Considering that each communication link has its own dedicated signaling scheme, suppliers of this equipment typically provide each system as an integrated unit. 
   One of the standard architectures employed by suppliers of such systems is the Versa Module Europa or VME bus. RF signaling circuits and digital signaling modules plug into discrete connector slots on the VME bus to avoid cross-talk and provide isolation between such components, and, to conform with the relatively tight dimensional spacing between the connector slots on the VME bus. Whether the communication system is intended for use in a mobile platform as noted above, or other applications, the enclosure or chassis which houses the VME bus and communication equipment components must be designed to withstand harsh environmental conditions including vibration, temperature variations and exposure to foreign matter. Consequently, VME bus specifications mandate ruggedized housing architectures which have the ability to cool circuit components and protect them from exposure to excess vibration and foreign material. 
   Initial efforts to meet VME bus specifications included chassis designs incorporating expensive and complex heat transfer elements. Alternatively, or in addition to these measures, the circuit card assemblies were provided with special, thermally robust circuit elements which added cost and unwanted bulk to the design. 
   These deficiencies were addressed to some extent in the system disclosed in U.S. Pat. No. 5,835,349 to Giannatto et al. This patent discloses a housing and cooling assembly which reduces the cost and overall size of the unit, while providing effective cooling of circuit components on the circuit card assemblies. A “U-pass” heat exchanger is mounted directly to each individual circuit card assembly which imparts structural rigidity to the cards, and isolates the circuit cards from the flow of cooling fluid, e.g. air, passing through the heat exchanger in a U-shaped flow path to and from an inlet/exhaust plenum. The circuit card assembly of each circuit card/heat exchanger combination or module is plugged into the VME bus, while the inlet and outlet of the heat exchanger is sealed with a gasket to elements of the chassis. 
   While the system of the U.S. Pat. No. 5,835,349 patent provides a number of advantages over prior approaches, it nevertheless has some limitations. Cooling air from outside of the heat exchanger circulates from the inlet of the plenum to the opposite end of the heat exchanger, and then reverses direction in order to flow to the exhaust portion of the same plenum. This U-shaped flow path creates a relatively large pressure drop that reduces the heat transfer performance of the heat exchanger. Additionally, the use of a gasket to seal the inlet and outlet of the heat exchanger reduces the reliability of the system, and creates a maintenance issue since the gaskets can be easily damaged and may require periodic replacement. Further, the circuit card/heat exchanger modules are provided with rails at each end which engage opposed slots formed in the end walls of the chassis to mount them in place. In order to readily permit installation of the modules in the chassis, the mating rails and slots cannot be constructed with tolerances which are too tight, and therefore the overall rigidity of the assembly is sacrificed to some extent and tolerance to vibration is reduced. 
   SUMMARY OF THE INVENTION 
   This invention is directed to an apparatus for housing and cooling circuit card assemblies employed in communication and other electronic systems. The apparatus includes a housing or chassis having opposed end walls, which are formed with a series of spaced inlet card guides and correspondingly spaced outlet card guides, respectively. A straight-pass heat exchanger is directly mounted to each circuit card assembly, and opposed ends of the heat exchanger are mounted by a wedge lock to respective inlet and outlet card guides. 
   One important aspect of this invention is the construction and mounting of the straight-pass heat exchangers. A significant amount of heat is produced by the circuit elements on each circuit card assembly which needs to be removed in order for them to operate properly. It is desired to employ standard VME bus architectures with commercial off-the-shelf (COTS) components to reduce cost and simplify construction. The heat exchanger of this invention is directly mounted to one side of each circuit card assembly to enhance the transfer of heat away from the circuit elements to the heat exchanger. In the presently preferred embodiment, the heat exchanger includes a section of corrugated fin stock mounted within a frame between an outer skin and a thermal interface sheet connected to the circuit card assembly. A flow of outside air is directed in essentially a straight path through the heat exchanger, from its inlet end mounted to the inlet card guide of the housing chassis, across the corrugated fin stock, to its outlet end mounted to the outlet card guide of the housing. Further, the air flowing through the inlet and outlet card guides is transmitted along nearly a straight flow path. Consequently, a minimal pressure drop is produced in the course of passage of cooling air through the chassis and heat exchanger, and a highly efficient transfer of heat away from the circuit card assemblies is provided. 
   Another advantage of this invention relates to the manner in which the interconnected heat exchangers and circuit card assemblies, or modules, are mounted within the housing or chassis. In the presently preferred embodiment, each of the inlet and outlet card guides has an angled surface which is formed to engage angled V-groove surfaces machined into the ends of the heat exchanger frame of each module. Wedge locks are coupled to the ends of the heat exchanger frame opposite the angled card guide surfaces. When each wedge lock is operated, the angled V-grooves of the heat exchanger frame are urged against the angled surfaces of respective card guides to securely mount the entire module within the housing. This forms a gasketless, air-tight interface between the heat exchanger and the inlet and outlet card guides, through which cooling air can flow. 
   The mounting arrangement described above has a number of advantages. The angled interface between the ends of the heat exchanger frame and the card guides is self-sealing which eliminates the need for gaskets, and thus greatly reduces ongoing maintenance of the unit. Such angled interface maximizes the module sealing area, maximizes rotational stiffness at the module-housing interface and minimizes the pitch of the module within the housing unit. The use of wedge locks to secure the modules within the chassis rigidifies the entire structure which greatly improves the vibration performance of the unit and allows for maximum use of COTS components as well as high density ball grid arrays (BGAs) in high vibration environments. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The structure, operation and advantages of the presently preferred embodiment of this invention will become further apparent upon consideration of the following description, taken in conjunction with the accompanying drawings, wherein: 
       FIG. 1  is a partially disassembled, perspective view of the apparatus of this invention from top to bottom; 
       FIG. 2  is a partially disassembled, perspective view of the apparatus of  FIG. 1  from end to end; 
       FIG. 3  is a partial perspective view of several modules mounted side-by-side to one end of the chassis of the apparatus of  FIG. 1 , with the side walls and opposite end of the chassis removed for ease of illustration; 
       FIG. 4  is an enlarged, segmented, cross sectional view between a module and inlet and outlet card guide; and 
       FIG. 5  is a disassembled, perspective view of a heat exchanger and circuit card assembly which form a module of this invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Referring now to the FIGS., the apparatus  10  of this invention includes a housing or chassis  12  having a hollow interior within which a number of modules  14  are mounted, each consisting of a straight-pass heat exchanger  16  directly connected to a circuit card assembly  18 . The construction of the chassis  12  is described initially, followed by a discussion of the modules  14  and the connection between the two. 
   As best seen in  FIGS. 1 and 2 , the chassis  12  comprises a top wall  20 , a bottom wall  22 , a front wall  24 , a back wall  26  and opposed side walls  28 ,  30  which are interconnected to form a hollow interior. The front wall  24  may be provided with a handle  25  and one or more apertures  27  to receive electrical connectors  29  as schematically depicted in the FIGS. An inlet end wall  32  is connected to the back wall  26  by an inlet plenum  34 , and an outlet end wall  36  is mounted to an exhaust plenum  38  connected to the front wall  24 . The back wall  26  is formed with an inlet port  40  which allows the passage of cooling air from outside of the chassis  12  into the inlet plenum  34 . The inlet plenum  34  channels the outside air into a series of openings  42  in the inlet end wall  32 . See also  FIG. 4 . In the presently preferred embodiment, the inlet plenum  34  is formed with a plurality of slots  35  each of which aligns with one of the openings  42  in the inlet end wall  32 . These slots  35  can be of different size to allow more or less cooling air to pass in respective openings  42  dependent on the degree of cooling requirements of individual modules  14 . Each of the openings  42  is connected to an inlet card guide  44 , the detailed structure of which is described below. Similarly, the front wall  24  is formed with one or more exhaust ports  46  at the point of connection to the exhaust plenum  38 . Preferably, the exhaust plenum  38  has an upper wall  48  and a lower wall  50  which taper inwardly, toward one another, from the location at which they connect to the outlet end wall  36  to the exhaust ports  46 . As described in more detail below, cooling air passing through the heat exchanger  16  of each module  14  is directed through openings  52  in outlet card guides  54  formed on the outlet end wall  36  and then into the exhaust plenum  38  for discharge from the chassis  12  through the exhaust ports  46  in the front wall  24 . 
   A number of modules  14  are mounted within the chassis  12  to the inlet and outlet card guides  44 ,  54 , in a manner described in detail below. When positioned within the chassis  12 , the circuit card assembly  18  of each module is plugged into a multi-pin connector  56  carried by a motherboard  58  mounted to the bottom surfaces of the inlet end wall  32 , outlet end wall  36 , and sidewalls  28  and  30  of chassis  12 . The motherboard  58  and connectors  56  are preferably compatible with standard VME architectures, although it is contemplated that other standard or custom architectures could be accommodated, as desired. 
   Referring now to  FIG. 5 , the detailed construction of modules  14  is shown. As noted above, each module  14  consists of a heat exchanger  16  and a circuit card assembly  18 . A compliant thermal interface material  59  may be used to couple the heat exchanger  16  to the circuit card assembly  18  if the circuit card assembly  18  contains electrical components on its secondary side. If no electrical parts are present on the secondary side, the circuit card assembly may be bonded directly to the heat exchanger to minimize the thermal path. The heat exchanger  16  includes a frame  60  having side walls  62 ,  64  connected to end walls  66 ,  68 , with a center support  70  extending between the end walls  66 ,  68 . The frame  60  forms a seat within which a section of corrugated fin stock  72 , preferably made of aluminum, is received and mounted between an outer skin  74  and a thermal interface sheet  76 . The corrugations of the fin stock  72  are oriented in a direction between the end walls  66  and  68  of the frame, which corresponds to the direction of air flow between the inlet and outlet ends of the chassis  12 . The heat exchanger  16  is characterized as a “straight-pass” unit because air flows from one end, along the corrugated fin stock  72  to the opposite end, in an essentially straight flow path. 
   The circuit card assembly  18  is of standard construction, the details of which form no part of this invention except as noted below. The assembly  18  has multi-pin connectors  78  at one end which plug into the multi-pin connectors  56  on the motherboard  58 . The circuit card assembly  18  may support the addition of PMC circuit card assemblies  82  attached to a heat sink often mounted to the primary side of the printed wiring board  80  as shown in  FIG. 5 . A standard wedge lock  84  is mounted to opposite ends of the circuit card assembly  18 , for purposes to become apparent below. Further, each end of the circuit card assembly  18  mounts an ejector  86  which is pivotal to allow the assembly  18  to be dislodged from the motherboard  58  and removed from the chassis  12 , as desired. 
   With reference now to  FIGS. 3 and 4 , details of the manner of mounting each of the modules  14  to the chassis  12  are shown. In the presently preferred embodiment, each of the inlet card guides  44  is machined in and protrudes outwardly from the inlet end wall  32 , extending substantially along the height dimension of the inlet end wall  32 , so that each opening  42  in the inlet end wall  32  is integral with one of the inlet card guides  44 . The term “height dimension” is intended to refer to the top to bottom dimension in the orientation depicted in the FIGS. The outwardly facing surface of each heat exchanger inlet  44  is an angled surface  88  extending at an acute angle of about 45° with respect to the inlet end wall  32 . An angled support edge  90  forms part of the inlet card guide  44 . Similarly, each of the outlet card guides  54  is integrally formed in the outlet end wall  36 , such as by machining, and extends to a height coextensive with that of the inlet card guides  44 . Each opening  52  formed in the outlet end wall  36  is integral with one of the outlet card guides  54 , and terminates at an angled surface  92  formed in the outlet card guide  54 . The angled surface  92  of each outlet card guide  54  preferably extends at the same acute angle as the tapered surface  88  of the inlet card guides  44 . An angled support edge  94  forms part of each outlet card guide  54 . 
   Opposite ends of the heat exchanger  16  portion of each module  14  are formed to mate and interlock with respective inlet and outlet card guides  44  and  54 . In the presently preferred embodiment, the frame  60  of heat exchanger  16  has one end formed with an angled surface  96  which mates with the angled surface  88  of the inlet card guide  44 , and a V-groove recess  98  which receives the support edge  90  of the inlet card guide. The opposite end of the heat exchanger frame  60  has similar structure. It includes an angled surface  100  which mates with the angled surface  92  of an outlet card guide  54 , and a V-groove recess  102  which receives the support edge  94  of the outlet card guide  54 . 
   Each module  14  is mounted within the chassis  12  as follows. As shown in  FIG. 1 , the top wall  20  of chassis  12  is removed to provide access to the hollow interior. Each module  14  is oriented so that its multi-point connectors  78  face toward the bottom wall  22 , and the support edges  90 ,  94  of the inlet and outlet card guides  44 ,  54  are received within the respective V-groove recesses  98  and  102  of the heat exchanger frame  60 . The module  14  is then slid along the inlet and outlet card guides  44 ,  54  until its multi-pin connectors  78  engage and connect to the corresponding multi-pin connectors  56  of the motherboard  58 . In the seated position of a module  14 , the angled surfaces  96  and  100  at opposite ends of the heat exchanger frame  60  contact substantially the entire surface area of the angled surfaces  88  and  92  on the inlet and outlet card guides  44 ,  54 , respectively. Each opening  42  in the inlet end wall  32  and one of the inlet card guides  44  aligns with a central passage  104  in the heat exchanger  16 , which is the area where the fin stock  72  is located, and the openings  52  in the outlet end wall  36  and outlet card guides  54  also align with central passage  104 . 
   This construction provides an essentially straight flow path from the inlet end wall  32 , through the heat exchanger  16  and out of the outlet end wall  36  ensuring a highly efficient transfer of heat from the circuit card assembly  18  mounted thereto and minimal pressure drop in the course of passage of cooling air through such flow path. Cooling air from outside of the chassis  12  enters its interior through the inlet port  40  in back wall  26  and is distributed by the inlet plenum  34  to each of the openings  42 . As noted above, a slot  35  is formed in the inlet plenum  34  for each opening  42 , and, hence, for each module  14 . It is contemplated that the size of such slots  35  can be varied depending on the circuit elements present on the circuit card assembly  18  of a particular module  14 . That is, a circuit card assembly  18  which produces 120 watts, for example, would require more cooling air and therefore a larger-size slot  35  than a 20 watt circuit card assembly  18 . The size of the slots  35  is therefore adjusted accordingly for a given group of modules  14 . After passing through the heat exchanger  16  along the flow path noted above, the now heated air exits the chassis  12  though the exhaust plenum  38  and exhaust ports  46  in the front wall  24 . 
   A series of modules  14  are placed side-by-side within the chassis  12  in the manner described above, and each is “locked” in place by operation of the wedge locks  84  located at either end of each module. An Allen wrench or the like is inserted into each wedge lock  84  and rotated causing the angled surfaces  88  and  92  of the heat exchanger frame  60  to bear against the angled surfaces  96  and  100 , and against the support edges  90 ,  94 , of the inlet card guide  44  and outlet card guide  54 , respectively. This creates an airtight seal at each end of the heat exchanger  16  which does not require a gasket, and, hence, avoids maintenance issues which can arise with seals that wear over time. Further, substantial rigidity is provided at the connection between the ends of the heat exchanger frame  60  and the inlet and outlet card guides  44 ,  54  due to the relatively large, angled area of contact between their angled surfaces and the force generated by the wedge locks  84 . This enhances the rotational stiffness at such interface and significantly improves the vibration performance of the chassis  12 . The modules  14  may be removed from the chassis  12  by loosening the wedge locks  84  and operating the ejectors  86  at ends of the module  14 . 
   While the invention has been described with reference to a preferred embodiment, it should be understood by those skilled in the art that various changes may be made and equivalents substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. 
   For example, wedge locks  84  are depicted in the drawings and described above as a means of locking or retaining the modules  14  in place within the chassis  12 . It is contemplated that other locking or retainer devices could be employed to releasably secure the modules  14  such as spring clips, screws or other devices. 
   Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.

Technology Category: 5