Patent Publication Number: US-6698079-B1

Title: Cardcage for circuit cards

Description:
This is a continuation of application Ser. No. 09/396,980, filed Sep. 15, 1999, now U.S. Pat. No. 6,230,541, which is a divisional of application Ser. No. 09/098,040 Jun. 16, 1998 now U.S. Pat. No. 6,128,187. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to cardcages. More particularly, the invention relates to an improved cardcage that increases flexibility, improves structural integrity, provides ease of insertion of circuit cards, reduces cost, and enhances airflow across circuit cards. 
     BACKGROUND OF THE INVENTION 
     Electronic components and integrated circuit packages are often mounted on printed circuit cards creating an electronic assembly of irregularly shaped packages bonded on a planer surface. Multiple circuit cards may be placed into an array (referred to as a “cardcage”) within an electronic enclosure thereby maximizing the packaging volume efficiency. Several types and configurations of electronic enclosures with the ability to support various numbers of circuit cards are readily available for connecting the circuit cards into a cardcage. Standard bus architectures have been defined for these cardcages. For example, the VME bus standard (ANSI/IEEE Std. 1014-1987) has been defined for the backplane bus interface for cardcages. Other bus standards include the VME64 Standard, which revises the VME bus standard into a 64-bit architecture, and the VXI (“VME eXtensions for Instrumentation”) bus, which is designed to allow low level signals to co-exist on a backplane with high speed digital and RF or microwave signals. 
     Currently available cardcages have significant complexity due to the number of components and fasteners used for assembly. These cardcages have typically been manufactured using four or five extruded metal rails and sheets, often aluminum, stamped and formed to the shape required to build the cardcage. The extruded rail design requires a pair of rails located at the inlet and exit of the cardcage. These rail extrusions are machined to length, which requires extreme precision to ensure accuracy. The sheets and rails are connected to each other by a suitable means, including rivets, screws, welds, or glue. A card guide is connected between each pair of rails for receiving a circuit card assembly into the cardcage. Both entrance and exit rails must be exact mirror images to ensure proper alignment of the card guides. The card guides are installed by snapping a plastic tab into an oval type slot in the extrusion. The card guides are usually mounted to the extrusion, through a mechanical means, such as screws, for holding them in place. The slots in the rail extrusions, which are used to locate the guides, are typically on 0.20″ centers. This spacing requires any card pitch changes (i.e., changes in the card spacing), to be limited to 0.20″ increments. 
     Next, a floating nutrail is slid into each extrusion to provide a means of attaching the associated cards and backplanes. Insulating strips must then be positioned between the backplane and the mounting surfaces prior to backplane attachment. 
     It has been recognized that this assembly process may be costly and time consuming due to the number of parts and connectors that must be used to connect the components and the complexity involved in aligning the various components. In particular, currently available cardcages require careful alignment of an associated backplane to ensure proper mating of the circuit cards with the backplane. In addition, the cardcage rail extrusion design requires significant tooling costs to produce the intricate features used to ensure proper alignment. 
     It has also been recognized that an extruded rail design may be deformed when placed under a moderate mechanical loading. The final cardcage assembly of an extruded rail design has a reduced structural integrity due to the number of parts and fastener locations that produce stress concentrations. Therefore, currently available cardcages are susceptible to deflection from a rectangular shape when forces are applied to the sides of the cardcage. 
     In addition, an extruded rail design does not lend itself to moderate changes or modifications due to the nature of the cardcage design. There is typically one mounting hole on each end of the extruded rails for attachment to the cardcage ends. These mounting holes are located in the extrusion and any change to the mounting holes would require significant tooling costs. 
     It has further been recognized that cardcages using sheets and extruded metal rails reduce the available airflow through the cardcage due to large obstructions present in the design. Cooling of the circuit cards is further hindered due to restrictive openings and blockage of airflow at the cardcage corners, which results in uneven distribution of air into a cardcage. Cooling of circuit cards in the cardcage is important for proper operation of electronic components on the circuit cards. As circuit cards are being developed with greater electronic component density per card, and electronic components continue to operate at higher frequencies and increased power, the cooling of circuit cards in these electronic enclosures becomes more critical for proper operation of these electronic components. Electronic components that operate above recommended thermal constraints may operate unreliably and cause circuit cards to prematurely fail due to thermal stress. The amount of cooling provided by an airflow source in a cardcage, such as an axial flow fan or a centrifugal blower, is based directly upon the velocity, temperature, and pressure of air provided over the components. If the circuit cardcage hinders airflow from the source due to restrictive openings, then total volumetric airflow is reduced, recirculation loops are created in the cardcage, and low velocity areas occur. As a result, the amount of cooling provided by the airflow source will decrease. Additionally, air that is not distributed evenly into a card slot by the cardcage will not cool all devices on the card evenly and adequately. 
     It has also been recognized that cardcage designs do not account for the problems associated with the use of multiple fans in front of a cardcage. The use of multiple fans in a cardcage causes significant disturbances in airflow due to the interactions of air from one fan interacting with air from an adjacent fan. 
     Therefore, there is a need for a cardcage assembly having an integrally formed cardcage side for inlet and exit airflow that reduces the cost and complexity of assembling a cardcage while increasing structural integrity. In addition, a need has arisen for a cardcage in which the cardcage sides inherently align circuit cards to a backplane and are not subject to deflection. An additional need exists for a cardcage assembly that provides for improved cooling within the cardcage and reduced airflow disturbances and/or loses. 
     SUMMARY OF THE INVENTION 
     The present invention provides an improved cardcage that enhances cooling, increases flexibility while adding structural rigidity, simplifies construction, and reduces assembly cost. In a broad aspect, the invention comprises a cardcage side for air inlet and exit. The cardcage side is specially designed to enhance airflow and aid in the alignment of the backplane with circuit cards. The unitary cardcage side includes flanges formed from a single metal sheet together with a plurality of generally parallel and laterally spaced card guide attachment locations extending between the flanges. Each flange is adapted to be aligned and fastened to the walls of a cardcage enclosure. 
     In an embodiment, the card guide attachment locations are configured to receive and secure a non-conductive member built to serve as a card guide. Alternatively, the inward surface of each card guide attachment location may be adapted to serve as a card guide. 
     In another aspect, the present invention comprises an improved assembly process for a cardcage side. In an embodiment, card guide attachment locations may be integrally formed with the cardcage side by punching or otherwise forming slots along a central portion of a metal sheet The metal sheet is further bent along one or more lines parallel to the slots and between the last slot and the end of the sheet to create flanges for fastening the sheet to a circuit card enclosure. The improved design simplifies the assembly process and reduces assembly time because the need for components to be fastened using numerous fastening elements is eliminated. Further, no special tools are required to connect the various components. Standard metal punches, drills, brakes, and other readily available tools are adequate to perform the principal manufacturing operations. 
     In another aspect, the invention provides an improved cardcage that improves airflow through the cardcage side. More particularly, airflow guides may be created integral with the cardcage side for directing, diverting, deflecting, or otherwise distributing the flow of air from an airflow source to improve airflow in the cardcage. 
     In another aspect, the present invention provides for more accurate alignment of the backplane and the card. In the present invention, tabs are located on the cardcage side, which mate with slots in the end plates to ensure that no alignment is required by the assembler. Since the cardcage side is formed from a single sheet of metal, the tolerances are drastically reduced and provide for a virtually fool proof alignment system. These tabs are used to set the distance between the inlet and exit sides. This dimension allows the cards to slide into the cardcage freely while providing a guide that ensures each card will mate with the backplane. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a more complete understanding of the present invention and the advantages thereof, reference is now made to the following description taken in conjunction with the accompanying drawings in which like reference numbers indicate like features and wherein: 
     FIG. 1A is an exploded view of a typical cardcage using an extruded rail design. 
     FIG. 1B illustrates the use of typical hardware to attach a card guide in an extruded rail design. 
     FIG. 1C is an exploded view of a typical extruded cardcage design with extruded rails, card guides, insulating strips, nutrails, backplane, and a circuit card. 
     FIG. 2 schematically illustrates the cardcage of the present invention. 
     FIGS. 3A-3C schematically illustrate the formed cardcage ends of the present invention. 
     FIG. 4 illustrates a side view of a circuit card inserted into the cardcage of the present invention. 
     FIGS. 5A-5B schematically illustrate an embodiment of the present invention in which a circuit card is attached using nutrails. 
     FIGS. 6A-6C illustrate three views of a circuit card installation showing the alignment required between the circuit card, card guide, and backplane. 
     FIG. 7A illustrates the airflow in a side view of an extruded rail cardcage with the circuit card, extruded rails, a backplane, and no ducting to direct the airflow. 
     FIG. 7B illustrates the airflow in a side view of a cardcage in an extruded rail system with a circuit card, extruded rails, a backplane, and air blockers above and below the cardcage. 
     FIG. 7C illustrates the airflow in a side view of a cardcage with the cardcage sides of the present invention with a circuit card, cardcage inlet and exit ducts, and a backplane. 
     FIG. 8 illustrates an entrance airflow guide of the present invention. 
     FIG. 9A is a top-front-right isometric view of an embodiment of the present invention showing a cardcage, dividers for separating airflow from multiple fans, and an integral duct to force the airflow to pass through the cardcage. 
     FIG. 9B is a side view of an embodiment of the present invention showing a fan, cardcage, inlet ducts to direct and bound the airflow, and exit ducts to direct the airflow out of the system. 
     FIG. 10 illustrates the incorporation of airflow diverters into the present invention. 
    
    
     DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS 
     An example of a prior art cardcage, assembled using traditional assembly techniques, is illustrated in FIGS. 1A,  1 B, and  1 C. The illustrated cardcage is a standard VME 6U Sub-rack with a depth of 160 mm. The cardcage is primarily comprised of two end plates ( 10  and  12 ) and four extruded rails ( 18 ,  20 ,  22 , and  24 ). The four extruded rails are connected to the side plates by a plurality of fasteners  26 , usually screws, extending through the holes in the end plates and into the extruded rails to form the cardcage. The four extruded rails include a plurality of holes spaced along the length of the rail used for attaching card guides. Card guides  34  (FIG. 1C) extend between extruded rail  18  and extruded rail  20  to form the side of the cardcage. Similarly, card guides  34  extend between extruded rail  22  and extruded rail  24 . These card guides provide support and alignment for a circuit card to be inserted into the cardcage. Additionally, the card guides  34  serve to guide a circuit card into the cardcage to engage a backplane connector as shown in FIG.  4 . The spaces between the card guides  34  are designed to allow the passage of air into and out of the cardcage. The backplane may be supported by a center rear extruded rail  28  that extends between the cardcage sides ( 10  and  12 ). Angle plates  14  and  16  may be provided for rack mounting the cardcage. 
     FIG. 1B shows a prior card guide that could be used with the cardcage in FIG.  1 A. The illustrated card guide  34  can be made to be any required length. Two end pieces  30  and  32  allow the card guide to be connected to the extruded rails. The length of the card guide  34  is determined by the number of extensions  31  placed between the end pieces  30  and  32 . The card guide  34  is connected to the extruded rails  18 ,  20 ,  22 , and  24 , through a hole by a screw  36 , washer  38 , and nut  40 . In the alternative, the card guide may snap into the extruded rails. 
     FIG. 1C illustrates the attachment of a circuit card  31  with mounting holes  33  and card connector  35  to a backplane  37  within the cardcage. The backplane  37  includes a plurality of backplane connectors  39  based on a standard bus architecture for connecting circuit cards. The backplane  37  is connected to the cardcage using mounting screws that extend through the backplane  37  and insulating strip  41  into floating nutrails  45 . The insulating strips  41  are provided to ensure that the backplane  37  is electrically isolated from the cardcage. A center extrusion rail  43  is provided in some systems to provide additional support for the backplane to prevent flexing under shock and vibration loads. A circuit card  31  is installed into the cardcage by sliding the circuit card  31  into a pair of associated card guides  34 . The circuit card connector  35  engages the backplane connector  39  when fully inserted. Circuit card mounting holes  33  are provided to securely attach the circuit card to the extruded rails ( 22  and  24 ) of the cardcage. 
     A plurality of nutrails  45  are provided in multiple locations for retaining multiple parts in the cardcage. The nutrails are small strips with tapped mounting holes on 0.20″ centers. Each of the locations provided for the nutrails  45  is part of the extrusions used to build the cardcage. Each extrusion has a slot formed into it which allows the nutrail  45  to slide from end to end, allowing alignment to be set during assembly. The nutrails  45  are used to mount the backplane to the cardcage, and to mount the front panel of the circuit card to the cardcage, thereby holding a circuit card in place after installation. 
     The cardcage illustrated in FIGS. 1A,  1 B, and  1 C provides some advantages in terms of flexibility in designing various sizes of cardcages. When building a cardcage, the cardcage extruded rails ( 18 ,  20 ,  22 , and  24 ) may be cut to an appropriate size. In other words, by using cardcage extruded rails and various length card guides  34 , a variety of sizes of cardcages with various heights and widths can be built. However, as previously discussed, cardcages using extruded rails and fasteners suffer from several disadvantages. In particular, these cardcage designs require a number of connections and components to build a cardcage. For example, as shown in relation to FIG. 1A, FIG. 1B, and FIG. 1C, there are 5 extruded rails; two card guides per circuit card; four screws, washers, and nuts per circuit card (shown in FIG.  1 B); four or six nutrails; four insulating strips; and a backplane. Due to the amount of hardware associated with each card guide, assembly time is a significant cost. Moreover, the extruded rail design requires careful alignment of the backplane, the cardcage, and the card guide rails. The failure to properly align the components may cause circuit cards to improperly connect to the backplane when inserted to the cardcage. In addition, the machining, drilling, tapping, and assembly of the various components is expensive and time consuming. 
     FIG. 2 illustrates an embodiment of the present invention that overcomes the problems associated with prior cardcages. The illustrated embodiment is a VME 6U-160 mm cardcage, which incorporates the present invention. Although the invention is illustrated using this particular size cardcage, it should readily be understood that other sizes and types of cardcages may be built without departing from the spirit and scope of the present invention. For example, cardcages may be built to support a variety of types of buses (e.g., VME and VXI standard buses), card heights (e.g., 3U, 6U, and 9U), card widths (e.g. 19 inches), or cardcage depths (e.g., 160 mm, 220 mm, and 400 mm). 
     The cardcage  50  includes two cardcage sides,  52  and  54 , and two end plates,  56  and  58 . Each cardcage side,  52  and  54 , is connected via a suitable connection means, including screws, bolts, or other acceptable connection, to sheets  56  and  58  through flanges  68 . The flanges  68  align with flange connection points  53  on the sheets  56  and  58 . Cardcage sides  52  and  54  also include alignment tabs  74  (FIG.  3 A). The alignment tabs  74  mate with slots  57  on sheets  56  and  58 . By providing cardcage ends as an integral unit with alignment tabs  74  and flanges  68 , the deficiencies with regard to alignment associated with extruded metal designs is substantially eliminated. Moreover, the use of fasteners is substantially reduced, resulting in reduced assembly time and increased shock and vibration tolerance because of the elimination of a number of attachments. 
     In an embodiment, the cardcage sides,  52  and  54 , are constructed using precision punched and formed aluminum construction instead of extrusions. These manufacturing procedures are well known in the art and result in a manufacturing process that ensures close tolerances between card guides, circuit card mounting features, and the cardcage backplane mounting features. It should readily be understood, however, that other metals and other techniques may be used to form the integral cardcage sides without departing from the spirit and scope of the present invention. 
     Each cardcage side includes a plurality of card guide attachment locations  62  that are laterally spaced and integral with a pair of flanges  67  and  69 . The card guide attachment locations may be conformed to receive and retain card guides  60 , which may be inserted into the integral cardcage sides,  52  and  54 . Normally, the cardcage attachment locations  62  are integral with the flanges ( 67  and  69 ). 
     The card guides  60  provide support and guidance for circuit cards while being inserted into the cardcage. In an embodiment, the card guides are made of nylon and are snapped in through holes punched into the guide attachment locations  62  of the integral cardcage sides,  52  and  54 . These card guides  60  are readily available from Bivar. Although the present invention is illustrated using card guides from Bivar, it should be understood that any suitable card guides may be used without departing from the spirit and scope of the present invention. 
     In an alternative embodiment, the card guides may be formed as part of the process used to form the card guide attachment locations. A numerically controlled punch tool can be used to form card guides instead of using off-the-shelf card guides. In this embodiment, no additional card guides need to be inserted into the formed cardcage side, thereby further reducing the required assembly. 
     FIGS. 3A-3C schematically illustrates an embodiment of a cardcage side  52  of the present invention. The cardcage side  52  is punched from a single piece of material. FIG. 3B shows the cardcage side  52  in its punched and unformed condition. After the cardcage side is punched to form a flat sheet as shown in FIG. 3B, the flat sheet is bent to the desired shape using a Numerical Controlled Brake. The resulting cardcage side  52 , illustrated in FIG.  3 A and FIG. 3C, includes a plurality of guide attachment locations  62 . Each card guide attachment location  62  includes a top opening  66  and a bottom opening  64  for receiving a card guide. The card guide attachment locations  62  extend between a first flange  67  and a second flange  69 . In an embodiment, the first and second flanges are formed at a 90 degree angle from the guide attachment locations  62 . The first flange  67  includes a plurality of holes  72  spaced along the flange for attaching devices to retain circuit cards. The second flange  69  includes a plurality of holes  73  (FIG. 3B) spaced along the flange  69  for attachment of a backplane. The first and second flange may also contain a plurality of holes spaced along the flange for attachment of airflow guides or ducts as shown in FIGS. 7C,  8 , and  9 A. In another embodiment, these features may be formed as part of the cardcage side. 
     In an embodiment, at both ends of each flange  67  and  69 , additional flanges  68  are formed perpendicular to the first flange  67  and the second flange  69 , and parallel to guide attachment locations  62 . Flanges  68  permit the cardcage sides,  52  and  54 , to be connected to sheets,  56  and  58 , to form the cardcage of the present invention. Additionally, tabs  74  may be included for alignment purposes of the cardcage sides when mated with slots  57  on the sheets  56  and  58 . 
     In the present invention, the cardcage side is formed at equal and opposite angles (45 degree) near the first flange  67  and the second flange  69 . This formation is pre-formed to meet the space requirements for hardware given in the specification that this embodiment is designed for. 
     The cardcage sides  52  and  54  also includes a plurality of flat tabs  70  at the front edge of front rail  67 . These tabs provide the surface for the circuit card to stop on when fully inserted into the cardcage. The rectangular openings allow access to the threaded insert for attaching the front panel of the card. 
     FIG. 4 provides a side view of a circuit card  80  inserted into an embodiment of a cardcage of the present invention. The circuit card  80  may be any well-known circuit card assembly that includes a circuit board  82  connected to backplane connectors  84 . The front of the typical circuit card includes a circuit card front panel  86  and handles  88  for inserting and removing the circuit card. 
     In operation, the circuit card  80  is inserted into the cardcage via card guides  60 , which are inserted into cardcage sides  52  and  54 . Card guides  60  provide alignment between the circuit card  80  and a backplane  98 . A backplane stiffener  100  may also be included between sheets  56  and  58  to provide additional torsional stiffness for inserting circuit card assembly  80  into backplane  98 . The backplane stiffener  100  is attached using nutrails  102  and  104 . 
     FIGS. 5A and 5B illustrate one method for connecting a circuit card to the cardcage side of the present invention. In particular, FIG. 5B provides a detailed view of connecting a nutrail  92  to the cardcage side using a nutrail compression bar  94 , and a spacer  90 . Comparably, FIG. 5A shows the backplane attachment which is handled though the use of a nutrail  96  which is attached to the second flange  69  by some suitable means, including screws or rivets, through mounting holes which have been punched to a close tolerance for alignment purposes. 
     The embodiment illustrated in FIGS. 5A and 5B is for connecting VME 6U-160 mm cards. Once again, it should be understood that the illustrated method for connecting may be used with any size cards. In the illustrated embodiment, a card fastener  91  extends through the circuit card front panel  93 , the spacer  90 , and the cardcage side flange  67 , and screws into the nutrail  92 . The spacer  90  is used to lower the nutrail  92  below the card fastener  91 . Since this fastener  91  is typically captive and protrudes below the card panel prior to insertion, the spacer  90  ensures that the captive fastener  91  does not hit the nutrail  92  prior to full card insertion. A nutrail compression bar  94  holds the nutrail  92  into place prior to card installation. Fasteners are used to join the nutrail compression bar  94  and the spacer  90  to the cardcage flange  67 . 
     Although the present invention is illustrated using the above method for connecting the circuit card, it should be understood that other connection methods may be used without departing from the spirit and scope of the present invention. 
     The alignment of the backplane to the cards is an important improvement made by the present invention. FIGS. 6A-C shows the alignment problems that must be overcome when using an extruded rail cardcage design. Misalignment can occur with the machining of the ends of the rails as referred to in FIGS. 6A-6C. These extrusion rails must be machined to the same length within a tight tolerance. Both upper and lower entrance rails and upper and lower exit rails must be exact mirror images of each other with respect to the location of the mounting features for the card guides. Any misalignment of card guide mounting locations from upper rail  22  and  24  to lower rail  18  and  20  (Alignment A) at the entrance or the exit will inhibit mating of circuit card connectors to backplane connectors due to the card guide  110  not being perpendicular to the backplane  112  (FIG.  6 B). The inaccuracies in cutting the extruded rails to length and in cutting the extruded rails at the proper location relative to the card guide mounting holes  114  (illustrated in FIG. 6B as DIM 1  and DIM 2 ) are due to random tolerance errors, errors in setup and fixturing the rails to be cut, and measurement errors in the cutting process. These inaccuracies add up in all extruded rail cardcage designs and cause poor repeatability in alignment. 
     In the present invention, the mounting holes ( 62  and  64  in FIG. 3A) for the card guides  60  and the alignment between card guide attachment locations  64  are all punched into a single sheet of metal relative to the mounting holes for the front panels and the backplane. Then, the metal is formed using a CNC brake machine. This procedure eliminates the tolerance build up that occurs in the machining process of the extruded rails. The only misalignment possible is the repeatability of the CNC punch (typically +/−0.004 inches). 
     In the extruded rail design in FIG. 6B, the card guide  110  is installed by snapping a plastic tab into an oval type slot and groove in the upper and lower extrusion rails ( 18 ,  20 ,  22 , and  24 ). The card guide may then be screwed to the extrusion to hold it in place. The card guides are typically mounted to the rails using screws, washers, and nuts through clearance holes in both the rail and the card guide. This process inherently adds misalignment due to the fact that all fasteners allow misalignment except for press fit pins and countersunk fasteners mating with a tapped thread form. 
     With the present invention, the card guides  60  (FIG. 2) are press fit into punched holes  64  and  66 , which are located with high accuracy by the CNC punch machine relative to the mounting holes  72  and  73  for the front panels and the backplane. This technique provides virtually no misalignment of the card guide to the intended hole location. 
     The typical extruded rail cardcage design, as shown in FIG. 6C, includes lower extruded rails with grooves extruded along their length that allows a floating nutrail  113  to be slid into place for attachment of the backplane  112 . Due to the non-fixed location of the nutrails  113  used for backplane attachment, the location of the backplane  112  is not fixed on one axis and, therefore, must be aligned (Alignment B). The alignment of the backplane  112  to the card is a manual alignment that must be performed during assembly for each backplane installed relative to its respective cards. This alignment procedure requires a precisely made alignment tool and is critical for the mating of the card to the cardcage and the card to the backplane. This alignment process can only correct misalignment between the backplane  112  and the associated extruded rail. It cannot correct misalignment from extruded rail to extruded rail (Alignment A), which causes a circuit card, when inserted, to approach the backplane at a non-perpendicular angle. 
     In the present invention, the nutrails  96  used for mounting the backplane are fixed to the cardcage side second flange  69  with countersunk screws into the tapped threads of the nutrail  96  to prevent misalignment. Additionally, the clearance holes through the second flange  69  are sized close to the backplane mounting screws, adding extra assurances that the backplane is fixed to the correct location and structurally held into place. In the present invention, alignment between mounting holes on the flanges, as shown in FIG. 3B, is assured due to the accuracy in the manufacturing processes previously mentioned and no manual alignment is required. 
     Referring to FIG. 6A, the typical extruded rail cardcage design uses extruded upper rails  22  and  24  with grooves extruded along their length that allows a floating nutrail  116  to be slid into place for attachment of the cards. Due to the non-fixed location of the nutrails  116  used for card attachment, the location of the threads in the nutrail used to attach a circuit card is not fixed on one axis and, therefore, must also be properly aligned (Alignment C). 
     In the present invention, the nutrails  92  (FIGS. 4,  5 A, and  5 B) used for mounting the circuit cards are fixed to the cardcage side first flange  67  and aligned via close tolerance alignment holes installed during assembly. The nutrail compression bar  94  ensures the nutrail  92  is fixed. This method prevents misalignment of the card fastener to the nutrail threads. In an alternate embodiment, the nutrail  92  may be attached to the cardcage first flange  67  with countersunk screws similar to the backplane nutrail  96 . 
     FIG. 7A illustrates the side view of a prior art enclosure  126  containing a cardcage built using extruded rails with a fan  120 , a circuit card  122 , and a backplane  124 . The airflow paths defined in this figure are typically what is seen in an extruded rail system. The preferred path for the airflow is path A, but as the figure shows, paths B and C are unrestricted areas and a large portion of the airflow may pass through these paths. Additional paths, D and E, allow air to avoid the cardcage by passing on the near side of the near end plate  125  or on the far side of the far end plate  127 . 
     FIG. 7A illustrates another problem associated with extruded rail designs. Prior cardcage sides built using extruded rails have a smaller airflow opening due to the size of the extrusions and the method of connecting extruded rails to card guides. The result is reduced height of the airflow opening into and out of the cardcage. On average, the height of typical cardcage openings for VME 6U-160 mm cards are approximately 70% of the card height (DIM 3  on FIG.  7 B). A specific example is the extruded rail system for VME 6U-160 mm cards made by SCHROFF INC. This example uses extruded rail part numbers 30819-332, 30819-333, and card guide part number 60817-053. The size of this airflow opening for this system is 4.2 inches by 0.6 inches. The height of a VME card is 6.293 inches. The airflow opening for this SCHROFF cardcage system is 67% of the card height. The converse is that 33% of the height available for airflow to enter and exit the cardcage is blocked and as a result, the airflow will attempt to flow through less restrictive paths (Paths B and C). 
     FIG. 7B illustrates the airflow in a side view of the cardcage in FIG. 7A, but with air blockers  130  and  132  for preventing airflow in undesired Paths B and C. The reduced inlet area on extruded rail designs creates places which can have low velocity air flow (or “dead” spots) at the recirculation loops located in the four corners of the card (locations C, D, E, and F). The reduced inlet area along with the typical airflow blockers  130  and  132  also creates locations (locations A and B) where turbulence can be created prior to the cardcage entrance. These recirculation areas (locations A and B) disturb the smooth flow of air through the cardcage. This turbulence outside the cardcage increases the pressure drop in the airflow path, reduces total airflow, and disturbs the smooth transition of air into the cardcage. These undesirable affects cause great concern with the increase of heat densities on state-of-the-art circuit cards and the increased packaging density of a circuit card&#39;s surface area with components requiring airflow. The recirculation loops shown can cause an increase in the local air temperature of 10 degrees C. or more. Since the cooling of the devices on circuit cards is based directly on temperature and velocity, any device located in the indicated recirculation areas will be considerably hotter and have a reduced life compared to devices in non-recirculation and low velocity areas. 
     The present invention, by providing an integral cardcage side that changes the attachment method for card guides, circuit cards, and backplanes, provides a larger opening for air from an airflow source to enter and exit the cardcage. The larger opening permits greater airflow and therefore greater cooling ability than prior cardcages. On average, the integral cardcage sides of the present invention provide openings of approximately 90% of the card height. FIG. 7C shows the new opening height (DIM  4 ). In the illustrated example, the opening on the present invention as used in a cardcage with VME 6U-160 mm cards, is 5.65 inches by 0.5 inches, which is 90% of the card height. This larger opening reduces or eliminates the recirculation and low velocity areas within a cardcage caused by blocked airflow in the extruded rail system. 
     FIG. 7C describes another embodiment of the present invention in which additional improvements in cooling are attained. In particular, FIG. 7C shows a side view of circuit card enclosure  110  with cardcage sides  52  and  54 . The cardcage sides are connected as previously discussed in relation to FIG.  3 . In addition, a fan  120  is provided for providing airflow across circuit cards within the cardcage. In an embodiment, the fan is a axial flow fan providing airflow above 0 cubic feet per minute. A typical axial flow fan is approximately 4.5 inch by 4.5 inch by 1.5 inch and is readily available from Rotron. It should be understood, however, that any source of airflow, including centrifugal blowers, may be used. The cardcage side further includes airflow entrance guides  134  and  135 , which are angled from the fan  120  to the cardcage side  52  of the present invention. The angled airflow entrance guides  134  and  135  direct airflow toward the desired area without the problems associate with traditional blockers. 
     FIG. 8 schematically illustrates an embodiment of the airflow entrance guides  134  and  135  with the cardcage side  52  according to the present invention. The air provided by the fan  120  is diverted toward circuit cards placed in the cardcage by integral airflow entrance guides  134  and  135 . The airflow entrance guides  134  and  135  prevent airflow from going outside the area desired to be cooled. Instead, the airflow entrance guides  134  and  135  divert air towards the circuit cards, thereby providing improved cooling. By replacing the traditional air blockers  130  and  132  (FIG. 7B) with the angled airflow entrance guides, the recirculation effects outside the cardcage may be substantially reduced or eliminated. In the illustrated embodiment, the airflow entrance guides  134  and  135  may be attached by a fastener to card cage side  52  by a fastener  91  and spacer  90 . In the alternative, the airflow entrance guides may be integral with the cardcage side. 
     As illustrated in FIG. 7C, further improvements are provided by including airflow exit guides (also referred to as ducts)  136  and  137 . Airflow exit guides ensure that airflow from the fan  120  exits the cardcage rather than re-circulating within the cardcage. It is an important aspect of the invention to eliminate these re-circulation effects and provide a more uniform airflow across circuit cards. The smooth transition airflow entrance and exit guides shown in FIG. 7C are integral with the cardcage sides  52  and  54 . In the shown embodiments, the exit guides (or ducts)  136  and  137  are an extended first and second flange. At the entrance, the entrance guides (or ducts)  134  and  135  are made as one part with the cardcage side. The upper entrance duct  135  is made as one part with the nutrail compression bar  94 . These re-circulation effects, along with other methods for providing an improved airflow in a cardcage, are discussed more fully in co-pending application Ser. No. 09/097,892, now U.S. Pat. No. 6,280,317, entitled “A Method and Apparatus for Controlling Airflow,” which was filed on Jun. 16, 1998 and is incorporated herein by reference. 
     FIG. 9A schematically illustrates a top-front-right isometric view of an embodiment of the present invention used in a specific application. Integral exit ducts  140  and inlet ducts  142  are shown attached to the cardcage. These integral ducts are used to prevent the airflow from passing above and below the cardcage. The end plates  144  and  146  shown in this embodiment are of two different varieties. On the left side of the cardcage is the side wall of the chassis  144 , which extends from the front to the back of the complete unit. On the right side is a smaller end plate panel  146  that is used as an extension to connect the cardcage to the right side of the chassis. Both of these panels are used to bound the airflow within the circuit card area. 
     Fan divider plates  148  and  150  are also used to prevent the interaction of airflow from multiple airflow sources. In the illustrated embodiment, two fan divider plates  148  and  150  are provided for use with 3 fans. The divider plates assist in removing the recirculation effects caused by placing fans side-by-side in a cardcage. The airflow out of a typical axial fan comes out in a spiral pattern. The interaction that occurs when multiple fans are mounted in close proximity to each other may be detrimental to the overall airflow path in a cardcage. Therefore, the fan divider plates  148  and  150  of the present invention prevent these detrimental effects by separating the airflow from each fan and directing it through the cardcage. 
     FIG. 9B illustrates a right side view of another embodiment of the present invention. In particular, FIG. 9B shows a fan  160  at the front of the unit  162 , integral cardcage inlet ducts  164  and  166  with flanges  168  and  170  above and below the fan to bound the airflow, a large left side panel  172 , a small right side panel  174 , and integral exit ducts  176  and  178  with flanges  180  and  182  to direct the airflow out of the system. The cardcage sides are connected as previously discussed in relation to FIG.  3 . The fan  160  is used to provide airflow across the card assemblies within the cardcage. In an embodiment, the fan  160  is a axial flow fan providing airflow. 
     FIG. 10 schematically illustrates the incorporation of airflow manipulation techniques, as shown in co-pending application Ser. No. 09/097,892, with the present invention. This figure shows the integral modification of the air inlet areas on the side plate. These modifications are made to adjust the airflow as it enters a cardcage. Modifications can be made to: increase airflow in a particular region of a card, even the airflow across the entire card, or stop the flow of air in part or all of a particular card slot. The use of these techniques may reduce the total volumetric airflow through a cardcage, but the distribution throughout the cardcage is usually more critical. With the present invention, these features can be integrally formed into the cardcage side with very little cost impact. With this technique, there is no assembly time or assembly hardware required. Although the present invention incorporates these features into the side of the cardcage, it should be understood that multiple pieces may be used without departing from the spirit and scope of the present invention. 
     All of the methods and/or apparatus disclosed and claimed herein can be made and executed without undue experimentation in light of the present disclosure. While the devices and methods of this invention have been described in terms of specific embodiments, it will be apparent to those of skill in the art that variations may be applied to the methods and/or apparatus and in the steps or in the sequence of steps of the method described herein without departing from the concept, spirit, and scope of the invention. Therefore, all such substitutions and modifications apparent to those skilled in the art are deemed to be within the spirit, scope, and concept of the invention as defined by the appended claims.