Patent Publication Number: US-7911796-B2

Title: Avionics chassis

Description:
GOVERNMENT LICENSE RIGHTS 
     This invention was made with Government support under Purchase Order No. 4CC1766 awarded by Department of the Air Force, Air Force Research Laboratory. The Government has certain rights in this invention. 
    
    
     CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is related to patent application Ser. No. 12/487,784, entitled Avionics Chassis, patent application Ser. No. 12/487,797, entitled Avionics Chassis, and patent application Ser. No. 12/487,834, entitled Avionics Chassis, filed concurrently herewith. 
     BACKGROUND OF THE INVENTION 
     Contemporary aircrafts use avionics in order to control the various equipment and operations for flying the aircraft. The avionics may be stored in an avionics chassis, which performs several beneficial functions, some of which are: electrically shielding the avionics from electromagnetic interference (EMI), protecting the avionics from lightning strikes, dissipating the heat generated by the avionics, and protecting the avionics from environmental exposure. 
     Weight is also a consideration for the avionics chassis. The avionics chassis should perform the beneficial functions without unnecessarily adding weight to the aircraft. 
     The performance of the beneficial functions is often inapposite to maintaining or reducing the weight of the avionics chassis, especially in light of newer avionics having faster processing speeds and higher frequencies, smaller size, and greater power densities. These avionics generate relatively large amounts of heat, but operate only under a certain range of threshold temperatures, which leads to an increased heat-dissipating requirement that has been previously addressed by increasing the size of the heat sink, leading to an increased weight. 
     Historically, commercially available avionics chassis are made of aluminum, which inherently has the desired shielding, heat dissipating, lightning strike protection, and environmental protection benefits. 
     BRIEF DESCRIPTION OF THE INVENTION 
     In one embodiment, an avionics chassis comprises a housing having a substantially thermally non-conductive frame comprising a composite of carbon fibers laid up in an epoxy matrix. The housing also includes at least two walls, at least one of which is a thermally conductive wall comprising a composite of carbon fibers in a carbonized matrix, and a plurality of spaced, thermally-conductive, card rails provided on the at least two walls. The at least two walls are mounted to the frame in opposing relationship such that corresponding card rails on the walls define an effective slot therebetween in which a printed circuit board may be received and the card rails and the at least one thermally conductive wall form a thermally conductive path from the interior to the exterior. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
       In the drawings: 
         FIG. 1  is a schematic view of an aircraft having an avionics chassis according to the invention. 
         FIG. 2  is a perspective view of the avionics chassis according to one embodiment of the invention, with a cover removed for clarity. 
         FIG. 3  is an exploded view of the avionics chassis shown in  FIG. 2 . 
         FIG. 4  is a cross-sectional view taken along the line  4 - 4  of a portion of the avionics chassis shown in  FIG. 2 . 
         FIG. 5  is a cross-sectional view taken along the line  5 - 5  of a portion of the avionics chassis shown in  FIG. 2 . 
         FIG. 6  is a cross-sectional view of a portion of the avionics chassis having an optional card rail mount for the card rails and forming a second embodiment of the invention. 
         FIG. 7  is a cross-sectional view taken along the line  7 - 7  of a portion of the avionics chassis shown in  FIG. 2 . 
         FIG. 8  is a bottom view of the thermal plane and stiffener shown in  FIG. 7 . 
         FIG. 9  is a cross-sectional view of a portion of the avionics chassis having an alternative thermal plane and thermal pad and forming a third embodiment of the invention. 
         FIG. 10  is a cross-sectional view of a portion of the avionics chassis having optional attachment structures for the printed circuit board forming a fourth embodiment of the invention. 
         FIG. 11  is an exploded view of a fifth embodiment of the avionics chassis according to the invention. 
         FIG. 12  is an exploded view of a sixth embodiment of the avionics chassis according to the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  schematically illustrates an aircraft  10  with an on-board avionics chassis  12  (shown in phantom) for housing avionics for use in the operation of the aircraft  10 . The avionics chassis  12  houses a variety of avionics elements and protects them against contaminants, electromagnetic interference (EMI), radio frequency interference (RFI), vibrations, and the like. While illustrated in a commercial airliner, the avionics chassis  12  can be used in any type of aircraft, for example, without limitation, fixed-wing, rotating-wing, rocket, commercial aircraft, personal aircraft, and military aircraft. The avionics chassis  12  may be located anywhere within the aircraft, not just the nose as illustrated. 
       FIG. 2  illustrates the avionics chassis  12  according to one embodiment of the invention, with a front cover  42  removed. The avionics chassis  12  includes a chassis housing  16  that defines an interior  18  and exterior  19 . Pluralities of thermally conductive card rails  20  define effective slots  21  (illustrated by the dotted lines) there between for receiving printed circuit boards (PCBs)  14 . Mounting feet  22  extend from the chassis housing  16  to facilitate mounting the avionics chassis  12  to the aircraft  10  by means of bolts or other conventional fasteners. Further, the mounting feet  22 , can function as an electrical ground for grounding the avionics chassis to the frame of the aircraft  10 . While mounting feet  22  are shown in this example the avionics chassis  12  can be used with any type of attachment mechanism. 
       FIG. 3  illustrates the avionics chassis  12  and the PCB  14  in more detail. For purposes of this description, it is noted that the PCB  14  may have negative characteristics for an avionics chassis environment, such as heat producing and radio wave sensitivity, which the chassis  12  is designed to address. The PCB  14  includes heat producing circuitry and/or at least one heat-producing component  24 , such as a semiconductor chip, that is mounted on and supported by a substrate  26 , which is generally thermally non-conductive. The PCB  14  may be provided with thermally conductive side strips  28  located along the exterior edges of the PCB  14 . Thermally conductive elements or interior paths  30  may be provided on the substrate  26  and/or in the interior of the PCB  14 . The interior paths  30  create a thermally conductive path from the heat producing component  24  to the thermally conductive side strips  28  to provide a direct thermal pathway from the interior to the periphery of the substrate  26 . The side strips  28  then provide a thermal pathway to the card rails  20 . The interior paths  30  can be one or more metal strips, typically copper, or other conductive material formed in or provided on the substrate  26 . 
     As illustrated in  FIG. 3 , the chassis housing  16  comprises a frame  34  having a top cover  36 , a bottom wall  38 , a back wall  40 , and opposing side walls  44  and  46 , collectively referred to as the walls. The side walls  44  and  46  have an interior surface  48  and an exterior surface  50 . A plurality of heat-dissipating fins  58  may project from the walls and are illustrated as projecting from the exterior surface  50  of side walls  44  and  46 . A removable front cover  42  includes openings  47  that may be configured for receiving a connector for connecting the one or more PCBs  14  to a wire harnesses or the like (not shown). 
     The frame  34  comprises both polyacrylonitrile (PAN) carbon fibers and pitch carbon fibers in an epoxy matrix. PAN fibers, compared to pitch fibers, have a very high strength and small diameter, which makes them suitable for use at the various radii of the frame  34 . However, PAN fibers, compared to pitch fibers, have a low thermal conductivity. Thus, the use of PAN fibers in the frame  34  results in the frame  34  being very strong, and satisfying the strength requirements for the avionics chassis  12 . The frame  34  has an undesirably low thermal conductivity, largely due to an insulative matrix, which is not capable in and of itself of conducting the heat that is anticipated to be generated by the PCBs  14 . 
     The walls are made with pitch fibers, which have a high stiffness to help meet mechanical requirements for the avionics chassis  12 . The pitch fibers are not as strong as PAN fibers, so they are more prone to breaking under stress or during manufacturing. While the walls are not as strong as the frame  34 , they need not be because the frame  34  provides the primary source of strength for the avionics chassis  12 . The use of pitch fibers helps reduce wall thickness with no loss in stiffness, and the PAN fibers in the frame  34  help maintain mechanical requirements. The pitch fibers have a higher thermal conductivity than the PAN fibers. Thus, the walls provide more thermal conductivity than the frame  34 . 
     The carbon composite has a lower density than traditionally-used aluminum, which reduces the material weight in the avionics chassis  12  while still providing the required strength and stiffness. Because the composite has a much lower density, the weight of the chassis housing  16  may be reduced a substantial amount. For example, avionics chassis made according to this embodiment have resulted in about a 40% weight reduction. The amount of reduction may vary depending on the mechanical requirements for a particular avionics chassis  12 . 
     In forming the frame  34 , top cover  36 , bottom wall  38 , back wall  40 , and opposing side walls  44  and  46 , a dry lay-up method or pre-preg process of constructing carbon composites may be used with both the pitch and PAN carbon fibers. In such a process, the carbon fiber material is already impregnated with the epoxy (pre-preg) and may be applied to a female or male mold. Pre-preg lay-up is a relatively inexpensive, common process that is low cost and well suited for handling thin walled parts. In this embodiment, pre-preg was applied to a female mold. 
     Bladder molding or other suitable techniques may be used to exert pressure on the pre-preg composite material in the female mold or on the male mold, thereby forcing the composite material to take the shape of the mold. Using bladder molding in a female mold the frame  34 , back panel  40 , bottom panel  38 , and side walls  44  and  46  of the avionics chassis  12  may be formed as an integral unit. 
     As an alternative to using bladder molding to exert pressure, an elastomeric male mandrel tool may be used. The elastomer expands when heated to create pressure and consolidate the composite in the female tool or mold. The heat-dissipating fins  58  may be separated by elastomeric spacers during cure and may thus be co-cured to the side walls  44  and  46  to achieve good consolidation, and walls flatness, eliminating seams, and improving thermal paths. Alternatively, the heat-dissipating fins  58  may be formed by machining. Any fittings or posts may be post-bonded to the interior  18 . 
     The top cover  36  and front cover  42  may be produced through compression molding with matched metal tooling and may be suitably joined to the frame  34  using any convenient method such as fasteners, solders, brazes, welds, adhesives, and the like. For example, a structural adhesive may be used to hold the top cover  36  and front cover  42  to the frame  34 . Then, to electrically seal the avionics chassis  12 , an electrically conductive adhesive may be placed right next to the structural adhesive on the interior  18  of the avionics chassis  12 . 
     The card rails  20  abut the interior surface  48  and may be fixedly mounted thereto. The card rails  20  can be attached to the interior surface  48  using any convenient method such as mechanical fasteners, solders, brazes, welds, adhesives, and the like. The card rails  20  may be arranged in pairs, with one card rail  20  residing on the side wall  44  and the other card rail  20  residing on the side wall  46  to effectively define a slot  21  extending between the pair of card rails  20 . Parallelism between the pair of card rails  20  is necessary to ensure that the PCB  14  will slide into the slot  21  properly. Each of the card rails  20  has two legs that define a groove or channel  52 , which partially defines the slot  21 . The card rails  20  should be centered such that when the PCB  14  is inserted into the slot  21 , the PCB  14  is supported by both of the card rails  20  forming the slot  21 ; this facilitates symmetric cooling of the PCB  14 . The card rails  20  may be made of any suitable thermally conductive material including either machined or extruded aluminum, copper, aluminum/beryllium alloy, machined silicon carbide or a metal matrix composite. 
     A radio wave shield  54  is provided on the housing  16  to render the avionics chassis  12  EMI/RFI resistant. The radio wave shield  54  may comprise a metallic layer  55  provided on the housing  16 . The radio wave shield  54  may be in the form of a metal deposition layer on the chassis housing  16 . The deposition layer may be formed by chemical vapor deposition, physical vapor deposition, or electrodeposition. Further, the radio wave shield  54  may be formed by other means such as thermal sprayed metal, the use of a co-cured mesh, or the use of a metal foil. To properly attenuate the electromagnetic interference, the radio wave shield  54  covers or overlies at least the entire exterior of the avionics chassis  12 . It may also be applied to the interior if needed. The radio wave shield  54  reflects the radio waves. While the composite material of the avionics chassis  12  absorbs some radio waves and provides some attenuation benefit, the wave shield  54  provides the necessary attenuation for practical purposes. The contemplated radio wave shield  54  attenuates the radio wave energy by at least 60 dB. The thickness of the metallic layer  55  for the selected material, is believed to be the main factor in attenuating the radio wave energy. A physical vapor deposition layer of aluminum having a thickness of 2-3 microns has been found to attenuate the radio wave energy at least 60 dB. 
     At least one lightning strike conductive path, comprising a metallic strip  56 , is provided on the chassis housing  16  in addition to the exterior metal layer. The metallic strip  56  is illustrated as overlying the metallic layer  55  forming the radio wave shield  54 . While illustrated as a single metallic strip  56 , multiple strips may be used and it may extend around corners and on multiple components of the assembly. The metallic strip  56  extends to one or more of the feet  22 , resulting in the metallic strip  56  forming a conductive path to the electrical ground. The single metallic strip  56  and/or the multiple metallic strips may extend to one or multiple feet  22  to provide multiple conductive paths to the electrical ground. 
     While the mounting feet  22  are illustrated as the grounding point for the avionics chassis  12 . Other suitable grounding points may be used and include: grounding studs, grounding surfaces, grounding straps, metallic spring fingers, etc. to provide a grounding path. These may all be done totally independently of the mounting feet  22 . It is contemplated that the avionics chassis  12  may not even have mounting feet  22  such as when mounting hooks and guide pins are used. 
     It has been contemplated that thermal sprayed aluminum, or another thermal sprayed metal, may be used to create the metallic strip  56 . Thermal sprayed aluminum is applied by propelling molten aluminum at the avionics chassis  12  with expanding gasses. The molten metal quenches at impact and adheres to the avionics chassis  12  by mechanical interlock and diffusion bonding. Subsequent impacting aluminum builds the metallic strip  56  thickness. The metallic strip  56  is relatively thick compared to the metallic layer  55  of the radio wave shield  54 , with a practical thickness of around 76 microns or greater. 
     The density and thickness of the metallic strip  56  should be selected to enable the current generated by a lightning strike to be quickly transmitted to the electrical ground without causing electro-migration or the fusing of the metallic strip  56 .  FIG. 4  illustrates a cross section of the metallic layer  55  and metallic strip  56  located on several of the heat-dissipating fins  58 . The thickness of the metallic strip  56  is shown schematically as being thicker than the thickness of the metallic layer  55 . 
     The thermal sprayed aluminum may also be applied over bonded joints on the avionics chassis  12 . For example, where the mounting feet  22  are attached to the chassis housing  16 . The thermal sprayed aluminum, or metallic strip  56 , creates a continuous, intimately bonded conductive path between the chassis housing  16  and the mounting feet  22  and this helps to avoid slight gaps between the conductive paths, which could enable sparking. The electrical resistance between any locations on the avionics chassis  12 , including the mounting feet  22 , may not exceed 2.5 milliohms. 
     Unlike its metal counterparts, the carbon composite avionics chassis  12  does not inherently attenuate radio wave energy or conduct away the extreme electrical currents generated by lightning strikes. This is because the carbon fiber composite chassis housing  16  is significantly less electrically conductive than an aluminum chassis because of an electrically insulative composite matrix. In a carbon fiber composite avionics chassis  12  current from a lightning strike seeks the metal paths available, which can damage and even destroy onboard electronics that have not been electromagnetic field shielded or lightning protected. The metallic layer  54  described above is not always thick enough to handle a lighting strike. Also, a thick enough metal layer to provide lightning strike protection greatly and unnecessarily increases the weight of the avionics chassis  12 . 
     The combination of different materials and thicknesses for the metallic layer  55  and metallic strip  56  provide for additional weight reduction, while still providing the desired radio wave shielding and lightning strike protection. The mixing of the metallic layer  55  and metallic strip  56  along with limiting their respective coverage area to that required to perform the desired function provides for a substantial weight reduction. 
       FIG. 5  illustrates that the card rail  20  may be attached to the interior surface  48 . The card rail  20  may be attached using fasteners, solders, brazes, welds, adhesives, or other attachment methods. If a structural adhesive is used, it will not have the necessary electrical conductivity and thus thermal sprayed aluminum, another thermal sprayed metal, or a metal applied by another means may be applied along the card rail  20  to increase electrical conductivity between the card rail  20  and the interior surface  48  of the side walls  44  and  46 . 
     The plurality of heat-dissipating fins  58  extend from the exterior surface  50  of the side walls  44  and  46 . Because the carbon fiber in the avionics chassis  12  is encased in the epoxy matrix, the resulting structure has the structural and weight benefits of the carbon fiber but not the thermal conductivity benefits. In this embodiment, the side walls  44  and  46  are integrated cold walls that help create a thermal management system to conduct heat from the interior  18  of the avionics chassis  12  to its exterior  19  where the heat may then be dissipated in the surrounding air through convection. 
     While other configurations are possible, the heat-dissipating fins  58  are illustrated in  FIGS. 2 and 5  as having the same orientation and commensurate in length to the card rails  20 . For example, the heat-dissipating fins may run perpendicular to the card rails. The heat-dissipating fins  58  increase the exterior surface area of the side walls  44  and  46  allowing more heat to be transferred to the surrounding air through convection. The heat-dissipating fins  58  are schematically illustrated in  FIGS. 4 through 6  as comprising a plurality of high-thermal conductivity carbon fibers  59  with isotropic orientation in the plane of the heat-dissipating fins  58 . The use of the oriented carbon fibers gives the heat-dissipating fins  58  several times the thermal conductivity, yet significantly less weight, than an aluminum part of similar dimensions. For example, the isotropic carbon fibers  59  can have a high-thermal conductivity of approximately 1100 W/m-K. 
     The heat-dissipating fins  58  can be co-cured to the side walls  44  and  46  eliminating seams and improving thermal paths. To further improve thermal conductivity, a plurality of isotropic fibers of the heat-dissipating fins  58  may be extended at discrete sites from an interior of the heat-dissipating fins  58  to create tabs  60 . These tabs  60  may be formed along the entire length of the heat-dissipating fin  58 . The tabs  60  go through the side walls  44  and  46  to contact the card rails  20  located on the interior surface  48 . The isotropic carbon fibers  59  form a direct conductive path from the card rail  20  to the heat-dissipating fins  58 . 
     Not all of the heat-dissipating fins  58  in abutting contact with the exterior surface  50  have tabs  60  extending through the side walls  44  and  46  to the card rail  20 . The plurality of isotropic carbon fibers  59  extending from the heat-dissipating fins  58  through the side walls  44  and  46  and in abutting contact with the card rail  20  is advantageous since it significantly improves heat transfer. Multiple tabs  60  from one heat-dissipating fin  58  may contact the card rail  20  down its entire length. Further, a plurality of tabs  60  from a plurality of heat-dissipating fins  58  are illustrated as abutting the single card rail  20  this also improves the amount of heat that may be conducted from the card rail  20 . 
       FIG. 6  illustrates an alternative mounting of the card rails  20 . More specifically a card rail mount  61  is provided on the card rail  20  and attached to the interior surface  48 . The card rail mount  61  is illustrated as a pedestal  62  having a grooved surface  64 . The card rail mount  61  may be adhered by at least one of a structural adhesive and a conductive adhesive to the card rail  20 . Depending on the application, the same adhesive may provide both the desired structural and conductive properties. 
     The grooved structure  64  defines intervening interstitial spaces  65  that may receive thermally conductive adhesive  67  when the card rail mount  61  is adhered to the interior surface  48 . This thermally conductive adhesive may touch the isotropic carbon fibers  59  to help form a conductive path from the card rail  20  to the heat-dissipating fins  58 . Additionally, a plurality of fasteners  66 , such as screws, may be inserted into the exterior surface  50  to provide mechanical reinforcement and ensure stability of the card rails  20 . 
       FIG. 7  illustrates a portion of the avionics chassis  12  including a circuit card assembly  68 , mounted in the slot  21 , and having a thermal plane  70 , a thermal pad  76 , and stiffeners  78 . The PCB  14  is illustrated as being mounted within the slot  21  with a thermal plane  70  also in the slot  21  and in overlying relationship with the PCB  14 . In this manner, the PCB  14  defines a first primary plane, the thermal plane  70  defines a second primary plane, and the spatial relationship between the PCB  14  and the thermal plane  70  is such that the first and second primary planes are located within the slot  21  when the circuit card assembly  68  is received within the slot  21 . 
       FIG. 8  better illustrates the thermal plane  70 , the thermal pad  76 , and the stiffeners  78 . The thermal plane  70  is used to conduct heat away from the PCB  14 . The thermal plane  70  can be comprised of a carbon fiber-reinforced composite as well as a carbon-carbon composite. For example, the thermal plane  70  may be comprised of pyrolytic carbon, which is highly thermally conductive. The carbon fibers may be laid up such that the thermal plane  70  is thermally conductive in the two-dimensional plane, that is it has in-plane (lateral) thermal conductivity that enables heat to dissipate in the x and y plane. It is also possible for the thermal plane  70  to have a lay-up of carbon fibers in 3D. The 3D lay-up would be more expensive but would facilitate the movement of heat away from the PCB  14 . It has been contemplated that a one-dimensional lay-up may also be useful. No matter its configuration, the thermal plane  70  is intended to thermally conduct heat from the PCB  14  towards the card rails  20 . 
     The thermal plane  70  may be attached to either the top or the bottom of the PCB  14 . The thermal plane  70  may be mounted directly to the PCB  14  or through the thermal pad  76 . The thermal pad  76  may be made of a carbon composite or any thermally conductivity material. For example, the thermal pad  76  may be made from 3D carbon-carbon composite. The thermal pad  76  may be located such that it directly contacts the heat-producing component  24 . 
     The stiffener  78  is operably coupled to the PCB  14  so that the PCB  14  will not flex or vibrate within the slot  21 . The stiffener  78  can be located between the PCB  14  and the thermal plane  70  when the circuit card assembly  68  is located within the slot  21 . The stiffener  78  can also be located within one of the card rails  20  when the circuit card assembly  68  is located within the slot  21 . The stiffener  78  may be comprised of aluminum or similar thermally conductive material and can have a variety of configurations to provide support for the PCB  14 . Although the thermal plane  70  has been illustrated as a plane, it has been contemplated that it may also be a bar or a strap. Furthermore, in alternate embodiments, any suitable shape stiffener  78  for strengthening the PCB  14  could be provided. For example, the stiffener  78  may be several bars that are not interconnected. The stiffener  78  can also be integral with the thermal plane  70 . 
     Referring back to  FIG. 7 , when the circuit card assembly  68  is in the slot  21 , the thermal plane  70  is conductively coupled to one of the card rails  20  to form a portion of a first conductive path  72  and the PCB  14  is conductively coupled to another of the card rails  20  to form a portion of a second conductive path  74 . The first conductive path  72  begins with the heat-producing component  24 ; heat is conducted through the thermal pad  76  to the thermal plane  70 , which in turn conducts that heat laterally to the card rails  20 . The first conductive path  72  continues through the card rails  20  to either the isotropic carbon fibers  59  in the tabs  60  or the side walls  44  and  46  themselves. The heat conducted through the isotropic carbon fibers  59  in the tabs  60  is directly conducted to the exterior of the heat-dissipating fins  58 . The heat conducted through the side walls  44  and  46  is conducted by the isotropic carbon fibers  59  in the heat-dissipating fins  58  to the exterior of the heat-dissipating fins  58 . Heat may then be dissipated through convection into the air surrounding the heat-dissipating fins  58 . 
     The second conductive path  74  begins with the heat-producing component  24 ; heat is then transferred through the interior paths  30  of the PCB  14  to the thermally conductive side strips  28 . Although the arrows illustrated in  FIG. 7  are offset from the interior paths  30 , this is done for illustrative purposes and the interior paths  30  are actually a portion of the second conductive path  74 . The arrow has merely been offset so that it does not obscure the interior paths  30  in the figure. The side strips  28  abut the card rail  20  and heat in turn conducts from the card rail  20  either through the side walls  44  and  46  to the exterior of the heat-dissipating fins  58  or through the tabs  60  to the exterior of the heat-dissipating fins  58 . Heat may then be dissipated through convection into the air surrounding the heat-dissipating fins  58 . Thus, the PCB  14  also acts as a heat spreader by itself. This allows the avionics chassis  12  to run much cooler with the additional conductive path provided by the thermal plane  70 . 
     The height of the PCB  14  is such that the PCB  14  and thermal plane  70  are both received within the channel  52 . As illustrated in  FIG. 7 , the PCB  14  is in direct contact with the main portion of the card rail  20 . The thermal plane  70  is in direct contact with the leg of the card rail  20  and in direct contact with the main portion of the card rail  20 . Alternatively, the contact between the PCB  14  and the card rail  20  or the contact between the thermal plane  70  and the card rail  20  could be indirect contacts. 
       FIG. 9  illustrates an alternative thermal pad comprising an adjustable thermal pad  80 . The adjustable thermal pad  80  is illustrated as a screw contact  82 . The lower portion of the screw contact  82  is adjustable relative to the PCB  14 . Thus, the screw contact  82  may be lowered and raised such that it may accommodate heat-producing components  24  of varying heights. 
       FIG. 10  illustrates an alternative mounting of the PCB  14  in the card rails  20 . More specifically, wedge locks  79  may be used to connect the PCB  14  and the thermal plane  70  to the card rails  20 . The wedge locks  79  may be made of aluminum or any similarly thermally conductive material. In this manner, the wedge locks  79  may become a portion of the first conductive path  72  and the second conductive path  74 . For example, the second conductive path then begins with the heat-producing component  24 ; heat is then transferred through the interior paths  30  to the thermally conductive side strips  28 . The side strips  28  abut the wedge locks  79 , which in turn conduct heat to the card rail  20 . The card rail  20  in turn conducts heat through the side walls  44  and  46  to the heat-dissipating fins  58 . Heat may then be dissipated through convection into the air surrounding the heat-dissipating fins  58 . Again, although the arrows illustrated in  FIG. 9  are offset from the interior paths  30 , this is done for illustrative purposes and the interior paths  30  are actually a portion of the second conductive path  74 . 
     With either embodiment, the height of the components of the circuit card assembly  68  should be selected such that the entirety of the circuit card assembly  68  is located within the slot  21 . This gives the circuit card assembly  68  a low profile design. This will allow more circuit card assemblies  68  to be placed in the avionics chassis  12 . As the amount of circuit card assemblies  68  in the avionics chassis  12  increases the presence of the two thermally conductive paths  72  and  74  will help provide additional heat distribution from the PCB  14  and allow the avionics chassis  12  to run cooler. 
       FIG. 11  is an exploded view of an avionics chassis  112  having cold walls  144  and  146  according to a fifth embodiment of the invention. The fifth embodiment  100  is similar to the first embodiment  10 . Therefore, like parts will be identified with like numerals increased by  100 , with it being understood that the description of the like parts of the first embodiment applies to the second embodiment, unless otherwise noted. 
     One difference between the first embodiment  10  and the fifth embodiment  100  is that the cold walls  144  and  146  are discrete in that they are formed separately from the frame from a thermally conductive material. More specifically, the cold walls  144  and  146  are formed from a composite of carbon fibers in a carbonized epoxy matrix. Carbonized epoxy matrix composites have relatively high thermal conductivity properties in each axes compared to epoxy matrix composites; thermal conductivity is increased in the axes depending on the carbon fiber lay-up. The carbon fibers in the cold walls  144  and  146  are laid up such that the cold walls  144  and  146  are more thermally conductive in a two-dimensional plane. The carbon fibers in the carbonized matrix have excellent thermal properties in the x and y plane due to the fiber lay-up much like the thermal planes described above. 
     This configuration provides that the cold walls  144  and  146  may be formed from a higher thermal conductivity material than the remainder of the avionics chassis  112  and frame  134 . The high thermal conductivity of the cold walls  144  and  146  results in the cold walls  144  and  146  being stiff but not strong. To make a whole avionics chassis out of the same material would require the whole avionics chassis  112  to be very thick to achieve the structural support necessary. Thus, the substantially thermally insulative frame  134  formed from carbon fibers laid up in an epoxy matrix gives the avionics chassis  112  its strength and the discrete cold walls  144  and  146  can provide the benefits of high thermal conductivity while not being required to provide such rigorous structural support. 
     Another difference is that card rails  120  are integrally formed on the interior surface  148  of the cold walls  144  and  146 . The cold walls  144  and  146  are mounted to the  134  frame in opposing relationship such that corresponding card rails  120  on the cold walls  144  and  146  define a slot  121  therebetween. Thus, the cold walls  144  and  146  should be aligned perfectly such that the circuit card assemblies may fit within the slots  121 . The discrete cold walls  144  and  146  may be assembled to the frame  134  using soldering, welding, brazing, adhesive, mechanical fasteners, or other similar attachment methods. Structural adhesive may be applied to fix the cold walls  144  and  146  to the frame  134  and an electrically conductive adhesive may be placed right next to the structural adhesive on the interior  118  of the avionics chassis  112  to electrically seal it. The cold walls  144  and  146  may also be metal plated, such as with nickel or aluminum, to provide better conductivity and to seal the carbon fibers against galvanic corrosion with aluminum wedge locks  179  on the PCBs  114 . 
       FIG. 12  is an exploded view of an avionics chassis  212  having cold walls  244  and  246  according to a sixth embodiment of the invention. The sixth embodiment  200  is similar to the fifth embodiment  100 . The difference being that the cold walls  244  and  246  include heat-dissipating fins  258  to increase the surface area of the exterior surface  250  of the cold walls  244  and  246 . The cold wall surface area may also be increased with pins or other similar methods. 
     From a weight perspective, a carbon fiber composite avionics chassis  12  is more desirable than a heavier aluminum version. However, the carbon fiber composite version is less desirable than an aluminum version because of the poorer thermal and electrical conductivity characteristics. Thus, the various embodiments of carbon fiber composite avionics chassis disclosed herein are beneficial for an aircraft environment because of their weight reduction. The reduced weight avionics chassis also addresses all requirements related to electromagnetic interference (EMI), dissipating the heat generated by the avionics, protecting the avionics from lightning strikes, and protecting against environmental exposure, while still achieving a relatively low weight avionics chassis. 
     This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal languages of the claims.