Abstract:
An avionics chassis for protecting against damage, dust, dirt and incidental moisture over an extended temperature range, EMI shielding to prevent radiation of internal circuit energy and preventing the entrance of external EMI. The chassis provides lower weight, lower levels of radiated emissions and improved resistance to incident external radiation. Electric and magnetic shielding is also provided.

Description:
BACKGROUND OF THE INVENTION 
     Traditionally electronic chassis are made from multiple forms of aluminum that surround circuit boards that contain RF, microwave, millimeter wave, or high speed digital circuitry. The chassis is designed to prevent internally generated radiation from escaping outside of the chassis and external radiation from entering the chassis. Aluminum does not damp electromagnetic resonances within a cavity (a lossy material must be added) and does not provide magnetic shielding. Aluminum has a high thermal conductivity and provides means for the removal of excess heat. Machining a chassis from aluminum is an expensive manufacturing process. 
     The demands by aerospace customers for weight and cost reductions and simpler operation (eliminate convective cooling) open up the opportunity for novel composite materials, structures, and manufacturing processes to replace traditional materials, particularly aluminum shaped by machining. Composites can achieve a 20-50% (typical 30%) weight reduction compared to aluminum but must also match certain very desirable properties of aluminum: high electrical conductivity, high thermal conductivity and low raw material costs. However, hand layup of composites is expensive, even compared to machined aluminum, so inexpensive manufacturing methods are needed. 
     SUMMARY OF THE INVENTION 
     An electronic chassis protects internal components against damage, dust, dirt and incidental moisture over a temperature range, electromagnetic resonances resulting from internal circuit energy and the entrance of external electromagnetic interference (EMI). Both electric and magnetic shielding is also provided. The chassis contains structures that facilitate heat removal. The composition of the chassis material to include a particular class of light weight additives further reduces the weight of the chassis. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Preferred and alternative embodiments of the present invention are described in detail below with reference to the following drawings: 
         FIG. 1  illustrates a perspective view of a conventional avionic electronic chassis for housing circuit board assemblies formed in accordance with an embodiment of the present invention; 
         FIG. 2  illustrates a cutaway view of the chassis shown in  FIG. 1 ; 
         FIGS. 3-1  and  3 - 2  illustrate an exploded perspective view of the chassis described in one embodiment of the present invention; 
         FIGS. 4-1  and  4 - 2  illustrate one embodiment of highly electrically and thermally conductive graphene plates; 
         FIG. 5  is an end cross-sectional view of a chassis formed in accordance with an embodiment of the present invention 
         FIG. 6  is side cross-sectional view of a portion of the chassis of  FIG. 5 ; and 
         FIG. 7  is a vertical cross-sectional view of the chassis of  FIG. 5 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     A composite electronics chassis with structure and material composition is disclosed herein. In one embodiment the composite avionics chassis is sufficiently electrically and thermally conductive to provide EMI shielding but sufficiently lossy to damp electromagnetic resonances, and also light weight resulting from a polymer matrix composite which may contain hollow carbon microspheres. An electronic chassis in one embodiment includes reinforcing layers capable of retaining high strength even with the use of hollow carbon microspheres which are known to degrade properties. The chassis includes a polymeric resin combined with composite fill material(s) that in one embodiment meet Federal Aviation Administration (FAA) Flammability, Smoke Density and Toxicity (FST) requirements for commercial aircraft applications. 
       FIGS. 1 and 2  illustrate a conventional chassis  20  made of machined aluminum with planes on the sides  22 , front  24 , back  26 , and bottom  28 . Openings  30  in the bottom  28  and top (not shown) are for air circulation to cool the cards in a conventional chassis. 
     In the composite chassis  40  in  FIGS. 3-1  and  3 - 2 , the larger side panels  42  are formed by compression molding of conventional prepreg layup or of a hollow microsphere core structure clad with prepreg sheets in one embodiment. The chassis  40  includes the side panels  42 , a top panel  53 , a bottom panel  48 , a back panel  55  and a front panel (not shown) that are made by injection, transfer or compression molding, which are very cost effective processes for high volume manufacturing. 
     A base plate that is attached to the bottom panel  48  includes highly electrically conductive (1050 W/m-K) Al-clad graphite plates  54  ( 70  in  FIG. 4-2 ) molded as inserts. As shown in  FIGS. 3-1 , the Al-clad graphite plates  54  rapidly transport heat along a path  49  from circuit boards (not shown) (see  FIGS. 5 and 7 ) and along a highly thermally conductive path  50  to an external heat sink at an end (heat sink  64  in  FIG. 4-2 ) without the need for convective cooling. The plates  54  can also serve as ground planes. 
     The heat conduction in graphene plates is highly anisotropic because of the two dimensional properties of graphene layers. Thermal conduction is very high in the planar directions indicated by arrows  61  and  62  in the as-manufactured plates shown in  FIG. 4-1  and orders of magnitude lower in the orthogonal path  49  through the plate  60 . To remove heat along a path  49  from the circuit boards and the path  50  to an external heat sink, efficient heat transfer is needed both through a first path  49  in the plate, where thermal conductivity is low in the as-manufactured plate  60 , and then along a second direction  50  in the same plate  60 , where thermal conductivity is high. One way to improve thermal conductivity into the plate would be to cut the graphene plate  60  into thin strips along its length as indicated by the dashed lines  63  in  FIG. 4-1 . The strips could then be reoriented and stacked as shown in  FIG. 4-2 . The reorientation involves rotating the surface marked X on strip  65  in the as-manufactured plate  60  to the surface marked X in strip  75  in the reassembled plate  70 . The reoriented strips may be bonded along planes  73 . In this embodiment, the high thermal conductivity directions both conduct heat away from the boards as indicated by the arrow in  FIG. 4-2  and along the length as indicated by the arrow of the plate  70  to the heat sink  64 . 
     Another way to transport heat in the low conductivity direction of the graphene plate is to drill small holes, known as vias (not shown), in the low conductivity direction and fill the holes with high conductivity material, such as aluminum. 
     The heat sink  64  is generally located outside the back wall  26  of the chassis. A portion  69  of the heat sink  64  may be an integral part of the chassis wall. Alternatively the heat sink  64  may be inserted through an opening in the chassis wall. The heat sink  64  disposes of heat removed from the graphene plate  70  by, for example, radiating from fins into an open environment, such as outer space, convective cooling by forced or free convention of air over finned surfaces, or transport by a pumped cooled liquid. 
     The use of polymers (density ˜1.3 g/cc), carbon fiber (˜1.8 g/cc), and crush resistant hollow microspheres (˜0.55 g/cc) compared to aluminum (2.7 g/cc) lower the overall weight of the assembled part. The large flat side panels  42  on the chassis  40  can be made by pressing laid up prepreg, or by applying cladding (carbon fabric prepreg) to a microsphere-resin composition (similar to sheet molding compound production). 
     The chassis  40  is molded from composite materials which include a base resin. Various materials can be combined within the chassis  40  to achieve different levels of conductivity, strength and weight. 
     In one embodiment, a microscale carbon material is hollow carbon microspheres which, below 50 microns in diameter, have sufficient crush strength for injection molding. The microspheres have the advantage of very light weight because of the low density of carbon and the fact they are hollow. Data on hollow microspheres generally show improved crush resistance as the microspheres decrease in diameter and increase in density; these trends in size and density are observed to occur naturally in the sphere manufacturing process. 
     The base resin may be polyetherimide (PEI) that is combined with one or more of the filler materials above. PEI is amorphous, amber transparent, high-performance thermoplastic that provides high heat resistance, high strength and modulus, and excellent electrical insulating properties. PEI, which maintains properties with continuous exposure to 340° F. (170° C.), is ideal for high strength/high heat applications, is hydrolysis resistant, is highly resistant to acidic solutions and is capable of withstanding repeated autoclaving cycles. PEI grades are available in an electrostatic dissipative grade, and FDA, and USDA compliant grades. Common trade names for PEI include Ultem®, Tecapei®, and Tempolux®. 
     Polyethersulfone (PES) (e.g., Ultrason® (BASF)) may be used instead of PEI. PES is also high temperature resistant (180° C. continuous use temperature) with good mechanical performance at high temperatures. 
     Polyphenylene sulfide (PPS) (e.g. Ryton®) is a highly crystalline (50-60% crystallinity) thermoplastic. PPS is fire resistant, impervious to aircraft fluids, and has a lower melt viscosity because it is a semicrystalline material. Its mechanical properties and temperature tolerance do not match PEI, but high temperature capability is not needed for electronics housings. 
     The chassis formed from the matrix materials described above may also include various forms of carbon materials to provide the required levels of electrical conductivity and greatly improved mechanical properties over the neat matrix resin. Nickel coated carbon or nickel particulates may be added in any combination to achieve higher conductivity and provide magnetic shielding. Desired conductivity with lowest weight and cost may also be achieved by using yet higher levels of carbon fiber to minimize the use of nickel, which is heavy and expensive. 
     In one embodiment, pellets for injection molding a chassis are made by mixing 20 wt % chopped carbon fiber (Fortafil® 219), 10 wt % nickel coated carbon fiber (Sulzer NiCF) and 70 wt % polyetherimide (Ultem® 1000). The pellets have an electrical resistivity of 3.7 ohm-cm and a density of 1.37 g/cc. Tensile properties (ASTM D-638-03) at room temperature were 28,000 psi tensile strength, 1,200,000 psi modulus, and 1.2% elongation. The corresponding flexural properties (ASTM D-790-07) are 35,000 psi flexural strength, and 3,000,000 psi flexural modulus. Other percentage mixtures may be used to obtain different properties. 
     In another embodiment the properties for the composite composition are electrical conductivity in the target range of 0.1 (1/ohm-cm) −1 to 2.0 (1/ohm-cm), density target of less than 1.1 g/cc, and flexural modulus target value greater than 600,000 psi for a composition based on an hand mixed epoxy resin. The matrix materials for these compositions advantageously pass the FST test. Compositions to be processed by injection molding, a rapid high pressure process, preferably have a low melt viscosity which allows filling of thin wall molds to allow both material cost and weight savings. Because of the harsh molding and compounding conditions these compositions require crush resistant microspheres with smaller diameters and relatively higher densities. Compression molding is a less demanding process which permits higher viscosity and requires lower molding pressures, allowing microspheres with thinner walls and lower density. 
     An example composition that meets these requirements for compression molding comprises 9.5 vol % milled carbon fibers and 19 vol % hollow carbon microspheres in a matrix of Epon™ 862 epoxy with Epikure™ W hardener. The microspheres used in this composition, which were not crush resistant, have a relatively larger diameter and a density of 0.38 to 0.43 g/cc as measured by a pycnometer. The microspheres lowered the density of this composition to 0.95 g/cc from 1.14 g/cc for a control with 19 vol % milled carbon fibers and no microspheres. The electrical resistivity for the composition was 0.59 ohm-cm which is within the target range of 0.5 to 10 ohm-cm. 
     Yet another embodiment suitable for injection molding comprised small diameter (40 micron), higher density 0.55 g/cc microspheres (EP36) at 8 wt % level compounded with 20 wt % chopped carbon fiber (Fortafil® 219) in a polyphenylene sulfide (Ryton® P6) resin. 
     In yet one more embodiment, compression molded panels were prepared by placing two layers of 5-harness carbon fiber prepreg as the top and bottom layers surrounding a core of bismaleimide matrix (Cycom® 5052-4) containing 17 wt % hollow carbon microspheres. These microspheres with a lower density of 0.38 to 0.43 as measured by a pycnometer would not be able to withstand the high pressures encountered in injection molding. The microsphere-matrix composition without other reinforcement is expected to have a modulus lower by a factor of 5 or more compared to a composite with continuous fiber reinforcement. The prepreg clad microsphere composite described in this example has a flexural modulus at 240° C. of 2,700,000 psi which is only about a 25% reduction compared to a BMI continuous fiber prepreg (Cycom® 5250HT) composite. The cladding substantially reduces the loss in properties associated with the use of hollow microspheres. 
       FIGS. 5-7  show a device  82  for electrically connecting the Al-Clad card guides (plates  54 ) to one another and then to a common ground connection on any rear connector  84  located at the back panel  55  of the chassis  40 . By themselves the Al-Clad guides (plates  54 ) are electrically and thermally conductive but electrically they do not go anywhere and make a poor ground connection. The rear connector  84  is in contact with an aircraft heat sink (not shown) for moving the heat away from the chassis  40 . 
     In one embodiment, the device  82  is a chemical etched metal piece that covers the plates  54  to provide the electrical ground. The device  82  may include straps to connect to a common ground stud  90  in  FIG. 6  located on the rear connector  84 . The device  82  may be formed into various configurations in order to connect each printed wiring assembly  80  to have a connection to an electrical ground and/or heat sink. 
     In another embodiment, an internal composite wall  86  in the chassis absorbs electromagnetic energy and also shields internal components in one region of the chassis from electrometric energy from components in another region. The shield wall materials may be of the compositions described heretofore and may be lighter and thinner than the chassis external walls because load and durability requirements may be less for internal walls. 
     The internal composite wall  86  and any circuit boards are held in place within the chassis  40  using known techniques, such as with a wedge device  92  and a second guide or wall  94 . The second guide or wall  94  may be the same as the plates  54  and attached to base plate  52  and bottom panel  48  in a similar manner. 
     The polymer resin is an at least one of polyetheretherketone, polyetherketoneketone, polyetherimide, polyphenylene sulfide, polyamideimide, and polysulfones. In another embodiment, the polymer resin is at least one of epoxy resin, bismaleimide resin, phenolic resin, polyimide resin, and cyanate ester resin. 
     The microspheres have been subject to a heat treatment in excess of 1400° C. 
     The composite panel includes more than one panels. An adhesive bonding agent bonds the panels together. 
     The reconfigured plates have thermal conductivity in excess of 800 W/m-K in at least in two directions. 
     While the preferred embodiment of the invention has been illustrated and described, as noted above, many changes can be made without departing from the spirit and scope of the invention. Accordingly, the scope of the invention is not limited by the disclosure of the preferred embodiment. Instead, the invention should be determined entirely by reference to the claims that follow.