Abstract:
An electromagnetic interference shield for use around access panels and doors in electronic equipment enclosures includes a base and profile manufactured from an electrically nonconductive solid material such as a thermoplastic resin polymer. An electrically conductive layer of a metallized fabric is bonded to the profile to provide effective shielding and grounding functions. The underlying polymer provides elastic compliancy and resiliency to the shield.

Description:
TECHNICAL FIELD 
     The present invention relates to electromagnetic interference (“EMI”) shields and, more specifically, to an EMI shield manufactured from an electrically nonconductive material and clad with an electrically conductive layer. 
     BACKGROUND 
     During normal operation, electronic equipment generates undesirable electromagnetic energy that can interfere with the operation of proximately located electronic equipment due to EMI transmission by radiation and conduction. The electromagnetic energy can by of a wide range of wavelengths and frequencies. To minimize the problems associated with EMI, sources of undesirable electromagnetic energy may be shielded and electrically grounded. Shielding is designed to prevent both ingress and egress of electromagnetic energy relative to a housing or other enclosure in which the electronic equipment is disposed. Since such enclosures often include gaps or seams between adjacent access panels and around doors, effective shielding is difficult to attain because the gaps in the enclosure permit transference of EMI therethrough. Further, in the case of electrically conductive metal enclosures, these gaps can inhibit the beneficial Faraday Cage Effect by forming discontinuities in the conductivity of the enclosure which compromise the efficiency of the ground conduction path through the enclosure. Moreover, by presenting an electrical conductivity level at the gaps that is significantly different from that of the enclosure generally, the gaps can act as slot antennae, resulting in the enclosure itself becoming a secondary source of EMI. 
     Specialized EMI gaskets have been developed for use in gaps and around doors to provide a degree of EMI shielding while permitting operation of enclosure doors and access panels. To shield EMI effectively, the gasket should be capable of absorbing or reflecting EMI as well as establishing a continuous electrically conductive path across the gap in which the gasket is disposed. Conventional metallic gaskets manufactured from copper doped with beryllium are widely employed for EMI shielding due to their high level of electrical conductivity. Due to inherent electrical resistance in the gasket, however, a portion of the electromagnetic field being shielded induces a current in the gasket, requiring that the gasket form a part of an electrically conductive path for passing the induced current flow to ground. Failure to ground the gasket adequately could result in radiation of an electromagnetic field from a side of the gasket opposite the primary EMI field. 
     In addition to the desirable qualities of high conductivity and grounding capability, EMI gaskets should be elastically compliant to compensate for variable gap widths and door operation, yet tough to withstand repeated door closure without failing due to metal fatigue. EMI gaskets should also be configured to ensure intimate electrical contact with proximate structure while presenting minimal force resistance per unit length to door closure, as the total length of an EMI gasket to shield a large door can readily exceed several meters. It is also desirable that the gasket be resistant to galvanic corrosion which can occur when dissimilar metals are in contact with each other for extended periods of time. Low cost, ease of manufacture, and ease of installation are also desirable characteristics for achieving broad use and commercial success. 
     Conventional metallic EMI gaskets, often referred to as copper beryllium finger strips, include a plurality of cantilevered or bridged fingers that provide spring and wiping actions when compressed. Other types of EMI gaskets include closed-cell foam sponges having metallic wire mesh knitted thereover or metallized fabric bonded thereto. Metallic wire mesh may also be knitted over silicone tubing. Strips of rolled metallic wire mesh, without foam or tubing inserts, are also employed. 
     One problem with metallic finger strips is that to ensure a sufficiently low door closure force, the copper finger strips are made from thin stock, for example on the order of about 0.05 mm (0.002 inches) to about 0.15 mm (0.006 inches) in thickness. Accordingly, sizing of the finger strip uninstalled height and the width of the gap in which it is installed must be controlled to ensure adequate electrical contact when installed and loaded, yet prevent plastic deformation and resultant failure of the strip due to overcompression of the fingers. To enhance toughness, beryllium is added to the copper to form an alloy; however, the beryllium adds cost and is a concern since beryllium is carcinogenic. Due to their thinness, the finger strips are fragile and can fracture if mishandled or overstressed, resulting in thin sharp edges which are a safety hazard to installation and maintenance personnel. Finger strips are also expensive to manufacture, in part due to the costs associated with procuring and developing tooling for outfitting presses and rolling machines to form the complex contours required. Changes to the design of a finger strip to address production or performance problems require the purchase of new tooling and typically incur development costs associated with establishing a reliable, high yield manufacturing process. 
     Metallic mesh and mesh covered foam gaskets avoid many of the installation and safety disadvantages of finger strips; however, they can be relatively costly to produce due to the manufacturing controls required to realize acceptable production yields. 
     SUMMARY OF THE INVENTION 
     A metallized fabric clad polymer EMI shield overcomes many of the limitations and disadvantages of conventional EMI shields. One method of manufacturing a metallized fabric clad polymer shield for shielding EMI from passing through a seam between first and second electrically conductive bodies includes forming a base and a profile of an electrically nonconductive solid material in a predetermined configuration. The base is designed to secure the shield to the first body while the profile is designed to contact the second body. An electrically conductive layer is then disposed on at least part of the profile so as to be interdisposed between the profile and the second body upon installation of the shield in a suitable gap of an electronic enclosure. In one exemplary embodiment, the profile and base may be an extrusion of a polymer such as polyvinyl chloride (“PVC”), a thermoplastic resin, and the conductive layer may be a metallized fabric bonded to the profile by a heat sensitive glue. The forming and deposition processes may be separate or may be substantially contiguous. After extrusion and cooling of the profile and base, the metallized fabric may be bonded to the profile in a separate operation. Alternatively, by employing an in-line crosshead extrusion method, the polymer base and profile may be formed and immediately thereafter the metallized fabric applied as a thermally activated glue-backed tape. Resultant thermal energy in the extrusion activates the glue on the fabric side of the tape, bonding the metallized fabric to the profile. As a subsequent step in either manufacturing method, the profile may be divided into a plurality of independently flexible cantilevered or bridged fingers to compensate for variable gap width along the length of the gap. 
     Another embodiment for manufacturing a metallized fabric clad polymer EMI shield according to the invention includes disposing an electrically conductive layer on an electrically nonconductive solid sheet material and then forming the sheet into a base and a profile of a predetermined configuration. The sheet may be a polymer such as PVC, the conductive layer may be a metallized fabric bonded to the sheet by a thermally activated glue, and the profile and base may be formed by a thermal process such as thermoforming. As a subsequent step in the manufacturing method, the profile may be divided into a plurality of independently flexible cantilevered or bridged fingers. 
     According to certain embodiments of the invention, a metallized fabric clad polymer shield for shielding EMI from passing through a seam between first and second electrically conductive bodies includes a base for securing the shield to the first body, a profile of an electrically nonconductive solid material attached to the base for contacting the second body, and an electrically conductive layer disposed on the profile. 
     The base and the profile may be formed integrally of the same material by extrusion or of different materials by co-extrusion. Alternatively, the base and the profile may be similar or distinct materials joined together by bonding. Additionally, a hinge of a material exhibiting different flexural characteristics may be disposed between the base and the profile, either by co-extrusion or bonding. As used herein, the term bonding includes chemical processes such as those using glues or solvents, as well as mechanical processes such as friction welding and interlocking mechanical cross-sections. 
     To facilitate installing the EMI shield in an enclosure, an adhesive strip may be attached to the base. Alternatively, the base may include apertures for mechanical fasteners or a return for insertion in a slot or for capturing a flange of the first body. The return may also include barbing to stabilize the shield once installed. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention, in accordance with preferred and exemplary embodiments, together with further advantages thereof, is more particularly described in the following detailed description taken in conjunction with the accompanying drawings in which: 
     FIG. 1A is a schematic plan view of a portion of an EMI shield in accordance with one embodiment of the present invention; 
     FIG. 1B is a schematic sectional view of the EMI shield depicted in FIG. 1A taken along line  1 B— 1 B; 
     FIG. 1C is an enlarged view of FIG. 1B rotated counterclockwise ninety degrees and including the metallized fabric. 
     FIG. 2 is a schematic sectional view of an EMI shield disposed in a gap in accordance with a another embodiment of the present invention; 
     FIG. 3 is a schematic sectional view of an EMI shield in an uninstalled state in accordance with yet another embodiment of the present invention; and 
     FIG. 4 is a schematic block diagram representation of a method of manufacture of an EMI shield by extrusion in accordance with one embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION 
     FIGS. 1A and 1B are schematic plan and sectional views, respectively, of a metallized fabric clad polymer EMI shield  10  in accordance with one embodiment of the present invention. The shield  10  is formed from an electrically nonconductive solid material to produce a substantially planar base  12  and a generally arcuate profile  14  having an offset tip  16 . The base  12  is configured with a series of apertures  18  so that the base  12  can be secured to a first body (not depicted) by a plurality of nuts and bolts, self-tapping machine screws, rivets, or other mechanical fasteners. 
     Referring now to FIG. 1C, an enlarged sectional view of the EMI shield  10  is depicted, including an electrically conductive layer of metallized fabric  20  disposed on the profile  14 . The relative thicknesses of the fabric  20  and the profile  14  in this and other embodiments are for illustrative purposes only. In some embodiments, as will be discussed in greater detail hereinbelow, the thickness of the fabric  20  is typically substantially less than that of the profile  14 . The metallized fabric  20  is bonded to the profile  14  with the fabric portion proximate the profile  14  and the metallic portion remote therefrom. The fabric  20  extends from the base  12  proximate the aperture  18  along the profile  14  and wraps around the offset tip  16 . Accordingly, when the EMI shield  10  is compressed between first and second bodies at contact zones shown generally as arrows B 1  and B 2 , the fabric provides an electrically conductive path therebetween. Since the EMI shield  10  is secured to the first body solely at the base  12  in a bridge configuration, the tip  16  is free to slide across the first body as the profile  14  is compressed by the second body. By offsetting the tip  16  slightly, the tip  16  slides readily over imperfections in the surface of the first body. Accordingly, the profile  14  elastically deforms during compressive loading, such as when the door is closed, instead of being jammed and crushed. 
     As is apparent from the depiction in FIG. 1C, the plane of the base  12  is offset from an end of the tip  16  so that when the EMI shield  10  is secured to the first body, a predetermined elastic deformation of the profile  14  results. This preloading of the EMI shield  10  ensures that the tip  16  remains in intimate contact with the first body, thereby providing a positive electrical ground path. In other embodiments, the fabric  20  may extend along the base  12  covering the aperture  18  so that a redundant electrical ground path exists when a fastener such as an aluminum rivet passes through the fabric and aperture, securing the EMI shield  10  to the first body. In yet another embodiment, the fabric may extend further, wrapping around the base  12  so that the fabric  20  is captured between the base  12  and the first body thereby providing a more positive electrical ground path. For any of these embodiments, the fabric  20  may stop short of the tip  16  or need not wrap around the tip  16 , if desired. 
     Referring again to FIG. 1A, the profile  14  of the EMI shield  10  may be divided at regular intervals into a plurality of independently flexible fingers  22 , each extending from the common base  12 . By subdividing the profile  14  in this manner, the EMI shield  10  can provide effective shielding in a gap which varies in width along its length. Reliefs  24  may be provided at the corners of the fingers  22  to blunt the fingers  22  and minimize catching of the tips  16  on surface imperfections in the first body. 
     Instead of having a compressive arcuate profile  14 , an EMI shield  100  may have a substantially planar profile  114  attached to a substantially planar base  112  by a hinge  26  in a cantilevered configuration so that the EMI shield  100  resembles a recumbent “W” as depicted in FIG. 2. A metallized fabric  120  envelops the entire exterior surface of the EMI shield  100  and wraps around respective tips of both the base  112  and profile  114 . The base  112  may be attached to a first body  28  by a thin layer of adhesive  32  along a portion thereof to retain the EMI shield  100  in a predetermined position. The adhesive  32  may be electrically conductive. As a second body  30  moves relative to the first body  28  varying the width of a gap G formed therebetween, the hinge  26  elastically deforms to keep the profile  114  in contact therewith. The V-shaped configuration of the hinge  26  is designed to accommodate a relatively large range of motion between the first and second bodies  28 ,  30  while maintaining contact for electrical ground and shielding purposes; however, any of a variety of hinge configurations may be used. While the fabric  120  is depicted as being bonded to the entire exterior surface of the EMI shield  100 , for ease of manufacture the fabric  120  need not be bonded to the hinge  26 . A sufficient amount of fabric  120  should be provided in the area of the hinge  26 , however, so as not to restrict the full range of compression of the EMI shield  100 . 
     In this EMI shield  100 , all of the base  112 , the hinge  26  and the profile  114  are manufactured from the same polymer material; however, different materials with different material properties may be used as depicted in FIG.  3 . The EMI shield  200  includes a substantially planar profile  214  which forms an acute angle with a substantially planar base  212  in a cantilevered configuration so that the EMI shield  200  resembles a recumbent “U.” Instead of terminating at a tip, however, the base  212  includes a return  34  which forms a throat  36  to permit mounting the EMI shield  200  on a planar flange of the first body. The return includes barbing  38  directed into the throat  36  to stabilize the EMI shield  200  once installed on the flange. The return  34  could alternatively be configured for insertion into a slot in the first body. Barbing  38  extending in opposing directions from both sides of the return may be used to stabilize the EMI shield  200  in such a configuration. 
     The profile  214  is attached to the base  212  by a hinge  126 , but instead of being manufactured from the material used to make the base  212  and the profile  214 , the hinge  126  may be manufactured of a more flexible material resistant to fatigue failure to extend the life of the EMI shield  200 . A metallized fabric  220  envelops the entire exterior surface of the EMI shield  200 , wrapping around the tip of profile  214  and extending into the throat  36  to provide a electrical ground path between the first and second bodies. While the fabric  220  is depicted as being bonded to the entire exterior surface of the EMI shield  200 , the fabric  220  need not be bonded to the hinge  126 . A sufficient amount of fabric  220  should be provided in the area of the hinge  126 , however, so as not to restrict the full range of compression of the EMI shield  200 . 
     A cost effective method of manufacture of the various EMI shields is by continuous extrusion of one or more selected polymers using a screw extruder  40  having a die  42  with the desired shape of the cross-section of the shield  10  as depicted schematically in FIG.  4 . In one embodiment, the shield  10  may be manufactured from high temperature rated PVC for flame retardancy, although any electrically nonconductive material with a suitable modulus of elasticity may be employed. Flame retardancy is desirable, however, since such EMI shields can be approved for installation in safety certified electronic equipment enclosures. If a more flexible hinge is desired, a compatible material such as polyester can be coextruded with the profile and base to form the hinge. A suitable polyester hinge material is Hytrel™, available from DuPont located in Wilmington, Del. Techniques of coextrusion are well known to those skilled in the art of polymer processing. 
     Once the polymer portion of the EMI shield  10  has been formed to the desired configuration, an electrically conductive layer is disposed on the profile  14  and any other portion of the EMI shield  10  desired. One method employs a metallized fabric  20  in tape form of suitable width backed with a thermally activated glue. The glue may cover substantially the entire backing or solely portions thereof such as along the edges. In a process known to those skilled in the art as in-line crosshead extrusion, as the polymer is being extruded through the die  42 , the hot extrusion passes through a second die  44  in which the metallized fabric tape  20  is mated thereto. The thermal energy in the extrusion and the second die  44  activates the glue, bonding the metallized fabric  20  to the profile  14 . Alternatively, the metallized fabric  20  can be disposed on the extrusion in a separate operation, such as by passing the formed polymer and metallized fabric tape  20  through a heated die after the polymer has been cut to a desired length. 
     As used herein, the term metallized fabrics include articles having one or more metal coatings disposed on woven, nonwoven, or open mesh carrier backings and equivalents thereof. See, for example, U.S. Pat. No. 4,900,618 issued to O&#39;Connor et al., U.S. Pat. No. 4,901,072 issued to Morgan et al.; U.S. Pat. No. 5,075,037 issued to Morgan et al., and U.S. Pat. No. 5,393,928 issued to Cribb et al., the disclosures of which are herein incorporated by reference. Metallized fabrics are commercially available in a variety of metal and fabric carrier backing combinations. For example, pure copper on a nylon carrier, nickel-copper alloy on a nylon carrier, and pure nickel on a polyester mesh carrier are available under the registered trademark Flectron® from Advanced Performance Materials located in St. Louis, Mo. An aluminum foil on a polyester mesh carrier is available from Neptco, located in Pawtucket, R.I. Other suitable metals include silver and tin. The choice of metal is guided, in part, by installation conditions of the EMI shield. For example, a particular metal might be chosen due to the composition of abutting body metal in the enclosure to avoid galvanic corrosion of the EMI shield which could increase electrical resistance and deteriorate electrical grounding performance. Metallized tapes are desirable both for ease of application as well as durability. Deposition of a metal layer directly on the polymer profile would wear away relatively quickly due to friction when compared to the metallized fabrics. 
     Once the metallized fabric has been bonded to the EMI shield, other features can be formed. For example, the EMI shield may pass through rotary drilling, stamping, or piercing tools to divide the profile into independently flexible fingers or create apertures in the base for mounting the shield. The EMI shield can be cut to the desired length before or after these finishing operations. 
     Instead of forming the polymer and then applying the metallized fabric, according to some embodiments of the present invention, the metallized fabric can be first bonded to either flat or preformed polymer sheet stock. Then, the sheet stock can be thermoformed to a final predetermined configuration by as known by those skilled in the art. Thermoforming can be accomplished on discrete pieces of sheet stock in a heated die or continuously by passing the sheet stock through heated rollers to produce the desired configuration. Additional manufacturing steps such as generating the mounting apertures or dividing the profile into fingers can be accomplished prior to, during, or after thermoforming, as desired. 
     By relying on the metallized fabric to provide an electrically conductive path and the underlying polymer to provide elastic compliancy and resiliency, lower electrical resistance is achievable since beryllium is not required to be added to copper for toughness. As a result, the electrical conductivity and the resultant EMI shielding capability of the EMI shield according to this invention is improved over conventional copper beryllium finger strips. Further, the polymer portion of the shield is less brittle than all metal copper beryllium finger strips, resulting in reduced handling and installation damage to the EMI shield, improved safety, and extended service life. 
     In an exemplary embodiment, the thickness of an EMI shield profile according to the invention may be between about 0.13 mm (0.005 inches) or less and about 1.5 mm (0.060 inches) or more. In a preferred embodiment, the thickness of the EMI shield profile may be between about 0.25 mm (0.010 inches) or less and about 1.0 mm (0.04 inches) or more. The metallized fabric may have a thickness between about 0.1 mm (0.004 inches) or less and about 0.5 mm (0.02 inches) or more. These ranges are considered exemplary in nature and specific dimensions for a particular application would depend on the mechanical properties of the polymer material selected for the profile and hinge, the overall configuration of the EMI shield, and the electrical properties of the metallized fabric. Accordingly, values outside these ranges are considered to be within the scope of the invention. 
     While there have been described herein what are to be considered exemplary and preferred embodiments of the present invention, other modifications of the invention will become apparent to those skilled in the art from the teachings herein. For example, instead of the fingers being arcuate or planar, more complex contours such as partially twisted fingers may be created by thermoforming when the profile is divided into fingers or thereafter. Additionally, the invention is not limited to linear EMI shields, but rather includes shaped EMI shields such as those in the form of annuli and other closed or open curves required for specific applications. The particular methods of manufacture of and geometries disclosed herein are exemplary in nature and are not to be considered limiting. 
     It is therefore desired to be secured in the appended claims all such modifications as fall within the spirit and scope of the invention. Accordingly, what is desired to be secured by Letters Patent is the invention as defined and differentiated in the following claims.