Abstract:
A substantially planar, laminated electromagnetic interference shield for use around I/O ports in electronic equipment enclosures such as computer cabinets and other electronics includes a resilient foam core, a metallized fabric or cover laminated to the core, and a stiffener bonded to at least one of the core and the fabric. The shield may be manufactured on a process line which laminates and bonds the elements together, die cuts the I/O port apertures, and prints identifying indicia on the shield.

Description:
RELATED APPLICATIONS 
     This application claims priority to and the benefit of International Application PCT/US99/06389, filed Mar. 23, 1999 (published in English under PCT Article 21(2) on Sep. 30, 1999 as WO 99/49712) and U.S. Provisional Application Serial No. 60/079,030, filed Mar. 23, 1998. 
    
    
     TECHNICAL FIELD 
     The present invention relates to electromagnetic interference (“EMI”) shields and, more specifically, to an EMI shield manufactured from a foam core laminated by a metallized fabric or other electrically conductive material or covering. 
     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 and connectors, 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. 
     An area of concern in electronic enclosures such as personal computers and the like, which connect to peripheral equipment, is the zone surrounding electrical connectors and electrical connections, generally referred to as an input/output (“I/O”) panel. Cutouts and other access are provided in a bezel in the enclosure to facilitate connection of cabling which connect a computer processor to a printer, a display, a keyboard, and other related equipment. The connector sockets are typically mounted on an I/O panel back plane of a printed circuit board. As with other gaps in the enclosure, these cutouts are preferably shielded with an EMI shield. 
     Specialized EMI shields 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 and fitting of connectors. To shield EMI effectively, the shield should be capable of absorbing or reflecting EMI as well as establishing a continuous electrically conductive path across the gap in which the shield is disposed. Conventional metallic shields 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 shield, a portion of the electromagnetic field being shielded induces a current in the shield, requiring that the shield form a part of an electrically conductive path for passing the induced current flow to ground. Failure to ground the shield adequately could result in radiation of an electromagnetic field from a side of the shield opposite the primary EMI field. 
     In addition to the desirable qualities of high conductivity and grounding capability, EMI shields should be elastically compliant and resilient to compensate for variable gap widths and door operation, yet tough to withstand repeated door closure and connector installation without failing due to metal fatigue. EMI shields should also be configured to ensure intimate electrical contact with proximate structure while presenting minimal force resistance to door closure and connector installation. It is also desirable that the shield 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 shields, 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 shields 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 sufficient compression 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 compressed, 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. 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 shields avoid many of the installation disadvantages of finger strips; however, they can be relatively costly to produce due to the manufacturing controls required to realize acceptable production yields. Further, due to manufacturing tolerances and the number of apertures or slots required for the connectors on an I/O panel, adequate shielding cannot always be ensured due to unshielded gaps. 
     SUMMARY OF THE INVENTION 
     An I/O port EMI shield according to the invention overcomes many of the limitations and disadvantages of conventional EMI shields. In one embodiment, a foam core is laminated by and bonded to an electrically conductive covering, such as a metallized fabric. A generally planar stiffener is then bonded to at least one of the core and the fabric. The stiffener may have an aperture formed in the stiffener prior to bonding with the core and/or fabric. Thereafter, one or more apertures may be formed in the shield, for example by a die cutting operation, to accurately size and locate desired connector openings in the shield. A silk screen or other printing operation may be used to mark the shield with indicia, such as the type of connector or cable to be employed with a particular port. 
     In an alternative embodiment, the foam core may be substantially fully wrapped by the metallized fabric or other electrically conductive covering. The stiffener may then be bonded to the fabric, the apertures formed, and the shield printed. In another embodiment, the foam core may be bonded to the stiffener and the assembly fully wrapped by the metallized fabric. The stiffener may have an aperture formed in the stiffener prior to bonding with the core and/or fabric. In any of these embodiments, one or more adhesive strips or spots may be provided to facilitate installation of the shield into the enclosure. 
     An EMI shield according to the invention combines effective EMI shielding, due to the compressible resilient nature of the foam core, with flexibility of manufacture to accommodate a wide variety of I/O port applications. In one embodiment, the shield may be manufactured by a continuous process, along a manufacturing line. A metallized fabric or other conductive covering in the form of a roll having an adhesive laminated along one side thereof is passed over a heated plate to thermally activate the adhesive. The fabric is then mated with a foam core dispensed from a roll. By initially adhering the foam to the fabric, stretching of the foam core is minimized. 
     Next, the foam and fabric are mated with the stiffener and passed through a folding die to wrap the fabric around at least opposite edges of the stiffener. In one embodiment, the stiffener may have an adhesive laminated to the side disposed proximate the foam core. In another embodiment, the stiffener may have an aperture formed prior to attaching the foam and fabric. The entire assembly is then pulled through a heated forming die to complete bonding of the fabric to the stiffener and the stiffener to the foam. The shield may then be passed through a rotary die cutter to form the desired apertures in the shield. The shield may also be passed through a silk screen or other printing process to label the apertures and the shield with suitable indicia, as desired. Alternatively, the stiffener could be pre-printed. Also, an adhesive tape or spotting may be applied to a front or rear face of the shield to facilitate installation in the enclosure. 
    
    
     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. 1 is a schematic plan view of an EMI shield in accordance with one embodiment of the present invention; 
     FIG. 2A is a schematic cross-sectional view of the EMI shield depicted in FIG. 1, taken along line  2 A— 2 A, in accordance with one embodiment of the present invention; 
     FIG. 2B is a schematic cross-sectional view of an EMI shield in accordance with an alternative embodiment of the present invention; 
     FIG. 2C is a schematic cross-sectional view of an EMI shield in accordance with another embodiment of the present invention; 
     FIG. 3 is a schematic view of a process line for manufacturing an EMI shield in accordance with an embodiment of the present invention; 
     FIG. 4 is a schematic cross-sectional view of an EMI shield in combination with a connector in accordance with an embodiment of the present invention; 
     FIG. 5 is a schematic cross-sectional view of an EMI shield in accordance with another embodiment of,he present invention; and 
     FIG. 6 is a schematic plan view of an EMI shield in accordance with yet still another embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION 
     Referring to FIG. 1, shown is a plan view of an I/O port EMI shield  5 . When the I/O port EMI shield  5  is constructed in accordance with the teachings of the present invention, the shield  5  can be used for sealing the ingress and egress of EMI transmissions to and from electronic equipment at connector apertures formed therein. The shield  5  is a lamination of a metallized fabric  15 , a foam core  30 , and a stiffener  10 . One or more apertures can be cut in the shield  5  to allow connection of support equipment or peripherals to the electronic equipment, such as a computer mouse connector aperture  25 , a computer monitor aperture  35 , a computer printer aperture  40 , a peripheral “A” aperture  50 , and a peripheral “B” aperture  45 . In addition, adhesive strips  20  may be attached to the shield  5  to facilitate installation of shield  5  in the electronic equipment. 
     FIG. 2A shows a cross-sectional view of FIG. 1, which illustrates the construction of the shield  5  according to one embodiment. The relative sizes and thicknesses of the various elements of the shield  5  in this and other figures are merely to facilitate depiction and should not be considered limiting. In this embodiment, the foam core  30  is attached to the stiffener  10 . The metallized fabric  15  is wrapped around the foam core  30  and around portions of the stiffener  10  along opposite edges thereof and bonded thereto. Optionally, conductive adhesive strips  20  may be attached to the metallized fabric  15  proximate the stiffener  10  to facilitate installation and form a conductive path between the shield  5  and the equipment enclosure. 
     Referring to FIG. 2B, shown is a cross-sectional view of another embodiment of the present invention. In FIG. 2B, the foam core  30  is substantially completely surrounded by the metallized fabric  15 . The stiffener  10  is then attached to the metallized fabric  15 , preferably covering the split line of the fabric  15 . Optionally, adhesive strips  20  are then attached to the stiffener  10 . 
     FIG. 2C shows a cross-sectional view of another embodiment of the present invention. In FIG. 2C, the foam core  30  is attached to the stiffener  10 . The metallized fabric  15  is then wrapped around both the foam core  30  and the stiffener  10 . Optionally, adhesive strips  20  are attached to the metallized fabric  15 . 
     FIG. 3 is a schematic diagram of one embodiment of a process line for manufacturing an I/O EMI shield. A roll of metallized fabric  204  provides a continuous source of metallized fabric  205  which passes over a heater  210  activate an adhesive backing thereon. The metallized fabric  205  is then passed through a nip formed between a pair of rollers  215 , where the metallized fabric  205  is mated with a foam core  200  supplied by a roll of foam  201 . The metallized fabric  205  and foam core  200  are then bonded together in a bonding die  220  which may wrap the fabric  205  wholly or partially about the core  200 . A roll  226  or other source is used to supply stiffener material  225 , which is passed through a folding/bonding die  230 , where the stiffener material  225  is bonded to the previously bonded fabric  205  and/or foam core  200 . A roll  236  of adhesive strips  235  may be used to feed one or more adhesive strips  235  to a nip formed between a pair of traction rolls  240  where the adhesive strips  235  are bonded to the stiffener  225  or metallized fabric  205  to form the I/O EMI shield  250 . A rotary die cutter  245  may be used to form one or more apertures  251 ,  252  in the EMI shield  250 . The I/O EMI shield  250  may be passed through a printer  255  for marking the EMI shield  250  to indicate connector types for the connector apertures  251 ,  252 . The exact order and type of manufacturing process steps may vary, depending on the configuration of EMI shield sought to be produced. For example, the stiffener  225  can be prepunched prior to attaching the metallized fabric  205  and foam core  200 , with the fabric  205  and core  200  being cut thereafter to form apertures therein of similar or dissimilar size. Alternatively or additionally, the stiffener  225  can be attached first to the foam core  200 , then subsequently be partially or fully wrapped by the fabric  205 . 
     FIG. 4 shows an aperture  190  formed in an I/O shield  108  having a connector  180  passing therethrough. The aperture  190 , which may be of substantially uniform size, is formed by cutting material from the stiffener  110 , the foam core  130 , and the metallized fabric  115 . The connector  180  can then be passed through the aperture  190 , such that a connector flange  185  abuts the metallized fabric  115 , compressing the foam core  130 , as necessary, to compensate for variability in axial location of the connector  180  and to provide a conductive electrical path between an outer ground shield of the connector  180  and the I/O shield  108 . 
     Referring now to FIG. 5, shown is another embodiment of an I/O EMI shield  109  having a connector  100  passing therethrough. In this embodiment, the stiffener  10  may be prepunched to form a stiffener aperture  105  having a first size. An aperture formed in the foam core  30  and the fabric  15  may be cut to a different size. For example, in the embodiment depicted, the aperture formed in the foam core  30  and fabric  15  is smaller than the aperture  105  formed in the stiffener  10 , such that a portion  60  of the foam  30  and a portion  65  of the fabric  15  covers at least a portion of the aperture  105  formed in the stiffener  10 . The unsupported overlapping foam  60  and fabric  65  may be configured to deform into the aperture  105  formed in the stiffener  10  to provide improved EMI shielding along a perimeter of the connector  100  once the connector  100  is inserted therethrough. 
     FIG. 6, shows a plan view of an I/O port EMI shield  300  for use with I/O cards such as those typically used in a computer. In this embodiment, the shield  300  is configured to form a plurality of adjacent I/O board slots  305  or apertures to accept a common number of I/O cards. The shield  300  includes a foam core, stiffener  310 , metallized fabric  315 , adhesive tape  320 , and adhesive spots  319 . The shield  300  may be manufactured according to any of the cross-sectional configurations discussed hereinabove, and variants thereof. 
     As used herein, the term aperture is meant to cover all sizes and shapes of holes, gaps, slits, openings, and other penetrations through the layers of the laminated EMI shields, including open ended slots, such as those depicted in FIG.  6 . 
     In general, according to the invention, the core is a fire retardant urethane foam, although other resiliently compressible cores may be used. For example, the core may be manufactured from other types of urethanes, thermally formable foams, or thermoplastic elastomer foams. Silicones, gels, natural or synthetic rubbers, and gas filled bladders may also be employed. 
     As used herein, the term electrically conductive covering is meant to cover all manner of electrically conductive structure capable of at least partially surrounding a stiffener or foam core, including metallized fabrics, foils, conductive polymers, flexible conductive ceramics, and the like. The term metallized fabrics includes 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., U.S. Pat. No. 5,082,734 issued to Vaughn, 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, copper on a nylon carrier, nickel-copper alloy on a nylon carrier, and 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, tin, zinc, palladium, gold, and platinum. Electrically conductive paints could also be used, as well as metallic vapor depositions. 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 to the foam core as well as durability. One method of manufacture employs the metallized fabric 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. 
     Any of a variety of stiffener materials may be utilized, including sheet metals, moderate to high durometer natural or synthetic rubbers, polymers such as polytetrafluoroethylene and polyvinyl chloride, and thermoplastic elastomers. In one embodiment, the stiffener is a polymer such as Lexan® resin, available from General Electric Company, Pittsfield, Mass. A relatively thin layer, such as 0.4 mm (0.017 inches) in thickness may be employed to provide structural integrity to the EMI shield. One function of the stiffener is to maintain dimensional stability of the EMI shield during both forming of the I/O port apertures and installation of the shield in the electronic enclosure. Another function of the stiffener is to provide a tough resilient support which guards against penetration of the electronic enclosure by a screwdriver or other tool during installation of electrical connectors to the I/O ports as well as the fingers of a user or technician. The stiffener also provides a smooth, flat surface on which indicia can be printed, labels may be bonded, or in which other markings may be embossed. 
     In one embodiment, the EMI shield exhibits a resistivity of less than about 0.2 ohms/cm 2  to about 5 ohms/cm 2 , or more. The thickness of the EMI shield may be between about 2.0 mm (0.080 inches) or less and about 3.2 mm (0.125 inches) or more. Length and width may be any suitable dimension, for example from about 38 mm (1.5 inches) or less to about 160 mm (6.25 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. The metallized fabric may overlap opposite edges of the stiffener from between about 2.5 mm (0.10 inches) or less to about 6.4 mm (0.25 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 foam core, the overall configuration of the EMI shield, the location tolerance of the connectors, and the electrical properties of the metallized fabric. Accordingly, values outside these ranges are considered to be within the scope of the invention. Further, the fabric may include one or more drain wires passing therethrough, for example, in a criss-cross pattern, to further facilitate grounding. 
     Manufacturing laminated I/O port EMI shields in accordance with FIG. 2A on an in-line process utilizes less metallized fabric than fully laminated designs, resulting in cost savings. Tight manufacturing tolerances and superior product appearance may also be maintained. Due to the compliant resilience of the foam core in combination with the structural rigidity of the stiffener, the EMI shield is easier and safer to install than metal shields, affords improved electrical grounding to the enclosure, and provides superior shielding effectiveness by conforming readily to variable width gaps and connector locations. 
     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. The particular methods of manufacture of and geometries disclosed herein are exemplary in nature and are not to be considered limiting. 
     For example, an extrudable polymer stiffener may be formed in situ on the fabric or foam core as the shield is produced. In another embodiment, all four edges of the shield can be wrapped by the fabric. In yet another embodiment, the thickness of the foam or other core material may vary as a function of location, so that the shield is thicker in one region than another to accommodate gaps of different thickness in the enclosure and connector locations. As will be readily appreciated, the EMI shield can be used effectively with elements other than connectors, such as cables and the like, when size and spatial location at installation can vary. 
     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.