Abstract:
A gasket device for shielding electromagnetic radiation is comprised of several alternating areas of two different elastomeric materials, one of which is conductive. The areas of conductive material are substantially thinner than the other areas and provide a conductive path from one side of the gasket to the other.

Description:
SPECIFICATION 
     This nonprovisional application claims priority to United States Provisional Application Ser. No. 60/112,333 filed Dec. 15, 1998, which is hereby incorporated by reference. 
    
    
     TECHNICAL FIELD OF INVENTION 
     The present invention relates generally to the shielding of electronic devices from electromagnetic interference and in particular, to a gasket which minimizes the amount of conductive material necessary to provide an effective electromagnetic seal. 
     BACKGROUND OF INVENTION 
     In the area of electromagnetic interference shielding, particularly for the computer industry, a problem has arisen. With reference to FIG. 1, on the backside of a typical personal computer or business small workstation  2 , is a relatively large opening covered with panel  4  to accept necessary connections, e.g., mouse, keyboard, printer, telephone line, etc. This panel presents extraordinary problems to the electromagnetic interference shielding engineer due to the variety of openings or seams which can radiate energy from within the device to the environment. 
     FIG. 2 is an overhead cut-away view of the back of a typical computer. An intermediate plate  7 , usually light-gauge stainless steel, is sandwiched between the frame of the case  14  and the connectors  10  which interface with the motherboard  17 . Due to irregularities in the frame  14 , the stainless plate  7 , and the connectors  10  themselves, a number of gaps and slot openings are created which tend to leak electromagnetic energy into the room. In the last few years, with the frequencies of the emissions becoming greater and greater, the need to shield these gaps and slot openings has increased. The conventional nickname which describes this area is the input/output plane, hereafter referred to as the “I/O backplane.” 
     The most popular fix at the current time is to seal this area with a gasket of the type illustrated in FIGS. 3-5. The gasket is comprised of thermoplastic foam  18  which has been wrapped with a conductive fabric  21 . The gasket is in a strip form with a width approximately equivalent to the width of the I/O backplane opening. The gasket thickness is approximately ⅛ inch, or enough to make contact between the connectors and the intermediate stainless steel plate. The holes  24  in the fabric strip shown in FIG. 4 are die-cut away so as to allow the connectors to be accessible to the end user. The termination to ground is made by the fabric  21  contacting the connector base, and grounding to the intermediate plate  7 . The intermediate plate  7  must make contact to the frame  14 , and this is usually achieved by several stainless steel fingers or divets  15  which protrude off the edges of the intermediate plate  7  and rub against the inner wall of the frame  14 . 
     The problems or shortcomings of the current approach to the problem are as follows: 
     1. The ground between the intermediate plate and the gasket allows energy to travel along the interface and can reach the end of the gasket which has an open slot. Energy can leak at either end and result in a concentration of energy, or an antennae effect. Presumably, this can also occur at the die-cut holes, since the action of making the hole exposes the non-conductive foam core. That is, the die-cut hole is another antennae. 
     2. The construction of the fabric over thermoplastic foam is only fair in its physical performance attributes. The foam core is prone to compression set, particularly in the case of urethane foam. After as little as thirty days in compression, the urethane can take up to a 50% set at room temperature. Furthermore, the fabric imparts a stiffness to the strip which increases the deflection force needed to compress the gasket. 
     3. Multiple manufacturing steps must be performed. The fabric is woven, and subsequently plated with conductive metal. The fabric must then be slit to the proper width. The foam portion of the gasket is made by many manufacturers in a separate step. Usually, the fabric is wrapped around the foam separately with an adhesive. Subsequently, the gasket has an outer pressure-sensitive adhesive applied which provides the end user a means to fasten the gasket to the intermediate plate. 
     4. The intermediate plate is relied upon to provide attachment and stability to the frame. Unfortunately, the intermediate plate itself is somewhat flimsy and not always the best support to handle the compressive load of the gasket. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, thin strips, or bars, of a conductive elastomeric material are interposed in a primary gasket material to provide a conductive path between conductive surfaces on either side of the gasket where the conductive bars are exposed. 
     In accordance with a further aspect of the invention, the gasket is generally planar and portions of the primary gasket material are cut out to permit connectors to pass through as is desirable in a gasket for an I/O backplane. The cut out portions expose the conductive bars to the connector thereby providing a conductive path to ground. 
     It is therefore an object of the present invention to provide a flexible conductive gasket for providing effective electromagnetic interference shielding while minimizing the amount of expensive conductive material used. 
     It is a further object to provide a flexible conductive gasket which solves the above-mentioned problems with the prior art. 
     For a better understanding of the present invention, together with other and further objects, reference is made to the following description, taken in conjunction with the accompanying drawings and its scope will be pointed out in the appended claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an elevation view of the backside of a typical personal computer or business small workstation; 
     FIG. 2 is a plan cross-sectional view of a typical personal computer or business small workstation; 
     FIG. 3 is a plan view of a prior art gasket; 
     FIG. 4 is a plan view of the prior art gasket of FIG. 3 with connector-receiving cut outs; 
     FIG. 5 is an end cross-sectional view of the prior art gasket of FIG. 4; 
     FIG. 6 is a plan view of one embodiment of the present invention; 
     FIG. 7 is a plan view of the inventive gasket of FIG. 6 with connector-receiving cut outs; 
     FIG. 8 is an end cross-sectional view of the inventive gasket of FIG. 7; 
     FIG. 9 is a perspective view of another embodiment of the present invention; and 
     FIG. 10 is a side elevation cross-sectional view of the gasket of FIG. 9 in an enclosure. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     With reference to FIGS. 6-8, according to the present invention, a gasket is made primarily from an easily compressible extruded foam or sponge  27  with intermittently spaced conductive bars  30   a ,  30   b ,  30   c  and  30   d . In view of the above-described shortcomings of thermoplastic foam, the primary gasket material is preferably a nonconductive thermoset rubber, such as a silicone, which exhibits superior compression set resistance. Superior compression set resistance aids in long term performance, since most every electromagnetic interference gasket construction relies on compressive load for maximum performance. Suitable silicone materials include commercially available heat curable sponge silicones such as C-1492 available from Wacker Chemie. The following is an illustrative formula for the primary gasket material using C-1492: 
     
       
         
               
               
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 Wacker C-1492 
                 97.5-99% 
                 by weight 
               
               
                   
                 Dicumyl peroxide 
                 1-2% 
                 by weight 
               
               
                   
                 tert-Butyl perbenzoate 
                 0-0.5% 
                 by weight 
               
               
                   
                   
               
             
          
         
       
     
     Alternatively, one may formulate a suitable primary gasket material from low vinyl silicones such as KE-76BS, available from Shin-Etsu, Inc., to which a suitable blowing agent, such as an azodicarbonamide available from Ritchem Co., Inc. is added. The following is an illustrative formula using these materials: 
     
       
         
               
               
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 KE-76 BS 
                 80% 
                 by weight 
               
               
                   
                 Ritchem blo-foam CF-910 
                 2-4% 
                 by weight 
               
               
                   
                 Dicumyl peroxide 
                 0.8-1.6% 
                 by weight 
               
               
                   
                 Cab-O-Sil (Cabot) M-350 
                 14.4-17.2% 
                 by weight 
               
               
                   
                   
               
             
          
         
       
     
     Other thermoset materials which might be utilized as the primary gasket material include fluorosilicone, EPDM, nitrile, epichlorohydrin, natural, chloroprene, isoprene, and butyl elastomers. Depending on the needs of a particular application, any of the above materials could be used as sponged or foamed materials, where sponge is medium density range, and foam would be in a low density range. Also of possible use are the thermoplastic materials, such as soft PVC, polyurethane and santoprene, particularly in the foamed condition. 
     A ground through the gasket to the intermediate plate is achieved using very thin conductive rubber bars  30   a ,  30   b ,  30   c ,  30   d . As seen in FIG. 8, the conductive bars protrude through the cross-section so as to effect grounding from top to bottom, and thereby separating the segments of the compressible extruded foam or sponge  27  from one another. The thickness of the extrusion will be the typical path length to grounding between the intermediate plate and the connector&#39;s housing, thereby minimizing the conductive path and improving shielding effectiveness values. In the prior art system of FIGS. 3-5, the wave must travel completely around the circumference of the gasket to reach ground, allowing for reflection, radiation, refraction, and escape to other surfaces. Additionally, the conductive bars offer superior shielding effectiveness when compared to the conductive cloth style of product. 
     The conductive bars will preferably be a rubberized conductive metal which exhibits good electrical performance and is flexible. Any rubber binder material will suffice, and the metals can include silver, nickel, aluminum, copper and stainless steel. A silicone binder with conductive silver or nickel particles has been found to work well. The following is an illustrative formula: 
     
       
         
               
               
             
           
               
                   
               
             
             
               
                 General Electric Gum No. SE-33 
                 100 parts per hundred 
               
               
                 Technic Silver flake No. 135 
                 250 to 1,00 pph, typical 300-350 
               
               
                 Cabot Cab-O-Sil M-5 
                 5 to 30 pph, typ 8-12 
               
               
                 Union Carbide Organosilane A174 
                 0.5 to 5 pph, typ 1-3 
               
               
                 Dicumyl peroxide 
                 0.5 to 5 pph, typ 1-2 
               
               
                   
               
             
          
         
       
     
     In the preferred embodiment, the distances between the conductive bars, i.e., the widths of the extruded foam or sponge segments  27 , are a function of the size of the connector holes,  34   a ,  34   b ,  34   c ,  34   d . The bars are spaced apart such that they will straddle the holes, such spacings measuring, by way of example only, approximately one-half inch. In FIG. 7, connector hole  34   a  is straddled by conductive bars  30   a  and  30   d ; connector holes  34   b ,  34   c  and  34   d  are each straddled by conductive bars  30   a  and  30   b ; and connector hole  34   e  is straddled by conductive bars  30   c  and  30   d . This straddling of the connector holes traps and grounds any emissions which might escape out of the connector portal area. This straddling also has an added benefit of minimizing many low- to mid-frequency radiation since the spacing will, in many cases, reduce the amplitude of the traveling wave. 
     Minimization of the width of the conductive bars will improve compressibility and reduce cost. Widths in the range of 0.5 mil to 20 mils, with a width of 2 to 6 mils being preferred, provide a good tradeoff between flexure and conductivity. 
     Enhancement of the electromagnetic shielding properties of the present invention may be obtained by applying a coating of a thin, conductive layer to the ends of the strip gasket  38   a ,  38   b  so as to prevent extremely high frequency radiation from escaping between the bars. This creates a simulated “cage” effect, capturing stray radiation in the x, y and z axes. 
     The conductive bars are preferably co-extruded with the remainder of the gasket in a single pass operation. In such a system, a master die holder which brings the two materials, conductive and nonconductive, together in the raw, unvulcanized (in the case of thermoplastics, molten) state is utilized. The primary material, i.e., the nonconductive material, is extruded in the manner of a typical single extrudate, i.e., the highly viscous material is pumped through the extruder into its head zone, and then into the master die holder. The initial set of forming/shaping dies are essentially the same as used in single die plate extrusion technology which is well-known, the die shape or pattern here being a series of individual rectangles with gaps between them. Several extruder manufacturers, such as Versa-Davis Standard and Troester Co., publish literature on how to accomplish single die plate extrusion. 
     The primary, nonconductive material is shaped and pushed into a blending or transition region, under the high pressures of extrusion. Typical pressure is about 1500 psi, but the pressure can range from 300 to 10,000 psi. In the short transition region, the secondary, conductive material is introduced. The delivery of this material to the precise areas in the transition zone may be accomplished using a series of runners which direct the secondary, conductive material to the desired locations. The material is injected from top and bottom into the areas lacking primary material. The upper surfaces of the primary, nonconductive rectangles are preferably protected by a series of blocks which interlock into the forward exit plate. These interlocks help ensure a nice, clean surface on the nonconductive element. 
     The runners discussed above may have additional control valves, known in the plastics industry as “gates.” Using the control valves, the width and, to a degree, the shape and stress achieved in the finished bar can be modulated. After the blending/transition zone, the now-married fluids enter an exit plate which resembles the finished part height and width dimensions. The exit plate allows for thermal shrinkage, stress relief, and sizing considerations relating to sponges and foams. The expansion and contraction of the part as relates to the sponging reaction chemistry used should also be taken into account. A master die holder heated uniformly or by zone may also be employed. The heating enables one to begin the crosslinking reaction under the pressure conditions of extrusion which tends to improve conductivity of the finished article. Those skilled in the art of co-extrusion, for example in making and designing striping tools for the medical extrusion community, will be familiar with the above-described co-extrusion techniques. 
     Alternative methods of manufacture include a method whereby the segments are manufactured individually. The conductive segments are extruded as thin conductive strips and the nonconductive segments are extruded as rectangles, the width of the rectangles determining the spacing distance required for matching to the connector layout. The segments are then laminated together, alternating the conductive and nonconductive segments to achieve the configuration which matches the connector layout. 
     Another alternative manufacturing method would be to calendar sheets of the two materials in wide formats, then laminate the sheets together in alternating fashion. Subsequently, the laminate would be sliced across the direction of the lamination to yield the desired construction. For example, a calendared sheet of conductive material can be manufactured at a desired thickness by whatever the practical width of a good calendar mill is available. The sheet would be wound into rolls and stored. Thereafter, the same calendar mill could be used to make the nonconductive sponge or foam material in a desired thickness and matching the width of the conductive sheet. Again, the sheet would be wound into rolls and stored. Subsequently, the sheets would be taken in alternating fashion and laminated together resulting in a “log roll.” The log roll would then be sliced perpendicular to the width dimension. The slice would be of a dimension so as to fill the gap between the connectors and the plate at the I/O backplane. This method would be useful for large volume applications where, for example, one to two million units are being produced. 
     Yet another manufacturing method involves extrusion of the nonconductive segments in long lengths or reels. The nonconductive extrusions are laid out in a template or bed, side by side, leaving a distance between the extrusions which corresponds to the desired width of the conductive bars. The bed might only be several feet in length, and is used to automatically dispense a slurry of conductive material of low viscosity to fill the vacant areas. In this way, a semi-automatic, robotic system can be utilized to make the product. The finished part is then cut down to the proper size. 
     While the invention has so far been described in conjunction with gaskets intended to address the particular problems of the I/O backplane of computers, the invention may also be used with smaller sealing gaskets, such that a traditional gasket such as a D-shaped bulb or P-shaped bulb might be replaced with a strip with one conductive bar through the center of the shape. This would offer cost savings over prior art conductive gaskets, such as disclosed in U.S. Pat. No. 4,968,854, since it would further minimize the amount of precious metal in a given shape. This embodiment of the present invention is shown in FIGS. 9 and 10. 
     FIG. 9 shows a strip gasket  41  of rectangular cross section having a conductive bar  55  running the length of the gasket. The primary gasket material  52  is on either side of the conductive bar  55 . FIG. 10 depicts the gasket  41  in an electronic enclosure  45  having a groove receptor  58  for the gasket  41 . The gasket provides a ground from the enclosure  45  to a lid  48 . 
     While this invention has been described with reference to several illustrative examples and embodiments, they should not be interpreted as limiting the scope or spirit of the invention. In actual practice many modifications may be made by those of ordinary skill in the art without deviating from the scope of the invention as expressed in the appended claims.