Abstract:
An EMI gasket ( 20 ) includes a plurality of serrations ( 34 ) along at least one edge ( 35, 36 ). The serrations ( 34 ) are arranged side-by-side so that each may be articulated individually. That is, one of the serrations ( 34 ) may be displaced significantly out of the plane of the gasket without affecting adjacent serrations. Thus, when the edge of the gasket ( 20 ) is compressed between two surfaces which include a discontinuity ( 40 ) such as a step or protuberance between them, the serration in contact with the discontinuity is displaced out of the plane of the gasket without displacing adjacent portions of the gasket. Allowing a portion of an EMI gasket ( 20 ) to be displaced out of the plane of the gasket without affecting adjacent portions of the gasket substantially reduces or eliminates gaps which occur due to the failure of the gasket to precisely follow the contour of a component.

Description:
TECHNICAL FIELD OF THE INVENTION 
     This invention relates to EMI shielding and, more particularly, to an EMI gasket structure which improves EMI shielding arrangements. The invention also encompasses a method of forming an EMI shield and a computer system employing the EMI gasket structure. 
     BACKGROUND OF THE INVENTION 
     Many types of electronic circuits, including high-frequency digital circuits, produce alternating electromagnetic fields in the course of operation. Also, an electronic circuit may be sensitive to certain electromagnetic fields arising from external sources, and may fail to operate properly within the presence of such fields. To avoid problems arising from electromagnetic fields emanating from, or incident upon, an electronic circuit, such circuits are commonly protected by an electrically conductive shield or barrier. This electrically conductive shield extends around the entire circuit and is commonly referred to as an electromagnetic interference or EMI shield. 
     Part of the electrical energy incident on an EMI shield is reflected while part of the energy induces alternating electrical currents in the material which forms the EMI shield. These alternating currents are dissipated by eddy currents in the EMI shielding material. Thus, the EMI shield attenuates the incident electromagnetic fields so that fields emanating from circuitry within the shielded area do not interfere with external circuitry. Similarly, the EMI shielding helps prevent fields emanating from outside the shielded area from interfering with the shielded circuitry. 
     EMI shielding for electronic circuitry is commonly formed by a housing or enclosure associated with the particular circuitry. In a computer system, for example, the enclosure which houses the system processor, random access memory, and related devices commonly includes structures which connect together to form an EMI shield. Other shielding may be used within the primary enclosure for shielding particular circuits in the system. Regardless of whether an EMI shield is included with a component in a shielded system or stands alone, conductive gaskets, referred to as “EMI gaskets,” may be used to help make good electrical contact between the various components which make up the shield. EMI gaskets generally include an electrically conductive material overlaying a resilient core material which is readily compressed as the pieces which make up an EMI shield are connected together. 
     FIG. 1 shows a portion of a prior art EMI gasket  10  compressed between two components which make up a portion of an EMI shield. The illustrated components comprise a base portion  11  of an input/output connector  12  associated with a computer system, and a wall  14  of the computer system chassis. Chassis wall  14  includes a cut out or opening for receiving the connector  12  so that the wall and base  11  cooperate to form a portion of the EMI shield for the computer system. In order to provide better electrical continuity between chassis wall  14  and connector base  11 , EMI gasket  10  is compressed between the two components as they are connected together. 
     However, FIG. 1 shows that a prior art EMI gasket may leave a gap  15  where a discontinuity in the components prevents the two components from perfectly abutting each other. Gaps between an EMI shielding component such as base  11  and the EMI gasket  10  may allow shorter wavelength electromagnetic energy to penetrate the shield. Such gaps are a particular problem in digital circuits which operate at high clock rates due to the short wavelength electromagnetic energy which may emanate from these circuits. At higher clock rates, clock rates over 300 MHz for example, even small gaps in the EMI shielding may be unacceptable. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to provide an EMI gasket which reduces gaps between components intended to cooperate to provide an EMI shield. It is also an object of the invention to provide a method for reducing or limiting gaps in an EMI shield. Another object of the invention is to provide a computer system with improved EMI shielding. 
     An EMI gasket according to the invention includes a plurality of serrations along at least one edge. The serrations are arranged side-by-side so that each may be articulated individually. That is, one of the serrations may be displaced significantly out of the plane of the gasket without affecting adjacent serrations. Thus, when the edge of the gasket is compressed between two surfaces which include a discontinuity such as a step or protuberance between them, the serration in contact with the discontinuity is displaced out of the plane of the gasket without displacing adjacent portions of the gasket. Allowing a portion of an EMI gasket to be displaced out of the plane of the gasket without affecting adjacent portions of the gasket substantially reduces or eliminates gaps which occur due to the failure of the gasket to precisely follow the contour of a component. 
     In the preferred form of the invention the gasket includes a resilient material which may form a core for the gasket. An electrically conductive material is associated with the resilient material and preferably comprises a cover which substantially covers the resilient core material. Serrations along an edge of the gasket are formed by making a number of spaced apart cuts along the gasket edge. Each cut extends substantially normal to the respective edge in which the cut is formed. 
     The method of forming an EMI shield according to the invention comprises placing the serrated EMI gasket between an electrically conductive first surface and a second surface. The method then includes compressing the gasket between the surfaces. As the gasket is compressed, at least one serration is displaced from the plane of the gasket over a protuberance on the first surface. Adjacent serrations which do not contact the protuberance remain in the plane of the gasket with no gap forming between the adjacent serrations and the first surface. 
     An EMI gasket according to the invention is particularly useful in a computer system using high clock rates. A computer system according to the invention includes a first structure and a second structure, at least one of which forms part of an EMI shield for the computer. A serrated EMI gasket is interposed between the first structure and second structure to form a good EMI shield despite any protuberances or other discontinuities which may be present on the structure forming part of the EMI shield. The serrations reduce or eliminate the gaps which the discontinuity would otherwise cause between the gasket material and the surface of the structure forming part of the EMI shield. 
     These and other objects, advantages, and features of the invention will be apparent from the following description of the preferred embodiments, considered along with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a view in section of a prior art EMI gasket, showing a gap resulting from a discontinuity in one of the conductive surfaces abutting the gasket. 
     FIG. 2 is an exploded view in perspective showing a printed circuit board, a computer chassis, and an EMI gasket embodying the principles of the invention. 
     FIG. 3 is a view in perspective similar to FIG. 2, but showing the printed circuit board and computer chassis in a connected position sandwiching the EMI gasket. 
     FIG. 4 is an enlarged view in perspective of a portion of the EMI gasket shown in FIG.  2 . 
     FIG. 5 is a view in section take along line  5 — 5  in FIG. 3, showing the EMI gasket embodying the principles of the invention accommodating a discontinuity between components to help reduce gaps in an EMI shield. 
     FIG. 6 is a view similar to FIG. 5, but showing the case where the serration does not align perfectly with a surface discontinuity. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIGS. 2 and 3 show an EMI gasket  20  embodying the principles of the invention, used in a computer system. The computer system includes a chassis  21  having a chassis wall  22 . The computer system also includes a printed circuit board  23  on which are amounted a number of electronic components of the computer system along with several input/output connectors  24 . Although each input/output connector is labeled with the common reference number  24 , it will be appreciated that the various illustrated connectors comprise different types of connectors including mouse and keyboard connectors, universal serial bus (USB) connectors, a monitor connector, and parallel and serial port connectors. 
     Chassis wall  22  includes a number of cutouts  25  for accommodating the input/output connectors  24 . Some of the connectors  24  may extend through the respective cutout  25 , while other connectors are recessed from the respective cutout. In any event, each input/output connector  24  includes a base portion  26  shown best in FIG.  2 . As indicated in both FIGS. 2 and 3, the printed circuit board  23  is mounted within the computer chassis  21  so that each connector  24  aligns with a cutout  25  and the connector base  26  of each respective input/output connector  24  generally abuts the inner surface of the chassis wall  22  around the respective cutout. Chassis wall  22 , together with the bases  26  of the various input/output connectors  24 , form a portion of the primary EMI shield for the computer system. EMI gasket  20  is used to help ensure good electrical continuity between each connector base  26  and the chassis wall  22 . Gasket  20  also extends to the periphery of chassis wall  22  where the wall connects with other components (not shown) of the computer system enclosure. These other components of the system enclosure continue the EMI shield around the system. 
     Referring now particularly to FIG. 4, the preferred EMI gasket  20  includes a core material  30  and a cover material  31 . Gasket openings  32  correspond generally to the cutouts  25  in chassis wall  22 . Core material  30  comprises a suitable resilient material such as a suitable foam rubber or plastic. Cover material  31  comprises a suitable conductive material such as a fabric formed from conductive carbon fibers or metal fibers. It will be appreciated by those skilled in the art that the illustrated resilient core/conductive cover type EMI gasket material is only one example of the types of EMI gasket materials in which the present invention may be employed. Other types of EMI gasket materials may include conductive fibers embedded in a resilient material. Still other types of EMI gasket materials may comprise conductive materials arranged to provide the resilient or crushable structure desired in an EMI gasket material. Each of these EMI gasket materials is to be considered equivalent to the resilient core/conductive cover type gasket material shown for purposes of example in the accompanying figures. 
     According to the invention, EMI gasket  20  includes a plurality of serrations  34  along at least one edge. In the illustrated form of the invention, each edge of gasket  20  includes serrations  34 , including both outer edges  35  and the inner edges  36  of openings  32  in the gasket. Serrations  34  are arranged in a side-by-side fashion and are formed by a plurality of spaced apart cuts  37 . Each cut  37  preferably extends substantially normal to the edge in which it is formed, although the cuts could be formed at other angles to the respective edge. As will be discussed below with respect to a specific example, each serration  34  is independently articulating. That is, each serration  34  may be moved laterally with respect to the plane P of the gasket. Plane P, shown both in FIGS. 4 and 5, may be defined for purposes of this disclosure and the accompanying claims as a plane bisecting the gasket through the center of the gasket material. In the illustrated case, plane P extends through the center of the resilient core material  30 . 
     The ability of EMI gasket  20  to prevent or reduce gaps in an EMI shield may now be described with particular reference to FIG.  5 . FIG. 5 comprises a section view taken along line  5 — 5  in FIG. 3, and shows an input/output connector  24  extending through a cutout  25  in chassis wall  22 . Gasket  20  is compressed between connector base  26  and chassis wall  22 . Gasket  20  is shown in FIG. 5 as a monolithic material to simplify the drawing. It will be appreciated that gasket  20  may still comprise a core and separate conductive cover. Also, section lines are omitted from gasket  20  in FIG. 5 so that cuts  37  are more readily seen. 
     Similarly to FIG. 1, described above to show the deficiencies of prior art EMI gaskets, connector base  26  includes a discontinuity or protuberance  40 . Due to this physical discontinuity  40 , the surface of connector base  26  and the inner surface of chassis wall  22  cannot abut each other perfectly. However, the group of serrations  34  residing along the length of the discontinuity  40  can be displaced somewhat out of the plane P of the gasket to accommodate the discontinuity. This group of serrations  34  is shown generally at reference letter D in FIG.  5 . In this example, each of the serrations  34  in group D is compressed more than adjacent serrations, and this additional compression moves the serrations in the group out of plane P. According to the invention, even though the group D of serrations  34  are displaced out of the plane P of the gasket, the serrations shown at reference letter E at the ends of discontinuity  40 , remain flush against both chassis wall  22  and connector base  26 . The ability of the serrations  34  to articulate independently of adjacent serrations allows the serrations to follow the contour of the discontinuity  40 . Thus, no gap is formed on either side of the discontinuity  40 . This is in contrast to gaps  15  which result from the prior art EMI gasket  10  shown in FIG.  1 . 
     The width of serrations  34  in an EMI gasket  20  according to the invention may be optimized for a particular application. Also, a single gasket  20  may include serrations of different widths. In any application, serration widths are chosen so that any gap in the EMI shield is less than one twentieth of the wavelength of the highest frequency in the shielded area. This maximum allowable gap may be accomplished in some applications with relatively wide serrations depending upon the frequencies employed by the circuitry in the shielded area and the nature of the enclosure. Generally, narrower serrations  34  are required for use in forming shields for higher frequencies. For example, serrations having a width of approximately 4 millimeters may be suitable for use with circuits which may emit electromagnetic radiation at frequencies on the order of 1 GHz. 
     The spacing between adjacent cuts  37  and resultant width of the serrations  34  is preferably chosen so that a cut will either directly align with or be very near the edge of the discontinuity, regardless of where discontinuity may be found. If cuts  37  are spaced too widely apart leaving the serrations  34  too wide, a cut may not align closely with a surface discontinuity. This failure of a cut  37  to align perfectly with an edge of a discontinuity may cause a small gap  42  at the discontinuity as shown in FIG.  6 . However, gap  42  is truncated by the next adjacent serration  34 . Thus, a gap resulting from the failure of a cut  37  to align perfectly with the edge of a discontinuity will still be smaller than a gap such as gap  15  shown in FIG.  1 . Furthermore, each serration  34  may articulate somewhat in the plane P of the gasket. In some cases this articulation may allow a cut  37  to be placed over discontinuity, thereby eliminating any gap between the EMI gasket material and the structure abutting the gasket. 
     The length of a particular cut  37  is preferably chosen based on the location of a discontinuity to be sealed by the gasket. For example, if a discontinuity is known to exist at a point located an inch from the edge of gasket  20 , the cut  37  at that point in the periphery of the gasket should extend into the gasket material at least an inch from the gasket edge. However, if discontinuities between the structures intended to be sealed by gasket  20  are known to include discontinuities only very near the edge of the gasket, then the cuts  37  may be relatively short, but long enough to extend past the area where discontinuities may be present. Deeper or longer cuts  37  generally allow the resulting serrations  34  to be displaced more easily to accommodate a discontinuity such as that shown in FIG.  5 . However, cuts  37  should not extend into the gasket material so far as to substantially interfere with the conductive properties of the gasket material. In any event, EMI gasket  20  is constructed so that there is sufficient gasket material at each edge to accommodate cuts  37  of the desired length. 
     The above described preferred embodiments are intended to illustrate the principles of the invention, but not to limit the scope of the invention. Various other embodiments and modifications to these preferred embodiments may be made by those skilled in the art without departing from the scope of the following claims. For example, the EMI shield interface between connector base  26  and a computer chassis wall  22  is shown only for illustrating the invention. The EMI gasket structure according to the invention is also helpful in eliminating gaps which may occur between two walls which fasten together to form a portion of an EMI shield. Furthermore, an EMI gasket within the scope of the invention may itself form a primary portion of an EMI shield rather than simply provide a good seal between components which form an EMI shield. To illustrate this point, chassis wall  22  in the figures could be formed from a nonconductive material. In this case, gasket  20  and the connector bases  26  would form the desired portion of the EMI shield.