Abstract:
An electromagnetic interference (EMI) gasket system which segregates EMI sensitive circuits from EMI generating circuits is disclosed. The gasket system includes a pair of deflectable leg members disposed relative to each other, which deflect vertically and horizontally when said gasket system is positioned between two opposing substrates and the substrates are drawn together. The leg members provide dual contact with one of the substrates to ensure EMI attenuation without any compression of said gasket.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates generally to providing electromagnetic interference (EMI) isolation of sensitive circuit areas on a circuit board and more particularly to a deflection-type gasket for providing EMI isolation by substantially attenuating EMI. 
     EMI is often encountered in the design of electronic circuits. Semiconductor devices such as metal oxide semiconductor field effect transistors (MOSFETs) and bipolar junction transistors (BJTs), which are widely used in electronic circuits, usually operate at frequencies ranging from several kilohertz to gegahertz. Unfortunately, operation at such frequencies produces EMI, which may cause malfunction in electronic equipment. 
     For example, an electronic device may contain several circuits some of which may generate EMI and others which do not generate EMI. Circuits which do not generate EMI may be adversely affected when EMI propagates to those areas. Circuits which generate EMI are herein referred to as EMI generating circuits and circuits which are affected by EMI propagation are herein referred to as EMI sensitive circuits. 
     EMI propagation between the EMI generating circuits and the EMI sensitive circuits is known as cross talk. Because it is necessary to prevent cross talk, both EMI generating circuits and EMI sensitive circuits are commonly shielded, and thus isolated from each other. 
     In one existing shielding arrangement, metal shield cans or lids are mounted on the printed circuit (PC) board to isolate the EMI generating circuits from the EMI sensitive circuits. 
     In another shielding arrangement, circuits which are susceptible to cross talk are fabricated on different circuit boards. The circuit boards are then mounted on a circuit board holder formed into a plurality of cells, each cell retaining a single circuit board. The circuit boards are thus isolated from each other in different cells of the circuit board holder. 
     These structures, while providing isolation between circuits susceptible to cross-talk, are complex, requiring additional material and manufacturing processes and are, therefore, costly to manufacture. Furthermore, they are inefficient in utilizing scarce space available on the circuit board and require the product into which they are incorporated to be larger to accommodate these larger, more complex structures. Efficient use of space can be a vital concern in some applications such as wireless communications and cellular telephony where smaller, more compact units or equipment are desired. 
     One prior art arrangement for isolating EMI generating circuits and EMI sensitive circuits is disclosed and claimed in U.S. Pat. No. 5,252,782 which describes a single board EMI isolation system comprising a “clam shell” to enclose either the EMI generating circuits or the EMI sensitive circuits or both on the circuit board. A simple compressible conductive gasket is positioned around the perimeter of the circuit to be isolated between the rim of the clam shell and the surface of the circuit board to enhance the EMI isolation. 
     While the above structure provides efficient utilization of space on the circuit board, it may fail to provide a satisfactory level of EMI attenuation because the circuits to be isolated must be substantially completely sealed off to prevent cross-talk. This is achieved by tightly securing the clam shell to the circuit board which could result in overly compressing the gasket between the rim of the clam shell and the circuit board. Typically, a plurality of fasteners are used to tightly seal such a clam shell and gasket configuration to a circuit board to provide effective EMI attenuation. If care is not taken during assembly, the fasteners may be over tightened or may require excessive tightening to provide an effective seal, causing the gasket to be overly compressed, and even possible deformation of the circuit board. In addition, excessive tightening of the gasket may result in a phenomenon known as “compression set” wherein the gasket permanently assumes a compressed state and loses its resilient property. These adverse conditions may result in gaps in the EMI shielding formed by the clam shell and gasket combination and resultant leakage of EMI between the EMI generating and EMI sensitive circuits. 
     Known prior art EMI isolation systems of the type just described utilize a gasket designed to be compressed to seal off the circuit areas susceptible to cross-talk and to achieve satisfactory attenuation of EMI propagation. These systems, however, only provide a single location or line of sealing contact where the gasket is compressed between the clam shell and the circuit board which can result in leakage or ineffective attenuation of EMI propagation from the EMI generating circuits to the EMI sensitive circuits, if not assembled correctly as just described. 
     In view of the foregoing disadvantages, there is a need for a new gasket structure which achieves a satisfactory level of EMI attenuation with a simple structure that is easy to assemble and less expensive to manufacture, and which does not cause compression of the gasket or deformation of the circuit board. 
     SUMMARY OF THE INVENTION 
     It is, accordingly, a primary object of the present invention to provide a novel gasket system for substantially attenuating the propagation of EMI from the EMI generating circuits to the EMI sensitive circuits on a circuit board, and which is not subject to the foregoing disadvantages. 
     It is another object of the present invention to provide a gasket system which substantially attenuates the propagation of EMI without requiring compression of the gasket, and thereby prevents any deformation of the circuit board. 
     It is yet another object of the present invention to provide a gasket system which requires a fewer number of fasteners to tightly seal off a desired circuit area and satisfactorily attenuate EMI than prior art gaskets. 
     In accordance with the present invention, a gasket system to substantially attenuate the propagation of EMI includes a pair of deflectable leg members disposed relative to each other to cause the leg members to deflect horizontally and vertically when the gasket system is positioned between two opposing substrates and the substrates are drawn together so that the leg members provide dual contact with one substrate. 
     In one embodiment of the present invention, a means is provided to limit the peak deflection of the leg members. In another embodiment of the present invention, a means for retaining the gasket system in a predetermined position on one of the substrates is provided. 
     These and other objects of the invention, together with the features and advantages thereof, will become apparent from the following detailed specification when read with the accompanying drawings in which like reference numerals refer to like elements. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an exploded perspective view of an unassembled electronic device including a gasket system in accordance with the present invention. 
     FIG. 2A is a detailed cross-sectional view of the gasket system in accordance with one embodiment of the present invention prior to final assembly and deflection of the leg members. 
     FIG. 2B is a detailed cross-sectional view of the gasket system of FIG. 2A after final assembly showing the deflection of the leg members in their operable position. 
     FIG. 3 is a detailed cross-sectional view of the gasket system in accordance with another embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring initially to FIG. 1, an enclosure assembly indicated generally by a reference numeral  10  for enclosing a circuit board  12  includes an enclosure base  14  and a cover shield  16 . EMI generating circuits  17  can be fabricated in an area  18 , while EMI sensitive circuits  19  can be fabricated in an area  20 . The circuit board  12  can be divided into several EMI generating circuit areas  18  and EMI sensitive circuit areas  20 . A cover gasket  22  is positioned between one surface  24  of the circuit board  12  and the cover shield  16 , and is correspondingly aligned to the perimeters of the EMI generating circuit areas  18  and EMI sensitive circuit areas  20 . A conductive trace  28  referred to herein as a “ground track” is fabricated on the circuit board  12  around the perimeters of the EMI generating and EMI sensitive circuit areas  18  and  20 . 
     Referring also to FIGS. 2A and 2B, the cover shield  16  is divided into a plurality of compartments  30  by grooved retainers  34  surrounding the compartments  30 . The retainers  34  may be integrally formed on the cover shield  16 . The retainers  34  are aligned with the ground track  28  when the circuit board  12  is assembled with the cover shield  16 . They retain the cover gasket  22  in a proper position so that it is aligned with and is in contact with the ground track  28  when the enclosure  10  is assembled. 
     The compartments  30  and the cover gasket  22  separate the EMI generating circuit areas  18  from the EMI sensitive circuit areas  20  by forming enclosed shields in cooperation with the cover shield  16  around both the EMI generating and EMI sensitive circuit areas  18  and  20 . The enclosed shields substantially attenuate EMI from propagating from the EMI generating circuit areas  18  to the EMI sensitive circuit areas  20  which could adversely affect the operation of the EMI sensitive circuits. 
     An enclosure gasket  36  is positioned between an opposite surface  38  of the circuit board  12  and the enclosure base  14 . The enclosure base  14  is divided into a plurality of compartments  40  by a groove or retainers  44  around the compartments  40 . A second ground track  46  (shown in FIG. 2A and 2B) is fabricated on the opposite surface  38  of the circuit board  12 , and is aligned with the first ground track  28  on the surface  24  of the circuit board  12 . The retainers  44  which are aligned with the ground track  46  retain the enclosure gasket  36  in a proper position relative to the ground track  46  during assembly. The enclosure gasket  36  will then be positioned in proper alignment with and in contact with the ground track  46  as best shown in FIGS. 2A and 2B after final assembly. 
     The gasket  36  and ground track  46  form a shield between compartments  40  and divide the opposite side  38  of the circuit board  12  into different areas corresponding to and positioned directly opposite to the EMI generating and EMI sensitive circuit areas  18  and  20 . This structure prevents EMI propagation from the EMI generating circuit  17  in area  18  from propagating through the circuit board  12  to the opposite side  38  of the circuit board  12  and then back through the circuit board  12  to the EMI sensitive circuit  19  in area  20 . 
     The cover gasket  22  is made from a mixture of silicone and electrically conductive fibers which provide electrical contact between the cover shield  16  and the ground track  28 ; this causes the cover shield  16  to be maintained at ground potential. 
     Likewise, the enclosure gasket  36  is also made from a mixture of silicone and electrically conductive fibers to provide electrical contact between the enclosure base  14  and the ground track  46  of the circuit board  12 ; this causes the enclosure base  14  to also be maintained at ground potential. 
     Accordingly, the opposing gaskets  22  and  36  and ground tracks  28  and  46  essentially form a barrier between cover  16  and base  14  to EMI propagation. 
     The gaskets  22  and  36  each include a main body member  48  and a pair of leg members  50  and  52  disposed relative to each other. The main body member  48  is sized to be retained by retainers  34  or  44  to hold gasket  22  or  36  in a predetermined position relative to the ground track  28  or  46  to cause the leg members  50  and  52  to contact the ground track  28  or  46  at two separate points of contact  54  and  56 . 
     While the present invention has been described with respect to the retainers  34  and  44  for holding the main body member  48  of gaskets  22  and  36 , an electrically conductive adhesive  78  (shown in FIG. 3) or similar arrangement could be used as well to retain the gaskets  22  and  36  in place relative to ground tracks  28  and  46  to provide the dual contact described above. 
     Fasteners  66 , such as a bolt or the like, are screwed through bosses  68  formed on the enclosure base  14  to secure the cover shield  16  to the enclosure base  14 . Likewise, fasteners  80 , such as a bolt or the like, may be used to attach the circuit board  12  to the enclosure base  14  by threading into bosses  70 . 
     When the circuit board  12  is drawn toward the enclosure base  14  by the fasteners  80 , the leg members  50  and  52  of the enclosure gasket  36  initially come in contact with the ground track  46  as shown in FIG.  2 A. As the circuit board  12  is further drawn toward the enclosure base  14  by tightening the fasteners  80 , the leg members  50  and  52  deflect horizontally and vertically while maintaining dual contact with the ground track  46  as shown in FIG.  2 B. 
     Similarly when the fasteners  66  are tightened, the leg members  50  and  52  of the cover gasket  22  initially come in contact with the ground track  28 . As the cover shield  16  and the cover gasket  22  are further drawn toward the circuit board  12  by tightening the fasteners  66 , the leg members  50  and  52  deflect horizontally and vertically while maintaining dual contact with the ground track  28 . 
     The bosses  70  maintain a predetermined space of height h 1  between the circuit board  12  and the enclosure base  14 . As the fasteners  80  are tightened, the enclosure base  14  is drawn closer to the circuit board  12  until the ends of the bosses  70  contact the circuit board  12 . After the ends of the bosses  70  contact the circuit board  12 , the fasteners  80  cannot be tightened any further. By limiting the extent to which fasteners  80  can be tightened, the predetermined space of height h 1  is maintained between the circuit board  12  and the enclosure base  14 . As a result, the peak deflection of the leg members  50  and  52  of the enclosure gasket  36  is limited. By limiting the peak deflection of the leg members  50  and  52 , compression of the enclosure gasket  36  is eliminated, and deformation of the circuit board  12  is prevented. Furthermore, limiting the peak deflection of the leg members  50  and  52  prevents them from being overly deflected and, thus, flattened out, which allows the leg members  50  and  52  to maintain dual contact with the ground track  46 . 
     Likewise, the bosses  68  maintain a predetermined space of height h 2  between the cover shield  16  and the circuit board  12  when assembled. As the fasteners  66  are tightened, the cover shield  16  is drawn closer to the circuit board  12  until the ends of the bosses  68  contact the circuit board  12 . After the ends of the bosses  68  contact the circuit board  12 , the fasteners  66  cannot be tightened any further. By limiting the extent to which fasteners  66  can be tightened, the predetermined space of height h 2  is maintained between the cover shield  16  and the circuit board  12 . As a result, the peak deflection of the leg members  50  and  52  of the cover gasket  22  is limited. By limiting the peak deflection of the leg members  50  and  52 , the compression of the cover gasket  22  is eliminated, and deformation of the circuit board  12  is prevented. Furthermore, limiting the peak deflection of the leg members  50  and  52  prevents the leg members from being overly deflected and, thus, flattened out, which allows the leg members  50  and  52  to maintain dual contact with the ground track  28 . 
     The gaskets  22  and  36  of the present invention may be referred to as deflection-type gaskets in contrast to prior art compression-type gaskets because the gaskets of the present invention incorporate a pair of leg members  50  and  52 , which deflect horizontally and vertically as the fasteners  66  and  80  are tightened and the enclosure base  14  and cover shield  16  are drawn to their respective predetermined spacing relative to the circuit board  12 . 
     Those skilled in the art will recognize that the deflection-type gasket of the present invention does not require a high degree of clamping force on the gasket compared to compression type gaskets to provide an effective seal for attenuation of EMI. Because there is no compression of the gaskets  22  and  36  deformation of the circuit board  12  is avoided. Additionally, because deflection type gaskets  22  and  36  use much less force compared to prior art compression type gaskets, fewer fasteners are required to provide a uniform seal to satisfactorily attenuate EMI which reduces assembly time and expense. 
     More importantly, the leg members of the gaskets  22  and  36  provide dual contact at locations  54  and  56  with the ground tracks  28  and  46 . The dual contact between the gaskets  22  and  36  and the ground tracks  28  and  46  insures more uniform sealing or isolation with minimal if any formation of leaks, and therefore, better and more reliable attenuation of EMI compared to prior art gaskets or gaskets with contact at only a single location. 
     Therefore, the leg members  50  and  52  of the gaskets  18  and  36  serve two important functions: first, they provide dual contact with the ground tracks  28  and  46 , which results in substantial attenuation of EMI and secondly, they deflect horizontally and vertically, which eliminates compression of the gasket and prevents deformation of the circuit board  12 . 
     Referring to FIG. 3, in another embodiment of the invention, the fasteners  66  are received into threaded holes or receptacles  72  which are formed into the enclosure base  14  to a predetermined depth d which limits the extent to which the fasteners  66  can be tightened. By limiting the extent to which the fasteners  66  can be tightened, the receptacles  72  maintain a predetermined height h 3  between the cover shield  16  and the circuit board  12  and between the enclosure base  14  and the circuit board  12 ; this eliminates the compression of the gaskets  22  and  36 , and the deformation of the circuit board  12 . 
     Those skilled in the art will recognize that other means for maintaining a predetermined space of height h 3  between the cover shield  16  and the circuit board  12  and between the enclosure base  14  and the circuit board  12  may be implemented such as a land or collar  82  attached to the fasteners  66 . The land or collar  82  formed on the fasteners  66  can limit the extent to which the fasteners  66  may be screwed into the enclosure base  14 , thus limiting the peak deflection of leg members  50  and  52  of the gaskets  22  and  36 . Such means can effectively eliminate the compression of the gaskets  22  and  36 , and prevent the deformation of the circuit board  12 . 
     While the present invention has been described with respect to its preferred embodiments, those skilled in the art will recognize that the present invention is not limited to the specific embodiments described and illustrated herein. Different embodiments and adaptations besides those shown herein and described as well as many variations, modifications and equivalent arrangements will now be apparent or will be reasonably suggested by the foregoing specification and drawings, without departing from the substance or scope of the invention. While the present invention has been described herein in detail in relation to its preferred embodiments, it is to be understood that this disclosure is only illustrative and exemplary of the present invention and is made merely for purposes of providing a full and enabling disclosure of the invention. Accordingly, it is intended that the invention be limited only by the spirit and scope of the claims appended hereto.