Abstract:
A removable RF shield cover that snap-fit attaches to RF shield fence walls without special tools or soldering. The RF shield cover engage the interior surface of the shield wall with a tensional force that holds the RF shield cover and provides for a continuous metal-to-metal seal to isolate electronic components to reduce RF emissions or protect against electo-magnetic interference. The snap-fit attachment enables the RF shield cover to be easily removed from the fence, without special tools, to access components within the RF shield enclosure.

Description:
RELATED APPLICATIONS 
     The present invention is related to U.S. patent application Ser. No. 09/396,921, filed concurrently herewith, entitled RADIO FREQUENCY SHIELD COVER WITH INTERLOCKING FINGERS, which is assigned to the same assignee and is incorporated by reference herein. 
    
    
     FIELD OF THE INVENTION 
     This invention is related to Radio Frequency (RF) shielding to reduce Electro-Magnetic Interference (EMI) caused by RF emissions. In particular the claimed invention simplifies the attachment of an RF shield cover onto an RF shield fence. 
     BACKGROUND OF THE INVENTION 
     RF circuits are well known in the art to produce, amplify and modulate radio frequency energy. RF energy radiating from the RF circuits is also known to increase the noise level that, in turn, affects the operation of adjacent electronic circuits. For example, the operation of adjacent electronic circuits becomes erratic and unpredictable as the RF energy, and the resultant noise level, ebbs and flows from the RF circuits. This RF interference, also known as electromagnetic interference (EMI), is an extremely important factor in determining the proper performance and functionality of electronic circuits adjacent to RF circuits. Accordingly, it is necessary to confine radiating RF energy to protect sensitive electronic components and prevent significant RF energy from radiating from a product. 
     One method to protect sensitive components from RF interference is to place RF shields around critical electronic components. An RF shield is a continuous conductive structure that surrounds and encloses components to prevent RF radiation from entering, leaving, or passing through the shield. Metallic RF shields, commonly referred to as ‘shield cans,” that separate RF circuits from one another and provide isolation between signal paths are well known in the art. RF shields are generally constructed in two parts—i.e., a thin continuous metallic wall that surrounds the circuitry and a cover that extends over the RF circuitry and attaches to the continuous metallic wall. The shield wall, commonly referred to as a fence, is typically soldered to a printed circuit board, or printed wiring board (PWB), and connected to the electrical ground of the board. In one implementation, the cover is held attached to the fence with copper tape. The copper tape retains the cover to the fence and prevents RF leakage as it seals any gap between the fence and the cover. This process, however, is both expensive and labor intensive. 
     In another implementation the cover is soldered to the fence creating a uniform seal around the RF circuitry, and insuring electrical contact with the electrical ground. Soldered shield covers provide excellent isolation, however the soldered shield cover is extremely difficult to remove when components within the enclosure require servicing. An example of this fixed attachment is U.S. Pat. No. 5,687,470 (Method for Forming an RF Shielded Enclosure) to Halttunen, et al., which discloses attaching an RF Shield cover with a conductive adhesive paste. 
     Other designs known in the prior art employ a cover with an openable lid to allow for the servicing of the components within the enclosure. U.S. Pat. No. 5,614,694 (One Piece Open and Closable Metal RF Shield) to Gorenz, et al., discloses such an enclosure formed from a single piece of conductive material in which the lid is hinged to the fence and swings open at a hinged point. 
     Other designs are known which employ removable covers. U.S. Pat. No. 5,365,410 (Electromagnetic Compatibility Enclosure) to Lonka, discloses opening the cover with a suitable tool and soldering it back or using a replacement cover. Other designs use compressive forces to grip the outer surface of the RF fence to retain the cover in place. This compressive force may typically be created by a set of tabs or finger-like projections that employ a spring-like compressive force to grip the outer surface of the fence. 
     The problem encountered with removable RF shield cover designs of this type is that the RF shield cover typically requires dedicated hard tooling to stamp a specific size and pattern of the cover and fingers from a thin metal. Typically metals having a width of 0.024 inches for fences and 0.015 through 0.020 inches for covers are used. Such thin metal for the cover is necessary to insure the tabs or fingers of the cover remain flexible and remain in contact with the shield walls. Attempts to fabricate the removable RF shield cover from thicker metal, for example, 0.024 inches, have proven to be not reliable and require expensive hard tooling. The tabs or fingers of these thicker covers are not resilient enough to guarantee good electrical contact with the fence walls. The thicker fingers are also prone to bending during assembly and do not spring back sufficiently to contact the fence wall. Thus, the effectiveness of the RF suppression is compromised because adequate contact is not maintained along the entire length of the fence. RF effectiveness is also compromised as the compressive forces of the fingers also tend to deflect the thin-walled fences inward. This deflection causes gaps between the fence and cover from which RF emissions can enter or escape the enclosure. 
     Thus, there is a need to provide a means of attaching RF shield covers to RF fences that allow for the simple removal of the cover without special tooling while providing effective suppression of RF emissions. 
     SUMMARY OF THE INVENTION 
     The invention utilizes standard RF shield fences soldered to a PWB and a removable RF shield cover, which incorporates a unique finger design feature. The fingers on the RF shield cover engage the inside wall of the RF shield fence. This is in contrast to the methods employed by the prior art that have covers fasten to the surface of the shield wall. 
     This design permits the use of thicker metal for the shield cover without compromising electrical contact with the fence. The use of thicker metal also eliminates the expense of dedicated tooling to fabricate the cover from thinner materials. 
     A further advantage of the invention is to allow for the removal, and reuse, of the RF shield cover to effectuate repairs on the electronic components within the enclosure. By being able to remove the cover without special tooling or having to unsolder either the cover from the fence or the fence from the PWB the cost of repairing components within the enclosure is reduced. 
    
    
     BRIEF DESCRIPTION OF THE FIGURES 
     A more complete understanding of the instant invention may be obtained from consideration of the following description in conjunction with the drawings in which: 
     FIG. 1 is a perspective view illustrating an exemplary embodiment of an RF shield can cover with internal fingers in accordance with the invention in which the RF shield cover is vertically displaced from the RF shield fence. 
     FIG. 2 is a perspective view illustrating an exemplary embodiment of an RF shield can cover with internal fingers in accordance with the invention in which the RF shield cover is engaging the RF shield fence. 
     FIG. 3 a  is a top view illustrating one embodiment of an RF shield cover with internal fingers in accordance with the invention; 
     FIG. 3 b  is a top view illustrating a second embodiment of an RF shield cover with internal fingers in accordance with the invention; 
     FIG. 3 c  is a top view illustrating another embodiment of an RF shield cover with internal fingers in accordance with the invention; 
     FIG. 4 is a side view through the cross-sectional line A—A of FIG. 3 a  illustrating an exemplary embodiment of a RF shield cover with internal fingers in accordance with the invention in which the RF shield cover is vertically displaced above the RF shield fence; 
     FIG. 5 is a side view through the cross-sectional line A—A of FIG. 3 a  illustrating an exemplary embodiment of a RF shield cover with internal fingers in accordance with the invention in which the RF shield cover engages the RF shield fence; 
     FIG. 6 is a side view through the cross-sectional line A—A of FIG. 3 a  illustrating another exemplary embodiment of a RF shield cover with internal fingers in accordance with the invention in which the RF shield cover is vertically displaced above the RF shield fence; and 
     FIG. 7 is a side view through the cross-sectional line A—A of FIG. 3 a  illustrating an another exemplary embodiment of a RF shield cover with internal fingers in accordance with the invention in which the RF shield cover engages the RF shield fence; 
     FIG. 8 is a side view through the cross-sectional line A—A of FIG. 3 a  illustrating another exemplary embodiment of a RF shield cover with internal fingers in accordance with the invention in which the RF shield cover engages the RF shield fence; 
     FIG. 9 is a perspective view illustrating an exemplary embodiment of an RF shield can cover with internal fingers in accordance with the invention in which the RF shield cover is vertically displaced from an RF shield fence that is ribbed; 
     FIG. 10 is a side view through the cross-sectional line A—A of FIG. 3 a  illustrating another exemplary embodiment of an RF shield cover with internal fingers in accordance with the invention in which the RF shield cover engages the ribbed surface of an RF shield fence; 
     FIG. 11 is a side view through the cross-sectional line A—A of FIG. 3 a  illustrating a ribbed surface of an RF shield fence; and 
     FIG. 12 is a side view through the cross-sectional line A—A of FIG. 3 a  illustrating dimpled surface of an RF shield fence. 
    
    
     It is to be understood that these drawings are for purposes of illustrating the concepts of the invention and are not to scale. It will be appreciated that the same reference numerals, possibly supplemented with reference characters where appropriate, have been used throughout to identify corresponding parts. 
     DETAILED DESCRIPTION 
     The invention is directed toward an improved methodology for attaching RF shield covers to RF shield fences, which are incorporated on circuit boards. FIG. 1 illustrates an exemplary embodiment of an RF shield cover  100  disengaged from, and vertically positioned above, an RF shield fence  102 . RF shield fence  102  is fixedly attached to a PWB  104  and encloses electronic components that either emit RF energy or require protection from RF energy. The RF shield cover  100  is comprised of a conductive material defining two regions, a top planar region  106  and a finger-like region  108 . As shown, finger region  108  is substantially perpendicular with respect to the planar region  106 . As will be understood, the height of the finger region  108  is sized according to the application. In the illustrated exemplary embodiment, finger region  108  is comprised of multiple fingers  110 . Each of fingers  110 , because of tension due to bending, is capable of providing a tensional force on RF shield fence  102 . When positioned within fence  102 , the tensional force of each finger  110  pushes against RF shield fence  102  to retain cover  100  in place. Subdividing the finger region  108  into multiple fingers  110 , in the manner shown, enables each individual finger to adjust to minor deformities in shield fence  102  while maintaining a constant tensional force against shield  102 . 
     Referring to FIG. 2, there is depicted an exemplary embodiment of the RF shield cover  100  where the RF shield cover  100  is installed upon RF shield fence  102 . In this position, the finger region  108  of the invention securely pushes against the inner surface of RF shield fence  102  to retain RF shield cover  100  in place. Tabs  210 , formed on top plate  106 , rest on the top edge of fence  102  and prevent cover  100  from falling into the enclosed space by providing a positive stop. RF shield cover  100  is also in continuous engagement with RF shield fence  102  and provides an uninterrupted seal to reduce RF emissions emanating from within the enclosure. 
     Referring to FIG. 3 a,  there is depicted a top view of an exemplary embodiment of the RF cover  100 . In this embodiment, there is illustrated a single internal finger  110  on each edge of cover  102 . However, the number of internal fingers is not limited to that number illustrated. Rather the number of fingers can be any number and is typically determined by the dimensions of the enclosure. In general, the longer the dimension of the enclosure, the greater number of fingers that will be employed. FIG. 3 b  illustrates a second embodiment of the invention in which includes a plurality of internal fingers  110   
     Referring to FIG.  4  and FIG. 5, there are shown cross sections of an exemplary embodiment of the RF shield cover  100  along the line A—A of FIG. 3 a.  FIG. 4 illustrates RF shield cover  100  above, and disengaged from, RF shield fence  102 , similar to the configuration illustrated in FIG.  1 . FIG. 5 illustrates RF shield cover  100  engaging the RF shield fence  102 , similar to that illustrated in FIG.  2 . 
     Referring to FIG. 4, it can be seen that fingers  110  extend downwardly from the top plate  106  of cover  100  and are directed inwardly toward the fence  102 . A slight compressive force is required to insert finger  110  into perpendicular alignment with the edges of cover  100  and fence  102 . This slight compressive force causes finger  110  to fit within the enclosure surrounded by fence  102 . In a spring-like response to the slight compressive force, finger  110  generates a tensional force that is exerted against fence  102 . These tensional forces hold cover  100  in contact with fence  102  without any adhesives such as solder or paste. 
     The unique shape of finger  110  serves to concentrate the tensional force upon fence  102  along a single engagement band  400 . The finger shape, extending downwardly from top plate  106 , is first directed away from RF fence  102 , and then directed toward fence  102 . This shape creates, along the width of finger  110 , a band  400  at the free edge of finger  110 , as illustrated in FIG.  4 . With this shape, fingers  110  contact fence  102  continuously along band  400 . Tensional forces generated by each finger are concentrated along band  400  and maintain continuous electrically contact between cover  100  and fence  102 . 
     FIG. 5 illustrates the engagement of cover  100  whereby finger  110  pushes against shield fence  102  to hold cover  100  in place. As illustrated, finger  110  engages and applies a tensional force, on the inner surface of fence  102 , at band  400 . Tab  210 , resting on edge  510  of RF fence  102  limits the depth that cover  100  extends into the enclosure and presents fence  102  from falling into the enclosure. Cover  100  is retained in position against RF shield fence  102  and is easily removed without the use of special tools. 
     In addition to concentrating the tensional force against fence  102 , the plurality of bands from each at least one finger  110  forms a continues belt of contact along the entire inner length of RF shield fence  102 . This belt of contact, in cooperation with tabs  210 , provides a continuous metal-to-metal electrical seal between fence  102  and cover  100  that inhibits RF emissions from escaping from, or entering into, the covered enclosure. 
     FIGS. 6 and 7 depict another embodiment of the invention with a different profile shape of finger  110 . In this embodiment, finger  110  extends downwardly toward fence  102  and then extends away from fence  102 . Similar to the shape illustrated in FIGS. 4 and 5, a band  600  is formed along each finger at which the tensional forces of fingers  110  are exerted against fence  102 . FIG. 8 depicts still another embodiment of the invention with another profile shape of finger  110 . In this embodiment, the extension away from fence  102  is more pronounced. 
     In addition to the ease of removal, a further advantage of the invention claimed is that the separation distance between RF fences may be reduced because cover  100  is contained completely within the enclosed area surrounded by fence  110 . Typically, RF shield covers extend beyond the shield fence and, as discussed previously, attach to the outer surface of the fence. To prevent contact between adjacent covers, the separation distance between the RF shield fences is made sufficiently large to prevent adjacent covers from contacting one another. Contact between RF shield covers is avoided as such contact may introduce interference within the enclosed RF shield from a mis-match in respective ground planes. An RF shield cover that fits within the RF shield fence, made in accordance with the method of the invention, does not extend beyond the fence and are not capable of contacting an adjacent RF shield cover. As no compensation is necessary to prevent adjacent covers from touching, the separation distance between adjacent RF shield fences may be reduced. With the reduction in spacing between RF shield fences, the fences may be placed closer together and more space is available on the PWB for the placement of electrical components. 
     Experimentation with fence thickness of 0.024 inches and cover thickness ranging from 0.015 to 0.020 inches have been shown to provide easily removable shield covers, with fingers flexible enough to engage the RF fence and to suppress RF emissions from RF emanating circuitry within the enclosure. 
     In another embodiment of the invention, RF shield fences may contain indentations, such as continuous ribs or individual dimples, to improve the contact between the surfaces of the RF shield cover fingers  110  and the RF shield fence  102 . FIG. 9 illustrates one embodiment of RF fence ribs  900  longitudinally inserted in the vertical walls of RF shield fence  102 . In this exemplary embodiment the internal fingers  110  engage the protruding surface of rib  900  to increase the tensional force finger  110  exerts upon RF shield fence  102 . 
     Referring to FIG. 10, there is depicted an exemplary embodiment of the RF shield cover  100  where the RF shield cover  100  is installed upon RF shield fence  102 . In this position, the finger  110  securely pushes against the protruding inner surface of RF shield fence  102  created by rib  900 . In this embodiment, finger  110  exerts a greater tensional force on RF shield fence  102  to retain RF shield cover  100  in place. RF shield cover  100  remains in continuous engagement with RF shield fence  102  at ribs  900  and provides additional surface area to which internal fingers  110  contact, thus improving the electrical contact between cover and shield. 
     FIG. 11 illustrates one embodiment of the ribbed shield fence wherein rib  900  is positioned horizontally and continuously along a vertical wall. In this illustration, rib  900  is positioned along a rear vertical wall of RF shield fence  102 . In another embodiment, illustrated in FIG. 12, the vertical wall of RF shield fence is dimpled at known locations horizontally along the vertical wall of RF fence  102 . In this embodiment, fingers  110  engage the protruding surfaces of dimples  1200 . 
     A novel method for attaching RF shield covers to RF shield fences is disclosed that provides for the suppression of emissions from enclosed RF components while simplifying the attachment and removal of an RF shield cover when it is necessary to operate upon the components within the RF shield enclosure. 
     Conductive materials suitable for use in fabricating the claimed invention include such metals as copper, aluminum, phosper bronze, BeCu, and stainless steel, which are presented here only as illustrative examples. It would be apparent to those skilled in the art to substitute any one conductive metal for another and therefore a listing of all-possible metal types and alloys is not presented herein. 
     Numerous modifications and alternative embodiments of the claimed invention will be apparent to those skilled in the art in view of the foregoing description. Accordingly, this description is to be construed as illustrative only and is for the purpose of teaching those skilled in the art the best mode of carrying out the invention and is not intended to illustrate all possible forms thereof. It is also understood that the words used are words of description, rather that limitation, and that details of the structure may be varied substantially without departing from the spirit of the invention and the exclusive use of all modifications which come within the scope of the appended claims is reserved.