Abstract:
In a printed circuit board ejector system, ejector levers are mounted on the outer surface of a faceplate that is constructed to withstand printed circuit board insertion forces without bending or breaking. The faceplate is then fastened to the printed circuit board. Since there is no opening in the faceplate through which the ejector lever passes, electromagnetic radiation cannot leak through the faceplate at this location.

Description:
RELATED APPLICATIONS  
       [0001]    This application is related to, and claims priority of, U.S. provisional application serial No. 60/258,001, entitled PRINTED CIRCUIT BOARD EJECTOR SYSTEM WITH IMPROVED EMI SHIELDING, filed Dec. 22, 2000 by Lorraine I. Duncan. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    This invention relates to apparatus that shields electromagnetic radiation emitted from printed circuit boards.  
         BACKGROUND OF THE INVENTION  
         [0003]    The density with which electronic circuitry can be integrated has been increasing dramatically for decades. As a consequence, the clock frequency at which electronic circuitry is operated has been increasing correspondingly and with it the problem of electromagnetic radiation emanating from the printed circuit boards (PCBs) used to interconnect that circuitry. Shielding to reduce electromagnetic interference (EMI) generated by such circuitry must be effective at least up to the fifth harmonic of the clock frequency to meet radiation emission restrictions imposed by national and international standards. Thus, the shielding for circuitry operating at 2.5 GHz must be effective up to at least 12.5 GHz. At such frequencies, an aperture as small as 0.25 inches can be an effective antenna.  
           [0004]    Another consequence of the increased density of electronic circuitry is the increased number of pins needed to connect PCBs to backplanes resulting in an increase in the force needed to mate the PCB and backplane connectors. It is common for well over 100 lbs. of insertion force to be required to make the connection. This necessitates the use of PCB injectors, that is, levers attached to the PCB that can engage the card cage into which the PCB is to be inserted, thereby providing a mechanical advantage which forces the board into the connectors. Since the same lever is generally used to aid in the ejection of the board, they are often referred to as ejectors, even when both functions are intended; that convention is used in the present disclosure.  
           [0005]    For EMI shielding purposes, PCBs are typically provided with a thin metal faceplate, mounted at the front of the board and perpendicular to it so that when a set of boards are inserted into a card cage, the PCB faceplates extend across the spaces between the boards and provide a nearly continuous metal shield across the front of the card cage. However, all prior art card PCB ejectors require a relatively large slot to be cut through the metal faceplate so that they can be attached directly to the printed circuit board. This direct attachment is necessary to provide the mechanical strength needed to withstand the required insertion force. Unfortunately, it is difficult if not impossible to keep the dimensions of these slots smaller than 0.25 in. and still accommodate levers of the size needed to provide the required mechanical advantage. Furthermore, again to withstand the required insertion force, the ejectors usually have to be made of metal. Consequently, the current flowing around the slot induces a high-frequency current in the metal ejector itself, which thereby becomes an effective radiator of the energy transferred to it. As the length of the ejector is increased to support greater insertion forces, it becomes an even more effective antenna at the frequencies of concern. As a result, prior art ejectors make it very difficult for high-speed electronic products to be compliant with emissions standards.  
         SUMMARY OF THE INVENTION  
         [0006]    In accordance with the principles of the present invention, ejector levers are mounted on the outer surface of a electrically-conductive PCB faceplate that is constructed to withstand the required insertion forces without bending or breaking. The faceplate is then fastened to the PCB. Since there is no opening in the faceplate through which the ejector lever passes, electromagnetic radiation cannot leak through the faceplate at this location.  
           [0007]    When the PCB and ejector assembly is inserted into a card cage, the ejector levers engage the card cage in order to provide the force necessary to insert and the PCB and seat its connectors and to remove the PCB.  
           [0008]    In one embodiment, the ejector levers are mounted on the faceplate surface by means of bolts that pass through the faceplate. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]    The above and further advantages of the invention may be better understood by referring to the following description in conjunction with the accompanying drawings in which:  
         [0010]    [0010]FIG. 1 is a perspective view of a printed circuit board incorporating the ejector according to the present invention.  
         [0011]    [0011]FIG. 2A is an enlarged perspective view showing the ejector lever, the faceplate and the manner of mounting the faceplate on the printed circuit board.  
         [0012]    [0012]FIG. 2B is a cross-section of one embodiment of a faceplate.  
         [0013]    [0013]FIG. 3 is an exploded view of the faceplate and ejector levers. 
     
    
     DETAILED DESCRIPTION  
       [0014]    A typical PCB assembly with an ejector constructed in accordance with the principles of the invention is shown in FIG. 1. The PCB itself  100  carries the electronic circuitry that communicates with a back plane (not shown) through one or more high-density connectors  101 . The ejector assembly comprises an electrically-conductive faceplate  103  that is fastened to the front edge of the PCB assembly  100 , both to provide a carrier for front-side connectors, displays and handles and to contain the electromagnetic radiation that is generated by the circuitry on the board. As with prior art designs, faceplate  103  has a sufficient width to extend across the spaces between adjacent printed circuit boards when the boards are mounted in a conventional card cage.  
         [0015]    In accordance with the principles of the invention, this faceplate  103  is much heavier in construction than the prior art faceplates used only for shielding purposes. In particular, the faceplate  103  is constructed to withstand the forces required for insertion and removal of the PCB into and from the card cage without bending or breaking. For example, in one embodiment, faceplate  103  can be constructed from metal with a heavy thickness. Alternatively, stiffeners or ribs can be added to prevent bending and breaking as discussed below.  
         [0016]    Since the high-density connectors  101  require significant force to be inserted into the mating connectors on the backplane (not shown), the ejector assembly further includes ejectors  102  are attached to the outer surface  104  of the faceplate  103  at the top and the bottom of the outer edge  104  rather than the PCB  100  assembly itself.  
         [0017]    The inventive ejectors  102  are shown in greater detail in FIG. 2A which enlarges the area “A” shown in FIG. 1. As illustrated, the faceplate  202  is fastened to the PCB  201  by means of brackets, of which bracket  210  is shown. Bracket  210  is attached to faceplate  202  by suitable means. For example, bracket  210  may be formed as part of faceplate  202 . In one embodiment, a plurality of brackets can be formed as integral legs extending perpendicularly from faceplate  202 . In still another embodiment, faceplate  202  can be substantially U-shaped with one leg of the U-shape being fastened to PCB  201 . The cross section of such a U-shaped design is shown in FIG. 2B. Here the bracket  210  comprises one leg of the U-shaped faceplate  202 . A stiffening rib  212  has also been added as the other leg. A faceplate  202  with the cross section of FIG. 2B is shown in the embodiments depicted in FIGS. 1 and 3. The faceplate shape would typically be made from a heavy gauge metal, for example, 0.040″ thick aluminum by suitable means, such as extrusion. In this embodiment, the bracket portion  210  is offset from the rib  214  that extends from the faceplate  202 . This offset brings the bracket  210  within 0.180″ of the force vector applied by the ejector lever, thereby not allowing the faceplate to bend or break. The force vector is also in-line with the neutral axis of the PCB which provides additional support of the faceplate and allows a direct translation of force to the backplane connectors.  
         [0018]    Additionally, bracket  210  may be a separate piece that is bolted to the faceplate  202  or attached by welding the bracket to the faceplate  202 . Bracket  210  may also be attached by means of a suitable adhesive, such as an epoxy adhesive.  
         [0019]    Bracket  210  is also fastened to the PCB  201 . In FIG. 2, bracket  210  is fastened to PCB  201  by through-bolts, of which bolt  208  is shown. Although only one bracket is shown, additional brackets (or alternatively a continuous rib) would be used. The brackets or rib form a secure connection to the PCB  201  that can withstand the insertion force required to fully seat the connectors  101 . However, other alternative arrangements can be used to fasten the brackets to the PCB  201 . For example, the brackets may be formed with two opposing sections. One bracket section can contain pins that pass through holes in the PCB  201  when the bracket sections are assembled. In another arrangement, the PCB  201  may have notches along its edge that are engaged by teeth in the brackets.  
         [0020]    Each ejector consists of a lever  206  attached by a hinge  207  to a mounting block  203 . The lever  206  is equipped with both an insertion tooth  204  and an extraction tooth  205 , the former tooth  204  engaging the inside of a track at the outer edge of the card cage (not shown) when the PCB assembly is inserted and the latter tooth  205  engaging the outside of the same track when the assembly is extracted.  
         [0021]    The mounting block  203  is designed to be fastened to the faceplate  202  as shown in FIG. 3, rather than to the PCB  201  as is conventionally done. In one embodiment, the mounting block is bolted to the faceplate  203 . In an embodiment in which the bracket  210  is bolted to the faceplate  202 , the bolt which is used to attach the bracket  210  to faceplate  202  may pass through a hole in faceplate  202  and thread into mounting block  203  so as to fasten the three parts together. Also, separate bolts can be used to fasten mounting block  203  to faceplate  202  and to fasten bracket  210  to faceplate  202 . Alternatively, the mounting block  203  could be fastened to the faceplate  202  by other suitable means, such as welding or by means of an adhesive, such as epoxy adhesive.  
         [0022]    As shown in FIG. 3, in an embodiment which the ejector assembly is bolted to the faceplate, screws  312  holding the ejector assembly  301  to the faceplate  303  pass through small holes  302  and fit into threaded holes (not shown) in the back surface of the mounting block  303 . Holes  302  are sealed by the mounting screws  312 , resulting in extremely low radiation loss, even at very high frequencies. In other embodiments in which the ejector assembly  301  is welded or fastened by adhesive, the faceplate  303  remains continuous thereby also resulting in extremely low radiation loss.  
         [0023]    A second ejector assembly  314  is attached to the faceplate  303  also by means of two screws  316 . In the specific example shown, only two holes  302 , each 0.090 in. in diameter, are required to attach an ejector  301  capable of exerting 66 lbs. of insertion force. In one embodiment, the handle  306  is 1.7 in. long and provides a 3 to 1 mechanical advantage, thereby enabling a PCB (not shown) requiring 132 lbs. of insertion force to be inserted by applying 22 lbs. of force to each of the two ejectors  302  and  314 .  
         [0024]    Although an exemplary embodiment of the invention has been disclosed, it will be apparent to those skilled in the art that various changes and modifications can be made which will achieve some of the advantages of the invention without departing from the spirit and scope of the invention. For example, it will be obvious to those reasonably skilled in the art that, although the description was directed to a particular hardware configuration, other hardware configurations could be used in the same manner as that described. Other aspects, such as conventional variations in the specific sizes of components recited in the specification to achieve a particular function, as well as other modifications to the inventive concept are intended to be covered by the appended claims.