Abstract:
The present invention relates to radio frequency and microwave connectors, and more particularly to grounding methods for printed wiring board edge-launch connectors. The grounding method comprises conducting tabs secured to a PWB and to an attached connector frame holding coaxial connectors. The conducting tabs thus provide a ground connection between the connector frame and one or more ground conductors on the PWB.

Description:
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
       [0001]    This invention was made with U.S. Government support under contract No. 06C2908 awarded by the Department of Defense. The U.S. Government has certain rights in this invention. 
     
    
     FIELD 
       [0002]    The present invention relates to radio frequency and microwave connectors, and more particularly to grounding methods for printed wiring board edge-launch connectors. 
       BACKGROUND 
       [0003]    Printed wiring boards (PWBs) are used extensively to produce electronic circuits. PWBs are typically formed as sandwiches of one or more layers of dielectric material and one or more layers of conductive material, in which the conductive material may be formed, by etching, into patterns including lines, known as traces, which form connections in a circuit. Holes with conductive walls, known as vias, may be formed in the dielectric layers to provide electrical connections between conductive layers. 
         [0004]    A circuit on a PWB may include connectors, and components such as resistors, capacitors, or transistors, which may be installed on the PWB by applying solder paste to the outer conductive layer at the locations where the components are to be installed, placing the components on the PWB, and heating the assembly in a solder reflow oven which melts the solder, soldering the components in place. Alternately, conductive epoxy may be used instead of solder. 
         [0005]    Coaxial connectors known as board edge-launch connectors may be installed at the edge of a PWB to provide connections to other parts of a system. For example, a PWB with an array of connectors along one edge may be installed in a system by sliding it into a chassis so that the connectors on the PWB connect simultaneously to an array of corresponding mating connectors in the chassis. Such an arrangement, in which there is no opportunity for a human operator or technician to align and connect the connectors individually and where the technician may not be able to see the connectors, is known as a blind-mate application. 
         [0006]    Coaxial connectors individually soldered to a PWB may be unsuitable for use in a blind-mate application because the process for soldering such connectors to a PWB may not produce sufficiently precise alignment to allow each connector to connect reliably with the corresponding connector in an array, such as in the chassis-based system described above. In such a case it may be helpful to use a single rigid part known as a connector frame to hold all of the connectors, and to maintain their alignment relative to each other and to a PWB. It may also be convenient to have the connector frame secured to the bottom surface of the PWB, providing a ground connection between the connector frame and a ground conductor on the bottom surface of the PWB. 
         [0007]    When a connector frame is used with coaxial connectors, it may be necessary to provide ground connections also between the outer conductors of the connectors and ground conductors on the top surface of the PWB. Moreover, when the connectors will be carrying high-frequency signals, such as radio frequency (RF) or microwave signals, it may be necessary to have a continuous connection from the connector frame to one or more ground conductors on the top surface of the PWB, forming a transmission line, so that the characteristic impedance of the signal path will be uniform and to prevent reflection or radiation of the signal. 
         [0008]    A connection between the connector frame and the top-layer ground conductors may be formed by bonding wires to the connector frame and to top-layer ground conductors near the edge of the PWB. A bond wire, however, generally follows a curved path through air between the bond pads it connects. This causes the corresponding part of the signal path to have a different, and generally high, characteristic impedance, and if the wire bonds are applied under manual control, the wire path and the characteristic impedance may suffer from poor repeatability. Moreover, wire-bonding machines may be designed to work with relatively small parts, and a PWB with a connector frame may be too large to fit into such a machine. 
         [0009]    Another means of forming a ground connection between the connector frame and a top-layer ground involves applying a globule of conductive epoxy manually to a ground conductor near the edge of the PWB and to a nearby surface of the connector frame, so that the epoxy bridges the gap between the connector frame and the top-surface ground conductor on the PWB. This method is unsatisfactory, primarily because of the conflicting requirements of (i) applying a sufficient quantity of epoxy to ensure that the gap is bridged by the epoxy and that contact is made reliably with both the connector frame and the PWB, and (ii) applying a sufficiently small quantity of epoxy that it will not flow to other nearby conductors, thereby forming unwanted short circuits. These difficulties may be compounded by variations in gap width resulting from fabrication tolerances, and from the poor repeatability of a manual process. 
         [0010]    Thus, there is a need for a system for providing connections between a conductive connector frame and one or more conductive areas on the top surface of a PWB. 
       SUMMARY 
       [0011]    Embodiments of the present invention provide a repeatable ground connection between a connector frame and conductors on the surface of a PWB. One aspect of embodiments of the present invention allows a signal path to maintain a uniform characteristic impedance between coaxial connectors and PWB transmission lines, by providing continuous ground paths from a connector frame to ground conductors on the PWB. Exemplary embodiments of the invention accomplish this by providing contact surfaces on the connector frame and on the PWB, and conductive tabs which may be soldered or adhered to both the connector frame and the PWB, to provide conductive ground paths from one to the other. 
         [0012]    In one embodiment, a system for forming a plurality of electrical connections to one or more conductive areas on a PWB comprises a connector frame attachable to a PWB, wherein the connector frame has at least one surface portion adjacent each of the conductive areas of the PWB, and each of the surface portions is electrically connectible to a conductive tab, to connect the surface portion of the connector frame to one of the conductive areas of the PWB. In one embodiment the system comprises flat tabs for connecting one or more of the surface portions of the connector frame to one or more of the conductive areas of the PWB. 
         [0013]    In one embodiment, a method of forming a plurality of ground connections between conductive areas on a PWB and a connector frame for holding coaxial connectors includes providing the connector frame with at least one surface portion adjacent each of the conductive areas on the PWB; securing one or more conductive tabs to one or more of the surface portions; and securing one or more of the conductive tabs to one or more of the conductive areas. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]    These and other features and advantages of the present invention will become appreciated as the same become better understood with reference to the specification, claims and appended drawings, wherein: 
           [0015]      FIG. 1  is a fragmentary front perspective view of a portion of a grounding system provided according to an embodiment of the invention; 
           [0016]      FIG. 2  is a rear perspective view of the grounding system of  FIG. 1 ; 
           [0017]      FIG. 3  is a rear perspective view of the grounding system of  FIG. 1  according to another embodiment of the invention; 
           [0018]      FIG. 4A  is a fragmentary top plan view of a portion of the embodiment of  FIG. 2 , showing an offset cutting plane used to generate  FIG. 4B ; 
           [0019]      FIG. 4B  is a cross-sectional view of the embodiment of  FIG. 2  taken along the offset cutting plane shown in  FIG. 4A ; 
           [0020]      FIG. 5A  is a top view of the top conductive layer of a PWB according to an embodiment of the invention; and 
           [0021]      FIG. 5B  is a top view of the middle conductive layer of a PWB according to an embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0022]    The detailed description set forth below in connection with the appended drawings is intended as a description of the presently preferred embodiments of an identification system provided in accordance with the present invention and is not intended to represent the only forms in which the present invention may be constructed or utilized. The description sets forth the features of the present invention in connection with the illustrated embodiments. It is to be understood, however, that the same or equivalent functions and structures may be accomplished by different embodiments that are also intended to be encompassed within the spirit and scope of the invention. As denoted elsewhere herein, like element numbers are intended to indicate like elements or features. 
         [0023]    As used herein, the term “PWB” means any combination of one or more insulating, dielectric, or semiconductor layers with one or more complete or partial conducting layers, and includes without limitation polymer on metal, ceramic substrates, GaAs and GaN chips, and combinations in which the dielectric material is glass reinforced epoxy, a Teflon-based material, or alumina, and in which the conducting material contains copper or copper and other metals. 
         [0024]    Referring to  FIG. 1  and  FIG. 2 , in the embodiment shown a connector frame  10  includes a plate portion  11  forming a PWB shelf  12  for supporting a PWB  14 , and also includes a wall portion  13  extending along one edge of the PWB  14 . The PWB  14  is secured to the PWB shelf  12 . Threaded holes  16  in the wall portion  13  accept coaxial connectors  17  with threaded bodies. In one embodiment, the threaded holes  16  may be through holes 0.035 inches in diameter, counterbored with a diameter of 0.148 inches to a depth of 0.167 inches, and the counterbored portion may be threaded with a 0.164-64 UNS-2B thread, to a minimum depth of 0.138 inches. The connector frame  10  may be fabricated from a single piece of metal or assembled from several pieces, and it may be formed of conductive materials other than metal, or of a combination of conductive and insulating materials. 
         [0025]    In the embodiment shown in  FIG. 2 , a relief cut  21  on each side of each threaded hole  16  forms a tab shelf  22  at the same height as the top surface of the top layer ground conductor  25 . On either side of each connector  17 , a ground tab  23  is secured to both the tab shelf  22  and to an adjacent area on the top layer ground conductor  25  using solder or conductive epoxy (element  26  of  FIG. 4 ). Except where it may cross a relatively small gap  27  between the PWB  14  and the connector frame  10 , the ground tab  23  does not produce an air gap between the signal conductor and ground conductors. As a result this system provides a signal path with a more uniform and more repeatable characteristic impedance than a pair of bond wires. 
         [0026]    For clarity of illustration,  FIG. 2  shows the system with ground tabs  23  installed at one of the coaxial connectors  17  and not yet installed at another, so that the tab shelves  22  are visible at the latter location. 
         [0027]    The invention is described herein in relation to an array of coaxial RF connectors  17 , but the invention is not limited to this application, and may be used in other types of connector assemblies, such as triaxial connectors or coaxial connectors intended for use at other frequencies. 
         [0028]    In one embodiment, the center conductors  18  of the coaxial connectors  17  extend just above the top surface of the top layer signal trace  30  on the PWB  14 . The distance between the PWB shelf  12  and the centerline of any of the threaded holes  16  may preferably be chosen such that when the PWB  14  is installed on the PWB shelf  12 , the clearance between the top layer signal trace  30  on the PWB  14  and the center conductor  18  of the connector  17  is sufficiently large to allow the connector  17  to be installed in the threaded hole  16 , and also sufficiently small to allow a reliable connection between the center conductor  18  and the corresponding top layer signal trace  30  to be formed. For example, it may be preferable to have the clearance be sufficiently small that during a soldering or gluing operation molten solder or conductive epoxy (element  26  of  FIG. 4 ) will bridge the gap between the center conductor  18  and the corresponding top layer signal trace  30 . In an exemplary embodiment the thickness of the conductive epoxy film (element  15  of  FIG. 4 ) between the PWB shelf  12  and the PWB  14  may be 0.005 inches, the thickness of the PWB  14  may be 0.036 inches, the diameter of the coaxial connector center conductor  18  may be 0.012 inches, and the distance between the center line of the threaded hole  16  and the PWB shelf  12  may be 0.049 inches, resulting in a nominal clearance between the center conductor  18  and the top layer signal trace  30  of 0.002 inches. 
         [0029]    The relief cuts  21  may be formed by any suitable method, in one embodiment as part of the process of machining the connector frame  10  using a milling machine under computer numerical control, also known as a CNC machine. In this case each of the relief cuts  21  may be formed using an end mill; the same end mill may also be used to machine other surfaces of the connector frame  10 . The width of the relief cut  21  in this case may be greater than or equal to the diameter of the end mill used for this operation. In embodiments of the present invention, the connector frame  10  may be made of a material having a coefficient of thermal expansion similar to that of the PWB  14 , such as an aluminum-silicon alloy containing 72% aluminum and 28% silicon. 
         [0030]    The PWB  14  may be fabricated from conductive layers made of copper and dielectric layers made of a Teflon-based material such as CLTE sold by Arlon-MED of Rancho Cucamonga, Calif., which may have a glass weave imbedded in it. In another embodiment, similar material sold by Rogers Corporation, of Chandler, Ariz., may be used. The glass weave may control the coefficient of thermal expansion of the dielectric layers so that it is similar to that of the copper conductive layers. 
         [0031]    In exemplary embodiments, after the PWB  14  has been secured to the connector frame  10 , connectors  17  with threaded bodies are installed in the connector frame  10  by threading them into the threaded holes  16  and tightening them to the torque specified by the manufacturer of the connectors  17 . The connectors  17  may in certain embodiments be SMPM connectors, with part number 18S103-500L5, sold by Rosenberger of North America, LLC, of Lancaster, Pa. In other embodiments they may be GPPO connectors, with part number B003-L33-02, sold by Corning Gilbert Incorporated of Glendale, Ariz. Similar or equivalent connectors may be available from other vendors including W. L. Gore &amp; Associates, Incorporated, of Newark, Del., and DDi Corporation of Anaheim, Calif. 
         [0032]    In one embodiment, the ground tabs  23  are oblong with a width of 0.025 inches, a length of 0.125 inches, and rounded ends with radii of curvature equal to half of the width. The relief cuts  21  may be slightly wider than the ground tabs  23  to permit the latter to fit into place easily. In such an embodiment the relief cuts  21  may have a width of 0.032 inches. 
         [0033]    In another embodiment, shown in  FIG. 3 , U-shaped ground tabs  23 ′ may be used in place of pairs of oblong ground tabs  23  of the kind illustrated in  FIG. 2 . The two arms of each U-shaped ground tab  23 ′ may have widths of 0.025 inches, rounded ends with radii of curvature of 0.0125 inches, and a gap of 0.055 inches between the arms of the U. Each U-shaped ground tab  23 ′ may have an overall width of 0.105 inches and an overall length, measured in the direction parallel to the arms of the U, of 0.1531 inches. 
         [0034]    The ground tabs  23  may, in an exemplary embodiment, be fabricated from a sheet of brass, 0.005 inches thick. In another embodiment, a sheet of another metal may be used. A metal having a coefficient of thermal expansion similar to that of the top conductive layer of the PWB  14  may minimize stresses that otherwise could result from differential thermal expansion or contraction with changes in temperature. It may be preferable to plate the ground tabs  23  with another metal or metals to provide a better bond during installation and to prevent galvanic corrosion. An etching process may be used to fabricate the ground tabs  23 . An etch-resistive film, in the shape that is to remain after etching, may be formed on both sides of a sheet of brass. After the formation of this film the sheet of brass may be etched from both sides. After etching, the sheet may contain a number of ground tabs  23 , each still connected to a supporting strip of the sheet by a narrow support finger of metal. In an exemplary embodiment, this etched sheet may then be plated with a layer of nickel 0.0001 to 0.0002 inches thick, and subsequently plated with a layer of gold 0.00001 to 0.00002 inches thick. Shearing the support fingers in such an embodiment releases the ground tabs  23  from the supporting strip, completing the process of fabricating the ground tabs  23 . In another embodiment, the ground tabs  23  may be punched from a sheet of metal, which may first have been plated with one or more other metals. 
         [0035]    Referring to  FIG. 4 , in one embodiment, the PWB  14  may be secured to the PWB shelf  12  using a conductive epoxy film  15  such as Ablestik ABLEFILM 561, a glass supported, modified epoxy adhesive film sold by Henkel Corporation, of Rocky Hill, Conn. The conductive epoxy film  15  may be applied to the PWB shelf  12 , the PWB  14  placed on the conductive epoxy film  15 , and the subassembly heated in an oven to cure the conductive epoxy film  15 . After the PWB  14  is secured to the connector frame  10 , a dab of conductive epoxy  26  may be applied to each tab shelf  22 , and to a point, on the top layer ground conductor  25 , adjacent to each tab shelf  22 . A ground tab  23  may then be placed across the gap  27  so that one end of the ground tab  23  is over the tab shelf  22  and the other end is over the top layer ground conductor  25 . In this embodiment the conductive epoxy  26 , both between the ground tab  23  and the tab shelf  22 , and between the ground tab  23  and the top layer ground conductor  25 , is sandwiched between closely spaced parallel surfaces, and prevented by its adhesion to these surfaces from flowing to other parts of the structure, where it could otherwise cause unwanted short circuits. The conductive epoxy  26  may be one that remains compliant after curing, to reduce the risk that differential thermal expansion of the parts joined by the conductive epoxy  26  may cause the conductive epoxy  26  to fracture. In one embodiment, the conductive epoxy  26  may be Ablestick 8175, which is sold by Henkel Corporation. In another embodiment, dabs of solder paste may be used in place of conductive epoxy  26 , and the subassembly may be subsequently heated in a reflow oven to form solder joints at the locations of the solder paste. The dabs of conductive epoxy  26  or of solder paste may, in an exemplary embodiment, be applied under computer control by a dispensing machine. In another embodiment the dabs may be applied manually. 
         [0036]    The ground tabs  23  may be sufficiently small and of sufficiently low mass for handling with a pick-and-place machine and in one embodiment may be placed on the PWB  14  using such a machine. In another embodiment the tabs may be installed manually. In yet another embodiment a comb-shaped strip of sheet of metal may include multiple ground tabs and may be installed on the PWB  14  and the tab shelves  22  in one manual operation. 
         [0037]    It may be possible to install the ground tabs  23  on the PWB  14  at the same time, and using the same equipment, as other components, improving the efficiency of the assembly process. For example, solder paste may be applied to the tab shelves  22  and to various points on the top surface conductors of the PWB  14 . The components may then be placed on the PWB  14  and the ground tabs  23  on the PWB  14  and on the tab shelves  22  in a subsequent step, and all of the solder joints formed simultaneously in a subsequent solder reflow step. 
         [0038]      FIG. 5  shows an exemplary arrangement of the top and middle conductive layers for an embodiment in which the PWB  14  has three conductive layers. A transition from coaxial transmission line to a transmission line geometry known as “coplanar-over-ground” is formed at the edge of the PWB  14 . As used herein the term “coplanar over ground” delineates a geometry of conductors used for a microwave transmission line including a top layer signal trace  30 , a top layer ground conductor  25 , or a pair of such conductors, extending to both sides of the top layer signal trace  30 , and a bottom layer ground  32  ( FIG. 4 ). A second transition to another transmission line configuration may be formed near the first transition. 
         [0039]    Referring to  FIG. 5 , the second transition may for example be from coplanar-over-ground to stripline. In this case, the signal path may be routed from the top layer signal trace  30  to the middle layer signal trace  34  using a signal via  28 . The signal via  28  may be back-drilled through the bottom layer with a drill bit having a diameter slightly larger than the diameter of the signal via  28 , to a depth extending almost to the middle conductive layer, to remove the conductive material from the lower half of the signal via  28 , where it would otherwise contact, or be unacceptably close to, the bottom layer ground  32  and the PWB shelf  12  ( FIG. 4 ). A signal via pad  35 , an annular region of conductor, may surround, or partially surround, the signal via  28 . A cage of ground vias  29  may be used for mode suppression as illustrated in the exemplary embodiment of  FIG. 5  to reduce loss in the structure. In an embodiment in which U-shaped ground tabs  23 ′ are employed ( FIG. 3 ), the top layer ground conductor  25  on the PWB  14  extends past the edge of the U-shaped ground tab  23 ′ at all edges of the U-shaped ground tab  23 ′ except at the edge of the PWB  14 . This ensures that the gap between the signal path and the nearest ground on the PWB  14  is determined everywhere by the edge of the top layer ground conductor  25 , and not by the placement of the U-shaped ground tab  23 ′ on the PWB  14 . In one embodiment the bottom layer ground  32 , shown in  FIG. 4 , may be a solid conductive sheet except for holes at the locations of vias. 
         [0040]    Adjustments to the dimensions of the conductors on the PWB  14  may be made to provide as uniform as possible a characteristic impedance along the signal path, and to minimize reflections and radiation along the path. These adjustments may be made using electromagnetic field simulation software such as Ansoft HFSS, sold by Ansys Incorporated, of Canonsburg, Pa. Using such software, a designer, in implementing the present invention, may define two ports in the system, one at the coaxial connector  17 , and one at a point on the PWB  14 . In an embodiment having a second transition from coplanar-over-ground to stripline, for example, the second port may be on the stripline transmission line. The designer may then use the simulation software to calculate the four complex S-parameters for this two port system, where the magnitudes of S 11  and S 22  indicate the return loss and the magnitudes of S 12  and S 21  indicate the insertion loss. If the insertion loss is larger than expected it may indicate that the signal path will radiate electromagnetic power, which may be undesirable. The designer may use the simulation software to display the impedance corresponding to S 11  or to S 22  on a Smith chart, on which the desired characteristic impedance is the center point, the upper half corresponds to impedances which are more inductive than the desired characteristic impedance, and the lower half corresponds to impedances which are more capacitive than the desired characteristic impedance. 
         [0041]    The designer may then, in a process known as tuning, adjust conductor dimensions until the design meets its requirements for return loss and insertion loss, over the frequency range of interest. To eliminate excess capacitance, the designer may for example reduce the width of the top layer signal trace  30 , increase the gaps between the top layer signal trace  30  and the regions of the top layer ground conductor  25  on both sides of the signal trace, decrease the diameter of the signal via  28 , decrease the diameter of the signal via pad  35 , enlarge the cage of ground vias  29 , or increase the gap between the signal via pad  35  and the adjacent top layer ground conductor  25 . When enlarging the cage of ground vias  29 , the designer may need to observe the insertion loss, which may become unacceptable if the ground vias  29  are moved too far from the transitions. To eliminate excess inductance, the designer may adjust, for example, any of these same parameters in the opposite direction. In a subsequent step, the designer may if necessary further reduce the capacitance of the structure by narrowing the middle layer signal trace  34  along a portion of its length, forming an inductive section  36 , and then adjust the length and width of the inductive section  36  to further improve the return loss and the insertion loss of the signal path. Alternatively, the designer may, instead of narrowing, widen a portion of the middle layer signal trace  34 , thereby forming a capacitive section, and adjust the length and width of the capacitive section for improved performance. 
         [0042]    When a system design employing the present invention has been adjusted for good performance over one range of frequencies, and it is desired to use the system over a different range of frequencies, it may be necessary to repeat the tuning process for the new frequency range. 
         [0043]    The grounding system of the present invention is described above, and illustrated in  FIG. 5 , in the context of a signal path having a first transition from coaxial transmission line to coplanar-over-ground, and a second transition from coplanar-over-ground to stripline. The invention, however, is not limited to such a pair of transitions. It may be used, for example, in a signal path without a second transition, or one in which the second transition is to microstrip transmission line. A transition from coplanar-over-ground to microstrip may be accomplished, for example, by flaring away the top layer ground, i.e., gradually increasing both the width of the top layer signal trace  30 , and the gaps between the top layer signal trace  30  and the ground conductor regions on both sides of the signal trace  30 , so as to keep the characteristic impedance constant, until the top layer ground conductor  25  is on both sides sufficiently distant from the signal trace  30  to have a negligible effect. 
         [0044]    The method for connector grounding of the present invention is not limited to PWBs with three conductive layers, also known as three-layer boards, but may be employed with single-layer boards, two-layer boards, four layer boards, or PWBs with an arbitrary number of conductive layers. In each case the ground tab or tabs  23  may be installed so as to connect the connector frame  10  to a top layer ground conductor  25 . The connection of the connector frame  10  to ground conductors in other layers may be accomplished by one of, or a combination of: tabs connecting the connector frame  10  to a top layer ground conductor  25 , vias from a top layer ground conductor  25  to ground conductors in other layers, vias from the bottom layer ground  32  to ground conductors in other layers, vias connecting ground conductors in intermediate layers, and direct contact, or adhesion using a conductive epoxy film  15 , between the PWB shelf  12  and bottom layer ground  32 . 
         [0045]    Although limited embodiments of a grounding system for an array of blind-mate coaxial connectors have been specifically described and illustrated herein, many modifications and variations will be apparent to those skilled in the art. Accordingly, it is to be understood that the grounding system constructed according to principles of this invention may be embodied other than as specifically described herein. The invention is also defined in the following claims.