Abstract:
A J-tail construction of a flexible interconnect between a membrane switch and auxiliary circuit boards allows for the elimination of an electrical joint between the membrane switch and the connector for circuit board orientations desirable in appliance manufacture.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application claims the benefit of U.S. Provisional Application 60/497,595 filed Aug. 25, 2003 hereby incorporated by reference. 
     
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT  
       [0002]     --  
       BACKGROUND OF THE INVENTION  
       [0003]     The present invention relates to membrane switches and in particular to an interconnection method useful for connecting membrane switches to other circuit elements.  
         [0004]     Membrane switches provide pairs of electrical contacts facing each other across opposed surfaces of spaced apart flexible membranes. Normally, the contacts are held in separation by a spacer layer and the natural elasticity of the membranes. Pressure on a front membrane, deforming the membrane inward toward a rear membrane, may cause the contacts to touch providing for a switching of electrical current.  
         [0005]     A flexible graphic fascia layer printed with button outlines may be adhered to the front face of the membrane switch and provides a continuous surface resistant to environmental contamination. For this reason, membrane switches are popular in many appliances where the surfaces of the switches may be splashed with liquid or washed during normal use.  
         [0006]     In a typical appliance, the membrane switch will be electrically connected by means of a wiring harness to a circuit board providing control functions for the appliance. The circuit board normally positioned behind the membrane switch in the limited space provided by the appliance housing, is usually formed of a rigid material amendable to standard printed circuit board manufacturing and assembly techniques. This is in contrast to the flexible circuit material of the membrane switch.  
         [0007]     Referring to  FIG. 2 , it is known in the art to use a flexible circuit strip  10  to connect the membrane switch  12  to the auxiliary circuit board  14  thereby avoiding the expense of discrete wiring. The flexible circuit strip  10  has a series of parallel conductors  16  exposed along one surface of a flexible insulating substrate and may be constructed using well known materials and techniques including, but not limited to, printing and etching fabrication methods.  
         [0008]     One end of the flexible circuit strip  10  may be attached to the membrane switch  12  and the other end received by an electrical connector  20  on the auxiliary circuit board  14  allowing the latter to be disconnected for manufacturing and repair.  
         [0009]     In a common appliance configuration, conductors  22  of the membrane switch  12  will be exposed on a front face of the rear membrane of the membrane switch  12  toward the user, and the electrical connector  20  on the circuit board  14  will open rearward from the circuit board  14  to connect with the parallel conductors  16  of the flexible circuit strip  10  on a lower side of the electrical connector  20 . These orientations, desirable for reasons of manufacture and access to the electrical connector  20 , require that the parallel conductors  16  of the flexible circuit strip  10  be connected to the conductors  22  of the membrane switch  12  in a face-to-face abutment. One method of making such a connection is by means of a “z-axis” adhesive which conducts electricity predominantly through rather than along the adhesive interface. The z-axis adhesive allows multiple parallel conductors  16  to be electrically connected with corresponding aligned conductors  22  without shorting between parallel conductors  16  or conductors  22 .  
         [0010]     This use of a z-axis epoxy adhesive increases the manufacturing complexity. Further, z-axis adhesives can be unreliable and by leaving a small gap between the flexible circuit strip  10  and the membrane switch  12  can promote a capillary attraction that draws moisture into the interface and degrades the electrical connection.  
       BRIEF SUMMARY OF THE INVENTION  
       [0011]     The present invention provides a flexible connector strip with a “J-tail” construction in which the flexible connector strip doubles back along its length while remaining in a plane of the flexible conductor strip. The J-tail effects a change of direction of the conductors without inverting the conductors. This in turn allows the flexible conductive strip to be a simple continuation of the flexible material of the membrane switch eliminating the need for a z-axis adhesive bond.  
         [0012]     Specifically the present invention provides an electrical interconnection system in which a flexible circuit providing conductors of a membrane switch may communicate with a circuit board having a connector. A flexible circuit strip has a first end attached to the flexible circuit and a second end receivable by the connector of the circuit board, the second end of the flexible circuit strip including a portion doubling back along the flexible circuit strip within a plane of the flexible circuit strip before being received by the connector on the circuit board.  
         [0013]     It is thus an object of at least one embodiment of the invention to provide an improved flexible circuit strip to interconnect membrane switches with associated circuit board.  
         [0014]     The flexible circuit strip may be a continuation of a flexible circuit forming part of the membrane switch and integral therewith. The same conductors of the flexible circuit may extend onto the flexible circuit strip.  
         [0015]     Thus it is therefore another object of at least one embodiment of the invention to provide a flexible circuit strip that prevents the need for a z-axis adhesive bond between the flexible circuit strip and conductors of the membrane switch. The invention may also eliminate the need for through-hole connections.  
         [0016]     The connector may include contacts contacting conductors on a surface of the flexible circuit strip.  
         [0017]     It is thus another object of at least one embodiment of the invention to provide a system that works with a connector that may directly receive the flexible circuit strip.  
         [0018]     The connector may include contacts contacting conductors on an outer surface of the flexible circuit strip.  
         [0019]     It is thus another object of at least one embodiment of the invention to allow convenient orientation of the membrane switch, the circuit board and the connector to allow the use of standard assembly techniques, connectors, and orientations of circuit boards.  
         [0020]     These particular objects and advantages may apply to only some embodiments falling within the claims and thus do not define the scope of the invention. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0021]      FIG. 1  is a simplified perspective view of an appliance showing a membrane switch exposed on a front door of the appliance such as may form part of the present invention;  
         [0022]      FIG. 2  is a perspective view of a prior art interconnection system for a membrane switch auxiliary circuit board using a z-axis adhesive as described above in the Background of the Invention;  
         [0023]      FIG. 3  is an exploded perspective view of an interconnection system of the present invention showing a J-tail flexible circuit strip such as eliminates the need for a z-axis adhesive;  
         [0024]      FIG. 4  is a cross-section along lines  4 — 4  of  FIG. 3  showing connections between conductors on the flexible circuit strip and the conductors of the circuit board; and  
         [0025]      FIG. 5  is a perspective view of the underside of the strip of  FIG. 3  showing surface mounted parallel conductors. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0026]     Referring to  FIG. 1 , an appliance  30  such as a dishwasher may include a housing  32  having a console area  34 , in this case the front of a door  36 , visible and accessible to a user of the appliance  30 . A membrane switch  38  may be positioned with a front face exposed to the user of the appliance  30  at the console area  34  so as to be operable by the user. Alternatively, the membrane switch  38  can be located on a surface not visible by the operator once the appliance is in use—for example—on the top of the door which is ultimately hidden by the countertop when the appliance is installed.  
         [0027]     Referring now to  FIG. 3 , the membrane switch  38  is composed of a transparent graphic layer  40 , through whose front surface may be visible button indicia  44  printed on the rear of the graphic layer  40 . The graphic layer  40  covers a front membrane  42  on whose rear surface may be placed shorting pads  45  as is understood in the art. A spacer layer (not shown) is positioned between the front membrane  42  and a rear membrane  46 , the latter having on its front surface interdigitated contact fingers  48  of a type well known in the art. The rear membrane  46  is flexible and provides a circuit with a flexible insulating substrate  17  supporting on its front face electrical conductors  50  communicating from the interdigitated contact fingers  48  to a lower edge of the rear membrane  46 .  
         [0028]     As will be understood to those of ordinary skill in the art, the interdigitated contact fingers  48  and conductors  50  may be printed of conductive inks or the like or created using subtractive etching techniques, and are shown in simplified form in  FIG. 3  in which the interdigitation and separate conductors for each of the finger pairs is not depicted.  
         [0029]     The flexible insulating substrate  17  of the rear membrane  46  may continue downward in a tab  52 , then bend rearward to provide the basis of the flexible circuit strip  10 . It will be understood, therefore, that the rear membrane  46  and flexible circuit strip  10  may share a common insulating substrate  17  and be manufactured together as an integral unit with conductors  50  becoming parallel conductors  16  exposed on a lower outer face of the flexible circuit strip  10 .  
         [0030]     Referring now to  FIGS. 3 and 5 , the flexible circuit strip  10  extends generally along a length  54  from the rear membrane  46  to the auxiliary circuit board  14  and the parallel conductors  16  run along that length and are spaced apart from each other across the width  56  of the flexible circuit strip  10 . The thickness  58  of the flexible circuit strip  10  is limited to provide flexibility. The width  56  and length  54  together define the plane  59  of the flexible circuit strip  10  being a two-dimensional planar or non-planar.  
         [0031]     Referring again to  FIG. 3 , the flexible circuit strip  10  in the present invention includes a J-tail section  60  in which the flexible circuit strip  10  curves back upon itself within the plane  59  of the flexible circuit strip  10  in contrast to the out-of-plane curvature shown in  FIG. 2 . As used herein, in-plane curvature is instantaneous curvature about an axis that is generally perpendicular to the instantaneous plane  59  of the flexible circuit strip  10 , whereas the out-of-plane curvature is instantaneous curvature about an axis that is generally parallel to the instantaneous plane  59  of the flexible circuit strip  10 . In the preferred embodiment, the J-tail section  60  provides two (2) parallel legs of the “J” so that the parallel conductors reverse direction by 180 degrees to be accepted by the rearwardly opening electrical connector  20 . In the J-tail section  60 , the parallel conductors  16  remain on the lower or outer surface of the flexible circuit strip  10 .  
         [0032]     Referring now to  FIG. 4 , the parallel conductors  16  on the lower surface of the flexible circuit strip  10  contact flexible fingers  64  contained within the shell of electrical connector  20  when the end of the flexible circuit strip  10  is inserted into the opening of the electrical connector  20 . The flexible fingers  64  extend upward from the shell of the electrical connector  20  and extend downward through the shell of connector  20  to join conductive traces  66  on the bottom or top of the circuit board  14 .  
         [0033]     Referring to  FIG. 3 , it will be understood to one of ordinary skill in the art, that the J-tail section  60  need not be planar but may, for example, have one or both legs curved out-of-plane to allow the circuit board  14  to be oriented at alternative angles as necessary. The term J-tail has been adopted herein for descriptive simplicity, but the present invention should not be considered to be limited (beyond as stated in the claims) to a connector strip that necessarily resembles a letter J.  
         [0034]     It is specifically intended that the present invention not be limited to the embodiments and illustrations contained herein, but include modified forms of those embodiments including portions of the embodiments and combinations of elements of different embodiments as come within the scope of the following claims.