FIELD OF THE INVENTION  
       [0001]     The invention relates to an interposer assembly and sub-assembly for forming electrical connections between two spaced circuit members.  
       DESCRIPTION OF THE PRIOR ART  
       [0002]     Interposer assemblies are used in modern electronic systems to form electrical connections between densely spaced contact pads on adjacent, parallel circuit members. The circuit members may be rigid, such as printed circuit boards and ceramic substrates or may be flexible. Flexible circuit members include flex cables having an electrically conductive core, often copper, and a surrounding insulator layer, often a polyimide coating. The contact pads on the two members connected by the interposer assembly are typically arranged in identical patterns. Two interposer assemblies and a short ribbon cable may by used to connect two rigid circuit members.  
         [0003]     Each interposer assembly includes an interposer plate, bolster plates and assembly screws. The interposer plate includes an insulating plate and a plurality of through-contacts carried in the plate and arranged in the same pattern as the pads on the circuit members. The interposer plate is held between upper and lower circuit members to form electrical connections between the opposed pairs of contact pads. The circuit members and interposer plate are sandwiched together between upper and lower bolster plates by screws to compress contacts in the interposer plate and form reliable connections with the contact pads on the circuit members.  
         [0004]     Interposer sub-assemblies include one bolster plate and one circuit member held together by screws extended through joining apertures in the bolster plate and circuit member, and a retention member for each screw. The sub-assembly is conventionally shipped as a unit from a manufacturer to a customer together with an interposer assembly and the other bolster plate. The customer assembles the sub-assembly, the interposer plate, another circuit member and the other bolster plate to form a complete interposer assembly.  
         [0005]     Double interposer sub-assemblies can include a short ribbon cable, two bolster plates, four screws and four retention members to form electrical connections between two rigid circuit members.  
         [0006]     Sub-assemblies are conventionally assembled using screws passed through the bolster plate and circuit member and a retention member to retain the screws in place and the subassembly together. The retention member may be threaded nuts, clips, nylon washers or a second bolster plate having threaded joining apertures. The retention members increase the cost of the sub-assembly.  
         [0007]     When using a retention member to retain the components of a sub-assembly, there is a risk that the sub-assembly will come apart when the consumer removes the retention member to install the sub-assembly on a circuit member. This complicates customer assembly.  
         [0008]     Therefore, there is a need for an interposer sub-assembly that allows for the easy installation of the sub-assembly onto a circuit member without the need to remove retention members. The sub-assembly should be easy for a customer to install, not require specialized retaining members, be inexpensive to produce and assemble, be usable with either flexible or inflexible circuit members and reduce or eliminate the chance of accidental separation of the sub-assembly components.  
       SUMMARY OF THE INVENTION  
       [0009]     The invention is an interposer assembly or sub-assembly having specialized screw retention members. The retention members allow sub-assembly components to be retained on two screws for easy installation of a circuit member, interposer plate and second bolster plate. The specialized retention members can be used in sub-assemblies having a flexible or an inflexible circuit member.  
         [0010]     The retention member is made by providing a stiffly flexible sheet over a joining aperture for each screw in the circuit member overlying the bolster plate. The stiffly flexible sheet may be a layer of polyimide tape, the polyimide coating of a flex circuit, or an elastomeric portion of a ribbon cable circuit member. A sheet aperture is cut into the sheet over the joining aperture to form one or more retention flaps extending into the aperture.  
         [0011]     When screws are extended through sheet apertures, the retention flaps are deformed and engage the threads to hold the screws in place. The joining apertures in adjacent sub-assembly components are enlarged to accommodate upwardly bent retention flaps. The retention flaps are flexible and permit retention of the screws into a top bolster plate during mounting of the sub-assembly on an interposer plate, second circuit member and top bolster plate.  
         [0012]     The retention flaps can be cut into a variety of shapes, including a cross cut aperture, an oval aperture, a semi-circular aperture, a star-shaped aperture or any other shape that defines one or more retention flaps capable of engaging a fastener thread extended through the sheet aperture. The retention flap can extend completely around the screw.  
         [0013]     The specialized aperture can also be formed directly out of a flexible circuit member. A flexible circuit retention member is cut through the flexible circuit member, away from circuit paths, to form flexible circuit retention flaps that act as the retention flaps described above.  
         [0014]     The specialized aperture can be located on an interposer assembly to form a sub-assembly comprised of a bolster plate, circuit member and interposer assembly.  
         [0015]     Other objects and features of the invention will become apparent as the description proceeds, especially when taken in conjunction with the accompanying drawings illustrating the invention.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]      FIG. 1  is a perspective view of two interposer sub-assemblies using a common flexible circuit;  
         [0017]      FIG. 2  is a top view of a portion of the flexible circuit of  FIG. 1 ;  
         [0018]      FIGS. 3   a ,  3   b , and  3   c  are top views of alternate flexible circuit retention members;  
         [0019]      FIG. 4  is a perspective view like that of  FIG. 1 , one sub-assembly mounted to an interposer assembly and the other mounted to a rigid circuit member and a bolster plate;  
         [0020]      FIG. 5  is a sectional view taken along line  5 — 5  in  FIG. 4 ;  
         [0021]      FIG. 6  is a top view of another flexible circuit retention member;  
         [0022]      FIG. 7  is a sectional view similar to  FIG. 5  showing a second embodiment flexible circuit retention member, and  
         [0023]      FIG. 8  is a top view of a another embodiment flexible circuit retention member. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0024]      FIG. 1  shows two interposer sub-assemblies  10  in accordance with the present invention. Each sub-assembly  10  has a flex circuit or circuit member  12 , a bolster plate  14  and two screws  16 . If desired, flex circuit  12  can be adhered to the bolster plate  14 .  
         [0025]     Flex circuit  12  has a resilient body  18 , a first end  20  and a second end  22 , each end having a plurality of contact pads  24 . Flex circuit joining apertures  26  extend through body  18  at each end  20  and  22 . Screw retention members  28  extend into the joining apertures  26 . Conductors join corresponding pads  24  on opposite ends of flex circuit  12 .  
         [0026]     Bolster plates  14  each extend along one end of the flex circuit  12  and include two bolster plate joining apertures  30  and two guide pins  32 . Guide pins  32  extend through flex circuit guide holes  34  to assure proper alignment between flex circuit  12  and bolster plate  14 .  
         [0027]     Screws  16  are extended through bolster plate joining aperture  30  and flex circuit joining apertures  26 . Screws  16  each include a head  36  and a threaded shaft. Retention members  28  engage threads on the screw shaft to hold sub-assembly  10  together with flex circuit  12  and bolster plate  14  between heads  36  and retention members  28 .  
         [0028]      FIG. 2  is a top view of retention member  28 . Retention member  28  is formed from polyimide film layer  38  that is continuous with flex circuit  12  and extends over flex circuit joining aperture  26 . Polyimide film layer  38  surrounds electrical conductor or copper core  40  at joining aperture  26 . Retention aperture  42  is cut into layer  38  to form flexible retention flaps  44  that extend from the perimeter of joining aperture  26  toward the center of joining aperture  26 . Core  40  does not extend into flaps  44 .  
         [0029]     A variety of retention apertures  42  can be used so long as they create one or more effective retention flaps  44 .  FIGS. 3   a ,  3   b  and  3   c  show retention apertures  42  that can be cut into layer  38  to form retention flaps  44 .  FIG. 3   a  shows an oval-shaped retention aperture  42   a  that forms two concave retention flaps  44   a .  FIG. 3   b  shows a star-shaped retention aperture  42   b  that forms five pointed retention flaps  44   b .  FIG. 3   c  shows retention aperture  42   c  that forms a single blunt tab retention flap  44   c.    
         [0030]      FIG. 4  shows two sub-assemblies  10 . End  20  of flex circuit  12  is mounted to interposer  46 . Interposer  46  is made up of an interposer plate  48  having through passages  50  and a contact  52  fitted in each passage. Interposer  46  may be an interposer assembly as described in U.S. Pat. No. 6,176,707. Bolster plate guide pins  32  pass through interposer guide holes  54  to assure proper alignment between contacts  52  and flex circuit contact pads  24  at end  20 .  
         [0031]     The second end  22  of flex circuit  12  is sandwiched between bolster plate  14  and a second interposer  46 . The second interposer forms electrical connections between contact pads  24  at end  22  and contact pads or points on rigid circuit member  56 . Rigid circuit member  56  includes joining aperture  58 . Bolster plate  60  includes joining apertures  62  and is placed above rigid circuit member  56 . Screws  16  extend through aligned joining apertures in the interposer plate, circuit elements and bolster plates. Bolster plate joining aperture  62  is threaded so that screw  16  engages bolster plate  60  to compress and hold the interposer assembly together.  
         [0032]      FIG. 5  is a cross sectional view of second end  22  of an interposer assembly  10  taken along the aligned joining apertures. Screw  16  is extended through retention aperture  42  so that retention flaps  44  are deformed upward and engage screw threads  64 . Screw  16  has a major diameter  66  and minor diameter  68 .  
         [0033]      FIG. 6  is a top view of another embodiment screw retention member  70 . Screw retention member  70  is formed from polyimide film layer  38  that is continuous with circuit member or flex circuit  12  and coats electrically conductive copper core  40 . Polyimide film layer  38  extends completely around the perimeter of circuit member joining aperture  26  to form a single retention flap  72 . Retention flap  72  defines a circular retention aperture  74 . Circular retention aperture  74  has a diameter less than major diameter  66  of screw  16 . Circular retention aperture  74  has diameter greater than the minor diameter  68  of screw  16 .  
         [0034]      FIG. 7  is a cross sectional view similar to  FIG. 5 . Screw  16  is extended into circular aperture  74  so that flap  72  is deformed to engage threads  64  on the shaft of screw  16 .  
         [0035]      FIG. 8  is a top view of a retention member  76  formed form a sheet  78  applied to the top surface of a circuit member over a circuit member joining aperture  80 . Sheet  78  may be a portion of polyimide tape or another suitably flexible substance. A retention aperture  82  is cut into sheet  78  to form one or more screw retention members or flaps  84  as described above.  
         [0036]     While the invention is directed to a interposer sub-assembly consisting of a bolster plate, a pair of screws and a flex cable having joining apertures with screw retention members, it is understood that the invention can also be directed to an interposer sub-assembly that includes an interposer, the interposer plate having joining apertures with specialized screw retention members adhered over plate joining apertures, as in  FIG. 8 .  
         [0037]     While I have illustrated and described preferred embodiments of my invention, it is understood that this is capable of modification, and I therefore do not wish to be limited to the precise details set forth, but desire to avail myself of such changes and alterations as fall within the purview of the following claims.
An interposer assembly including specialized fastener retention apertures for use with a threaded fastener. The interposer assembly components are held together by the threaded fastener and aperture.