Abstract:
An electrical connector includes contact pads on a printed circuit board and contact members on a substrate. The contact members are pressed against the contact pads by a compression mat having compressor fingers. A clamping arrangement forces the compressor fingers against the substrate and thereby presses the contact members against the connector pads. The compression mat is made of elastomeric material, which has a tendency to relax and thus reduce the pressure after the clamping arrangement is tightened. A restrainer member is used to offset this tendency of the polymer to relax. The restrainer member has holes through which the compressor fingers of the compression mat extend.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
   This is a continuation of application Ser. No. 10/691,294, filed Oct. 22, 2003, U.S. Pat. No. 6,814,589 the entire disclosure of which is incorporated herein by reference. 

   BACKGROUND OF THE INVENTION 
   The present invention is directed to an electrical connector for use with printed circuit boards. More particularly, the invention is directed to a connector of the type that uses a compression mat made out of elastomer material in order to press contact members against contact pads. 
   Connectors are in widespread use in the electronics industry. One class of electrical connectors employs a first mechanical support that holds first contacts and a second mechanical support that holds second contacts. In use, the first and second contacts are either pressed against one another or inserted one inside the other. One disadvantage of this class of connector is that at least one of the first and second mechanical supports must typically be mounted on a housing or other structure, and the contacts must be soldered to conductors. 
   In another class of connectors, printed circuit wiring extends to the edge of a printed circuit board. The edge of the printed circuit board is inserted into a fixture having contacts that engage the wiring. 
   In a further class of connectors, contact members on a ribbon cable are pressed against contact pads on a printed circuit board. Pressure is exerted on the back of the ribbon cable by a compression mat having compressor fingers that are aligned with the contact members and contact pads. The compression mat is clamped to the printed circuit board. The compression mat and its compressor fingers are made of elastomer materials, and the compressor fingers act somewhat as springs. When the clamping arrangement is tightened, the compressor fingers are placed under state of compression and bulge outward, like small barrels. A connector of this type is disclosed in U.S. Pat. No. 6,607,120. 
   Connectors of this latter type have a drawback in that the elastomer material of the compression mat has a tendency to relax after the clamping arrangement has been tightened to a desired state. The compressor fingers bulge outward and assume a shape that becomes more barrel-like with the passage of time. The relaxation of the material reduces the pressure forcing the contact members against the connector pads, and thus may lead to faulty connections. 
   One might consider adjusting the geometry or hardness of a compression mat in an attempt to minimize this stress relaxation. However, as the hardness of a compression mat increases, so does the actuation load that is required to compress the compressor fingers to the necessary degree. Furthermore, attempts might be made to shorten the compressor fingers in an attempt to minimize stress relaxation, but shorter compression fingers pose reliability concerns due to assembly tolerance stack (e.g., compressor fingers that are not quite long enough but are still within tolerance may not press the contact members against the contact pads with sufficient force to ensure a reliable connection). 
   Another problem with conventional compression mats is that the compression fingers are prone to off-axis loading, so that one or more compressor fingers may skew to one side. This phenomenon, too, is detrimental to reliability. 
   SUMMARY OF THE INVENTION 
   The primary object of the present invention is to provide a connector which employs an elastomeric compression mat for pressing contact members against contact pads on a printed circuit board, but without the drawbacks discussed above with respect to the prior art. 
   A related object is to improve the reliability of a connector which employs a compression mat to force contact members against contact pads. 
   A further object is to provide a restrainer member that reduces the tendency of a compression mat to relax after initial compression and which also reduces skewing of the compressor fingers of a compression mat. 
   In accordance with one aspect of the invention, these and other objects that will become apparent in the ensuing detailed description can be attained by providing a connector that includes connector pads in a contact region on a printed circuit board and contact members that are disposed in a contact region on a first side of an insulating substrate. The contact region of the substrate is aligned with the contact region of the printed circuit board. The connector also includes a compression mat having compressor fingers that contact a second side of the substrate in alignment with the contact members on the first side, and a clamping arrangement to press the compression mat toward the printed circuit board. In order to counteract the inherent tendency of the compressor fingers to undergo stress relaxation after the compressor mat has been clamped, the connector also includes a restrainer member having holes through which the compressor fingers of the compressor mat extend. 
   In accordance with another aspect of the invention, a compression mat having compressor fingers is used in conjunction with a restrainer member in a method for electrically connecting pads that are disposed within a contact region on a printed circuit board to contact members that are disposed within a contact region on a first side of an insulating substrate. The method includes the step of bracing the compressor fingers with the restrainer member, which has holes through which the compressor fingers extend. The method also includes the step of placing the contact region of the substrate in a face-to-face relationship with the contract region of the printed circuit board and the step of positioning the compression mat adjacent a second side of the substrate, with the contact fingers being aligned with the contact members on the first side of the substrate. Finally, the method includes the step of pressing the compression mat toward the printed circuit board. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a top view schematically illustrating two printed circuit boards and a ribbon cable that connects them by way of two connectors in accordance with the present invention; 
       FIG. 2  is a top view of a contact region on a broken-away portion of one of the printed circuit boards shown in  FIG. 1 ; 
       FIG. 3  is a bottom view of a contact region on a broken-away portion of the ribbon cable shown in  FIG. 1 ; 
       FIG. 4  is a cross-sectional view of a connector in accordance with the present invention; and 
       FIG. 5  is a cross-sectional view of a compressor mat and restrainer member in accordance with a modified embodiment of the invention. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The present invention is directed to an improved connector that can be used, for example, to connect a ribbon cable to contact pads on an integrated circuit board.  FIG. 1  illustrates a first printed circuit board  10  having circuitry such as integrated circuits  12  and a second printed circuit board  14  having circuitry such as integrated circuits  16 . A ribbon cable  18  having a plurality of parallel conductors (not shown in  FIG. 1 ) carries signals between the circuitry of the first and second printed circuit boards  10  and  14 . Reference number  20  designates a clamping plate that is part of a connector  22  (see  FIG. 4 ) that connects the left end of cable  18  to the circuitry on printed circuit board  10 . Similarly, the right end of cable  18  is connected to the circuitry on printed circuit board  14  by a connector  22  that includes a clamping plate  20 . 
     FIG. 2  illustrates a contact region  24  on the top side of printed circuit board  10 . The region  24  contains an array of contact pads  26 . Dotted lines that are shown in  FIG. 2  between the contact pads  26  are intended to indicate that more contact pads are typically present in the contact region  24  than are shown in FIG.  2 . Printed wiring  28  connects the contact pads  26  to the circuitry carried by the printed circuit board  10 . The printed circuit board  10  is provided with alignment holes  30  adjacent the ends of the contact region  24 . 
     FIG. 3  illustrates the bottom side of the left end of the ribbon cable  18 . It includes a flexible plastic strip  30  with an array of contact members  32  that are grouped within a contact region  34 . Dotted lines are used between the contact members  32  in  FIG. 3  in order to indicate that more contact members are typically present than are actually shown in the drawing. Printed wiring  36  is carried by the strip  30  and connected to the contact members  32 . Although the printed wiring  36  in  FIG. 3  is located on the same side of strip  30  as the contact members  32 , the wiring  36  may be provided on the reverse side of the strip  30  and connected electrically to the contact members  32  by plated through-holes (not shown). The strip  30  is provided with an alignment hole  38  adjacent each end of the contact region  34 . 
   When the ribbon cable  18  is inverted and the holes  38  are aligned with the holes  30  in the printed circuit board  10 , the contact region  34  will be aligned with the contact region  24  and the contact members  32  of the ribbon cable  18  will be positioned directly above corresponding contact pads  26  on the printed circuit board  10 . 
   With reference to  FIGS. 2-4  together, the connector  22  includes the contact pads  26 , the contact members  32 , a compression mat  40  having an array of compressor fingers  42  that are located so as to press the contact members  32  against the contact pads  26 , and a clamping assembly  44  which presses the compression mat  40  toward the printed circuit board  10 . This exerts a compressive force on the compressor fingers  42 , which act somewhat as springs. However, the compression mat  40  and its compressor fingers  42  are made of a rubbery, elastomeric material such as thermal silicon rubber. As was discussed in the “Background of the Invention” section of this document, the elastomeric material has a tendency to relax over a period of time after it has been placed in a state of compression. It is believed that this tendency for the elastomeric material to relax is accompanied by a slight increase in the bulge of the compressor fingers  42  or possibly a redistribution of the bulge. At any rate, the result is that the pressure forcing the contact members  32  against the contact pads  26  would ordinarily be reduced after the clamping assembly  44  is originally tightened. In order to reduce the tendency of the elastomeric material to relax, the connector  22  also includes a restrainer member  46 . It is made of a pliable material having a stiffness, or durometer measurement, which is smaller than that of the elastomeric material of the compression mat  40 . 
   The purpose of the clamping assembly  44  is to force the compression mat  40  toward the printed circuit board  10 . It will be apparent that there are many possible ways to achieve this purpose and that the clamping assembly  44  may take many forms. In the form shown in  FIG. 4 , the clamping assembly  44  includes a clamp member  48  having two cylindrical alignment arms  50 . Threaded metal bolts  52  are embedded in the alignment arms  50  and have outer portions that extend above them. The clamp member  48  may be made by an injection molding process. 
   The clamping assembly  44  also includes nuts  54  that screw onto the bolts  52  and cap elements  56  beneath the nuts  54 . The cap elements  56  have disk-shaped upper surfaces with holes in them for passage of the bolts  52 , and cylindrical skirts that extend downward to press against the clamping plate  20 , which is also part of the clamping assembly  44 . The clamping plate  20  has holes (not numbered) for passage of the alignment arms  50 . 
   During assembly, the alignment arms  50  are threaded through the alignment holes  30  (see  FIG. 2 ) of the printed circuit board  10 , the alignment holes  38  (see  FIG. 3 ) of the ribbon cable  18 , alignment holes  58  and  60  in the compression mat  40  and the restrainer member  46 , respectively, and the holes in the clamping plate  20 . The exposed outer portions of the threaded bolts  52  are threaded through the holes in cap elements  56  and nuts  54  are screwed on to the bolts  52 . The nuts  54  are then tightened to compress the compression fingers  42  so as to force the contact members  32  tightly against the contact pads  26 . 
   Compression mats with compression fingers are commercially available from InterCon Systems, Inc., of Harrisburg, Pa. as part of their C-BYTE™ connector system. Alternatively, the compression mat  40  can be made by injection molding, from (for example) thermal silicon rubber. Other suitable materials include polyurethane, flexibilized epoxies, and thermoplastic elastomers. 
   The restrainer member  46  can be made by placing the restrainer member  46  in a tray and then filling the tray to a suitable height with a polyurethane compound that has been heated to (for example) 60° C. The polyurethane is not poured over the compressor fingers  42  themselves, but is, instead, introduced through a nozzle at their base and allowed to rise to a suitable height. A suitable polyurethane compound is commercially available from Chemical Innovations Limited of Preston, England, under their trademark MONOTHANE A30. After it is poured, the polyurethane can be cured by heating it at approximately 150° C. for about 2 hours. 
   The inventors have confirmed the effectiveness of a restrainer member, made as discussed above, in an experiment. The results of this experiment are shown in the following Table: 
   
     
       
             
             
             
           
             
             
             
             
           
             
             
             
             
           
         
             
               TABLE 
             
           
           
             
                 
             
             
               Relaxation 
               Normal Force @ 31 mils compression (pounds) 
                 
             
           
        
         
             
               time 
               Compression mat w/o 
               Compression mat with 
               % Load 
             
             
               (hours) 
               restrainer member 
               restrainer member 
               Loss 
             
             
                 
             
           
        
         
             
               0 
               32.39 
               35.75 
               47.3 
             
             
               48 
               17.07 
               30.80 
               13.8 
             
             
                 
             
           
        
       
     
   
   In this experiment, the force initially required to compress the compressor fingers by 31 mils was 35.75 pounds when the compressor fingers  42  were fortified by a restrainer member  46 , and this fell to 30.80 pounds after a relaxation time of 48 hours. In contrast, for a compressor mat  40  without a restrainer member  46 , the force initially required to compress the compressor fingers by 31 mils was 32.39 pounds, and this fell to 17.07 pounds after a relaxation time of 48 hours. In short, the amount of stress relaxation of the compressor fingers  42  with the restrainer member was less than a third of the stress relaxation without the restrainer member  46 . 
   The relaxation-retarding properties of the restrainer member  46  can be varied by varying the Shore hardness of the material from which a restrainer member  46  is made (in this example, MONOTHANE A30 polyurethane) and also by varying the thickness of the material in the interstitial spaces between the compressor fingers  42 . Although it has been found that polyurethane with a hardness of Shore A30 and a height (in the interstitial spaces between the compressor fingers  42 ) of about 50 to 75% of the length of the compressor fingers  42 , counteracts the relaxation of the compression mat  40  admirably, it is believed that other hardnesses and heights would also provide satisfactory results. 
     FIG. 5  illustrates a different technique for making the retainer member  46 . First, in this technique, the retainer member  46  is fabricated by injection molding as a separate element having the alignment holes  58  and holes  62  for receiving the compressor fingers  42 . This separate element can then be lubricated (for example, by silicone oil) and inserted onto the compressor fingers  42 , as indicated schematically by the arrow  66 . 
   In addition to polyurethane, the restrainer member  46  could be made from silicone, flexibilized epoxies, or thermoplastic elastomers. If a thermoplastic elastomer is used, though, it would need to be molded around the compression mat  40  in either a secondary molding operation or a two-shot process (or as a separate, pre-formed element as in FIG.  5 ). Regardless of the material that is used for the restrainer member  46 , it should be softer than the material used for the compression mat  40 . 
   It will be apparent to those ordinarily skilled in the art that the embodiments described above are susceptible to various changes, modifications, and adaptations, and it is intended that such changes, modifications, and adaptations be covered by the appended claims.