Abstract:
A conductive filled polymer contact which is molded at an aperture through a carrier sheet includes an elongated conductive frame introduced prior to the molding process as an insert which is held captive in the molded contact and which extends from at or near the upper contact surface, through the aperture and terminates at the opposite end at or near the lower contact surface to provide a continuous conductive path through the length of the contact, whereby the sequence of particle to particle interfaces within the molded polymer contact is reduced in number to increase reliability.

Description:
FIELD OF THE INVENTION 
     This invention pertains to electrical connectors and more particularly to conductive particle filled elastomer contacts mounted on an interposer. 
     BACKGROUND OF THE INVENTION 
     The performance requirements of modern electronic systems has required the use of increased interconnect contact densities. For high density connector array applications, a common connector of choice is the land grid array (LGA) connector to achieve direct electrical interconnection between such typical devices as a module and a printed circuit board. Electrical engagement between the contact array of the module and the mating contact array presented by the circuit board is enabled by a contact carrying interposer, wherein connection is effected by aligning the interposer contact array with the respective mating surfaces of the module and circuit board and applying a clamping force to mechanically compress the interposer between module and circuit board. 
     The contacts mounted on the interposer must be closely adjacent one another to comply with the density of the aligned grids on the component and circuit board which are to be connected. While meeting the density requirement, the individual contacts must provide a reliable electrical connection between the confronting device contact surfaces and be wholly isolated from the adjacent contacts to preclude any shorting between signal lines. This must be accomplished despite any relative movement resulting from the variation of the coefficients of thermal expansion of the module and circuit board. 
     Several types of LGA interposer contacts are used including metal springs, fuzz buttons (a column of kinked, wadded wire filaments) and conductive elastomers. The present invention is an improvement of the conductor filled elastomer contacts. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to conductive filled polymer (CFP) contacts wherein a grid of contacts is created through transfer molding of a base elastomer that has been mixed with a curing agent and conductive particles. The LGA interposer is formed by molding conductive filled polymer contacts at apertures in an electrically insulating carrier sheet that is interposed between the mold portions during the molding sequence. The interposer is used to connect two arrays of contact surfaces typically presented by a module and a printed circuit board on which the module is to be mounted. The contact surface interconnections are established by clamping the module to the circuit board (with the interposer therebetween) with a force of sufficient magnitude to compress each interposer contact between the confronting contact surfaces to ensure electrical interconnection. Although the force required to be applied to each contact may be only 1 to 4 ounces, a grid of a few thousand contacts can require a significant compressive force. 
     Conductive particle filled polymer contacts are a common contact structure of choice. Such contacts are subject to the occasional occurrence of a glitch caused by an interruption of particle to particle electrical contact. Although the occurrence of a glitch is infrequent, in a grid including thousands of contacts which must always provide a reliable electrical path, even rare occurrences are not acceptable. A typical condition that may cause a glitch is the difference between the coefficients of thermal expansion of the module and the printed circuit board which are being interconnected. The differing coefficients can introduce a substantial lateral shear force which can cause particle to particle electrical contact to be interrupted or glitch. The present invention discloses a means to reduce the risk of glitching. 
     The glitching problem is reduced and the reliability of the conductive filled elastomer contact is enhanced by a conductive frame which is introduced into the mold cavity, prior to the molding of the contact, as an insert extending through the carrier aperture to provide a conductor extending from adjacent one contact surface on the contact body to the other contact surface on the contact body. The reliability of a CFP contact can be estimated based on the number of particle contact interfaces in parallel versus the number of particle contacts in series. With a conductive filled polymer there are many contacts across any given cross section of the CFP contact. For contacts having the same height and diameter with particles of uniform size, the ratio of the parallel to series contacts would be one or the contact reliability would be that of a single interface. However, since a contact bodies height is greater than the diameter, the ratio will actually be greater than one meaning the contact reliability has actually been reduced (i.e. the failure rate has been increased). 
     Use of a conductive frame reduces the number of series electrical connections between the contact upper and lower surfaces which engage the device contact surfaces to be interconnected. The presence of a conductive frame extending through the length of the contact provides a reliable electrical path through the length of the contact. The path between the contact surfaces being connected (the height of the connector) can be 8 to 10 times the length of the path to the conductive frame. The short particle to particle electrical path to the conductive frame increases contact reliability by effectively reducing the number of connector particle interfaces through the contact height. This reduces the contact bulk resistance through the contact height. 
     The conductive frame does not have to have spring properties since the contact body elastic properties are controlled by the elastomer properties. This means the electrical conductivity of the conductive frame base material can be pure copper or other highly conductive malleable material. Inclusion of a conductive frame increases the reliability achieved by reducing the dependence on particle surface contacts and further enables smaller contact cross sectional size and closer contact spacing. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an exploded view, illustrating a typical of a land grid array socket hardware. 
         FIG. 2  is an assembled elevation of the socket hardware of  FIG. 1 . 
         FIG. 3  is a section view of two adjacent conductive filled polymer contacts taken along a plane defined by the center lines of the contacts shown in cooperation with portions of the module and circuit board interconnected by the contacts. 
         FIG. 4  is a section view of a conductive filled polymer contact taken through the vertical center line of the contact which includes the conductive frame of the invention. 
         FIGS. 5 and 6  show embodiments of the conductive frame used in the practice of the invention. 
         FIG. 7  illustrates a second embodiment of the invention which employs a conductive frame which is longer than the height of the molded contact. 
     
    
    
     DETAILED DESCRIPTION 
       FIGS. 1 and 2  illustrate a typical land grid array (LGA) socket assembly  10 .  FIG. 1  is an exploded view of the assembly and  FIG. 2  is an assembled view of the elements of  FIG. 1 . The LGA module  12  carries a pattern or grid of contact surfaces on the lower surface which confronts the printed circuit board  14  that carries a pattern or grid of contact surfaces adapted to be aligned with and electrically connect the module to the circuits on the printed circuit board. The electrical connection is effected by an LGA interposer with a pattern or grid of contacts which are aligned between the LGA module contact surfaces and the printed circuit board contact surfaces. Each interposer contact extends through and is captured at an aperture in the electrically insulating interposer carrier substrate and is conductive and compressible to establish an electrically conductive path between the confronting contact surfaces presented by the module  12  and the printed circuit board  14 . A rigid upper stiffener  18  engages the upper surface of LGA module  12  and a rigid backside stiffener  20  abuts the lower surface of printed circuit board  14 . Backside stiffener  20  is electrically isolated from printed circuit board  14  by a backside insulator  21 . Beneath the backside stiffener  20  is a spring plate  22 . Load posts  24 , secured to the upper stiffener  18 , pass respectively through apertures  26  in the LGA interposer  16 , apertures  27  in the printed circuit board  14 , apertures  28  in the backside insulator  21 , apertures  29  in the backside stiffener  20  and are affixed to the spring plate  22  to secure the LGA module  12  to the printed circuit board  14  and align the connector grid on the interposer  16  with the grid of contact surfaces on printed circuit board  14 . The spring plate  22  slides laterally onto load posts  24  allowing keyhole openings  17  and open ended slots  19 , formed in the spring plate, to engage peripheral grooves  25  in the load posts. The LGA module grid of contact surfaces is aligned with the respective interposer contacts by the interposer marginal frame portion  31  that surrounds the LGA module  12  in the assembled condition of  FIG. 2 . The compressive force that urges the LGA module  12  toward the printed circuit board  14  is effected the load screw  32  which extends through a threaded opening in spring plate  22  and engages backside stiffener  20 . 
       FIG. 3  is a sectional view which illustrates a pair of typical prior art conductive filled polymer (CFP) contacts  34  mounted on a polyimide carrier sheet  36  at apertures defined by cylindrical walls  37 . Contacts  34  are compressed between pads or contact surfaces  38  at the surface of module  40  and the pads or contact surfaces  42  presented by printed circuit board  44  by a force F which is applied using a socket assembly such as that shown in  FIGS. 1 and 2 . The contacts  34  are each formed as a polymer body filled with metal particles that have a concentration that causes particles to engage adjoining particles and establish a conductive path from the lower surface  46  to the upper surface  48 . The lower surface  46  is the contact body surface below and most remote from the carrier sheet  36  which, as shown, engages the printed circuit board contact surface  42 . Similarly, upper surface  48  is the contact body surface above and most remote from the carrier sheet  36 . The conductive filler is commonly silver powder. The contacts are formed by transfer molding and are held captive at apertures in the polyimide carrier  36  by enlarged diameter portions adjacent the carrier. 
     The showings of  FIGS. 3 ,  4  and  7  are greatly enlarged for illustration. In current practice, typical contacts  34  have an overall height and pitch between contacts (arrow B) of 1.0 mm and a diameter adjoining the carrier of 0.7 mm. The carrier sheet thickness (arrows D-D) is about 0.125 mm. 
       FIG. 4  shows a contact  50  molded at an aperture in carrier sheet  51  which includes the conductive frame  52  of the present invention. The conductive frame  52  is inserted in the mold cavity prior to the transfer molding of the conductive filled polymer contact to become an insert captured in the completed contact. Since the conductive frame is captured within the polymer contact body, manufacturing tolerances are controlled by the polymer molding process, not the conductive frame manufacturing process, placement or deformation. Signal integrity is also controlled by the molding process. The contact surface is conductive and controls the electrical characteristics with respect to the next adjacent contacts. The conductive frame may be formed of any highly conductive material. Since the elastomer forming the body of the contact, rather than the frame, provides the compressive and elastic characteristics of the contact, the frame may be formed of pure copper or another malleable metal conductor. 
     The conductive frame  52  may take many physical forms. For example, it may be a cylindrical conductor, a continuous sheet formed as a split cylinder  56  (as seen in  FIG. 5 ), a split cylinder  58  which has an apertured wall (as in  FIG. 6 ), formed of metal screen material, or other configuration that serves as an elongated conductor providing a continuous conductive path. The conductive frame functions to provide a continuous conductive path from adjacent the contact top surface  53  to adjacent the contact bottom surface  54 . Since the conductive particle to particle path from either top surface  53  or bottom surface  54  is shorter to frame  52  than between top surface  53  and bottom surface  54 , the reliability of the contact is enhanced. The use of a tubular conductor frame  53 , in the form of a single piece cylinder, split cylinder or a perforate form of either having a generally circular cross section, optimizes the reduction of the length of particle to particle conductive paths to the frame&#39;s continuous conductive path. The reduced contact bulk resistance that provides improved reliability can also be utilized to reduce both the contact diameter and the pitch between contacts to achieve greater contact density when required. 
       FIG. 7  shows a second embodiment of the invention wherein the conductive frame, that is captured as an insert in the molded contact, is longer than the height of the molded contact. The longer conductive frame  61  engages the top and bottom of the mold to form a continuous conductive path from the top surface  63  to bottom surface  64 . To further accommodate its length, the conductive frame  61  can be formed to encourage buckling at the mid portion or could be prebuckled. This form of conductive frame would further assure a reliable electrical path through the contact length by an even greater reduction of the required particle to particle contact. 
     While the invention has been shown and described with reference to preferred embodiments thereof, it will be understood that various changes in form and details may be made therein without departing from the spirit and scope of the invention. For example, the conductive frame could take many shapes other than those shown and described, such as a helical coil, twisted rectangle or other shape that provides the conductive path through the contact and is compatible with the compression or transfer molding of the contact.