Abstract:
Resilient electrical interconnects are provided which have a non-uniform cross section which achieves an increased range of deflection and a predetermined relationship between compression force and deflection of the interconnect, and between resistance and deflection of the interconnect. A smaller cross sectional area decreases the spring rate, or compression force, of the interconnect during compression. Increased range of deflection and reduced spring rate enables improved compensation for surface irregularities and facilitates mounting of integrated circuit or other devices having large arrays of interconnects. The non-uniform cross section is provided by a single or compound slope, or alternatively a nonlinear curve, from the end of smaller cross-section to the end of larger cross-section.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     Not Applicable 
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not Applicable 
     BACKGROUND OF THE INVENTION 
     This invention relates generally to electrical interconnection devices, and more particularly to resilient electrical interconnection devices. 
     Resilient interconnects that provide electrical connection between opposing planar surfaces are known, and generally comprise an elastomeric or polymeric material containing conductive particles disposed in the material to provide a conductive path between a first surface and a second surface. In one version the interconnects are shaped as elongated columns providing a conductive path between the end faces or surfaces thereof, as shown in co-pending application Ser. No. 08/736,830, filed Oct. 28, 1996 now U.S. Pat. No. 5,949,029, issued Sep. 7, 1999. 
     Such resilient interconnects can be used to provide electrical connection between a printed circuit board and an integrated circuit or other device. For such a use, the resilient interconnects are disposed in respective openings in a substrate which are arranged in a predetermined pattern. Such a substrate can be a circuit board itself or a separate substrate attachable to a circuit board. The integrated circuit or other device has an array of contacts that correspond to the pattern of resilient interconnects disposed in the substrate. The integrated circuit or other device is mounted in the substrate by aligning the contacts with the resilient interconnects, applying force against the integrated circuit or device so that the resilient interconnects are compressed against the contacts, and then securing the integrated circuit or device to maintain intended contact force. 
     Complications may occur when a relatively large array of resilient interconnects is employed. Because each resilient interconnect must be deflected to a sufficient degree to establish and maintain electrical connection, the amount of force required to seat the integrated circuit or other device on the printed circuit board increases in proportion to the number of resilient interconnects that are employed. The amount of force that is required may hinder assembly. Further, as the number of interconnects employed is increased, there is an increased risk that satisfactory connection will not be established with all the interconnects, especially where a mating circuit board surface is warped or irregular. In some instances, the resistant interconnects will not be sufficiently compressed to make electrical contact or proper electrical contact, while in other instances the resilient interconnects may be excessively compressed such that misalignment can occur. The misalignment of a resilient interconnect and a mating contact can result in improper engagement of the interconnect with an adjacent contact or short circuiting of one or more contacts. 
     BRIEF SUMMARY OF THE INVENTION 
     In accordance with the present invention, resilient electrical interconnects are provided which have a non-uniform cross section which achieves an increased range of deflection and a predetermined relationship between compression force and deflection of the interconnect, and between resistance as a function of deflection. A smaller cross sectional area decreases the spring rate, or compression force, of the interconnect during compression. Increased range of deflection and reduced spring rate enables improved compensation for surface irregularities and facilitates mounting of integrated circuit or other devices having large arrays of interconnects. The non-uniform cross section is provided by a single or compound slope, or alternatively a nonlinear curve, from the end of smaller cross-section to the end of larger cross-section. 
     Various embodiments as disclosed herein encompass shapes of tapered columns having a straight line contour along the entire height, a contour following a plurality of straight lines at different angles, and a contour following a curve. Conductive material used to make the interconnect, such as a polymeric material with conductive particles, provide the necessary conductivity between the opposed ends of the interconnect. The end of the interconnect having the larger cross-section provides a base for mounting on a fixed contact surface, such as a circuit board or socket, while the end having the smaller cross section provides a contact end for contacting a conductive pad or terminal on the mating device. The broader base stabilizes the interconnect and maintains alignment of the contact end to minimize lateral movement of the interconnect to avoid adjacent interconnects from contacting each other or from contacting an incorrect conductive pad on the mating device. 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
     The invention will be more fully understood in view of the Detailed Description of the Invention, and the Drawing of which: 
     FIG. 1 a  is a side view that illustrates resilient electrical interconnects in accordance with the invention; 
     FIG. 1 b  is a side view that illustrates the interconnects of FIG. 1 a  in a deflected state; 
     FIG. 1 c  is a side view that illustrates contact pads attached to an uneven contact surface; 
     FIG. 2 a  is a pictorial view of one embodiment of a resilient electrical interconnect; 
     FIG. 2 b  is a side view of the embodiment of FIG. 2 a;    
     FIG. 2 c  is a curve of the resistance vs. deflection characteristic of the embodiment of FIGS. 2 a  and  2   b;    
     FIG. 2 d  is a curve of force vs. deflection for the embodiment of FIG. 2 a  and  2   b;    
     FIG. 3 a  is a pictorial view of a second embodiment of the invention; 
     FIG. 3 b  is a side view of the embodiment of FIG. 3 a;    
     FIG. 3 c  is a curve of resistance vs. deflection for the embodiment of FIGS. 3 a  and  3   b;    
     FIG. 3 d  is a curve of force vs. deflection for the embodiment of FIGS. 3 a  and  3   b;    
     FIG. 4 a  is a pictorial view of a third embodiment of the invention; 
     FIG. 4 b  is a side view of the embodiment of FIG. 4 a;    
     FIG. 4 c  is a curve of resistance vs. deflection for the embodiment of FIGS. 4 a  and  4   b;    
     FIG. 4 d  is a curve of force vs. deflection for the embodiment of FIGS. 4 a  and  4   b;    
     FIG. 5 a  is a pictorial view of another embodiment of the invention having a curved wall; 
     FIG. 5 b  is a side view of the embodiment of FIG. 5 a;    
     FIG. 6 is a cutaway side view of an embodiment employing a pair of interconnects in accordance with the invention; 
     FIG. 7 a  illustrates a double ended version of a resilient interconnect in accordance with the invention; 
     FIG. 7 b  is a cutaway side view of the embodiment of FIG. 7 a  mounted in a substrate; 
     FIG. 7 c  is a pictorial view of the embodiment of FIG. 7 a  mounted in a substrate; 
     FIG. 8 a  is another embodiment of a double ended version of the invention; 
     FIG. 8 b  is a sectional side view of the embodiment of FIG. 8 a;    
     FIG. 8 c  is a sectional side view of the embodiment of FIG. 8 a  mounted in a substrate; 
     FIG. 8 d  is a pictorial view of the embodiment of FIG. 8 a  mounted in a substrate; 
     FIG. 9 a  is a pictorial view of yet another embodiment of the invention; 
     FIG. 9 b  is a sectional side view of the embodiment of FIG. 9 a;  and 
     FIG. 9 c  is a sectional side view of the embodiment of FIG. 9 a  mounted in a substrate. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to FIG. 1 a  resilient electrical interconnects  10  are mounted on a substrate  11  such as a printed circuit board for providing electrical interconnection between mating contacts of the substrate  11  and conductive areas  12  of an integrated circuit  14  or other device. Each interconnect  10  is formed of a resilient non-conductive material such as a polymeric material having conductive particles dispersed therethrough to provide an intended level of conductivity. Each interconnect includes a body  16  of truncated conical shape having a top area of smaller cross-section terminating in a contact surface  18  and a larger base area from which extends a mounting stem  20 . The mounting stem  20  is inserted in a hole provided in substrate  11  and the bottom surface of the body  10  is disposed on the confronting surface of the substrate surrounding the hole. The stem  20  is sized in relation to the associated hole to be press fit into the hole for retention of the body  10  on the substrate. The substrate has conductive areas  22  on a surface thereof on which the bottom surface of body  16  is supported in mechanical and electrical contact. The mounting holes in the substrate may have plated walls to also electrically contact the stem portion of the interconnect disposed therein. Preferably the body is chemically bonded to the conductive areas of the substrate or plated hole to provide good electrical connection. 
     The composition of the resilient electrical interconnects can be as disclosed in co-pending application Ser. No. 08/736,830 filed Oct. 28, 1996, now U.S. Pat. No. 5,949,029, and issued Sep. 7, 1999 and incorporated herein by reference. The interconnects can also be of other conductive resilient compositions, for example a thermoplastic elastomer having carbon particles disposed therein. 
     In FIG. 1 a,  the interconnects  10  are illustrated prior to electrical engagement with the mating contacts  12 . To provide electrical engagement, the interconnects are brought into engagement with the mating contacts and are retained in contact engagement by suitable mechanical fixturing which is usually part of an interconnect socket which can take a variety of configurations known to the art. When engaged the interconnects  10  are compressed as illustrated in FIG. 1 b  to provide electrical connection between contacts  12  of the integrated circuit or other device and the contacts  22  of the substrate. 
     The resilient electrical interconnects have a range of deflection sufficient to provide proper contact engagement with mating contacts without introducing excessive force on the interconnects or the mating contacts which could have deleterious results such as unwanted lateral deflection of the interconnects and misalignment of the interconnects in relation to the mated contacts. The distance between upper and lower contacts to be interconnected via the resilient interconnect can vary due to warped or irregular circuit boards or other substrates as shown in FIG. 1 c.  The upper contact  12   a  is spaced from lower contact  22   a  by a distance less than the distance separating contacts  12   b  and  22   b.  The difference in separation distance can be caused for example by a warped or bent circuit board as depicted in exaggerated form by reference  24 . 
     The range of deflection provided by the interconnects of the present invention having non-uniform cross-section is greater than a resilient interconnect of uniform cross-section while maintaining an acceptable range of compressive forces for providing the contact engagement. 
     The height, cross-sectional area and contour of the interconnect body between the upper and lower contact surfaces can be specified to provide an intended electrical resistance vs. deflection characteristic and force vs. deflection characteristic. 
     FIG. 2 a  and  2   b  illustrate a conically shaped resilient electrical interconnect similar to that shown in FIG. 1 a.  The taper of the conical surface and the height and width of the body can be dimensioned to suit particular operational requirements. A radiussed area  19  can join the contact end  18  and the tapered body. A typical resistance vs. deflection curve for this embodiment is shown in FIG. 2 c.  A typical force vs. deflection curve is shown in FIG. 2 d.    
     In FIGS. 3 a  and  3   b  there is shown a resilient interconnect having a truncated conical body of two different slopes. The body  30  has a taller upper portion  32  of one slope and a shorter base portion  34  of greater slope. The compound sloped surfaces of the interconnect body  30  provide a relatively steep force vs. deflection characteristic as shown in FIG. 3 d.  The resistance vs. deflection curve is shown in FIG. 3 c  which depicts the resistance quickly declining from an initial value to its final value with a relatively small amount of deflection. The relatively broader area of the base portion  34  of the body  30  provides a stable interconnect structure which can accommodate relatively large deflection without excessive force being imposed. 
     Another resilient interconnect having compound sloped surfaces is shown in FIGS. 4 a  and  4   b  in which the upper portion  42  and lower portions  44  are of approximately equal height. The resistance vs. deflection characteristic is depicted in FIG. 4 c,  and the force vs. deflection characteristic is depicted in FIG. 4 d.    
     Another embodiment of the resilient interconnect is shown in FIGS. 5 a  and  5   b  wherein the interconnect body has a curved surface  45  which provides a smoother force vs. deflection characteristic and which can provide more evenly distributed forces within the body during compression. 
     A pair of interconnects can also be provided in an opening of a substrate and extending in opposite directions as shown in FIG.  6 . The base area of each interconnect is in electrical engagement with a conductive surface area on the substrate. The stem portions of the interconnects can also be in electrical engagement with a conductive wall of a plated through hole in the substrate, as noted above. 
     The interconnect can also be formed in a double ended version as shown in FIG. 7 a.  Each portion of the interconnect is of truncated conical form. The upper portion  70  terminates in contact surface  74 , and the lower portion  72  terminates in contact surface  76 . An intermediate portion  78  is of reduced cross-sectional area and is sized to be accommodated within a corresponding opening in a substrate  77  as shown in FIGS. 7 b  and  7   c.  The tapered body portions  70  and  72  are joined to the surface areas by intermediate beveled peripheries  80  and  82 . The resistance and force characteristics as a function of deflection are generally similar to the characteristic curves shown in FIG. 3 c  and  3   d.    
     The resilient interconnects are typically arranged in an array mounted on a substrate, as shown in FIG. 7 c,  which corresponds to the array of contacts of a mating device. 
     FIGS. 8 a  and  8   b  show another double ended version of an interconnect, this version having compound tapered surfaces similar to the compound version shown in FIG. 3 a  and FIG. 4 a  above. These are similarly mounted on a substrate as shown in FIG. 8 c  and  8   d.    
     Another embodiment is shown in FIGS. 9 a  and  9   b  in which the upper end  90  of the interconnect is larger than the lower end  92 . The upper end  90  is of truncated conical shape having a single slope. The lower end  92  has a compound conical slope defined by broader portion  94  and narrower portion  96 . The interconnect is mounted on a substrate as shown in FIG. 9 c.  The upper end has a conical contact area  92 , and the bottom end has a planar contact area  99 . 
     The interconnects can be individually molded or otherwise formed and installed into respective openings of a substrate such as by press fitting the resilient elements into the substrate openings. Alternatively the resilient interconnects can be integrally molded or otherwise formed in the substrate to provide an interconnection array of intended configuration for particular purposes. 
     The contact surfaces of the interconnects may be of planar configuration as shown above, or may be of other configurations to mate with particular types of contacts, such as a conical contact area to engage a ball contact of a ball grid array. The resilient interconnect contact areas can be of other forms as may be desired to suit specific purposes. 
     Having the described the preferred embodiments of the invention, other embodiments which incorporate concepts of the invention will now become apparent to those skilled in the art. Therefore, the invention should not be viewed as limited to the disclosed embodiment but rather should be viewed as limited only by the spirit and scope of the appended claims.