Abstract:
An electronic interconnect includes a flexible circuit which has a plurality of contacts formed thereon. A plurality of springs is positioned so as to apply a compressive force to at least one of the contacts when the electronic interconnect is mated. A resilient material is configured to apply a compressive force to at least one of the springs.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to electrical interconnects. The present invention relates more particularly to a microelectronic contact assembly having a plurality of metallic springs and an elastomeric pad which cooperate to effect reliable electrical contact between raised interconnection features, such as bump contacts, formed upon a flexible circuit and contact pads of an electronic device. 
     BACKGROUND OF THE INVENTION 
     It is frequently desirable to attach electronic devices, such as integrated circuits or printed circuit boards, to flexible circuits. Flexible circuits typically have bump contacts or the like formed thereon to facilitate such attachment. The bump contacts of the flexible circuit provide electrical contact to corresponding contact pads of the electronic device so as to facilitate electrical communication between the flexible circuit and the electronic device. 
     According to contemporary practice, compressive force is applied to the flexible circuit so as to urge the bump contacts thereof into intimate contact with the corresponding contact pads of the electronic device. This compressive force is typically applied via a clamp and/or fastener arrangement which defines an attachment device or connector for maintaining the desired mechanical contact of the bump contacts of the flexible circuit with the contact pads of an electronic device. Occasionally, an elastomeric pad is provided on the opposite side of the flexible circuit, with respect to the bump contacts, in an attempt to maintain adequate compressive force over time. Thus, the elastomeric pad is compressed substantially by the clamping device and/or the fasteners which hold the flexible circuit in position with respect to the electronic device. 
     In this manner, the elastomeric pad is captured within the attachment device and is compressed as the bump contacts of the flexible circuit are urged into intimate contact with the corresponding contact pads of the electronic device. The compressed elastomeric pad generally continues to apply a compressive force to the bump contacts of the flexible circuit in an attempt to maintain the intimate contact of the bump contacts with the contact pads for a prolonged period of time. 
     As those skilled in the art will appreciate, such elastomeric pads additionally provide compliance by allowing the bump contacts of the flexible circuit to move with respect to the plane within which they are nominally formed. That is, the bump contacts are permitted to move generally at right angles with respect to this plane, so as to compensate for non-coplanar alignment of the corresponding contact pads of the electronic device. In this manner, the elastomeric pad tends to assure reliable contact of the bump contacts of the flexible circuit with the contact pads of the electronic device, even though there may be imperfections in the planar alignment of the contact pads of the electronic device. 
     Further, such elastomeric pads tend to concentrate the compressive force generally at the locations of the bump contacts, where the compressive force is needed. Since the compressive force tends to be concentrated in such a manner, the total clamping force may be reduced, thus facilitating construction of the attachment device of lighter and less expensive materials. Thus, the use of a resilient pad assures that the compressive force is selectively concentrated upon the contact feature, instead of being distributed across the entire terminus array. 
     However, as those skilled in the art will appreciate, such elastomeric pads tend to take on a mechanical compression set when compressed for a prolonged period of time, whereby such elastomeric pads tend to lose at least a portion of their resiliency. Thus, the compressive force applied by such elastomeric pads tends to degrade or lessen over time. This effect is particularly pronounced when elastomeric pads are subjected to harsh environments, such as elevated temperatures. 
     Frequently, modern high-density interconnects are used to communicate signals at comparatively high frequencies. The operation of interconnects at such high frequencies may result in heat build up, which is detrimental to the lasting effectiveness of elastomeric pads. Additionally, such interconnects are frequently used in automotive and other electronic applications, wherein the temperature of the environment is inherently elevated. For example, such interconnects may find applications in facilitating electrical communication between engine transducers and the dashboard of an automobile. In such applications, it is generally desirable that the electronic devices for which communication is facilitated by the interconnect operate reliably. 
     In view of the foregoing, it is desirable to provide an electronic interconnect having a compliant structure for facilitating reliable electrical contact of the bump contacts of a flexible circuit with the contact pads of an electronic device, wherein the electronic interconnect is not substantially susceptible to mechanical compression set. 
     SUMMARY OF THE INVENTION 
     The present invention specifically addresses and alleviates the above-mentioned deficiencies associated with the prior art. More particularly, the present invention comprises an electronic interconnect which includes a flexible circuit having a plurality of contacts formed thereon, a plurality of springs positioned so as to apply a compressive force to at least one of the contacts when the electronic interconnect is mated, and a resilient material configured to apply a compressive force to at least one of the springs. 
     According to the preferred embodiment of the present invention, the contacts formed upon the flexible circuit comprise bump contacts. The bump contacts are preferably formed upon the flexible circuit so as to define an array of bump contacts. The array of bump contacts may be of any desired shape. For example, the array of bump contacts may be generally circular, triangular, square, rectangular, hexagonal, octagonal, etc. 
     The springs preferably comprise metallic springs. The metallic springs are preferably formed so as to define an array thereof. The array of metallic springs will generally conform in shape to the array of bump contacts. Thus, for example, if the array of bump contacts is square, then the array of springs will typically be square as well. Generally, each pad possessing bump contacts (some pads have more than one bump formed thereon) has a dedicated metallic spring, so that there is a one-to-one correspondence between pads and metallic springs. 
     It is important to understand that each pad may optionally comprise more than one bump contact. For example, a single pad may have two, three, four or more separate bump contacts formed thereon. The use of a plurality of bump contacts for a single pad is generally done so as to enhance the reliability of the connection formed thereby. As those skilled in the art will appreciate, the use of such a plurality of bump contacts substantially enhances the reliability of electrical contact by providing a plurality of surfaces through which contact is made. 
     Thus, according to the preferred embodiment of the present invention, the contacts comprise an array of bump contacts and the springs comprise a corresponding array of springs, wherein each spring is configured so as to apply a compressive force to a dedicated pad possessing bump contacts when the interconnect is mated. The springs are disposed on an opposite side of the flexible circuit with respect to the contacts. 
     The resilient material preferably comprises a sheet of elastomer, e.g., an elastomeric pad, which is configured so as to apply compressive force to the springs when the interconnect is mated. The elastomeric pad applies a compressive force which is substantially localized to each individual bump contact. However, the use of an elastomeric pad alone would be subject to the problems associated with the elastomeric pad eventually taking a mechanical compression set, thereby losing at least some portion of the compressive force applied thereby. According to the present invention, the array of metallic springs continues to provide a compressive force to the bump contacts, even in the event of the elastomeric pad undergoes a mechanical compression set. 
     Preferably, the elastomeric pad is configured to facilitate substantially independent movement of the metallic springs with respect to one another, so as to provide the desired compliance. That is, the combination of the elastomeric pad and the array of metallic springs is flexible or bendable, such that the elastomeric pad and the metallic springs can move so as to accommodate imperfections in the planarity of the contact pads of the electronic device. 
     Thus, according to the present invention, each spring is configured so as to apply compressive force to only a dedicated pad possessing bump contacts when the electronic interconnect is mated. The elastomeric pad is preferably configured so as to facilitate substantially independent movement of the metallic springs with respect to one another, wherein such independent movement is generally orthogonal to the plane of the springs. Such orthogonal movement of the springs thus provides compliance which facilitates a reliable interconnection of a plurality of closely spaced bump contacts of a flexible circuit with corresponding contact pads of an electronic device, particularly when the contact pads of the electronic device are not substantially coplanar with respect to one another. 
     The combination of the elastomeric pad and the metallic springs tends to concentrate compressive force generally at the locations of the bump contacts, such that less overall compressive force is required to effect reliable contact between the bump contacts of the flexible circuit and the contact pads of the electronic device. Because less overall compressive force is required, the clamping structure of the connecting device (which effects mechanical attachment of the flexible circuit to the electronic device) can be made of less expensive materials. As those skilled in the art will appreciate, contemporary clamping structures are generally constructed of metal, such that they are capable of applying the necessary, comparatively high, compressive force required to effect reliable connection of the bump contacts of flexible circuits to the contact pads of an electronic device. 
     These, as well as other, advantages of the present invention will be more apparent from the following description and drawings. It is understood that changes in the specific structure shown and described may be made within the scope of the claims without departing from the spirit of the invention. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a semi-schematic fragmentary side view of an electronic interconnect, including a clamping structure, having metallic springs and having a resilient material disposed upon one side of the springs according to one configuration of the present invention; 
     FIG. 2A is an enlarged semi-schematic fragmentary side view of the electronic interconnect of FIG. 1, having the clamping structure removed therefrom; 
     FIG. 2B is an enlarged semi-schematic fragmentary side view of an alternative configuration of the present invention, wherein resilient material is disposed upon both sides of the springs; 
     FIG. 3 is a semi-schematic fragmentary perspective view of another embodiment of a metallic spring according to the present invention; 
     FIG. 4 is an enlarged semi-schematic fragmentary top view of the metallic spring of FIG. 3; and 
     FIG. 5 is a semi-schematic fragmentary side view of the metallic spring of FIG. 3, also showing an elastomeric pad in laminar juxtaposition with respect to the base of the metallic spring. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The detailed description set forth below in connection with the appended drawings is intended as an description of the presently preferred embodiment of the invention and is not intended to represent the only form in which the present invention may be constructed or utilized. The description sets forth the functions of the invention and the sequence of steps for constructing and operating the invention in connection with the illustrated embodiment. It is to be understood, however, that the same or equivalent functions and sequences may be accomplished by different embodiments that are also intended to be encompassed within the spirit and scope of the invention. 
     Referring now to FIGS. 1 and 2A, an electronic interconnect  100  comprises a clamping device  101  and a flexible circuit  103  to which it is desired that an electronic device  105  be placed in electrical communication. This is, the primary purpose of the electronic interconnect of the present invention is facilitating electrical communication between the flexible circuit  103  and the electronic device  105 . The clamping device  101  maintains desired positioning and alignment of the flexible circuit  103  and the electronic device  105  and also applies a compressive force to the flexible circuit  103  and electronic device  105 , as well as other components of the electronic interconnect  100 , as discussed in detail below. 
     The clamping device  101  more particularly comprises a first clamp member  107 , a second clamp member  109 , and a plurality (generally two) of bolts  111  and their corresponding nuts  113  (each of which only a representative one is shown). FIG. 1 is a fragmentary view, showing only a representative portion of the electronic interconnect of the present invention. Typically, two bolts  111  and their corresponding nuts  113  will be utilized to effect clamping of the first clamp member  107  and second clamp member  109 . Typically, the two bolts  111  and their corresponding nuts  113  are disposed at opposite ends of the clamping device  101 . However, those skilled in the art will appreciate that the number of fastening devices, such as bolts  111  and corresponding nuts  113 , as well as the position of fasteners, may be varied as desired. 
     The flexible circuit  103  has a plurality of raised features, such as bump contacts  115 , formed upon one surface (the upper surface as shown in FIGS. 1 and 2A) thereof for facilitating electrical contact with the electronic device  105 . Examples of methods for the construction of such bump contacts  115  are disclosed in U.S. Pat. No. 5,245,750, issued on Sep. 21, 1993 to Crumly et al. and entitled METHOD OF CONNECTING A SPACED IC CHIP TO A CONDUCTOR AND THE ARTICLE THEREBY OBTAINED, and in U.S. Pat. No. 5,790,377, issued on Aug. 4, 1998 to Schreiber et al. and entitled INTEGRAL COPPER COLUMN WITH SOLDER BUMP FLIP CHIP, the contents of both of which are hereby incorporated by reference. It is important to note that the two bump contacts  115  shown in FIGS. 1 and 2A are only representative examples of a larger number of such bump contacts which define an array thereof. Only two representative bump contacts  115  are shown since FIG. 1 is a fragmentary view. Such bump contacts  115  are preferably formed of gold, silver, platinum, lead, or solder. Generally, the bump contacts  115  will comprise a metal which is at least somewhat resistant to surface oxidation. A plurality of conductive conduits or traces formed upon the flexible circuit may terminate at such bump contacts  115  to facilitate the desired interconnection. Similarly, vias may be formed in the flexible circuit  103 , so as to facilitate electrical interconnection of the bump contacts  115  and conductive conduits formed upon the opposite side (the bottom side as shown in FIGS. 1 and 2A) of the flexible circuit  103 . 
     The electronic device  105  has a plurality of contact pads  117  formed upon one surface (the lower surface as shown in FIGS. 1 and 2A) thereof. The bump contacts  115  formed upon the flexible circuit  103  and the contact pads  117  formed upon the electronic device  105  are both preferably formed in an array, such as a rectangular array. The array may be of any desired shape, such as circular, triangular, square, rectangular, hexagonal, octagonal, etc. 
     An optional elastomeric pad  121   a  is disposed within the clamping device  101 . The elastomeric pad  121   a  is positioned within the clamping device  101  such that the elastomeric pad  121   a  applies a compressive force which urges the flexible circuit  103  toward the electronic device  105 , so as to assure continued reliable electrical contact of the bump contacts  115  with the contact pads  117 . However, as discussed above, the use of an elastomeric pad  121   a  alone is not adequate to assure such reliable contact of the bump contacts  115  with the contact pads  117  over an extended period of time due to the undesirable effects of mechanical compression set. 
     According to the present invention, an array of springs, preferably metallic springs  125   a,  is disposed intermediate the elastomeric pad  121   a  and the flexible circuit  103 , so as to assure that desired reliable contact of the bump contacts  115  with the contact pads  117  is maintained over prolonged periods of time. Like the bump contacts  115 , only representative examples of the metallic springs  125   a  are shown in FIGS. 1 and 2A. The representative metallic springs  125   a  of FIGS. 1 and 2A are thus part of an array defined by such metallic springs  125   a.    
     According to the preferred embodiment of the present invention, each bump contact  115  has a dedicated metallic spring  125   a,  and each metallic spring  125   a  is disposed generally in alignment with each bump contact  115 . Thus, each spring  125   a  applies a compressive force to a dedicated pad with bump contacts  115 , so as to assure desired reliable electrical contact between the bump contact  115  and its corresponding contact pad  117 , and the springs  125   a  also are capable of moving with respect to one another so as to facilitate desired compliance. 
     As discussed above, such compliance is necessary so as to assure adequate electrical contact between the bump contacts  115  and the contact pads  117  when the contact pads  117  of the electronic device are not generally coplanar with one another. That is, by allowing the springs  125   a  to move in a direction orthogonal to the plane of the array of springs  125   a  (in the Z direction as shown in FIGS.  1  and  2 A), substantial differences in the thicknesses and/or Z-axis positioning of the contact pads  117  can be accommodated. 
     With particular reference to FIG. 2A, according to one aspect of the present invention, the springs  125   a  extend upwardly from a shim stock base  123   a  and comprise a plurality, e.g., four, spokes  131  which radiate outwardly from a central hub  133  in a generally circular pattern (like the spokes of a wheel), such that when a spring  125   a  is compressed, the spokes  131  deform to provide a compressive force. 
     The elastomeric pad  121   a  is preferably disposed upon the opposite side of the array of springs  125   a  with respect to the flexible circuit  103 . However, the elastomeric pad  121   a  may alternatively be disposed intermediate the array of springs  125   a  and the flexible circuit  103 , if desired. 
     Referring now to FIG. 2B, as a further alternative two elastomeric pads,  121   a  and  121   b,  may be utilized, such that the array of springs  125   a  is sandwiched between the two elastomeric pads  121   a  and  121   b.  The elastomeric pads  121   a  and  121   b  may, for example, be formed of Silastic, Dow Corning part number 94-595-HC or Poron, Rogers Corp. of Chandler, Ariz. part number HT 840. Those skilled in the art will appreciate that various other elastomers are likewise suitable for use in the formation of the elastomeric pads  121   a  and  121   b.    
     The discussion and illustration of the contacts of the flexible circuit  103  being formed as bump contacts  115  is by way of example only, and not by way of limitation. Those skilled in the art will appreciate that the contacts of the flexible circuit  103  must be compatible with the complementary contacts (such as the contact pads  117 ) of the electronic device  105  to which the flexible circuit  103  is to be electrically connected, but the contacts may be of various different configurations. Thus, for example, if bump contacts are provided on the electronic device  105 , then the flexible circuit  103  may alternatively comprise contact pads. Indeed, the contacts of the flexible circuit  103  and/or the electronic device  105  need not all be of a single configuration. According to the present invention, various different configurations and combinations of configurations of the contacts of the flexible circuit  103 , as well as of the electronic device  105 , are contemplated. 
     Various different configurations of the springs  125   a  are likewise suitable. According to a further aspect of the present invention, another preferred configuration of the springs  125   a  is shown in FIGS. 3-5 and is discussed in detail below. 
     Referring now to FIGS. 3-5, another configuration of the springs  125   b  comprises generally S-shaped structures disposed generally within openings  302  formed in the base  123   b.  Each spring  125   b  comprises a central hub  133   b  from which two legs  303  extend toward and connect with the base  123   b.    
     According to the present invention, the springs  125   a,    125   b  are preferably etched from a non-relieved sheet of metal, e.g., a metallic shim stock. The springs  125   a,    125   b  may be formed by etching a metallic shim formed of non-relieved sheet steel. The springs  125   a,    125   b  may also be formed by stamping or die cutting. Those skilled in the art will appreciate that various other methodologies for forming the springs  125   a,    125   b  are likewise suitable. The sheet steel preferably has a thickness of between approximately 0.001 inch and approximately 0.003 inch, preferably approximately 0.002 inch. Those skilled in the art will appreciate that the springs  125   a  and  125   b  may be formed from sheet steel having various different desired thicknesses. 
     Referring again to FIG. 1, in use, the electronic device  105 , such as a printed circuit board or an integrated circuit, is placed in reliable electrical communication with a flexible circuit  103  by clamping at least a portion of the electronic device  105  to at least a portion of the flexible circuit  103  using the clamping device  101 . The elastomeric pad  121   a  (and/or the elastomeric pad  121   b  of FIG.  2 B), along with an array of metallic springs  125   a,  which are preferably attached to the elastomeric pad(s)  121   a  and/or  121   b,  are clamped along with the electronic device  105  and the flexible circuit  103 . 
     The array of metallic springs  125   a  is preferably attached to the laminary juxtaposed elastomeric pad  121   a.  Such attachment of the array of metallic springs  125   a  to the elastomeric pad  121   a  may be accomplished via adhesive bonding, thermal bonding (wherein the surface of the elastomeric pad  121   a  is at least partially melted so as to adhere to the array of metal springs  125   a  ), via fasteners such as rivets or by any other suitable method. Alternatively, the array of metallic springs  125   a  may be separate from, i.e., not attached to, the elastomeric pads  121   a  and/or  121   b.    
     The bump contacts  115  of the flexible circuit  103  are generally aligned with the contact pads  117  of the electronic device  105 . Preferably, the springs  125   a  are similarly aligned with the bump contacts  115  of the flexible circuit  117 . The clamping device  101  is configured to effect compression of the stack defined by the flexible circuit  103 , electronic device  105 , elastomeric pad  121   a,  and springs  125   a.  Such compression of the stack causes the bump contacts  115  to deform as they are urged toward the contact pads  117  and to thus provide reliable electrical contact between the flexible circuit  103  and the electronic device  105 . 
     As shown in FIG. 1, such compression may be effected by tightening the bolts  111  with respect to their corresponding nuts  113 , such that the first  107  and second  109  clamp members are moved toward one another. Those skilled in the art will appreciate that various other clamping structures are likewise suitable. For example, various clip, clamp and/or spring arrangements may similarly be utilized. 
     Because the springs  125   a  provide a compressive force to the bump contacts  115 , wherein the compressive force is concentrated generally at that portion of the flexible circuit  103  where the bump contacts  115  are located, less compressive force is required than is the case when the metallic springs  125   a  are not utilized. Since less compressive force is required, the clamping device  101  does not need to be as structurally strong as when no metallic springs  125   a  are utilized. Therefore, the clamping device  101  may be comprised of a less expensive material, such as plastic, rather than metal. Not only is the cost of the clamping device  101  reduced, but also the weight thereof is similarly reduced. This reduction in weight of the clamping device  101  may be particularly significant in weight sensitive applications, such as satellites and space craft, particularly when a large number of such electronic interconnects  100  are utilized. 
     The use of the metallic springs  125   a  of the present invention provides continued compressive force to the bump contacts  115  and their corresponding contact pads  117  in the event that the elastomeric pad  121   a  takes a compression set and fails to provide compressive force. Indeed, the metallic springs  125   a  may be configured so as to provide all of the compressive force necessary to ensure adequate contact of the bump contacts  115  with the contact pads  117 , such that the elastomeric pad  121   a  may be entirely omitted, if desired. 
     The elastomeric pad  121   a  provides compliance. That is, the elastomeric pad  121   a  can bend and move so as to allow the springs  125   a  to move in a manner which accommodates non-coplanar alignment of the contact pads  117 . 
     It is understood that the exemplary electronic interconnect described herein and shown in the drawings represents only a presently preferred embodiment of the invention. Indeed, various modifications and additions may be made to such embodiment without departing from the spirit and scope of the invention. For example, various different configurations of the springs are contemplated. Thus, the springs need not necessarily be circular in configuration, but rather may be of any other desired shape, e.g., triangular, rectangular, square, hexagonal, octagonal, etc., which is desirable. It is understood that the springs  125   a  of FIGS. 1 and 2A are considered to be generally functionally equivalent to the springs  125   b  of FIGS. 3-5. Thus, either the springs  125   a  or the springs  125   b  may be utilized, as desired. Indeed, various different configurations of the springs are contemplated. For example, the springs may have any desired number of legs, e.g., one, two, three, four, five, six, seven, or eight. Further, the springs may be comprised of any desired material which has the necessary spring characteristics. Thus, these and other modifications and additions may be obvious to those skilled in the art and may be implemented to adapt the present invention for use in a variety of different applications.