Abstract:
A system and method is described which provides for electrical signal transmission and a secure mechanical attachment in a flexible circuit assembly. The inventive mechanism combines the electrical features of discrete wiring with the mechanical features of etched pads connected to plated vias on flex circuits in order to achieve robustness in both the mechanical and electrical properties. A discrete wire is preferably securely bonded to a conductive pad which pad is then securely attached to a plated via. In this manner, the sequence of connections is made mechanically secure by either ultrasonically bonding or welding the discrete wire to the pad and employing the traditionally robust connection between the pad and the via. The arrangement achieves high quality electrical signal transmission by employing discrete wiring for signal transmission along any path of significant length.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application is a divisional of commonly assigned and co-pending U.S. application Ser. No. 09/702,461, entitled FLEXIBLE CIRCUIT USING DISCRETE WIRING, filed Oct. 31, 2000, which is incorporated herein by reference. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    This invention relates in general to electrical interconnection systems and in particular to reliable multiple path flexible circuit connections.  
         BACKGROUND OF THE INVENTION  
         [0003]    In many industrial systems, such as computer and telecommunications systems, there is a need for making a large number of electrical interconnections between a plurality of points of interest. It is generally desirable to achieve reliable connections (i.e. with low connection failure rates), with good electrical properties, and to be able to dispose such connections within a small area to address space limitations. Various prior art approaches have been attempted which experience limitations in one or more of the above-mentioned characteristics.  
           [0004]    One prior art approach is that of Printed Wire Board (PWB) flex circuits. PWB flex circuits are generally easy to manufacture if they have wide lines and traces. However, such circuits are generally difficult to manufacture in volume when fine lines, high density, and/or tight impedance controls are required. Moreover, if tight impedance controls are required, yield losses are generally high in volume production due primarily to variations in the etch processes.  
           [0005]    [0005]FIG. 1 is a section view of a strip line flex circuit  100  for high speed high density applications according to a prior art solution. Conductive traces  102  are shown with a dashed line. Ground planes  101  and  103  are shown running parallel to traces  102 .  
           [0006]    [0006]FIG. 2 is a top view of a center line cross-section of the strip line flex circuit depicted in FIG. 1. A number  200  of parallel traces  201  may be seen in the top view of FIG. 2. Generally, the width of and spacing between traces  201  may be important features in determining electrical properties of the structure such as characteristic impedance, resistance, skin effect losses, and crosstalk between the traces.  
           [0007]    [0007]FIG. 3 depicts sectional views  300  of four possible conductive trace geometries. The three columns separated by ideal spaces  306  are generally equivalent. Accordingly, the four trace geometries depicted in the rightmost column  301  will be discussed herein. Trace  302  generally represents an ideal trace geometry, which although highly desirable, is very difficult to achieve in a typical etch process. Trace  303  generally represents a desirable trace geometry which may be achieved with careful process control and with some yield loss. Trace  304  depicts a trace from which an excessive amount of conductive material, such as copper, has been removed. Trace  304  will generally experience excessive direct current (d.c.) resistance, increased skin effect losses, and undesirably high characteristic impedance. Element  305  depicts an under-etched trace. Such an under-etched trace will generally have excessive crosstalk and lower than ideal characteristic impedance.  
           [0008]    High speed data cables have been employed to provide interconnection having superior electrical signal characteristics. However, the use of such cabling is generally more expensive to implement for transmission of a given set of signals than is printed wire board flex circuit. Moreover, connectors used to connect such data cables to a board generally provide lower signal density than do flex circuits. Furthermore, such cable connectors are commonly the cause of impedance mismatches, crosstalk, and skew.  
           [0009]    In certain cases, multiple rigid PCBs (printed circuit boards) are used to establish electrical connections to system or subsystem units which are not on the same plane. As a result, multiple connectors may be introduced into the signal path, the addition of which generally degrades the quality of signal transmission. In addition, implementing a plurality of rigid PCBs generally adds to system cost, and takes up additional space.  
           [0010]    [0010]FIG. 4A is a section view  400  of a via connected to a wire according to a prior art multiwire connection arrangement. Employing this arrangement, via  401  is generally formed onto the end of wire  402 . Wire  402  may be coated with TEFLON® (polytetrafluoroethylene) for insulation purposes. In a connection employing the arrangement depicted in FIG. 4A, the available area for establishing a connection between wire  402  and via  401  is generally limited to the cross-sectional area of wire  402 . Some additional contact area may become available where the polytetrafluoroethylene coating is etched back for a finite distance along wire  402  from the outside diameter of via  401 . FIG. 4B is a top view of the same connection.  
           [0011]    Although the embodiment depicted in FIGS. 4A and 4B generally provides the superior electrical properties of discrete wiring, the attachment of wire  402  to via  401  provides a weak mechanical connection between wire  402  and via  401  which is subject to failure if wire  402  is pulled or moved in any direction.  
           [0012]    [0012]FIG. 5A is a section view  500  of a via  501  connected to a trace  502  in a printed circuit board arrangement. Employing this arrangement, a hole is drilled in a conductive pad connected to trace  502 , and plating material added to create via  501 . FIG. 5B is a top view of the connection depicted in FIG. 5A. This connection generally provides for a 360 degree connection between plating on via  501  and the copper of trace  502 . This trace-via connection offers a more robust mechanical connection than the connection between the discrete wire and via depicted in FIG. 4. However, the trace-via connection of FIG. 5 is subject to the inconsistency in dimensional tolerance and electrical properties discussed in connection with FIG. 3.  
           [0013]    [0013]FIG. 6 is a section view of a rigid circuit employing discrete wiring  602  between stripline shield layers  601  according to a prior art embodiment. FIG. 7 is a top view  700  of a center line cross-section of wires  602  depicted in FIG. 6. Returning to FIG. 6, generally, the cross sectional area between stripline shield layers  601  is filled with rigid dielectric material  603 , such as FR4 or G-Tec. The rigid circuit embodiment of FIG. 6 benefits from the advantageous electrical performance properties of discrete wiring. However, the embodiment employs the mechanically vulnerable wire to via connection discussed in connection with FIG. 4A.  
           [0014]    Accordingly, it is a problem in the art that PWB flex circuits are difficult to manufacture in high volume and experience and high yield losses due to etch variations.  
           [0015]    It is a further problem in the art that it is difficult to generate consistent trace geometries, resulting in inconsistent electrical properties for conductive traces.  
           [0016]    It is a still further problem in the art that cables are generally more expensive to implement than flex circuits for the same number of signals.  
           [0017]    It is a still further problem in the art that connectors used to attach cables to boards offer lower signal density than PWB flex circuits, and commonly cause impedance mismatches, crosstalk, and skew.  
           [0018]    It is a still further problem in the art that the deployment of multiple PCBs cause added cost, take up additional space, and cause degraded performance because of the implementation of multiple connectors along individual signal paths.  
           [0019]    It is a still further problem in the art that the connection of discrete wires to plated vias generally mechanically weak and subject to failure when the wire is pulled or moved.  
         BRIEF SUMMARY OF THE INVENTION  
         [0020]    The present invention is directed to a system and method which provides consistently high quality electrical performance characteristics in combination with robust mechanical attachment between conductors and electrical junctions. Preferably, discrete wires are connected to conductive pads employing either laser welded or ultrasonically bonded wire joints between the wires and pads. A plurality of layers having conductive pads may be combined to form a multiple layer flex circuit. Holes may then be drilled through the conductive pads in the traditional manner and then plated to create vias.  
           [0021]    Preferably, electrically reliable discrete wires may be employed and still benefit from the mechanically robust connection created by the attachment of the pad to the via. Therefore, there is a generally a two stage connection which includes a connection between a wire, or other high performance conductor, to a conductive pad, and then a robust connection between the conductive pad and the via. A robust connection between the high quality conductor and the conductive pad may be formed by a number of mechanisms including but not limited to: laser welding and ultrasonic bonding. In this manner, a mechanically flexible circuit is provided which enjoys the high quality electrical performance of discrete wiring conduction and robust mechanical attachments between the wire, pad, and via, thereby enabling the circuit to be flexed at will without causing electrical connection failure.  
           [0022]    In a preferred embodiment, a wire may be placed in contact with a conductive pad and then be ultrasonically bonded or welded thereto. Thereafter, a hole may be drilled in the conductive pad, and optionally, through a plurality of other layers in a flex circuit. The drilled hole may then be plated, thereby enabling the plating material to form a solid conductive and mechanical connection to the conductive pad. In this manner, the resulting connections are mechanically robust and benefit from the superior electrical properties of discrete wiring.  
           [0023]    In a preferred embodiment, the system and method described above allow the creation of a high speed flexible circuit that combines high speed signal handling ability with robust mechanical connections. Since the bulk of the distance traveled by the signals is covered by a high quality conductor, such as, for instance, drawn wire, rather than etched trace, the conductor surface is preferably smooth, thereby enabling low loss skin effect parameters to be realized.  
           [0024]    In a preferred embodiment, the wires may be individually formed, thereby enabling such wires to avoid the electrical performance problems associated with under-etch and over-etch to be avoided. Preferably, the wire to pad connection, which may be accomplished by welding or ultrasonic bonding, allows robust signal vias to be created employing standard drill and plating processes. Preferably, tighter impedance, resistance, and skin effect loss parameters may be realized employing a process which allows consistent manufacturing of a large number of circuits.  
           [0025]    Therefore, it is an advantage of a preferred embodiment of the present invention that electrical performance characteristics of high quality conductors such as discrete wiring may be combined with robust mechanical characteristics in a flex circuit.  
           [0026]    It is a further advantage of a preferred embodiment of the present invention that a flex circuit connection with robust mechanical and electrical characteristics may be reliably and consistently manufactured.  
           [0027]    It is a still further advantage of a preferred embodiment of the present invention that impedance mismatch and other electrical performance problems associated with introduction of connectors into the signal path may be minimized.  
           [0028]    It is a still further advantage of a preferred embodiment of the present invention that high quality conductors may be employed which do not suffer the yield losses associated with fine line etching of conductive traces.  
           [0029]    The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims. The novel features which are believed to be characteristic of the invention, both as to its organization and method of operation, together with further objects and advantages will be better understood from the following description when considered in connection with the accompanying figures. It is to be expressly understood, however, that each of the figures is provided for the purpose of illustration and description only and is not intended as a definition of the limits of the present invention.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0030]    For a more complete understanding of the present invention, reference is now made to the following descriptions taken in conjunction with the accompanying drawing, in which:  
         [0031]    [0031]FIG. 1 is a section view of a strip line flex circuit for high speed high density applications according to a prior art solution;;  
         [0032]    [0032]FIG. 2 is a top view of a center line cross-section of the strip line flex circuit depicted in FIG. 1;  
         [0033]    [0033]FIG. 3 depicts sectional views of four possible conductive trace geometries;  
         [0034]    [0034]FIG. 4A is a section view of a via connected to a wire according to a prior art multiwire connection arrangement;  
         [0035]    [0035]FIG. 4B is a top view of a via connected to a wire according to a prior art multiwire connection arrangement;  
         [0036]    [0036]FIG. 5A is a section view of a via connected to a trace in a printed circuit board arrangement;  
         [0037]    [0037]FIG. 5B is a top view of a via connected to a trace in a printed circuit board arrangement;  
         [0038]    [0038]FIG. 6 is a section view of a rigid circuit employing discrete wiring between stripline shield layers;  
         [0039]    [0039]FIG. 7 is a top view of a center line cross-section of the rigid circuit depicted in FIG. 6;  
         [0040]    [0040]FIG. 8 is a top view of a printed circuit board pad;  
         [0041]    [0041]FIG. 9 is a side view of the printed circuit board pad depicted in FIG. 8;  
         [0042]    [0042]FIG. 10 is a top view of an attachment of a wire to the printed circuit board pad of FIG. 8 according to a preferred embodiment of the present invention;  
         [0043]    [0043]FIG. 11 is a side view of an attachment of a wire to the printed circuit board pad of FIG. 8 according to a preferred embodiment of the present invention;  
         [0044]    [0044]FIG. 12 is a section view of a wire connected to one of a set of pads connected to a via according to a preferred embodiment of the present invention;  
         [0045]    [0045]FIG. 13 depicts a pad pattern available using the inventive connection technology according to a preferred embodiment of the present invention;  
         [0046]    [0046]FIG. 14 is a section view of a plurality of inventive wire-to-pad connections according to a preferred embodiment of the present invention;  
         [0047]    [0047]FIG. 15 is a top view of a center line cross-section of a plurality of wires according to a preferred embodiment of the present invention;  
         [0048]    [0048]FIG. 16 is a top view of a ground shield layer for use in a flex circuit according to a preferred embodiment of the present invention;  
         [0049]    [0049]FIG. 17 is a top view of signal layer within a flex circuit according to a preferred embodiment of the present invention;  
         [0050]    [0050]FIG. 18 depicts a section view of a flex circuit according to a preferred embodiment of the present invention;  
         [0051]    [0051]FIG. 19 depicts multiple layers composing a flex circuit according to a preferred embodiment of the present invention; and  
         [0052]    [0052]FIG. 20 depicts two printed circuit boards connected by a flex circuit according to a preferred embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0053]    [0053]FIG. 8 is a top view of a printed circuit board pad  800 . FIG. 9 is a side view of the printed circuit board pad of FIG. 8.  
         [0054]    [0054]FIG. 10 is a top view  1000  of an attachment of a wire  1001  to the printed circuit board pad  800  of FIG. 8 according to a preferred embodiment of the present invention. Preferably, wire  1001  is connected to pad extension  801  which is preferably aligned with wire  1001 . Pad extension  801  is preferably dimensioned to allow for variation in the length of and placement of wire  1001  and still permit secure attachment of wire  1001  to pad  800 . Variation in the length of wire  1001  may be introduced by minor inconsistencies in the manufacturing process for wire  1001 , and variation in the placement of wire  1001  may arise from the dimensional repeatability of robotic or other assembly equipment employed to place wire  1001  in contact with pad  800 . Wire  1001  is preferably welded or ultrasonically bonded to pad  800 .  
         [0055]    [0055]FIG. 11 is a side view  1100  of an attachment of a wire  1001  to the printed circuit board pad of FIG. 8 according to a preferred embodiment of the present invention. Wire  1001  and pad  800  are preferably connected employing attachment region  1101 . As mentioned in connection with FIG. 10, wire  1001  is preferably affixed to pad  800  employing methods including but not limited to welding and ultrasonic bonding.  
         [0056]    In a preferred embodiment, a highly robust mechanical attachment is established between wire  1001  and pad  800  in attachment region  1101 . This robust mechanical attachment preferably enables wire  1001  to maintain solid mechanical contact with pad  800  even when the flex circuit of which both are a part is flexed, causing wire  1001  to be pulled and moved in various directions.  
         [0057]    In a preferred embodiment, in addition to the previously discussed robust mechanical attachment, the connection between wire  1001  and pad  800  preferably enables transmission of signals through attachment region  1101  without degradation of the high performance electrical properties of wire  1001 . This is in contrast with the effects of employing connectors as practiced in the prior art which connectors commonly introduce impedance mismatches, crosstalk, and skew. Accordingly, attachment region  1101  preferably provides a combination of robust mechanical and electrical connection properties not simultaneously available in the prior art.  
         [0058]    [0058]FIG. 12 is a section view  1200  of a wire  1205  connected to one of a set of pads connected to a via  1201  according to preferred embodiment of the present invention. Pads  1202 ,  1203 , and  1204  preferably surround via  1201  and are preferably in substantially continuous contact with via  1201  along the circumference of via  1201 . Accordingly, a highly robust mechanical and electrical connection is preferably established between via  1201  and each of conductive pads  1202 ,  1203 , and  1204 .  
         [0059]    In a preferred embodiment, wire  1205  generally corresponds to wire  1001  depicted in FIG. 11. Wire  1205  is preferably rigidly bonded to pad  1202  employing one of a plurality of mechanisms including but not limited to: ultrasonic bonding and welding. Wire  1205  preferably makes contact with pad  1202  along attachment region 1208 . Wire  1205  preferably includes inner conductive portion  1206  and an outside insulating layer  1207  which may be composed of polytetrafluoroethylene. Securely bonding wire  1205  to pad  1202  which is in turn rigidly bonded to plated via  1201  provides for an indirect connection of wire  1205  to via  1201  which indirect connection preferably boasts superior mechanical properties to those present in a direct wire to via connection of the prior art.  
         [0060]    In a preferred embodiment, the advantage in mechanical robustness of the indirect attachment of wire  1205  to via  1201  is caused by both the secure bonding of wire  1206  to pad  1202  and by the traditionally robust attachment of pad  1202  to via  1201  owing to the substantially continuous circumferential contact between pad  1202  and via  1201 .  
         [0061]    In a preferred embodiment, a portion of a flex circuit such as that shown in FIG. 12 may be created employing the following steps. Layers of the flex circuit may be created by etching various conductive pads such as conductive pad  1202 . A selection of pads may be disposed so as to be vertically aligned such as with pads  1202 ,  1203 , and  1204 . Pads  1202 ,  1203 , and  1204  are preferably arranged such that one or more holes may be drilled through the pads and contact each of the pads at a desired location. Upon completing the drilling operation, plating material is preferably added to the drilled hole, thereby establishing robust mechanical and electrical connectivity between the plated hole or via  1201  and the pads  12021204  in communication with via  1201 . Connection of wires to the pads, such as wire  1205  to pad  1202 , may be accomplished prior to drilling the hole through pads  1202 - 1204  or after the drilling and plating of the hole and creation of via  1201 .  
         [0062]    In the prior art, conductive traces and pads were used together because the conductive pads were generally merely geometric enlargements of the traces which were employed as a mechanism for establishing conduction over a significant distance. Where traces were employed as a conduction mechanism for significant distances, discrete wires are generally not present since traces and wires would represent redundant functionality. Since wires and traces were generally not deployed in the same systems at the same time, little opportunity would arise to connect the two together.  
         [0063]    In the prior art, discrete wires were generally employed in high speed data applications for which rigid circuit boards were employed. The problems of “wire pull” and movement of wires are generally not present in a rigid circuit environment. Accordingly, although the mechanical connection between discrete wires and vias was not robust in the rigid circuit board environment, potentially destructive events such as “wire pull” which might cause rupture of the wire-via connection generally did not occur.  
         [0064]    [0064]FIG. 13 depicts a pad pattern  1300  available using the inventive connection technology according to a preferred embodiment of the present invention. It will be appreciated that numerous other arrangements of wires and pads are available, and all such variations are included within the scope of the present invention. One factor which may be varied in alternative embodiments is the angle formed between pad extension  801  and wire  1001  upon attachment of the two. For instance, there may be a 45 degree angle between the pad extension  801  and wire  1001 . Other implementations include deployment of multi-row high-density connection patterns.  
         [0065]    [0065]FIG. 14 is a section view of a plurality  1400  of inventive wire-to-pad connections disposed within a flex circuit according to a preferred embodiment of the present invention. It may be seen that wire  1001  sits immediately above pad  800  to which it is preferably bonded employing a welded connection, ultrasonically bonded connection, or other secure attachment means. FIG. 15 is a top view  1500  of a center line cross-section of the same plurality of wires depicted in the section view of FIG. 14. Although the wires shown in FIGS. 14 and 15 are shown running in parallel, it will be appreciated that the wires  1001  may be arranged in a range of different orientations and locations with respect to each other and that all such variations are included within the scope of the present invention.  
         [0066]    In a preferred embodiment, the plurality of wires  1400  is disposed between two stripline shield layers  1401  and  1402 . Flexible dielectric material  1403  is preferably disposed between pad  800  and lower stripline shield layer  1402 , between wires  1400  and upper stripline shield layer  1401  and in regions in between the wires  1400  themselves.  
         [0067]    In a preferred embodiment, the wires  1400  may be adhered to flexible dielectric material  1403  employing a thermosetting adhesive. It will be appreciated that other methods for adhering wires  1400  to dielectric material  1403  may be employed, and all such variations are within the scope of the present invention. The ends of wires  1400  are preferably affixed to pads  800  employing either welding or ultrasonic bonding as discussed in connection with FIG. 10 and elsewhere herein. Therefore, a flex circuit may be constructed which combines thermosetting adhesion of the wires along the majority of their length to a flexible dielectric and a secure ultrasonic bond or welded connection between the ends of wires  1400  and conductive pads  800 . A flex circuit employing the described combination of connection mechanisms preferably enjoys the mechanical robustness arising from the described connection mechanisms as well as advantageous electrical characteristics arising from the deployment of discrete wires for conduction purposes. Moreover, stacking of various layers is available in the inventive discrete wired flex circuit as with traditional flex circuits.  
         [0068]    [0068]FIG. 16 is a top view of a ground shield layer  1600  for use in a flex circuit according to a preferred embodiment of the present invention. One possible embodiment of shield layer  1600  is depicted as element  1401  in FIG. 14.  
         [0069]    [0069]FIG. 17 is a top view of signal layer  1700  within a flex circuit according to a preferred embodiment of the present invention. One possible embodiment of signal layer  1700  is represented by the plurality of wires  1001  connected to pads  800  in a horizontal array as depicted in FIG. 14.  
         [0070]    [0070]FIG. 18 depicts a section view of a flex circuit  1800  according to a preferred embodiment of the present invention. Generally, each of the elements disposed vertically includes wire  1001  in contact with conductive pad  800 . Also depicted is via  1201  generally having horizontal conductive portions at the upper and lower layers of flex circuit  1800  and a plated hole depicted as a solid black line running between the upper and lower layers of flex circuit  1800 .  
         [0071]    [0071]FIG. 19 depicts multiple layers composing a flex circuit  1800  according to a preferred embodiment of the present invention. Ground shield layers  1901  and  1902  generally correspond to ground shield layer  1600  depicted in FIG. 16 and generally form the upper and lower layers of flex circuit  1800 . Signal layer  1903  generally corresponds to signal layer  1700  depicted in FIG. 17. Signal layer  1903  is generally disposed in between ground shield layers  1901  and  1902 , thereby benefiting from the insulation provided by layers  1901  and  1902 .  
         [0072]    [0072]FIG. 20 depicts two printed circuit boards connected by flex circuit  1800  according to a preferred embodiment of the present invention. Flex circuit  1800  is shown attached at one end to PCB “A” employing attachment means  2003  and at a second end to PCB  2002  employing attachment means  2004 . Attachment means  2003  and  2004  may include but are not limited to: attachment bolts, clips, clamps, mating pin and hole attachments, and socket connections.  
         [0073]    Although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims. Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, composition of matter, means, methods and steps described in the specification. As one of ordinary skill in the art will readily appreciate from the disclosure of the present invention, processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized according to the present invention. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps.