Abstract:
A flexible circuit for bonding to another circuit includes a film having a first conductor layer fabricated upon a topside of the film and a second conductor layer fabricated on an underside of the film, the first conductor layer being insulated by a first insulator layer fabricated thereover, the second conductor layer being insulated by a second insulator layer fabricated thereover; wherein the first conductor layer terminates in at least one bonding pad for the bonding to the another circuit and the second conductor layer terminates in at least one finger lead for the bonding to another circuit. A method for fabricating the flex circuit is provided.

Description:
TRADEMARKS 
       [0001]    IBM® is a registered trademark of International Business Machines Corporation, Armonk, N.Y., U.S.A. Other names used herein may be registered trademarks, trademarks or product names of International Business Machines Corporation or other companies. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    This invention relates to making electrical connections between flexible circuits and devices. 
         [0004]    2. Description of the Related Art 
         [0005]    It is known that ultrasonically bonding wires to flexible circuit conductors can be problematic. For example, unreliable connections may occur when the conductors are supported by an adhesively joined insulating layer. In this situation, the conductors tend to deform under the force of an ultrasonic bonding tip. One attempt in the prior art to solve this problem has been to experiment with different adhesives and methods for applications. However, experience has shown that the best epoxies and the thinnest layers are still a compromise. 
         [0006]    What are needed are flexible circuit conductors that provide reliable wire bonding connections. 
       SUMMARY OF THE INVENTION 
       [0007]    The shortcomings of the prior art are overcome and additional advantages are provided through the provision of a flexible circuit for bonding to another circuit, the flexible circuit including: a film having a first conductor layer fabricated upon a topside of the film and a second conductor layer fabricated on an underside of the film, the first conductor layer being insulated by a first insulator layer fabricated thereover, the second conductor layer being insulated by a second insulator layer fabricated thereover; wherein the first conductor layer terminates in at least one bonding pad for the bonding to another circuit and the second conductor layer terminates in at least one flying lead for the bonding to another circuit. 
         [0008]    Also disclosed is a method for fabricating a flexible circuit including bonding pads and finger leads, the method including: disposing upon a film a first conductor layer on a topside and a second conductor layer on an underside of the film; disposing upon the first conductor layer a first insulator layer and upon the second conductor layer a second insulator layer; trimming back the first insulator layer to expose the first conductor layer; trimming back the first conductor layer to expose the film and create the bonding pads; removing a portion of the film and the second insulating layer to provide flying leads. 
         [0009]    Further disclosed is a tape unit including: a flexible circuit for bonding to a device, the flexible circuit including a film having a first conductor layer fabricated upon a topside of the film and a second conductor layer fabricated on an underside of the film, the first conductor layer being insulated by a first insulator layer fabricated thereover, the second conductor layer being insulated by a second insulator layer fabricated thereover; wherein the first conductor layer terminates in a plurality of bonding pads for bonding to the device and the second conductor layer terminates in a plurality of finger leads for bonding to the device; wherein the plurality of bonding pads are ultrasonically bonded to the bonding wire coupled to the device and the plurality of finger leads are ultrasonically bonded to the device. 
         [0010]    Additional features and advantages are realized through the techniques of the present invention. Other embodiments and aspects of the invention are described in detail herein and are considered a part of the claimed invention. For a better understanding of the invention with advantages and features, refer to the description and to the drawings. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    The subject matter, which is regarded as the invention, is particularly pointed out and distinctly claimed in the claims at the conclusion of the specification. The foregoing and other objects, features, and advantages of the invention are apparent from the following detailed description taken in conjunction with the accompanying drawings in which: 
           [0012]      FIG. 1  illustrates a tape data recorder with a device and a flexible circuit; 
           [0013]      FIGS. 2A ,  2 B,  2 C,  2 D, and  2 E, collectively referred to as  FIG. 2 , illustrate various aspects of a double-sided flexible circuit; 
           [0014]      FIG. 3  illustrates a double-sided flexible circuit with an end prepared for making electrical connections; 
           [0015]      FIG. 4  illustrates an end view of an embodiment of a flexible circuit used as a cable; 
           [0016]      FIG. 5  is a flow chart of an exemplary process for fabricating flying leads and finger leads; 
           [0017]      FIG. 6  illustrates flexible circuit connections to a device; 
           [0018]      FIG. 7  is a flow chart of the wire bonding process; 
           [0019]      FIG. 8  is a cross section of a double-sided flexible circuit; 
           [0020]      FIG. 9  is a top view of a double-sided flexible circuit. 
       
    
    
       [0021]    The detailed description explains the preferred embodiments of the invention, together with advantages and features, by way of example with reference to the drawings. 
       DETAILED DESCRIPTION OF THE INVENTION 
       [0022]    Shown in  FIG. 1  is a tape data storage unit  1 . The tape data storage unit  1  includes a flexible circuit  10 , a device  11 , an electronic circuit board  12  and a tape  13 . In typical embodiments, the tape  13  provides for data storage. As discussed herein, the flex circuit  10  is an embodiment of a cable. The teachings herein are applicable to flexible circuits  10  and cables fabricated from flexible circuits. In the exemplary embodiment, the flexible circuit  10  electrically connects the device  11  to the electronic circuit board  12 . The flexible circuit  10  provides a flexible connection and thus allows the device  11  to articulate when performing its function. Connection to other types of circuits and devices may be realized using the flexible circuit  10  disclosed herein. Use of the flexible circuit  10  as disclosed herein is particularly advantageous for coupling to other circuits having movement relative to another component. 
         [0023]      FIGS. 2A  thru  2 E, collectively known as  FIG. 2 , illustrate various aspects of preparation of a double-sided flexible circuit. Referring to  FIG. 2 , the flexible circuit  10  includes a plurality of layers. In this example, the flexible circuit  10  is made up of five layers. 
         [0024]    In the embodiment depicted, the flexible circuit  10  includes a topside and an underside. One skilled in the art will recognize that many configurations for flexible circuits and cables fabricated from flexible circuits may be realized. Accordingly, the embodiments discussed herein are merely illustrative and not limiting of the teachings herein. 
         [0025]    Referring to  FIG. 2A , a flexible film  20  provides a dielectric surface to which a first conductor layer  22  is attached. Typically, the flexible film  20  is a polymer film, however, other flexible dielectric materials may be used. In the embodiment depicted, the first conductor layer  22  is attached to a topside  21  of the film  20 . A second conductor layer  24  is attached to an underside  23  of the film  20 . As shown herein, the first conductor layer  22  and the second conductor layer  24  include wires as circuit components. However, one skilled in the art will recognize that the conductor layers  22 ,  24  may also provide other circuitry. 
         [0026]    A first insulating layer  25  insulates the first conductor layer  22 . Typically, the first insulating layer  25  is adhesively attached to the first conductor layer  22 . The first insulating layer  25  may insulate the topside  21  of the film  20  on surface areas not covered by the first conductor layer  22 . 
         [0027]    Similarly, a second insulating layer  26  insulates the second conductor layer  24 . Typically, the second insulating layer  26  is adhesively attached to the second conductor layer  24 . The second insulating layer  26  is adhesively attached to the underside  23  of the flexible film layer  20  on surface areas not covered by the second conductor layer  24 . 
         [0028]    Shown in  FIG. 2B  is another view of the flexible circuit  10  depicted in  FIG. 2A . In  FIG. 2B , the first insulator layer  25  is trimmed back exposing the first conductor layer  22 . Trimming is merely illustrative of a process to expose a portion of the first conductor layer  22  and not meant to limit the teachings herein. Trimming may include at least one of mechanical trimming, laser trimming, and chemical trimming. Trimming may also include having the flexible circuit manufactured with a portion of the first conductor layer  25  exposed. 
         [0029]    Shown in  FIG. 2C  is a side view of the flexible circuit  10  depicted in  FIG. 2B . As depicted, the first conductor layer  22  is trimmed back exposing the film  20 . The first conductor layer  22  is trimmed a portion of the way back to an edge of the first insulator layer  25 . The first conductor layer  22  extends beyond the first insulator layer  25  by about 0.25 mm. The 0.25 mm dimension is typical and not meant to limit the teachings herein. 
         [0030]    Shown in  FIG. 2D  is a top view of the flexible circuit  10  depicted in  FIG. 2C . In  FIG. 2C , a portion of the film  20  exposed in  FIG. 2C  is removed along with a portion of the second insulator layer  26 . The portion of the second insulator layer  26  removed corresponds to the portion of the film  20  removed. The result is at least one circuit element of the second conductor layer  24  is exposed. The exposed circuit elements are known as flying leads  41 . The flying leads  41  bridge an opening in the polymer film layer  20  and the second insulator layer  26 . The flying leads  41  are typically supported on two ends. As discussed herein, the flexible circuit  10  includes a plurality of flying leads  41 . 
         [0031]    Shown in  FIG. 2E  is a top view of the flexible circuit  10  depicted in  FIG. 2D  in which the plurality of flying leads  41  are parted at a generally perpendicular angle (with respect to an orientation of the flying leads  41 ), such as along line A-A (shown in  FIG. 2D ). The parting results in the creation of a plurality of finger leads  42 . Parting may be performed using any one or more of a variety of techniques for separating portions of electrical circuits. Sections of the film  20  and the second insulator layer  26  are also parted in the vicinity of the flying leads  41 . 
         [0032]    Shown in  FIG. 3  is a side view of the flexible circuit  10  depicted in  FIG. 2E . Referring to  FIG. 3 , the first conductor layer  22  extends beyond the first insulator layer  25  and provides a plurality of bonding pads  43 . The bonding pads  43  are supported from below by the film  20 . The finger leads  42  extend beyond the film  20  and the second insulator layer  26 . The finger leads  42  are unsupported from above and below. The second insulator layer  26  is incorporated into the flexible circuit  10  via adhesive. The second insulator  26  and associated adhesive may be trimmed back so as not to be under the bonding pads  43 . The elimination of this adhesive for some embodiments of the flexible circuit  10  provides an improvement over the prior art for wire bonding applications. 
         [0033]    Shown in  FIG. 4  is an end view of the flexible circuit  10  depicted in  FIG. 3 . As shown in  FIG. 4 , the bonding pads  43  and the finger leads  42  are staggered. However, this arrangement is merely illustrative and non-limiting. 
         [0034]    Shown in  FIG. 5  is a flow chart depicting aspects of an exemplary method  50  used for preparing the flexible circuit  10  for making external electrical connections. A first step  51  calls for selecting a multiple conductor flexible circuit. In the embodiment discussed, a double-sided flexible circuit is selected as the flexible circuit  10 . A second step  52  calls for trimming back the first insulator layer  25  exposing the first conductor layer  22 . A third step  53  calls for trimming back the first conductor layer  22  exposing the film  20 . The first conductor layer  22  is not trimmed back completely to the first insulator layer  25 . The first conductor layer  22  extends beyond the first insulator layer  25  and creates the bonding pads  43 . A fourth step  54  calls for fabricating the flying leads  41  by removing a portion of the film  20  and the second insulator layer  26 . The removal exposes the second conductor layer  24  from which the flying leads  41  are fabricated. The flying leads  41  are supported on two ends. A fifth step  55  calls for parting the flying leads  41  and thus creating the finger leads  42 . Portions of the film  20  and the second insulator layer  26  are also cut in the vicinity of the finger leads  42 . 
         [0035]    Bonding of the flexible circuit  10  may advantageously male use of multiple techniques.  FIG. 6  illustrates a side view of connections of the flexible circuit  10  to the device  11 . The device  11  includes a plurality of device bonding pads  60 . Also shown is bonding wire  61  used in the stitch bonding process. Referring to  FIG. 6 , in the embodiment discussed, the plurality of bonding pads  43  are ultrasonically bonded to the corresponding bonding wires  61  coupled to the device  11 . One bonding process that may be used is known as stitch bonding. Typically, the film  20 , which is supported from below by an external hard surface, provides enough support for an ultrasonic bonding tip to work correctly. In the embodiment discussed, the bonding pads  43  are gold-plated copper and the bonding wire  61  is aluminum. 
         [0036]    Bonding the circuitry of the second conductor layer  24  may be accomplished with either the flying leads  41  or the finger leads  42 . The flying leads  41  and the finger leads  42  are ultrasonically bonded to the device  11  without any intervening wire. As bonding the finger leads  42  is directly to the device  11 , adequate support for the ultrasonic bonding tip to work correctly is realized. One skilled in the art will realize that ultrasonic bonding is merely illustrative. Other bonding processes (for example, thermosonic, thermocompressive and other such techniques) may also be used. 
         [0037]      FIG. 7  presents a flow chart depicting an exemplary cable wire bonding process  70 . A first step  71  calls for preparing the flexible circuit  10  for bonding, which has been fabricated according to a method  50  such as the one presented in  FIG. 5 . A second step  72  calls for the finger leads  42  be bonded directly to the device bonding pads  60  without any of the bonding wire  61 . A third step  73  calls for the bonding pads  43  to be bonded to the device  11  using the stitch bonding process with the bonding wire  61 . The bonding wire  61  used in the process of bonding to the conductor layer  22  bridges over the direct bonds of the finger leads  42  made in Step  72 . 
         [0038]      FIG. 8  depicts exemplary aspects of a double-sided flexible circuit  10  prepared with flying leads  41  and bonding pads. In the embodiment depicted in  FIG. 8 , a variety of materials are used. For example, the flexible circuit  10  uses copper for the conductor layers  22  and  24 . Gold plating is used on copper in areas where the conductor layers  22  and  24  are not protected by insulator layers  25  and  26 , respectively. Nickel plating is applied to the copper in the bonding pads  43  before gold plating is applied. Polyimide is used in the film  20  and insulator layers  25  and  26 . As shown in  FIG. 8 , additional bonding pads  43  (and bonding wires  61  for that matter) may be included. In the embodiment depicted  FIG. 8 , the plurality of device  11  bonding pads  60  are arranged in two rows, a first row  81  and a second row  82 . 
         [0039]      FIG. 9  depicts exemplary aspects of a double-sided flexible circuit  10  prepared with flying leads  41  and bonding pads  43 . 
         [0040]    While the preferred embodiment to the invention has been described, it will be understood that those skilled in the art, both now and in the future, may make various improvements and enhancements which fall within the scope of the claims which follow. These claims should be construed to maintain the proper protection for the invention first described.