Abstract:
A wire bonded electrical interconnection includes a first electrical contact, a second electrical contact, and a bond wire having a first end bonded to the first electrical contact, a second end bonded to the second electrical contact, and a central portion connecting the first and second ends. The central portion includes notches formed during a wire bonding process, with each notch having at least two corners.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The present invention is directed to a method of wire bonding electrical contacts and, more particularly, to a method of wire bonding electrical contacts on flexible substrates. 
         [0002]    Electronic devices such as smart phones, tablets, and the like are trending toward being flexible for a myriad of reasons. In addition, electronic devices have become more compact, and less space is available in manufacturing for the placement of electronics and circuitry. Thus, the use of flexible substrates and chips has become more commonplace in the industry. 
         [0003]    However, standard wire bonds are not particularly suitable for use with flexible chips. Stresses on the wire loop, the bond at the bond pad, and the stitch bond at the lead contact can be too great due to the flexibility of the substrate, which can lead to breakage of the bond wire or wire bond. In addition, electrical interconnection through solder balls is not a suitable method for use with flexible substrates. 
         [0004]    It is therefore desirable to provide a method of electrically interconnecting electrical contacts on a flexible substrate that will minimize stresses and reduce breakage of the bond wire, and which will add few additional steps or components to the wire bonding process. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0005]    The present invention is illustrated by way of example and is not limited by embodiments thereof shown in the accompanying figures, in which like references indicate similar elements. Elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. Notably, certain vertical dimensions have been exaggerated relative to certain horizontal dimensions. 
           [0006]    In the drawings: 
           [0007]      FIG. 1  is a side perspective view of an electrical interconnection in accordance with an embodiment of the invention; 
           [0008]      FIG. 2  is a schematic view of an embodiment of a wire bonder for use in forming the electrical interconnection of  FIG. 1 ; 
           [0009]      FIG. 3  is an enlarged top plan view of a wire shaping tool from the wire bonder of  FIG. 2 ; and 
           [0010]      FIG. 4  is a partial schematic view of an alternate embodiment of a wire bonder for use in forming the electrical interconnection of  FIG. 1 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0011]    Referring to the drawings, wherein the same reference numerals are used to designate the same components throughout the several figures, there is shown in  FIG. 1  an electrical interconnection  10  in accordance with a preferred embodiment of the invention. A first electrical contact  12  is provided, preferably on a substrate  14  or some other form of support. The substrate  14  may be made from flexible materials, such as polyimide, polyether ether ketone (PEEK), polyester, other flexible polymer-based materials, or the like, and may have a thickness in the range of about 0.2 to about 0.4 millimeters (mm) in exemplary embodiments. 
         [0012]    The first electrical contact  12  may be, as shown in the example of  FIG. 1 , in the form of a bond pad on the substrate  14 . The bond pad  12  may be made from copper (Cu) and/or other conductive materials, and may be coated, alloyed or pre-plated with a metal layer or layers such as gold (AU), nickel (Ni), palladium (PD), tin (Sn) or the like. 
         [0013]    A second electrical contact  16  may also be provided. Although the second electrical contact  16  is shown in  FIG. 1  as being located on the same substrate  14  as the first electrical contact  12 , the two contacts  12 ,  16  may alternatively be disposed on different substrates  14  or supports. In the example of  FIG. 1 , the second electrical contact  16  is in the form of a conventional lead, for a semiconductor package or the like, and may be made of copper that is coated, alloyed or pre-plated with a metal layer or layers such as gold, nickel, palladium, tin, or the like. However, other like conductive materials may be used to form the second electrical contact  16 . 
         [0014]    The first and second electrical contacts  12 ,  16  are electrically interconnected with a bond wire  18 . The bond wire is preferably made from gold, although other conductive materials such a copper or aluminium may be used as well. At least a portion of the bond wire  18  may also include an insulating coating (not shown), which can be an insulating organic or polymeric material surrounding at least a portion of the conductive core. 
         [0015]    The bond wire  18  includes a first portion  18   a  that is attached to the first electrical contact  12 . A first bond  20  at the first electrical contact  12  is preferably a ball bond, which may be formed by forming a ball at an end of the bond wire  18  with a hydrogen flame or a spark and pressing it against the first electrical contact  12  by a capillary  58  ( FIG. 2 ) holding the bond wire  18 . Next, thermocompression, thermosonic or ultrasonic wire bonding is performed to bond the first portion  18   a  of the bond wire  18  to the first electrical contact  12 . However, other conventional methods of forming the first bond  20  may also be used. 
         [0016]    The bond wire  18  further includes a second portion  18   b  that is attached to the second electrical contact  16 . A second bond  22  at the second electrical contact  16  is preferably in the form of a stitch bond. The second bond also may include a security bond formed over the stitch bond. The stitch bond may be formed by pressing the second portion  18   b  of the bond wire  18  against the second electrical contact  16  by the capillary  58  ( FIG. 2 ), and performing thermocompression, thermosonic or ultrasonic wirebonding to bond the metal of the bond wire  18  to the second electrical contact  16 . The capillary  58  is then lifted off of the second electrical contact  16 , which breaks the bond wire  18 . In one embodiment, a ball bump is thereafter welded over top of the stitch bond using conventional techniques to complete the second bond  22 . 
         [0017]    The bond wire  18  further includes a third portion  18   c  that connects the first and second portions  18   a,    18   b  together. The third portion  18   c  is formed of a plurality of bent sections that form notches  24 , which better enables the bond wire  18  and the first and second bonds  20 ,  22  to alleviate stresses from elongation and compression through flexing of the substrate  14  or bond wire  18 . Each notch  24  is defined by at least two corners  26 , which are locations where adjacent lengths of the bond wire  18  meet at a generally right angle. 
         [0018]    For example,  FIG. 1  shows the notches  24  as each having a generally rectangular shape, defined by four corners  26  each having an angle of about 90 degrees. Although this configuration is preferred for its rigidity, other shapes for the notches may also be used. For example, the notches  24  may take the shape of triangles, each defined by three corners  26 . The notches  24  may also have two corners  26  with a curve extending therebetween. Other shapes, including irregular shapes and configurations can be used as well. The notches  24  are also shown in  FIG. 1  as being periodic and evenly spaced apart from one another. However, the present invention also contemplates that the shapes of the notches  24  need not correspond to one another and no periodicity or set spacing is required. 
         [0019]    Multiple bond wires  18  having notch configurations can be used to string together multiple electrical contacts (not shown) in series or in parallel. 
         [0020]    Referring now to  FIG. 2 , a wire bonder  50  that may be used for making the electrical interconnection  10  of  FIG. 1  is shown. The wire bonder  50  includes a bond wire supply or feed  52 , which is typically a spool of the bond wire  18 . An air guide  54  receives the bond wire  18  from the feed  52  and is provided to ensure a smooth and continuous wire supply to the downstream components. A tensioner  56  may be provided to supply a constant, pre-determined amount of tension to the bond wire  18  during predetermined timeframes of a wire bonding cycle. A capillary  58  is provided to receive the bond wire  18  for application to a bonding site (i.e., the first or second electrical contact  12 ,  16 ). Other components may be provided for the wire bonder  50  as is conventionally known. 
         [0021]    A wire shaping tool  60  is provided for forming the notches  24  within the third portion  18   c  of the bond wire  18 . In a preferred embodiment, the wire shaping tool  60  includes at least a first die  62  and a second die  64  spaced apart from one another along a feed direction of the bond wire  18  and disposed on opposite sides of the bond wire  18 . The first and second dies  62 ,  64  are repetitively moved into and out of a feed path of the bond wire  18  to shape the third portion  18   c  of the bond wire  18  into the plurality of notches  24  (see e.g.,  FIG. 4 ). 
         [0022]      FIG. 3  is a top plan view of the first and second dies  62 ,  64  and shows that each die  62 ,  64  is in the form of a plate  61  including an edge  63  facing the bond wire  18 . An opening  65  is formed in each edge  63  facing the bond wire  18  for providing the desired shape for the notches  24 . The dies  62 ,  64  are preferably made from halide-free composite, ceramic, polymeric, metal, or like materials. 
         [0023]    More preferably, as shown in  FIG. 2 , a third die  66  similar to the first and second dies  62 ,  64  is also provided for creating the rectangular shape of the notches  24  shown in  FIG. 1 . The third die  66  is spaced apart from both the first and second dies  62 ,  64  along the feed direction of the bond wire  18  and is disposed on an opposing side of the bond wire  18  from the second die  64 . The first and third dies  62 ,  66  are preferably coupled together, although this is not required. 
         [0024]    The wire shaping tool  60  is disposed between the bond wire feed  52  and the bonding site  12 ,  16 . More particularly, the wire shaping tool  60  is shown in  FIG. 2  as being disposed between the bond wire feed  52  and the capillary  58 , specifically between the air guide  54  and the tensioner  56 , although other locations are contemplated by the invention. Alternatively,  FIG. 4  shows the wire shaping tool  60  disposed between the capillary  58  and the bonding site  12 ,  16 . 
         [0025]    Preferably, one or more components of the wire bonder  50  are in operative communication with a processor (not shown) which is programmed or configured to calculate the length of the bond wire  18  necessary for the formation of each interconnection  10 , identify the first, second, and third portions  18   a,    18   b,    18   c  of the bond wire  18  along the calculated length, and prompt the wire shaping tool  60  to move in and out of the feed path of the bond wire  18  as the third portion  18   c  passes therethrough. 
         [0026]    Referring again to  FIG. 1 , the first and second electrical contacts  12 ,  16  and the bond wire  18  may be encapsulated or covered with a halide-free elastomeric or soft and flexible polymeric material  28  (in phantom). The cover material  28  is used to provide additional flexibility during bending or compression of the bond wire  18  and/or substrate  14 . The cover material  28  may be applied by conventional deposition, coating, or spin-on, or other techniques. 
         [0027]    In the foregoing specification, the invention has been described with reference to specific examples of embodiments of the invention. It will, however, be evident that various modifications and changes may be made therein without departing from the broader spirit and scope of the invention as set forth in the appended claims. 
         [0028]    Those skilled in the art will recognize that boundaries between the above-described operations are merely illustrative. The multiple operations may be combined into a single operation, a single operation may be distributed in additional operations and operations may be executed at least partially overlapping in time. Further, alternative embodiments may include multiple instances of a particular operation, and the order of operations may be altered in various other embodiments. 
         [0029]    The terms “front,” “back,” “top,” “bottom,” “over,” “under” and the like in the description and in the claims, if any, are used for descriptive purposes and not necessarily for describing permanent relative positions. It is understood that the terms so used are interchangeable under appropriate circumstances such that the embodiments of the invention described herein are, for example, capable of operation in other orientations than those illustrated or otherwise described herein. 
         [0030]    In the claims, the word ‘comprising’ or ‘having’ does not exclude the presence of other elements or steps then those listed in a claim. Further, the terms “a” or “an,” as used herein, are defined as one or more than one. Also, the use of introductory phrases such as “at least one” and “one or more” in the claims should not be construed to imply that the introduction of another claim element by the indefinite articles “a” or “an” limits any particular claim containing such introduced claim element to inventions containing only one such element, even when the same claim includes the introductory phrases “one or more” or “at least one” and indefinite articles such as “a” or “an.” The same holds true for the use of definite articles. Unless stated otherwise, terms such as “first” and “second” are used to arbitrarily distinguish between the elements such terms describe. Thus, these terms are not necessarily intended to indicate temporal or other prioritization of such elements. The fact that certain measures are recited in mutually different claims does not indicate that a combination of these measures cannot be used to advantage.