Abstract:
Enhanced ACF bonding pads for use in conjunction with anisotropic conductive film (ACF) in electronic devices, such as, liquid crystal display panels and plasma display panels have at least two finger-like portions. Such bonding pads, typically provided on a flexible wiring lead, when bonded to other metal structures via the ACF film, make better electrical contact with the other metal structures because the spaces between the finger-like portions of the improved bonding pads allow the ACF film&#39;s binder material to reside between the finger-like portions preventing the bonding pad metal in the center region of the bonding pad from separating away from the other metal structures.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application claims benefit under 35 U.S.C. § 119(e) of U.S. Provisional Patent Application No. 60/519,186, filed on Nov. 11, 2003, which is incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The present invention relates to the design of metal terminal pads on devices such as a liquid crystal display to enhance electrical connection made using anisotropic conductive films.  
       BACKGROUND OF THE INVENTION  
       [0003]     Anisotropic conductive films (ACF) are used to make electrical connections in miniature and high-performance electronic equipment that require fine and dense wiring implementations for connecting the terminals of circuit boards. Descriptions of such ACF films and their use can be found in U.S. Pat. No. 5,770,305 and United States Patent Application Publication No. 2002/0111055 A1, the entire disclosure of which are incorporated herein by reference.  
         [0004]     Electrical connections made between glass substrates of display devices, such as, liquid crystal displays (LCD) and plasma display panels (PDP) are examples of ACF film applications. In these applications, ACF films are often used to electrically connect flexible wiring leads to one or more metallic terminal pads on the periphery of the display device&#39;s glass substrate. The flexible wiring leads are usually made of a flexible polymer and typically have thin metal foil wirings that terminate into ACF bonding pads on the surface of the flexible wiring leads.  
         [0005]      FIG. 1  is a cross-sectional view of an ACF film  10  in a typical application. The ACF film  10  is positioned between an ACF bonding pad  20  of a flexible wiring lead  70  and a base terminal pad  30  of a glass substrate  80 . The bonding pad  20  and the base terminal pad  30  are generally made of thin metal alloy foils (generally a copper based alloy). The ACF film  10  comprises conductive particles  14  that are coated with an insulation material and dispersed within a binder material  12 . When the bonding pad  20  and terminal pad  30  are compressed together, squeezing the ACF film  10 , the ACF film  10  acts as the bonding agent bonding the two metal pads together and at the same time r also provides the electrical connection between the bonding pad  20  and the terminal pad  30 . This bonding method will be referred to herein as pressure bonding.  
         [0006]      FIG. 2  is a cross-sectional schematic illustration of the assembly of  FIG. 1  after the pads  20  and  30  have been pressure bonded together by a compression force, represented by arrows  50 . The electrical connection between the pads  20  and  30  is made by the conductive particles  14  dispersed within the ACF film  10 . When the pads  20  and  30  are compressed together, the conductive particles  14  trapped between the pads are squeezed. The compression force  50  should be sufficiently high to crush and deform the conductive particles  14  breaking their coating of insulation material. Thus, metal-to-metal contacts are formed between the crushed conductive particles  14   a  and the bonding pad  20  and the base terminal pad  30  establishing electrical conduction paths between the pads  20  and  30 .  
         [0007]     In ACF film  10 , the binder material  12  is the adhesive that bonds to the pads  20  and  30  to hold them together. But, when the pads  20  and  30  are compressed together to activate the conductive particles of the ACF film, much of the binder material  12  is squeezed out from between the pads  20  and  30 , leaving behind very little binder material  12  between the pads  20  and  30 . Thus, as illustrated in  FIG. 3 , once the compression force  50  is removed, the elasticity of the conductive particles  14   a  exert force on the pads  20  and  30  causing them to separate. But the abundantly present binder material  12  in the regions C, immediately adjacent to the pads  20  and  30 , will keep the periphery regions A of the pads  20  and  30  from separating. Thus, the result is that the peripheral regions A of the bonding pads generally stay bonded together while the bonding pads  20  and  30  are separated in the center region. Therefore, the conductive particles  14   a  sandwiched between the bonding pads  20  and  30  near the peripheral regions A remain under sufficient compression to form good electrical conduction paths between the bonding pads  20  and  30 .  
         [0008]     The ACF bonding pads are generally thin metal foils formed on flexible wiring leads so in this example of a typical application, it would be the bonding pad  20 , whose center region bows outward and separate away from the base terminal pad  30 . Because the base terminal pad  30  is on a rigid glass substrate, it would not flex or bow. In some applications, ACF film may be used to bond to thin ACF bonding pads together. For example, two flexible wiring leads having ACF bonding pads on them may be connected together using an ACF film. In those cases, both of the two mating ACF bonding pads will bow out in the center region since they are both flexible.  
         [0009]     Because the center regions of the bonding pads  20  and  30  have separated and are not exerting sufficient compression force on to the conduction particles  14   b  in the center region, the electrical connection between the bonding pads  20  and  30  are pretty much limited to the peripheral regions A. This problem is also illustrated in  FIG. 4 , which is a plan view microphotograph of the conventional ACF bonding pad configuration of  FIG. 3 . The view is taken from above the bonding pad  20 . The outline of the conductive particles  14   a  remaining under compression in the peripheral regions of the bonding pad  20  can be seen through the bonding pad  20 .  
         [0010]     Thus, an enhanced bonding pad designs in this type of application is needed to improve the electrical contact between the bonding pads utilizing ACF films.  
       SUMMARY OF THE INVENTION  
       [0011]     According to an aspect of the present invention, improved interconnection bonding pad configuration for use with anisotropic conductive films (ACF) is disclosed. Interconnection bonding pads for use with ACF films according to an embodiment of the present invention have at least two finger-like portions.  
         [0012]     According to another embodiment of the present invention, a flexible wire lead comprises an interconnection bonding pad for bonding to an anisotropic conductive film, wherein the bonding pad comprises at least two finger-like portions.  
         [0013]     The finger-like portions may be defined by at least one notch extending from an edge of the bonding pad toward the center region of the bonding pad. 
     
    
     BRIEF DESCRIPTION OF THE DRAWING  
       [0014]      FIG. 1  is a schematic cross-sectional view of a conventional ACF bonding pad ready to be pressure bonded to a second metal pad structure using an ACF film;  
         [0015]      FIG. 2  is a schematic cross-sectional view of the assembly of  FIG. 1  while a compression force is applied to form a bond between the ACF bonding pad and the second metal pad structure;  
         [0016]      FIG. 3  is a schematic cross-sectional view of the assembly of  FIG. 2  after the compression force has been removed;  
         [0017]      FIG. 4  is a plan view photograph of and example of the ACF bonding pad of  FIG. 3  illustrating the outline of the conductive particles near the peripheral regions of the conventional ACF bonding pad;  
         [0018]      FIG. 5  is a schematic illustration of the outline shape of a typical conventional ACF bonding pad;  
         [0019]      FIGS. 6 through 14  are schematic illustrations of ACF bonding pads according to various embodiments of the present invention;  
         [0020]      FIG. 15  is a schematic cross-sectional view of the ACF bonding pad of  FIG. 7  ready to form an electrical connection with a base terminal pad using an ACF film; and  
         [0021]      FIG. 16  is a schematic cross-sectional view of the ACF bonding pad of  FIG. 15  after the ACF bonding pad has been pressure bonded to the base terminal pad using the ACF film. 
     
    
       [0022]     The features shown in the above referenced drawings are not intended to be drawn to scale. Like reference numerals are used to denote like parts throughout the various figures.  
       DETAILED DESCRIPTION  
       [0023]      FIG. 5  is a schematic illustration of a conventional ACF bonding pad  120  showing the conventional rectangular outline shape of the bonding pad provided here for the purpose of comparison.  
         [0024]      FIGS. 6-14  are schematic illustrations of ACF bonding pads having an improved outline shapes according to various embodiments of the present invention. ACF bonding pads are electrical interconnection pads, generally provided on flexible wiring leads, for pressure bonding using ACF films. The bonding pads are generally made from foil of copper-based alloys or aluminum-based alloys. The methods of forming such metal bonding pad patterns on flexible wiring leads are well known in the art.  
         [0025]     The improved ACF bonding pads of  FIGS. 6-14  have at least two finger-like portions. For example, the ACF bonding pad  130  of  FIG. 6  has two finger-like portions  132  and  134 . The finger-like portions  132  and  134 , in this example, are oriented parallel to the conductive line  135  connected to the bonding pad  130  but they may be oriented in other directions also. The ACF bonding pad  140  of  FIG. 7  has three finger-like portions  142 ,  143 , and  144 . The finger-like portions in this example are also oriented parallel to the conductive line  145  connected to the bonding pad  140  but hey may be configured to have different orientation relative to the conduction line  145 . The ACF bonding pad  150  of  FIG. 8  has three finger-like portions  152 ,  153 , and  154  oriented orthogonally to the conductive line  155  connected to the bonding pad  150 . The ACF bonding pad  160  of  FIG. 9  has four finger-like portions  161 ,  162 ,  163  and  164  that are oriented orthogonally to the conductive line  165 . The ACF bonding pad  170  of  FIG. 10  has six finger-like portions  172 ,  173 ,  174 ,  176 ,  177 , and  178 . These finger-like portions are also oriented orthogonally to the conductive line  175  connected to the bonding pad  170 . Three of the finger-like portions  172 ,  173 , and  174  extend away from the longitudinal axis of the bonding pad defined by the conductive line  175  while the remaining three finger-like portions  176 ,  177 , and  178  extend away from the longitudinal axis in the opposite direction. The ACF bonding pad  180  of  FIG. 11  has five finger-like portions  182 ,  183 ,  184 ,  186 , and  187 . These finger-like portions are oriented parallel to the conductive line  185 . The ACF bonding pad  310  of  FIG. 12  has four finger-like portions  312  that radiate outwardly from the center of the geometric center of the bonding pad  310 . The ACF bonding pad  320  of  FIG. 13  has three finger-like portions  322 . The ACF bonding pad  330  of  FIG. 14  has four finger-like portions  332 . These various shapes for improved ACF bonding pads discussed herein are only exemplary illustrations and are not intended to limit the present invention to those particular shapes.  
         [0026]     Another way of defining the finger-like portions of the ACF bonding pads of the present invention is that the finger-like portions may be said to be defined by the spacing or the notches between the finger-like portions. Thus, an improved ACF bonding pads according to an embodiment of the present invention may comprise at least two finger-like portions that are defined by at least one notch extending from an edge of the bonding pad toward the center region of the bonding pad. For example, the ACF bonding pad  130  of  FIG. 6  has a notch  133  defining the finger-like portions  132  and  134 . The ACF bonding pad  310  of  FIG. 12  has three notches  313  defining the four finger-like portions  312 . The ACF bonding pad  330  of  FIG. 14  has three notches  333  defining the four finger-like portions  332 .  
         [0027]     Whether the improved ACF bonding pads of the present invention are defined by the finger-like portions or the notches, the desired effect of the novel improved shapes of the ACF pads of the present invention is that unlike the conventional ACF bonding pad, the improved ACF bonding pad is divided into two or more smaller finger-like portions having spacings or notches between them so that the notched areas provide room for the ACF film&#39;s binder material to exist within the outline of the bonding pads near the center regions of the bonding pads after the ACF film is compressed between the ACF bonding pad and a mating surface. The mating surface is usually the base terminal pads of a glass substrate in LCD or PDP devices or another ACF bonding pad.  
         [0028]     The improvement achieved by the use of the ACF bonding pads of the present invention will now be described with reference to  FIGS. 15 and 16 .  FIG. 15  is a schematic cross-sectional illustration of the embodiment of the ACF bonding pad  140  of  FIG. 7  taken along the line A-A. The bonding pad  140  of flexible wiring lead  500  is about to be compressed with an ACF film  110  to form electrical connections with a base terminal pad  200  of a substrate  600 . The three finger-like portions  142 ,  143 , and  144  of the terminal ACF bonding pad  140  are shown. The ACF film  110  comprises insulator coated conductive particles  114  dispersed within a binder layer  112 .  
         [0029]      FIG. 16  is a cross-sectional schematic illustration of the ACF bonding pad  140  of  FIG. 15  after the bonding pad  140  has been bonded to the base terminal pad  200  using the ACF film  110  to form an electrical connection. Each of the three finger-like portions  142 ,  143 , and  144  of the ACF bonding pad  140  makes electrical connection with the base terminal pad  200  through the conductive particles  114   a  squeezed near the peripheral regions of the finger-like portions  142 ,  143 , and  144 . Thus, there are substantially more conductive particles  114   a  that are squeezed and crushed between the terminal pad  200  and the finger-like portions  142 ,  143 , and  144  when compared to the conventional configuration illustrated in  FIG. 3 . This provides more electrical conduction paths compared to the conventional ACF bonding pads.  
         [0030]     Also, as illustrated in  FIG. 16 , in this configuration according to an aspect of the present invention, binder materials  112   a  and  112   b  remain in the center region of the ACF bonding pad in between the three finger-like portions  142 ,  143  and  144 . The binder materials  112   a  and  112   b  are bonded to the flexible wiring lead  500 , the base terminal pad  200  and the finger-like portions  142 ,  143  and  144 . Thus, the binder material  112   a  and  112   b  will hold the finger-like portions in place against the base terminal pad  200  preventing the finger-like portions from separating away from the base terminal pad  200 . This keeps the conduction particles  114   a  under compression and electrically connecting the three finger-like portions to the base terminal pad  200 . The result is that a substantially more electrical conduction paths are formed between the three finger-like portions of the ACF pad  140  and the base terminal pad  200  compared to the conventional configuration of  FIG. 3 .  
         [0031]     The particular outline shapes illustrated in  FIGS. 6-11  are also for illustrative purposes only and are not intended to limit the ACF bonding pads of the present invention to those particular shapes. The ACF bonding pad configurations discussed in reference to  FIGS. 6-11  may be formed in any dimensions appropriate for a particular ACF film application. The particular width and length of the finger-like portions of a particular ACF bonding pad that is optimal for a given application should be determined in consideration of various factors. Among these factors are the conductive particle size of the ACF film, the thickness of the ACF film, the thickness of the ACF bonding pad metallurgy.  
         [0032]     While the foregoing invention has been described with reference to the above embodiments, various modifications and changes can be made without departing from the spirit of the invention. Accordingly, all such modifications and changes are considered to be within the scope of the appended claims.