FIELD OF THE INVENTION  
       [0001]     The present invention relates to a flexible printed circuit board, particularly to a flexible printed circuit board with an anisotropic conductive film, which has even and low impedance, even breakage of conductive particles, even pressure, and thus has good reliability of the anisotropic conductive film for use in handheld electrical devices.  
       BACKGROUND OF THE INVENTION  
       [0002]     Please refer to  FIG. 1A  and  FIG. 1B . They are schematic views showing the liquid crystal display devices provided in the industry with the prevalence of handheld electrical devices, such as cellular phone, PDA, smart phone, stock manager, etc. In the drawings, the flexible printed circuit boards  61 ,  71  can be built with a copper circuit print and can be fixed with integrated circuits  615 ,  715  respectively. The surface mounting devices (SMD)  716  (such as surface mounting capacitor, surface mounting resistor, LED and etc.) can be mounted on the flexible printed circuit board  71  as well. The flexible printed circuit boards  61 ,  71  are electrically connected to the upper liquid crystal displays (LCD)  62 ,  72  via the anisotropic conductive films  612 ,  712  with the pitch under 0.1 mm respectively.  
         [0003]     Please refer to  FIGS. 2A and 2B .  FIG. 2A  is a side view showing the main portion of the flexible printed circuit board according to the prior art, and  FIG. 2B  is a diagram illustrating the structure of the conventional liquid crystal display of the handheld electrical device according to the prior art. In  FIG. 2A , the main portion  811  of the flexible printed circuit board  81  includes the plastic layer  812 , the first copper layer  813 , and the second copper layer  814 . The plastic layer  812  not only can be built with the first copper layer  813  and the second copper layer  814  of the copper circuit print, but also can be fixed with an integrated circuit and surface mounting devices, such as surface mounting capacitor, surface mounting resistor, LED, and etc. The main portion  811  of the flexible printed circuit board  81  is electrically connected to the upper LCD  82  via the anisotropic conductive film  8121  with the pitch under 0.1 mm.  
         [0004]     In  FIG. 2B , the anisotropic conductive film  8121  correspondingly bonds to the indium-tin oxide layer  821  of the LCD  82  for transferring a liquid crystal control signal to the LCD panel via the indium-tin oxide layer  821 . The bottom end of the flexible printed circuit board  81  is the golden-finger region  817 . Under the golden-finger region  817 , it is the LCD circuit board  83  made up of the main portion  831  and the connector portion  832 . The connector portion  832  is also called welding portion and designed to be connected with the golden-finger region  817 .  
         [0005]     The anisotropic conductive film  8121  contains plural conductive particles. The conductive particles must be broken in order to conduct electricity, if not, they cannot conduct electricity. However, the conductive particles are often broken unevenly. Though the conductive particles of uneven breakage can still conduct electricity, they could reduce the reliability of the anisotropic conductive film  8121 . In the conventional technique, the pitch of the anisotropic conductive film  8121  is about 0.1 mm, as shown in the A and B portions of  FIG. 2B . As the pitch of the anisotropic conductive film  8121  is kept under 0.1 mm, it is easier to reach the requirement of smooth for the anisotropic conductive film  8121 . However, in order to reduce the impedance and adapt to the standards of the integrated circuit industry, the pitch of the anisotropic conductive film  8121  is broadened and wider than 0.5 mm. As a result, the anisotropic conductive film  8121  is not smooth, which causes the uneven breakage of the conductive particles.  
         [0006]     In order to overcome the drawbacks in the prior art, a flexible printed circuit board is provided. In the particular design, the anisotropic conductive film with broadened pitch has the features of even and low impedance, even breakage of conductive particles, even pressure, and thus the anisotropic conductive film for use in handheld electrical devices will have good reliability.  
       SUMMARY OF THE INVENTION  
       [0007]     It is an object of the present invention to provide a flexible printed circuit board with an anisotropic conductive film for use in handheld electrical devices.  
         [0008]     It is another object of the present invention to provide a flexible printed circuit board with an anisotropic conductive film, which has the features of even and low impedance, even breakage of conductive particles, even pressure, and thus a good reliability of the anisotropic conductive film for use in handheld electrical devices is achieved.  
         [0009]     In accordance with an aspect of the present invention, a flexible printed circuit board includes a substrate layer; at least a circuit layer formed on the substrate layer; and a conductive film layer formed on one end of the circuit layer, characterized in that a pitch of the conductive film layer is broadened to be ranged from 0.5 mm to 3.0 mm.  
         [0010]     Preferably, the other end of the circuit layer is formed to be a golden-finger region and is electrically connected to the LCD circuit board.  
         [0011]     Preferably, the anisotropic conductive film includes conductive particles and sticky polymers.  
         [0012]     Preferably, the conductive particles are one of metal-plated polymer particles and nickel particles.  
         [0013]     Preferably, the metal of the metal-plated polymer particles is selected from a group consisting of a nickel, a copper, a gold, and a silver.  
         [0014]     Preferably, the substrate layer is a plastic layer.  
         [0015]     Preferably, the plastic layer includes a polyimide layer and a polypropylene/epoxy resin layer.  
         [0016]     Preferably, the plastic layer includes a polyimide layer.  
         [0017]     Preferably, the circuit layer is a copper circuit layer.  
         [0018]     Preferably, the circuit layer further includes an integrated circuit disposed thereon which is packaged by using one of a tape carrier package and a chip on film.  
         [0019]     Preferably, the circuit layer further includes surface mounting devices.  
         [0020]     Preferably, the conductive film is an anisotropic conductive film.  
         [0021]     Preferably, the flexible circuit board is connected to a liquid crystal display via the conductive film layer.  
         [0022]     In accordance with another aspect of the present invention, a flexible printed circuit board includes at least two substrate layers; at least a circuit layer formed between every adjacent two substrate layers; and at least a conductive film layer formed on one end of the circuit layer, characterized in that a pitch of the conductive film layer is broadened to be ranged from 0.5 mm to 3.0 mm.  
         [0023]     Preferably, the other end of the circuit layer is formed to be a golden-finger region and is electrically connected to the LCD circuit board.  
         [0024]     Preferably, the anisotropic conductive film includes conductive particles and sticky polymers.  
         [0025]     Preferably, the conductive particles are one of metal-plated polymer particles and nickel particles.  
         [0026]     Preferably, the metal of the metal-plated polymer particles is selected from a group consisting of a nickel, a copper, a gold, and a silver.  
         [0027]     Preferably, the substrate layer is a plastic layer.  
         [0028]     Preferably, the plastic layer includes a polyimide layer and a polypropylene/epoxy resin layer.  
         [0029]     Preferably, the plastic layer includes a polyimide layer.  
         [0030]     Preferably, the circuit layer is a copper circuit layer.  
         [0031]     Preferably, the circuit layer further includes an integrated circuit disposed thereon which is packaged by using one of a tape carrier package and a chip on film.  
         [0032]     Preferably, the circuit layer further includes surface mounting devices.  
         [0033]     Preferably, the conductive film is an anisotropic conductive film.  
         [0034]     Preferably, the flexible circuit board is connected to a liquid crystal display via the conductive film layer.  
         [0035]     The foregoing and other features and advantages of the present invention will be more clearly understood through the following descriptions with reference to the drawings, wherein: 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0036]      FIGS. 1A and 1B  are schematic views showing the conventional liquid crystal display of the handheld electrical devices according to the prior art;  
         [0037]      FIG. 2A  is a side view showing the main portion of the flexible printed circuit board according to the prior art;  
         [0038]      FIG. 2B  is a diagram illustrating the structure of the conventional liquid crystal display of the handheld electrical device according to the prior art;  
         [0039]      FIG. 3A  is a side view showing the main portion of the flexible printed circuit board according to a preferred embodiment of the present invention;  
         [0040]      FIG. 3B  is a diagram illustrating the structure of the liquid crystal display of the handheld electronic device according to a preferred embodiment of the present invention;  
         [0041]      FIG. 4A  is a side view showing the main portion of the flexible printed circuit board according to another preferred embodiment of the present invention;  
         [0042]      FIG. 4B  is a diagram illustrating the structure of the liquid crystal display of handheld electronic device according to another preferred embodiment of the present invention; and  
         [0043]      FIG. 5  is a side view of  FIG. 3B . 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0044]     The present invention will now be described more specifically with reference to the following embodiments. In the first embodiment of the present invention, the flexible printed circuit boards  1 ,  1 ′ with the broadened anisotropic conductive film respectively have the main portions  11 ,  11 ′. Please refer to  FIGS. 3A and 4A , which are side views showing the main portions of the flexible printed circuit boards according to the preferred embodiments of the present invention. The main portions  11 ,  11 ′ are respectively made up of the first circuit layers (circuit layer)  13 ,  13 ′, the second circuit layers (circuit layer)  14 ,  14 ′, and the plastic layers (substrate layer)  12 , 12 ′. Preferably, the plastic layers  12 ,  12 ′ can be polyimide having good flexibility and mechanical property, i.e. all-polyimide flexible printed circuit board. In addition, a polypropylene/epoxy resin layer can be spread on the polyimide layer. Preferably, the first and the second circuit layers  13 ,  13 ′,  14 ,  14 ′ are printed copper layers regarding the double flexible printed circuits.  
         [0045]     Please refer to  FIG. 3B  which is a diagram illustrating the structure of the liquid crystal display of the handheld electronic device according to the preferred embodiment of the present invention. In  FIG. 3B , the upper end of the main body  11  of the flexible printed circuit board  1  of the present invention is electrically connected to the indium-tin oxide layer  21  of the liquid crystal display  2  via the anisotropic conductive film  121 . The bottom end of the main body  11  of the flexible printed circuit board  1  of the present invention is electrically connected to the connector  32  (also called welding portion) of the LCD circuit board  3  via the golden-finger region  17 . In addition, the flexible printed circuit board  1  can be fixed an integrated circuit  15  by using a tape carrier package.  
         [0046]     Please refer to  FIG. 4B . The upper end of the main body  11 ′ of the flexible printed circuit board  1 ′ of the present invention is electrically connected to the indium-tin oxide layer  21 ′ of the liquid crystal display  2 ′ via the anisotropic conductive film  121 ′. The bottom end of the main body  11 ′ of the flexible printed circuit board  1 ′ of the present invention is electrically connected to the connector  32 ′ (also called welding portion) of the LCD circuit board  3 ′ via the golden-finger region  17 ′. In addition, the integrated circuit  15 ′ and surface mounting devices  16  (such as surface mounting capacitor, surface mounting resistor, LED, and etc.) can be fixed on the flexible printed circuit board  1 ′ by using a tape carrier package or a chip on film.  
         [0047]     In the second embodiment of the present invention, the flexible printed circuit boards  1 ,  1 ′ are all-polyimide flexible laminae. For example, it has two layers, three layers, four layers, and six layers.  
         [0048]     Please refer to  FIG. 5  which is a side view of  FIG. 3B  showing the liquid crystal display of the handheld electronic devices. In the left side, the drawing is the liquid crystal display  2 , whose up-right portion is the indium-tin oxide layer  21 ,  21 ′. The indium-tin oxide layer  21 ,  21 ′ is electrically connected to the anisotropic conductive film  121  located at down-left portion of the flexible printed circuit board  1  by the bonding process. Moreover, the substrate layer  12 , the circuit layer  13 , and integrated circuit  15  are also shown in the drawing.  
         [0049]     In the anisotropic conductive adhesive or anisotropic conductive films  121 ,  121 ′ of the present invention, usually the conductive particles are dispersed and mixed in the polymers in a random fashion. The sticky polymers are subsequently transformed into the solid thin films. The conductive particles, typically as small as a few microns in diameter, are usually gold-plated polymers or nickel particles. The interconnection, between the anisotropic conductive adhesive or anisotropic conductive films  121 ,  121 ′ of the LCD display circuit board and the Indium-tin oxide layers  21 ,  21 ′ of the LCDs  2 ,  2 ′, is achieved by causing the break of the conductive particles with the hot bar. Hence the electricity is conducted along the films. The above process is called thermocompression process. Furthermore, during the thermocompression process the space between the Indium-tin oxide layers  21 ,  21 ′ and the anisotropic conductive films  121 ,  121 ′ are filled with the sticky polymers.  
         [0050]     The core substance of the conductive particles is the thermosetting and the thermoplastic polymers. Further, the surface of core particles is processed by the method of surface metalization so as to make conductive particles. The requirement of the product is that the fine pitch is under 0.1 mm. The present invention can be applied to a fine pitch interconnection in the following fields: anisotropic conductive film (ACF), anisotropic conductive adhesive (ACA), liquid crystal display (LCD)/TAB, liquid crystal display (LCD)/FPC, chip of glass (COG), chip on film (COF), electrolumine scence (EL) electrode, flip chip, and etc. Thus, the present invention with a fine pitch interconnection is indispensable in the LCD and semiconductor industries.  
         [0051]     The above-mentioned polymers of conductive particles can be plated with a layer of nickel, copper, gold, or silver so as to form 0.5-4 mm particles in diameter. Furthermore, the techniques of emulsification synthesis and surface transforming are used for metalizing the surface of the polymers. Accordingly, the cores of the conductive particles are polymers with even particles having same diameters so that they can enhance the conductivity while connected. The anisotropic conductive adhesive or anisotropic conductive films  121 ,  121 ′ has an area the same with the area of the indium-tin oxide layers  21 ,  21 ′ so that the impedance is reduced. Therefore, since the conductive particles on the anisotropic conductive film  121 ,  121 ′ have even sizes and good conductivity. The anisotropic conductive film  121 ,  121 ′ provided in the present invention can be adapted to the standards in the IC industry. In other words, even if the pitch is broadened up to 0.5 mm owing to the product requirement, the anisotropic conductive film is still smooth and has excellent conductive interconnection.  
         [0052]     While the invention has been described in terms of what is presently considered to be the most practical and preferred embodiments, it is to be understood that the invention needs not be limited to the disclosed embodiments. On the contrary, it is intended to cover various modifications and similar arrangements included within the spirit and scope of the appended claims which are to be accorded with the broadest interpretation so as to encompass all such modifications and similar structures.
A flexible printed circuit with a broadened anisotropic conductive film is provided. The flexible printed circuit includes a substrate layer and a circuit layer, characterized in that a pitch of the anisotropic conductive film is broadened to be ranged from 0.5 mm to 3.0 mm. It has even and low impedance, even breakage of conductive particles, even pressure, and thus has good reliability of the anisotropic conductive film for use in handheld electronic devices.