BACKGROUND OF THE INVENTION 
       [0001]    1. Field of Invention 
         [0002]    The present invention relates to an PCB (printed circuit board) structure, in particular to an PCB structure using a silicone layer to combine the metal layer and the substrate. 
         [0003]    2. Related Art 
         [0004]    Since a flexible printed circuit board (FPC) assembly has flexibility by which the flexible printed circuit board assembly can be bent by itself, the flexible printed circuit board assembly has been used in various kinds of electrical appliances. For example, a flexible printed circuit board can be installed between a main body and an openable display unit of a notebook computer to transmit an electrical signal between the main body and the display unit, or can be installed between a main body and a movable pickup assembly of an optical disc drive in a computer to transmit a signal therebetween. 
         [0005]    A flexible copper clad laminate (FCCL) is main product of the FPC. The FCCL is manufactured by gluing together a metallic foil such as a copper foil to the surface of a heat-resistant film represented by a polyimide film via an adhesive formed of an epoxy system resin adhesive, such as epoxy resin, polyester, or acrylic resin. The FCCL is normally produced by hot-pressing the layered structure at around 180° C. Due to the main chain of the epoxy system resin adhesive is rigid, there has a problems known as warping phenomenon of the laminate itself during the hot-pressing. 
         [0006]    To solve this problem, the adhesive layer of the above three-layer flexible copper clad laminate is eliminated, so it is called “two-layer flexible copper clad laminate” (2L FCCL). It is manufactured by a casting method, sputtering method, or lamination method to form the copper foil on the surface of the substrate film represented by a polyimide (PI) film. 
         [0007]    However, because of the characteristic of the material, the FCCL, including three-layer and two-layer, has low ion-migration resistance. The material of the substrate, polyimide, and the adhesive, such as epoxy resin, or acrylic resin, are high polarity and hydrophilic. For example, the FCCL is utilized in a fine pitch wiring and high voltage appliances, such as LCD (liquid crystal display), OLED (Organic Light-Emitting Diode), or plasma display. Please refer to  FIG. 1 , it shows a three-layer flexible copper clad laminate. A first lead  43  and a second lead  44  are formed on the adhesive layer  41 ,  42  and the substrate  40 . During the high voltage operation, these devices are known to have the drawback that a short distance S between leads  43 ,  44  causes an ion migration phenomenon to the copper used as the wiring or electrode. It often results in electrical failures such as increased leakage current flow and short-circuiting between leads  43 ,  44 . On the other hand, for the two-layer flexible copper clad laminate, due to the substrate, PI, is also high polarity and hydrophilic. The drawback, the ion migration phenomenon, still exists. 
       SUMMARY OF THE INVENTION 
       [0008]    It is an objective of this invention to provide a PCB structure. The silicone is utilized to be the adhesive between the metal layer and the substrate. The silicone is sufficiently soft to absorb the deformation due to heat-shrink to prevent warping and bent during high temperature processing. 
         [0009]    Another objective of this invention is to provide a PCB structure. The ion-migration resistance is raised. The problems of unclear signal and voltage drop caused by moisture are overcame due to the characteristic of non-polar and non-hydrophile of the silicone. 
         [0010]    It is an objective of this invention to provide a PCB structure. The silicone layer has two modified silicone layers disposed two sides thereon to enhance the adhesion between the silicone and substrate/metal layer. The problems of producing bubbles or easy to peel off are solved. 
         [0011]    This invention discloses a PCB structure. The PCB structure includes a substrate, a silicone layer, and a metal layer. The silicone layer is disposed on the substrate and utilized to adhere the metal layer to the substrate. The silicone layer is sufficiently soft to absorb the deformation of the metal and the substrate due to heat-shrink to prevent warping and bent during high temperature processing. Due to the characteristic of non-polar and non-hydrophile of the silicone layer, the problems caused by the high polarity and hydrophilic material, such as polyimide (PI), epoxy resin, or acrylic resin, are solved. The ion-migration resistance of this PCB structure is raised. The problems of unclear signal and voltage drop caused by moisture are overcame due to the characteristic of non-polar and non-hydrophile of the silicone. 
         [0012]    Furthermore, to enhance the adhesion between the silicone and substrate/metal layer, the silicone layer has two modified silicone layers disposed two sides thereon. The problems of producing bubbles or easy to peel off are solved. The modified silicone layer is modified the interfacial tension and the polarity thereof to enhance the adhesion for different materials. 
         [0013]    Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]    The present invention will become more fully understood from the detailed description given hereinbelow illustration only, and thus are not limitative of the present invention, and wherein: 
           [0015]      FIG. 1  illustrates the example of the conventional PCB structure when utilizing in a fine pitch wiring and high voltage appliances. 
           [0016]      FIG. 2  illustrates the PCB structure of this present invention. 
           [0017]      FIG. 3  illustrates the PCB structure according to the present invention when utilizing in a fine pitch wiring and high voltage appliances. 
           [0018]      FIG. 4  illustrates the example of the conventional coating silicone layer. 
           [0019]      FIG. 5  illustrates another embodiment of the PCB structure according to the present invention. 
           [0020]      FIGS. 6A and 6B  illustrate embodiments of PCB structure of this present invention, which show only one modified silicone layer is disposed. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0021]    Silicones are synthetic compounds with a variety of forms and uses. Typically heat-resistant and rubber-like, they are used in sealants, adhesives, lubricants, medical applications, cookware, and insulation. Some of the most useful properties of silicones include low thermal conductivity, low chemical reactivity, low toxicity, and thermal stability. Also, silicones have excellent resistance to oxygen, ozone, and ultraviolet (UV) light and good electrical insulation. Because silicone can be formulated to be electrically insulative or conductive, it is suitable for a wide range of electrical applications. The silicone exhibits good fluidity and adhesion when melting, and is sufficiently soft after cured. Also, the silicone is non-polar and non-hydrophile. Therefore, it is suitable to replace epoxy resin as a material of adhesive. 
         [0022]    Please see  FIG. 2 , the PCB (printed circuit board) structure includes a substrate  11 , a silicone layer  12  and a metal layer  13 . The material of the substrate  11  includes polyimide (PI), polyethylene terephthalate (PET), polyethylene naphthalate (PEN), glass, glass fiber, and liquid crystal polymer to form a flexible PCB or an inflexible PCB. The silicone layer  12  is disposed between the substrate  11  and the metal layer  13  to be as adhesive therein. The silicone exhibits good fluidity and adhesion when melting. It is suitable to be an adhesive. For example, the silicone is disposed on the substrate  11  by coating to form the silicone layer  12 . The metal layer  13  is disposed on the silicone layer  12  and then is cured by a baking process. After cured, the silicone layer  12  is sufficiently soft to be as a buffer to absorb the deformation due to heat-shrink between the metal layer  13  and the substrate  11  to prevent warping and bent during high temperature processing. The wiring may be formed by on the metal layer  13  by etching or other processing. The further protective layer, solder mask, and etc. may also be formed thereon. The material of the metal layer  13  is copper (Cu), aluminum (Al), nickel (Ni), gold (Au), silver (Ag), tin (Sn) or an alloy comprised of at least one of the foregoing metals. 
         [0023]    Moreover, the silicone is non-polar and non-hydrophile. Please refer to  FIG. 3 , when the PCB structure is utilized in a fine pitch wiring and high voltage appliances, such as LCD (liquid crystal display), OLED (Organic Light-Emitting Diode), or plasma display, at least one first lead  131  and one second lead  132  are formed on the metal layer  13 . Unlike a conventional PCB structure, the silicone layer  12  is non-polar and non-hydrophile. Even the substrate  11  represented by a polyimide (PI) is high polarity and hydrophilic, the silicone layer  12  will stop the ion migration phenomenon between the first lead  131  and the second lead  132 . The problems of unclear signal and voltage drop will not be happened. 
         [0024]    However, the silicone has a problem for adhesion. The silicone will perform both the condensation reaction and the addition reaction during curing. The structure cured by a condensation reaction has less adhesion force than an addition reaction. Also the byproduct of the condensation reaction is hydrogen, which is easy to produce bubbles. As refer to  FIG. 4 , the silicone layer  22  is disposed on the substrate  21  with different material. During polymerization, the gas, i.e. hydrogen, is moved randomly. When blocking by the substrate  21 , due to the substrate  21  is made of compact material, such as metal, glass, or polymer, the gas is heaped up to form bubbles  23  at the interface therebetween. When there have bubbles  23  at the interface, the structure is easy to peel off. 
         [0025]    General speaking, the silicone is utilized in fill. In other words, one side of the silicone layer  22  is free, as shown in  FIG. 4 . If the polymerization is performed slowly, the produced gas can be exhausted slowly. However, in this invention, the silicone layer  12  is placed between the substrate  11  and the metal layer  13 , as shown in  FIG. 2 . It has to perform hot pressing or thermal polymerization for curing. That will produces more gases. When the gas is moved randomly and blocked by the substrate  11  and the metal layer  13 . The heaped gas bubbles would crack the adhesion interface therebetween. Also, the gas bubbles will be merged to a larger one to make the adhesion be weaker. 
         [0026]    Please refer to  FIG. 5 , to solve this problem, the silicone layer  12  has two modified silicone layers  31 ,  32  disposed two sides thereon. The first modified silicone layer  31  is disposed between the silicone layer  12  and the substrate  11 . The second modified silicone layer  32  is disposed between the silicone layer  12  and the metal layer  13 . The modified silicone layers  31 ,  32  are modified the interfacial tension and the polarity thereof depending on the materials of the substrate  11  and the metal layer  13 . Therefore, the good adhesion situations are presented on the interfaces between the first modified silicone layer  31  and the substrate  11 , and the second modified silicone layer  32  and the metal layer  13 . Also, the amounts of the produced gas bubbles are decreased and the size of the gas bubbles is lessened. The modified silicone layers  31 ,  32  are modified by adjusting a proportion of condensation-type silicone and addition-type silicone and/or by adding Epoxy, Acrylic Acid or a combination thereof into silicone. 
         [0027]    Take the following forming method for example, the modified silicone layers  31 ,  32  are formed on the substrate  11  and the metal layer  13  respectively. Then the polymerization is performed slowly for curing. Due to one side is free, the produced gas can be exhausted. Also, the modified silicone layers  31 ,  32  are modified depending on the materials of the substrate  11  and the metal layer  13 . The good adhesion situations are presented on the interfaces between the first modified silicone layer  31  and the substrate  11 , and the second modified silicone layer  32  and the metal layer  13 . Then the silicone layer  12  is disposed therebetween for curing to form the above-mentioned structure. 
         [0028]    Due to the silicone layer  12  is disposed between the modified silicone layers  31 ,  32 , which made of the same or substantially the same material, the adhesion force therebetween is high. Even the gas is produced, it is not easy to weaken the adhesion structure. Also, due to the silicone is not a compact material as the substrate  11  or the metal layer  13 . In micro-view, the silicone has larger hole inside. Even the silicone layer  12  is disposed between the modified silicone layers  31 ,  32  for curing, the produced gas is easily to be exhausted from the modified silicone layers  31 ,  32 , and is not easy to heap up to form bubbles. The intermolecular forces between the silicone layer  12  and the modified silicone layers  31 ,  32  are equal. The gas flows inside are uniform. The gas bubbles will not easy to merge to a larger one. Therefore, the good adhesion situations are presented on the interfaces between the modified silicone layers  31 ,  32  and the silicone layer  12 . 
         [0029]    It is also disposed a modified silicone layer to one of the interfaces, which the adhesion situation is worse. For example, it only has the first modified silicone layer  31  to be disposed between the substrate  11  and the silicone layer  12 , see  FIG. 6A . Or, it only has the second modified silicone layer  32  to be disposed between the metal layer  13  and the silicone layer  12 , see  FIG. 6B . 
         [0030]    The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.
A PCB structure is provided. The material of the adhesion is replaced by silicone from conventional Epoxy or Acrylic acid. The problems of unclear signal and voltage drop caused by moisture are overcame due to the characteristic of non-polar and non-hydrophile of the silicone. Also, the silicone layer has two modified silicone layers disposed two sides thereon to enhance the adhesion between the silicone and substrate/metal layer. The problems of producing bubbles or easy to peel off are solved.