Abstract:
The present invention relates to a high density printed circuit board and a method of manufacturing the same which enable a thin printed circuit board to be manufactured and can overcome problems occurring in a conventional method of manufacturing a printed circuit board because a conventional CCL is not used as a raw material. The high density printed circuit board includes a first insulating layer having a constant thickness, and a pair of first circuit layers embedded in two sides of the first insulating layer, respectively.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a U.S. divisional application filed under 35 USC 1.53(b) claiming benefit of U.S. Ser. No. 11/599,435 filed in the United States on Nov. 15, 2006 now abandoned, which claims earlier benefit to Korean Patent Application No. 10-2005-0110805 filed with the Korean Intellectual Property Office on Nov. 18, 2005, the disclosure of which is incorporated herein by reference in its entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a printed circuit board and a method of manufacturing the same, and, more particularly, to a high density printed circuit board and a method of manufacturing the same which enable a thin printed circuit board to be manufactured and can overcome problems occurring in a conventional method of manufacturing a printed circuit board because a conventional copper clad laminate (CCL) is not used as a raw material. The present invention relates to a printed circuit board. 
     2. Description of the Related Art 
     With the development of the electronic industry, electronic parts having high functionality and small size have been required. In particular, the demand for decreasing the thickness of electronic parts which are installed in portable terminals, in order to decrease the thickness of the portable terminals, is increasing. 
     In this situation, the reduction of the height of a package is one of the principal issues. 
     As various services in the mobile communication field are increasing, the number of electronic parts which are installed in a mobile phone is also increasing. Since the reduction of the area of the mobile phone is one of the demands of the final user, the trend that several chips are mounted on one interposer is increasing. 
     A Chip Scale Package (CSP), which is used as an interposer for ICs, has been widely used in mobile phones. Recently, almost all packages are using CSP, and almost all lCs are being stacked in such a CSP. 
     However, in order to stack more ICs, since the total height of the package is constant, methods of mounting the ICs while maintaining the height of the package have been sought in two directions. 
     That is, the methods include a method of decreasing the thickness of the ICs and a method of decreasing the thickness of the interposer. 
     The thickness of ICs ranges from 50 μm to 75 μm, and high level techniques for mounting them have already been established. Although researches into ICs thinner than the thickness range are being conducted, it is considered that such techniques have reached the limit of current technology. 
     The interposer is also becoming extremely thin, but it is considered that the techniques for decreasing the thickness of the interposer have also reached limit of current technology. Accordingly, in order to further decrease the thickness of the interposer, the technique for decreasing the thickness of components of the interposer by approaching the lower limit values of the components has been studied. 
     Meanwhile, in a conventional process of manufacturing a printed circuit board, methods of forming a circuit are classified into a tenting method (an etching method) and an additive method. 
     The tenting method is a method of forming a circuit pattern by forming an etching resist pattern on a copper foil formed on a CCL to a constant thickness, and etching portions other than the circuit by dipping a substrate, on which the etching resist pattern is formed, into an etching solution. 
     The additive method, which has recently been widely used, is a method of realizing a circuit pattern by forming a plating resist pattern on a CCL, plating only portions to form a circuit, and removing the plating resist pattern. 
     The tenting method has a low manufacturing cost, but has a limitation in forming fine circuit patterns. Accordingly, to overcome that limitation, the additive method is employed. 
       FIGS. 1A to 1D  show a conventional method of manufacturing a printed circuit board using a semi-additive method. 
     In  FIG. 1A , a plating resist pattern is formed by applying a plating resist  13  on the surface of the copper foil  12  of a CCL, which includes the copper foil  12  and a reinforced substrate  11 , and developing the plating resist  13 . 
     The thickness of the copper foil  12  of a typical CCL is about 0.5 μm to 3 μm. Typically, a photosensitive dry film is used as the plating resist  13 . 
     In  FIG. 1B , a plating layer  14  is formed through electrolytic plating. During plating, the copper foil  12  serves as a seed layer. However, in this case, the thickness of the plating layer  14  formed through electrolytic plating is not constant throughout the entire region due to deviation occurring during plating. 
     In  FIG. 1C , the remaining plating resist  13  is separated after the plating is completed. 
     When the plating resist  13  is separated, it is removed by dipping the resulting CCL into a separating liquid. In this case, there is a problem in that the plating resist  13  is not completely removed, thus some of the plating resist  13  remains on the side walls of the plating layer  14 . 
     In  FIG. 1D , when the portions of the copper foil  12  that do not form a circuit pattern are removed through soft etching, and thus only a desired circuit pattern remains, the circuit pattern is formed. 
     However, even when the additive method is employed, it is still impossible to obtain a desired thickness. 
     SUMMARY OF THE INVENTION 
     Accordingly, the present invention has been made keeping in mind the above problems occurring in the prior art, and an object of the present invention is to provide a method of manufacturing a printed circuit board without using a conventional CCL as a raw material. 
     Another object of the present invention is to provide a method of manufacturing a thin printed circuit board. 
     In order to accomplish the above object, the present invention provides a high density printed circuit board, including a first insulating layer having a constant thickness; and a pair of first circuit layers embedded in two sides of the first insulating layer, respectively. 
     Further, the present invention provides a method of manufacturing a high density printed circuit board, including preparing a pair of carrier films, each of which has a copper foil formed on one side thereof; forming plating resists, each having a circuit pattern, on the copper foils of the pair of carrier films, and then forming plating layers thereon; removing the plating resists from the pair of carrier films, and then attaching the carrier films to each other by causing the plating layers to face each other and interposing an adhesive between the pair of carrier films; removing the pair of carrier films, forming via holes, forming a seed layer, and then forming the plating resists to open only the via holes; and performing plating, and then separating the plating resists and removing the seed layer through an etching process. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other objects, features and advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which: 
         FIGS. 1A to 1D  are views showing a method of manufacturing a printed circuit board based on a conventional semi-additive method; 
         FIG. 2  is a cross-sectional view showing a high density printed circuit board according to an embodiment of the present invention; 
         FIGS. 3A to 3M  are views showing a method of manufacturing a double-sided high density printed circuit board according to an embodiment of the present invention; and 
         FIGS. 4A to 4H  are process diagrams showing a process of forming via holes using a laser drill in the method of manufacturing a high density printed circuit board according to the embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention will be described in more detail with reference to the accompanying drawings below. 
       FIG. 2  is a cross-sectional view of a printed circuit board according to an embodiment of the present invention. 
     Referring to  FIG. 2 , in the printed circuit board according to the embodiment of the present invention, circuit patterns  22  are embedded in an epoxy resin layer  21 . Here, the thickness of the circuit patterns  22  ranges from 15 to 25 μm, so that the total thickness of the printed circuit board can be decreased to a value in a range of 30 to 50 μm, compared to the conventional art. The thickness of solder resists  24  also serves to decrease the total thickness of the printed circuit board because the solder resists  24  may be applied to such an extent that they do not cause a problem in mounting, due to the flatness of the surface of the printed circuit board. 
       FIGS. 3A to 3M  show a method of manufacturing a high density printed circuit board according to an embodiment of the present invention. The method of manufacturing a high density printed circuit board according to the embodiment of the present invention will be described with reference to  FIGS. 3A to 3M  below. 
     First, as shown in  FIG. 3A , the manufacturing process begins with two structures, each of which includes a carrier film  31  and a thin copper foil  32  attached to the carrier film  31 . 
     Materials commonly used in manufacturing a printed circuit board are used as materials for the copper foil and the carrier film. It is preferred that the thickness of the copper foil  32  be 3 μm. Furthermore, it is preferred that the thickness of the carrier film  31  be 15 to 100 μm, and it is possible to use a metal, such as copper or aluminum, or an epoxy resin as the raw material of the carrier film  31 . 
     The thin copper foil  32  and the carrier film  31  are attached to each other using an organic material or an inorganic material, and can be easily removed after a high temperature and high pressure process. 
     Then, as shown in  FIGS. 3B and 3   c , etching resists  33   a  are layered on two sides of one of the two structures, and etching resists  33   b  are layered on two sides of the other of the two structures, wherein each of the structure includes the copper foil  32  attached to the carrier film  31 . Subsequently, circuit patterns are formed on any one of the etching resists  33   a  and any one of the etching resists  33   b  through an exposing and developing process. 
     The method of forming the etching resists  33   a  and  33   b  is classified as a photographic method or a screen printing method. The photographic method is classified as either a D/F method, which uses a dry film as the etching resist, or a liquid phase photosensitive material method, which uses a liquefied photosensitive material as the etching resist. 
     Then, as shown in  FIGS. 3D and 3E , copper plating circuit layer  34   a  and  34   b  are formed on the etching resists  33   a  and  33   b , respectively, by performing electrolytic plating using the copper foils  32   a  and  32   b  as a plating lead lines, and then the etching resists  33   a  and  33   b  are removed. The electrolytic copper plating is used for the copper plating for the etching resists  33   a  and  33   b . A thick plating film is easily formed through the electrolytic copper plating, and the physical properties of a plating film are also excellent, compared to electroless copper plating. 
     Then, as shown in  FIG. 3F , the two base substrates, on each of which the copper plating layer circuit pattern  34   a  or  34   b  is formed, are attached to each other using an adhesive  35  interposed therebetween, through a high temperature and high pressure process. 
     In this case, the surface of each of the copper plating circuit layers  34   a  and  34   b  is subjected to a surface treatment in order to be easily adhered to the adhesive  35 , and it is preferred that the surface treatment be a black treatment, a brown-black treatment or the like. 
     It is preferred that a prepreg that is used for interlayer insulation and has a suitable adhesive force when heat is applied thereto at the time of manufacturing a multi-layered printed circuit board be used as the adhesive  35 . 
     The prepreg is a material in which an adhesive is impregnated into a glass fiber material, and is interposed between the copper plating circuit layers  34   a  and  34   b  in which the circuit patterns are formed, to serve as an adhesive layer between the copper plating circuit layers as well as an insulator therebetween. 
     When the adhesive  35  is aligned as shown in  FIG. 3F , and then is pressed from the upper and lower surfaces thereof while being heated, as shown in  FIG. 3G , both surfaces of the adhesive  35  are forced into the copper plating circuit layers  34   a  and  34   b  and, simultaneously, the copper plating circuit layers  34   a  and  34   b  are attached to each other due to the adhesive  35  interposed therebetween. 
     Then, as shown in  FIG. 3H , the carrier films  31   a  and  31   b  are removed. In this case, it is possible to remove them by hand. 
     Next, as shown in  FIG. 3I , via holes  36  are formed in the attached structure to connect signals between the first layer and the second layer through a drilling process. 
     In this case, it is preferred that the drilling process be a mechanical drilling process using a Computer Numerically Controlled (CNC) drill, because the copper plating circuit layers  34   a  and  34   b  and the adhesive  35  must be drilled. 
     Then, as shown in  FIG. 3J , a seed layer  37  is formed on the entire substrate to a thickness of 1.0 μm to 3.0 μm by electroless copper plating. 
     The copper plating for the inner walls of the via holes is performed in the sequence of electroless copper plating and electrolytic copper plating, because the inner walls of the drilled via holes are made of epoxy resin, serving as an insulator, and thus the electrolytic copper plating due to electrolysis cannot be performed. Accordingly, first, the electroless copper plating is performed, and then the electrolytic copper plating is performed, thereby completely copper plating the inner walls of the via holes. 
     The electroless copper plating is an exclusive method of providing a surface of a nonconductor such as a resin, a ceramic and glass with conductivity. 
     Since the inner holes of the via holes are provided with conductivity by performing the electroless copper plating, the electrolytic copper plating can be performed. In this case, plating resists  38  are formed on the portion other than the via portion in order to perform copper plating of only the via portion. That is, after the plating resists  38  are applied, only the via portion is exposed. 
     Then, a copper plating layer  39  is formed by performing electrolytic copper plating of only the via portion. In this case, a thick plating film is easily formed through the electrolytic plating, and the physical properties of the plating film are also excellent, compared to the electroless plating. 
     The inner walls of the via holes  36  are plated using the copper plating layer  39 , and simultaneously the inner portions of the via holes  36  is filled with the copper plating layer  39 . 
     Next, as shown in  FIG. 3K , after the copper plating layer  39  is formed, the plating resists  38  are removed to expose the seed layer  37 . 
     Then, as shown in  FIG. 3L , the seed layer  37  and the copper foils  32   a  and  32   b  are removed through an etching process. Here, the etching process is continuously performed until the adhesive  35  is exposed. 
     That is, the pattern of the adhesive  35  is externally exposed by etching the surfaces of the seed layer  37  and the copper foils  32   a  and  32   b . Through these processes, when a four layered printed circuit board is formed later, a core layer, on the upper and lower surfaces of which a first layer circuit pattern and a second layer circuit pattern are formed, is formed. 
     Furthermore, as shown in  FIG. 3M , solder resists  40  are applied, the portion to be surface-treated (gold plated or Organic Surface Protection (OSP) treated) is formed through exposing and developing processes, and the surface treatment is performed, thereby completely manufacturing an interposer. 
       FIGS. 4A to 3H  are process diagrams showing a process of forming via holes using a laser drill in the method of manufacturing a high density printed circuit board according to an embodiment of the present invention. 
     First, referring to  FIG. 4A , the portion, in which via is to be formed, of the upper copper plating circuit layer  44   a  of the two copper plating circuit layers  44   a  and  44   b  forms an annular ring  40 . 
     The two base substrates, on which are formed the upper copper plating circuit layer  44   a  including the annular ring  40  formed on the portion in which the via is to be formed, and the lower copper plating circuit layer  44   b , are attached to each other using an adhesive  45  through a high temperature and high pressure process. 
     In this case, the surface of each of the copper plating circuit layers  44   a  and  44   b  is subjected to a surface treatment in order to be easily adhered to the adhesive  45 , and it is preferred that the surface treatment be a black treatment, a brown-black treatment or the like. 
     It is preferred that a prepreg that is used for interlayer insulation and has a suitable adhesive force when heat is applied thereto at the time of manufacturing a multi-layered printed circuit board be used as the adhesive  45 . 
     When the adhesive  45  is aligned as shown in  FIG. 4A , and then is pressed from the upper and lower surfaces thereof while being heated, both surfaces of the adhesive  45  are forced into the copper plating circuit layers  44   a  and  44   b  and simultaneously the copper plating circuit layers  44   a  and  44   b  are attached to each other through the adhesive  45 . 
     Then, as shown in  FIG. 4B , the carrier films  41   a  and  41   b  are removed. In this case, it is possible to remove them by hand. 
     Next, as shown in  FIG. 4C , the carrier films  41   a  and  41   b  are removed, and then the seed layers  42   a  and  42   b  are removed through an etching process. 
     Next, as shown in  FIG. 4D , via holes  46  are formed in the resulting structure to connect signals between the first layer and the second layer through a laser drilling process. In this case, the laser drill is used so as not to affect the lower copper plating circuit layer  44   b . Since the annular ring  40  is already formed on the upper copper plating circuit layer  44   a , and the copper plating circuit layer  44   a  of the portion in which the via is to be formed is removed, it is not necessary to conduct the process of removing the copper plating circuit layer  44   a.    
     Then, as shown in  FIG. 4E , a seed layer  47  is formed throughout the substrate, on which the via  46  is formed, to a thickness of 1.0 μm to 3.0 μm by electroless copper plating. 
     The copper plating for the inner walls of the via holes is performed in the sequence of electroless copper plating and electrolytic copper plating, because the inner walls of the drilled via holes is made of epoxy resin, serving as an insulator, and thus the electrolytic copper plating cannot be performed. Accordingly, first, the electroless copper plating is performed, and then the electrolytic copper plating is performed, thereby entirely copper plating the inner walls of the via holes. 
     In this case, plating resists  48  are formed on the portion other than the via in order to perform copper plating of only the via portion. That is, after the plating resists  48  are applied, only the via portion is exposed. 
     Then, as shown in  FIG. 4F , a copper plating layer  49  is formed by performing electrolytic copper plating of only the via portion. In this case, a thick plating film is easily formed through the electrolytic plating, and the physical properties of the plating film are also excellent, compared to the electroless plating. 
     The inner walls of the via holes  46  are plated using the copper plating layer  49 , and, simultaneously, the inner portions of the via holes  46  are filled with the copper plating layer  49 . 
     Next, as shown in  FIG. 4G , after the copper plating layer  49  is formed, the plating resists  48  are removed to expose the seed layer  47 . 
     Then, the seed layer  47  and the copper foils  42   a  and  42   b  are removed through an etching process. In this case, the etching process is continuously performed until the adhesive  45  is exposed. 
     That is, the pattern of the adhesive  45  is externally exposed through etching the surfaces of the seed layer  47  and the copper foils  42   a  and  42   b . Through these processes, when a four layered printed circuit board is formed later, a core layer, on the upper and lower surfaces of which a first layer circuit pattern and a second layer circuit pattern are formed, is formed. 
     Furthermore, as shown in  FIG. 4H , solder resists  50  are applied, the portion to be surface-treated (gold treated or OSP treated) is formed through exposing and developing processes, and the surface treatment is performed, thereby completely manufacturing an interposer. 
     As described above, according to the present invention, it is possible to decrease the total thickness of the interposer by about 60 μm to 110 μm. 
     Furthermore, according to the present invention, since the circuit is formed using a seed layer, resulting in a fine pattern, the adhesive force of the circuit is increased, poor quality owing to insufficient adhesive force does not occur during subsequent processes, and the breaking of a wire does not occur. 
     Furthermore, according to the present invention, since the seed layer is removed and the removed portion is a reference for flatness, the flatness of the pattern is better than that of conventional process. 
     As described above, although the preferred embodiments of the present invention have been disclosed for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims.