Abstract:
Disclosed is a method of fabricating rigid flexible PCBs. In the method, a self-detachable adhesive tape is used to separate a wafer and a substrate from each other in the course of conventionally fabricating a semiconductor wafer, is employed to avoid inherent problems occurring in the conventional method of fabricating rigid flexible PCBs.

Description:
INCORPORATION BY REFERENCE 
   The present application claims priority under 35 U.S.C. §119 to Korean Patent Application No. 2005-5440 filed on Jan. 20, 2005. The content of the application is incorporated herein by reference in its entirety. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a method of fabricating a rigid flexible printed circuit board (PCB). More particularly, the present invention pertains to a method of fabricating a rigid flexible PCB, wherein a self-detachable adhesive tape used to separate a wafer from a substrate in the course of conventionally fabricating a semiconductor wafer, is employed to avoid inherent problems occurring in a conventional method of fabricating the rigid flexible PCB. 
   2. Description of the Prior Art 
   In accordance with the recent trend toward miniaturized, light, and slim electronic devices, light, slim, and high density PCBs are in demand. Accordingly, the use of a multilayered flexible PCB capable of satisfying the above demand is rapidly growing. However, a process of fabricating the multilayered flexible PCB is complicated and has many stages compared to a process of fabricating a general rigid PCB, thus costs and defective fractions are high. Of multilayered rigid PCBs, recently, a rigid flexible PCB having a flexible portion has been employed to produce folding-type mobile phones, thus it is expected that demand for rigid flexible PCBs will rapidly grow. 
   The flexible portion of the rigid flexible PCB reduces workability in a process of producing goods and increases the defective fractions. The development of special material and a process employing same contribute to solving the above problems, thereby assuring superiority over rival companies. 
   In order to fabricate the flexible PCB, the formation of microcircuits using thin and strong material must be feasible. In other words, the selection of material and process is important. Polyimide resin has mainly been used as the material because there is no material usable as a substitute for polyimide resin with regard to heat resistance, mechanical strength, fire retardancy, and electrical properties. However, currently, material capable of being used as a substitute for polyimide resin has been developed with regard to high frequency response, moisture resistance, dimensional stability, and cost. 
   With respect to technology of processing microcircuits of the flexible PCB, a pitch of a connection part of a flat panel display, such as an LCD, becomes fine so as to be changed from 33 μm to 25 μm in accordance with an increase in resolution, thus slimness of copper foils and base films has been achieved so that they are both 5 μm in thickness. According to data from JEITA (Japan Electronic and Information Technology Industries Association), it is predicted that the flexible PCB must have a minimum conductor width/minimum gap (L/S) of 15 μm/12 μm in the year 2006 and L/S of 10 μm/10 μm in the year 2008, and that a tape substrate must have L/S of 7 μm/7 μm in the year 2006 and L/S of 5 μm/5 μm in the year 2010. 
   To satisfy the above conditions, there remains a need to develop technologies for forming microcircuits or to develop novel processes or novel materials. 
     FIGS. 1   a  to  1   i  are sectional views illustrating a procedure of fabricating a rigid flexible PCB, according to conventional technology. 
     FIG. 1   a  is a sectional view of a flexible base substrate  101  on which circuit patterns  102  are formed. 
   In  FIG. 1   b , a coverlay  103  is partially or totally applied on the substrate  101 . 
   In  FIG. 1   c , prepregs  104  or bonding sheets are layered on both sides of the substrate  101 . Since the prepregs  104  or the bonding sheets are physically rigid, prepreg portions  105   a ,  105   b  of the resulting structure become rigid portions, and the remaining portion  106  of the resulting structure, on which the prepregs  104  are not layered, becomes a flexible portion  106 . 
   In  FIG. 1   d , thin copper foils  107  are layered on both sides of the substrate  101 . At this stage, since a height difference exists between the rigid portions  105   a ,  105   b  in which the prepregs  104  are layered and the flexible portion  106  in which the prepregs  104  are not layered, a portion of the copper foil layered on the flexible portion  106  sinks, causing a gap  108   a  between the copper foil  107  and the coverlay  103  on the flexible portion  106 . On a portion of the substrate on which the coverlay  103  is not applied, a larger gap  108   b  is formed between the flexible substrate  101  and the copper foil  107 . 
   In the layering of the copper foils  107 , heat and pressure are simultaneously applied to the prepregs  104  after the copper foils  107  are layered so that the copper foils  107  are stuck to prepregs  104 . At this time, the prepregs  104  are melted and thus flow from the rigid portions  105   a ,  105   b  to the flexible portion  106 . 
   If the prepregs flow downward as described above, the prepreg material is present on the flexible portion, thus the minimum curvature radius of the flexible portion is not the same as the set design value. 
   In  FIG. 1   e , a via hole  109  is formed through a portion of the rigid portions  105   a ,  105   b , and copper plating layers  112  are formed on a wall of the via hole  109  and on external layers of the substrate through a plating process. The formation of the via hole  109  is mostly conducted using laser drilling or mechanical drilling. Typically, after the via hole  109  is formed, a desmear process is implemented to remove impurities from around the via hole. 
   In  FIG. 1   f , a copper plating process is conducted on both sides of the substrate to form the copper plating layers  112  on surfaces of the substrate and on the wall of the via hole  109 . 
   In  FIG. 1   g , dry films  111  are applied on both sides of the substrate. A portion of the copper foil  107  on the flexible portion  106  has already sunk, and portions of the dry films  111  on the sunk portions of the copper foils  107  sink, forming additional gaps  110   a ,  110   b  between the copper foils  107  and the dry films  111 . 
   In  FIG. 1   h , the dry films  111  are exposed and developed to form etching resist patterns. At this stage, since the dry films  111  are not flat but have recesses, the dry films are nonuniformly exposed during an exposure process, resulting in the formation of undesirable etching resist patterns. 
   In  FIG. 1   i , etching is conducted to form circuit patterns on the copper foils  107 , and etching resists are stripped, thereby creating the rigid flexible PCB. 
   In the fabrication of the rigid flexible PCB according to conventional technology, as shown in  FIG. 1   i , a height difference exists between the rigid portions  105   a ,  105   b  and the flexible portion  106 , thus the prepregs flow down on the flexible portion  106  during the heating and pressurizing processes of  FIG. 1   d , causing gaps  108   a ,  108   b ,  110   a ,  110   b  between the copper foils  107  and the coverlay  103  and between the copper foils  107  and the dry films  111 , resulting in defective products. 
   In order to avoid the problems of nonuniform exposure of the dry films  111  as shown in  FIG. 1   h , a laser direct imaging (LDI) process, in which exposure is conducted with a laser device used in conventional laser drilling, may be employed so as to expose recesses or corners, onto which light is not easily radiated, during the exposure. However, this process is problematic in that productivity is very poor. 
   With respect to this, Japanese Patent Laid-Open Publication No. 2000-332416 discloses a method of fabricating a PCB, in which an external base film is layered on a cable portion as well as on a rigid portion so as to protect the cable portion corresponding to a flexible portion in a rigid flexible multilayered PCB. The external base film is removed from the cable portion after various chemical treatments, making the requirement for a cover film, which protects the cable portion corresponding to the flexible portion from the chemical treatments conducted during the fabrication of the PCB, unnecessary. 
   Having been layered on only the rigid portion, the external base film is layered on the flexible portion as well as on the rigid portion in the above method, thereby avoiding problems caused by a height difference between the rigid portion and the flexible portion. In this regard, the external base film layered on the flexible portion must be removed afterwards, but the above patent does not suggest a method of removing the external base film. The external base film has adhesive strength due to heat and pressure in the course of fabricating the PCB, thus it is very difficult to remove the external base film from the cable portion corresponding to the flexible portion. Additionally, when it is removed, a product may be damaged, or residues may remain on the product. With respect to this, the patent describes that it is preferable to attach the external base film to the flexible portion so that the attachment is not very strong. However, this is not a fundamental solution. 
   An adhesive material disclosed in Japanese Patent Laid-Open Publication No. 2003-231875 may be employed to produce an adhesive tape used in the present invention. 
   SUMMARY OF THE INVENTION 
   Therefore, an object of the present invention is to avoid problems of defective products resulting from a gap, which is caused by a height difference between rigid and flexible portions, in a conventional method of fabricating a rigid flexible PCB. 
   Another object of the present invention is to avoid problems of undesirable exposure due to the height difference between the rigid and flexible portions in the conventional method of fabricating the rigid flexible PCB. 
   Still another object of the present invention is to avoid problems of flow of an insulating base film onto the flexible portion, which is caused by the height difference between the rigid and flexible portions, in the conventional method of fabricating the rigid flexible PCB. 
   Yet another object of the present invention is to provide a method of completely removing the insulating base film layered on the flexible portion, so as to avoid the problem with respect to the height difference between the rigid and flexible portions. 
   A further object of the present invention is to provide a method of fabricating the rigid flexible PCB, in which reliability and defective fractions of products are significantly improved. 
   The above objects can be accomplished by providing a method of fabricating a rigid flexible PCB. The method includes the steps of forming first circuit patterns on both sides of a flexible base substrate; applying a coverlay on a portion of the circuit patterns or on whole circuit patterns; attaching self-detachable adhesive tapes to flexible portions of both sides of the flexible base substrate; layering rigid insulating layers on rigid portions of both sides of the flexible base substrate; forming via holes and second circuit patterns on the rigid portions of the flexible base substrate; conducting a predetermined treatment process for the self-detachable adhesive tapes to separate the self-detachable adhesive tapes from the flexible base substrate; and removing the self-detachable adhesive tapes. 
   In the method of fabricating the rigid flexible PCB according to the present invention, the self-detachable adhesive tapes are layered on the flexible portions so that the tapes have the same thickness as the rigid portions, and external layer circuits are formed on the rigid portions through a conventional process. Subsequently, ultraviolet rays are radiated onto the adhesive tapes layered on the flexible portions, or the tapes are subjected to a predetermined treatment process so that the tapes are easily removed. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other objects, features and other 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. 1   a  to  1   i  are sectional views illustrating a procedure of fabricating a rigid flexible PCB, according to conventional technology; 
       FIGS. 2   a  to  2   d  are sectional views illustrating attachment of an adhesive tape in the course of fabricating a rigid flexible PCB according to the present invention; 
       FIGS. 3   a  to  3   e  are sectional views illustrating formation of a circuit pattern and a via hole on a rigid portion in the course of fabricating the rigid flexible PCB according to the present invention; and 
       FIGS. 4   a  to  4   c  are sectional views illustrating removal of the adhesive tape in the course of fabricating the rigid flexible PCB according to the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Hereinafter, a detailed description will be given of the present invention, with reference to the drawings. 
     FIGS. 2   a  to  2   d  are sectional views illustrating the attachment of an adhesive tape in the course of fabricating a rigid flexible PCB according to the present invention. 
     FIG. 2   a  is a sectional view of a flexible base substrate  201  on which circuit patterns  202  are formed. It is preferable that the flexible base substrate  201  be made of polyimide resin having excellent heat resistance, mechanical strength, fire retardancy, and electric properties. 
   To fabricate the substrate shown in  FIG. 2   a , copper foils are layered on the flexible substrate made of the polyimide resin, proper etching resist patterns are formed on the copper foils, and etching is conducted to form circuit patterns  202 . 
   In  FIG. 2   b , a coverlay  203  is partially or totally applied on the substrate  201 . The coverlay  203  serves to protect the circuit patterns  202 , and is flexible. Additionally, the coverlay must have insulation properties. The coverlay  203  may be applied on the entire surface of, or a portion of, the substrate  201 . 
   In  FIG. 2   c , self-detachable adhesive tapes  206  are layered on flexible portions  205  of the substrate  201 . The self-detachable adhesive tapes  206  must easily be separated from the coverlay  203  or from the base substrate  201  when ultraviolet rays are radiated thereonto or the resulting structure is subjected to predetermined treatments. Furthermore, each adhesive tape  206  must have desirable resistance to subsequent chemical and heat treatments. The adhesive tape  206  must have a thickness that is the same as that of an insulating base film, such as a prepreg, to be layered on rigid portions  204   a ,  204   b.    
   It is preferable to use an adhesive tape  206  disclosed in Japanese Patent Laid-Open Publication No. 2003-231875, and “Selfa” (commercial name), manufactured by Sekisui Chemical Co., Ltd. in Japan, may be employed. 
   Selfa is an adhesive tape which is easily separated from an adhesion surface due to the generation of nitrogen gas at the adhesion surface when ultraviolet rays are radiated thereonto. 
   There are many types of Selfas, but it is preferable to use Selfa, which is capable of withstanding heat at 170-180° C. for 1-2 hours, so as to endure pressure and heat used during the fabrication of the PCB as will be described later. Additionally, it is preferable to use tape- or film-type Selfa because solution-type Selfa has a problem related to washing of dissolved components. 
   Instead of Selfa, any material may be used to produce the adhesive tape  206  if the material is separated from the adhesion surface of the tape by ultraviolet rays or other treatments, is flexible, and has sufficient heat resistance. 
   The adhesive tape  206  may be attached through a printing process, a photography process, or a lamination process, and must be attached so as to have a thickness that is the same as that of the insulating base film to be layered on the rigid portions  204   a ,  204   b.    
   In  FIG. 2   d , the insulating base films  207  are layered on the rigid portions  204   a ,  204   b . The insulating base films  207  must be hard so as to support the rigid portions, and be properly melted by heat so as to act as an interlayer adhesive. It is preferable to use a prepreg, which is most frequently used as an insulating layer in the course of fabricating a multilayered PCB, as the insulating base film  207  satisfying the above requirements. Subsequently, a circuit layer is layered on the insulating base film  207 , and a circuit pattern and a via hole are formed. 
     FIGS. 3   a  to  3   e  are sectional views illustrating the formation of the circuit pattern and the via hole on the rigid portions  204   a ,  204   b  in the course of fabricating the rigid flexible PCB according to the present invention. 
   In  FIG. 3   a , copper foils  208  are layered on both sides of the substrate using a press. When the insulating base film  207  is heated and pressed by the press, the insulating base film has fluidity due to heat, but the insulating base film  207  does not flow down onto the flexible portion  205  because of the adhesive tape  206 . 
   In  FIG. 3   b , a via hole  209  is formed using a laser drilling process or a mechanical drilling process, and copper plating layers  211  are formed on a wall of the via hole  209  and on external layers by plating. After the via hole is formed using the laser drilling process or mechanical drilling process, it is preferable to conduct a desmear process so as to remove impurities or residues from around the via hole  209 . 
   In  FIG. 3   c , photosensitive etching resists  210  are applied or laminated, and etching resist patterns are formed during exposure and development processes. It is preferable to use a dry film as the etching resist  210 . 
   In  FIG. 3   d , the substrate is etched to form circuit patterns on the copper foils  208 . 
   In  FIG. 3   e , the etching resists  210  are removed. The substrate is immersed in a stripping solution to strip the etching resists  210 . 
   Subsequently, the adhesive tape  206  is removed. The adhesive tape  206  is separated from the adhesion surface of the substrate by ultraviolet rays or predetermined treatments as described above, and a description will be given of the removal of the adhesive tape, below.  FIGS. 4   a  to  4   c  illustrate the removal of the adhesive tape  206 . 
   In  FIG. 4   a , ultraviolet rays are uniformly radiated onto the flexible portion  205  of the substrate of  FIG. 3   e , to which the adhesive tape  206  is attached, or predetermined treatments are conducted to separate the adhesive tape  206  from the substrate. 
   If “Selfa” is used as the adhesive tape  206 , nitrogen gas is generated at the adhesion surface of the adhesive tape  206 , thus the adhesive tape  206  is separated from the substrate to form a cavity  206   b  between a separated adhesive tape part  206   a  and the substrate as shown in  FIG. 4   b.    
   In  FIG. 4   c , a release ink or a release film is removed. Since the adhesive tape  206  is already separated from the substrate, the removal of the adhesive tape is very easy. If Selfa, manufactured by Sekisui Chemical Co., Ltd. in Japan, is used as the adhesive tape  206 , the adhesive tape is separated from the substrate due to nitrogen gas generated at the adhesion surface. Accordingly, the adhesive tape  206  is very easily removed without using additional force. 
   As described above, in a method of fabricating a rigid flexible PCB according to the present invention, a problem of a gap between layers, which is caused by a height difference between a rigid portion and a flexible portion, is avoided, thereby reliably exposing an etching resist, resulting in precise formation of a circuit pattern. 
   Furthermore, in the method of fabricating the rigid flexible PCB according to the present invention, problems of flow of an insulating base film onto the flexible portion, which are caused by the height difference between the rigid and flexible portions, are avoided in the course of fabricating the rigid flexible PCB, thereby realizing a minimum curvature radius of the flexible portion that is the same as a set design value. 
   Additionally, the present invention provides the method of fabricating the rigid flexible PCB, in which reliability and defective fractions of products are significantly improved. 
   The present invention has been described in an illustrative manner, and it is to be understood that the terminology used is intended to be in the nature of description rather than of limitation. Many modifications and variations of the present invention are possible in light of the above teachings. Therefore, it is to be understood that within the scope of the appended claims, the invention may be practiced other than as specifically described.