Abstract:
According to one mode of the present invention, a method of producing an electronic circuit, comprising forming an integrated resin layer having a prescribed thickness by repeating a resin layer forming process a number of times so that resin layers are layered to be integrated with all the resin layers on a substrate, wherein the resin forming process comprises charging the surface of a photoconductor; forming an electrostatic latent image having a prescribed pattern on the surface of the charged photoconductor; forming a visible image by electrostatically attaching charged particles composed of resin on the surface of the photoconductor on which the electrostatic latent image is formed; transferring the visible image formed on the surface of the photoconductor and composed of the charged particles onto the substrate; and fixing said visible image transferred onto said substrate on said substrate to form the resin layer on said substrate, is provided.

Description:
CROSS-REFERENCE TO THE INVENTION 
   This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2003-435756, filed on Dec. 26, 2003; the entire contents of which are incorporated herein by reference. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a method of producing an electronic circuit and to an electronic circuit substrate. 
   2. Description of the Related Art 
   Conventionally, when producing an electronic circuit substrate, a conductor pattern is formed by performing resist coating on a thin metal film, exposure, development, etching, and the like (refer to Japanese Patent Laid-open Application No. Hei 7-263841). However, this method requires exposure masks for respective layers, the design and production thereof require a plenty of time and costs. Besides, when alternation, modification or the like of an electronic circuit substrate becomes necessary, a great influence is exerted upon the time of delivery or costs of the electronic circuit substrate. 
   Because of these disadvantages, a method of forming an electronic circuit substrate by printing using electrophotography is proposed instead of the above method. In this method, an underlying layer for electroless plating having an arbitrary pattern is first prepared by using electrophotography with a charged particle containing fine metal particles in the resin. A plating layer is formed on the underlying layer by electroless plating, and an insulating layer is formed by electrophotography using charged particles made of resin only, so that the electronic circuit substrate is formed. 
   Note that since the above-described insulating layer requires sufficient electric insulation, it is necessary for it to have a thickness of 20 μm or more. Here, the thickness of the insulating layer formed by printing charged particles in one printing depends on the average particle size of the charged particles. Accordingly, it is conceivable to use charged particles having a large average particle size capable of obtaining a thickness of 20 μm or more in one printing to form an insulating film. However, there are a couple of disadvantages to form an insulating layer by using such large charged particles. One is the generation of a void in the insulating layer. Another is that in the case of using a large charged particle, a pattern cannot be formed because the resolution of a pattern largely depends on the particle size. 
   BRIEF SUMMARY OF THE INVENTION 
   According to one mode of the present invention, a method of producing an electronic circuit, comprising forming an integrated resin layer having a prescribed thickness by repeating a resin layer forming process a number of times so that resin layers are layered to be integrated with all the resin layers on a substrate, wherein the resin forming process comprises charging the surface of a photoconductor; forming an electrostatic latent image having a prescribed pattern on the surface of the charged photoconductor; forming a visible image by electrostatically attaching charged particles composed of resin on the surface of the photoconductor on which the electrostatic latent image is formed; transferring the visible image formed on the surface of the photoconductor and composed of the charged particles onto the substrate; and fixing said visible image transferred onto said substrate on said substrate to form the resin layer on said substrate, is provided. 
   According to another mode of the present invention, an electronic circuit substrate, comprising a substrate; a plating layer formed on said substrate; and a resin layer formed on said plating layer using the resin particles having the average particle size from 7 to 18 μm, is provided. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a flow chart showing a flow of a production process of an electronic circuit substrate relating to an embodiment of the present invention. 
       FIG. 2  is a flow chart showing a flow of forming process of a resin layer relating to the embodiment of the present invention. 
       FIGS. 3A to 3E  are schematic diagrams of a production process of the electronic circuit substrate relating to the embodiment of the present invention. 
       FIG. 4  is a view showing the operation of an underlying layer forming apparatus relating to the embodiment of the present invention. 
       FIG. 5  is a view showing the operation of an insulating layer forming apparatus relating to the embodiment of the present invention. 
       FIG. 6  is a graph showing relations between number of printings and thickness of the resin layer. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Hereinafter, an embodiment will be explained.  FIG. 1  is a flow chart showing a flow of a production process of an electronic circuit substrate relating to an embodiment of the present invention, and  FIG. 2  is a flow chart showing a flow of forming process of a resin layer relating to the present embodiment.  FIGS. 3A to 3E  are schematic diagrams of a production process of the electronic circuit substrate relating to the present embodiment.  FIG. 4  is a view showing the operation of an underlying layer forming apparatus relating to the present embodiment, and  FIG. 5  is a view showing the operation of an insulating layer forming apparatus relating to the present embodiment. 
   First, as shown in  FIG. 1  and  FIG. 3A , an underlying layer  2  for electroless plating is formed by printing using electrophotography on a substrate  1  (step  1 ). The underlying layer  2  can be formed by using an underlying layer forming apparatus  10  as shown in  FIG. 4 . Concretely, the underlying layer forming apparatus  10  mainly comprises a photoconductive drum  11 , an electrostatic charger  12 , a laser generator and scanner  13 , a developing machine  14 , a transfer printing machine  15 , and a fixing apparatus  16 . 
   In order to form the underlying layer  2 , while the photoconductive drum  11  is turned along the arrow direction first, a surface potential of the photoconductive drum  11  is uniformly charged at a fixed potential (for instance, a minus charge) by the electrostatic charger  12 . As a concrete method of charging, a Scorotron method, a roller method, and a brush method can be cited. 
   Next, a laser beam  13 A is irradiated to the photoconductive drum  11  in response to an image signal by a laser generator and scanner  13  removing the minus charge in the irradiated portion to form a charged image (electrostatic latent image) of a prescribed pattern on the surface of the photoconductive drum  11 . 
   Then, charged resin particles containing fine metal particles  2 A stored in the developing machine  14  are electrostatically attached on the electrostatic latent image on the photoconductive drum  11  by means of a feeder to obtain a visible image. A dry or wet toner transfer technology in a well known electrophotography type copy system can be applied to the developing machine  14 . 
   When the developing machine  14  is a dry type, the metal-containing resin particles  2 A having a particle size from 3 to 50 μm are stored in the developing machine  14 . A more desirable particle size of the metal-containing resin particles  2 A is from 5 to 10 μm. When the developing machine  14  is a wet type, the metal-containing resin particles  2 A having a particle size of 3 μm or less are stored in the developing machine  14  together with a liquid which serves as a solvent. 
   The metal-containing resin particles  2 A stored in the developing machine  14  are supplied to the photoconductive drum  11  by means of the feeder to be developed. At this time, a charged-area development or a discharged-area development can be used. 
   A B-stage thermosetting resin which is solid at room temperatures is used as a resin composing a metal-containing resin particle here. The B-stage refers to a state in which at least one portion of the thermosetting resin is not hardened but melted when prescribed heat is applied. As the b-stage thermosetting resin, epoxy resin, polyimide resin, phenol resin, bismaleimide resin, cyanate ester resin, bismaleimide-triazine resin, benzicyclobutene resin, polyimide resin, polybenzoxazol resin, butadiene resin, silicone resin, polycarbo-di-imide resin, polyurethane resin and so on can be used, and a charge control agent can be added thereto as necessary. 
   The metal-containing resin particle  2 A is mainly composed of a B-stage thermosetting resin, in which conductive fine metal particles having, for instance, a particle size ranged from 0.05 to 3 μm are contained at a rate from 15 to 70 wt %. A more desirable content of the fine metal particles contained in the metal-containing resin particle  2 A is from 30 to 60 wt %. Here, as the fine metal particles, at least one kind of fine metal particles selected from the group consisting of platinum (Pt), palladium (Pd), copper (Cu), gold (Au), nickel (Ni), and silver (Ag) is desirably used. These fine metal particles serve as kernels for electroless plating to be described later and have a catalytic function for progress of a plating reaction. Among these metal elements, especially palladium or copper is desirably used. 
   Then, the visible image (pattern) formed with the metal-containing resin particles  2 A on the surface of the photoconductive drum  11  is electrostatically transferred onto a desired substrate  1  from the photoconductive drum  11  by the transfer printing machine  15 . The photoconductive drum  11  is recovered after the transfer by removing the metal-containing resin particles  2 A left on the surface of the photoconductive drum  11  with a cleaning apparatus (not shown). 
   Then, the B-stage metal-containing resin particles  2 A, which are transferred onto the substrate  1 , are passed through the fixing apparatus  16  which emits heat or light, so that a thermosetting resin composing the metal-containing resin particles  2 A is melted to form a metal-containing resin layer  2 B. Thereafter, the metal-containing resin layer  2 B is heated or irradiated with light by the fixing apparatus  16  to be hardened so that the metal-containing resin layer  2 B is fixed on the substrate  1 . Through these processes, an underlying layer  2  is formed. 
   After forming the underlying layer  2  on the substrate  1 , a plating layer  3  is formed on the underlying layer  2  by electroless plating using the fine metal particles contained in the underlying layer  2  as kernels (step  2 ) as shown in  FIG. 3B . It should be noted that though the plating layer  3  is formed by electroless plating in the present embodiment, the plating layer  3  can be formed by both of electroless plating and electroplating. 
   In order to effectively perform the electroless plating, it is recommendable to treat at least some of the fine metal particles to project on the surface of the underlying layer  2  before performing the electroless plating to the underlying layer  2 . As such a treatment, for instance, etching with a solvent such as aceton, isopropanol acid or alkali or the like, or shot blasting, airblasting and so on can be cited. 
   After forming the plating layer  3  on the substrate  1 , an electrically insulative insulating layer  4  is formed on the substrate  1  by printing using the electrophotography (step  3 ). The insulating layer  3  can be formed using an insulating layer forming apparatus  20  nearly similar in structure to the underlying layer forming apparatus  10 . A resin particle  4 A is stored in the developing machine  14  in place of the metal-containing resin particle  2 A. 
   In order to form the insulating layer  4 , first, as shown in  FIG. 2 , while the photoconductive drum  11  is turned along the arrow direction, a surface potential of the photoconductive drum  11  is uniformly charged at a fixed potential (for instance, minus charge) by the electrostatic charger  12  (step  31 ). 
   Next, after charging the surface of the photoconductive drum  11 , a laser beam  13 A is irradiated to the photoconductive drum  11  in response to an image signal by the laser generator and scanner  13  removing the minus charge in the irradiated portion to form a charged image (electrostatic latent image) of a prescribed pattern on the surface of the photoconductive drum  11  (step  32 ). 
   After the electrostatic latent image is formed on the surface of the photoconductive drum  11 , the resin particles  4 A, which are charged by the developing machine  14 , are electrostatically attached on the surface of the photoconductive drum  11  to form a visible image on the surface of the photoconductive drum  11  (step  33 ). A dry or wet toner transfer technology in a well-known electrophotography copying system can be applied to the developing machine  14 . 
   The resin particles  4 A having an average particle size from 7 to 18 μm, or more desirably from 8 to 15 μm, are stored in the developing machine  14 . When forming the insulating layer  4 , the insulating layer should have enough thickness to provide electric insulation, and therefore, the particle size of the resin particle  4 A should be larger than that of the metal-containing resin particle  2 A. 
   The resin particles  4 A stored in the developing machine  14  are supplied to the photoconductive drum  11  by a feeder to be developed. At this time, a charged area development or a discharged area development can be used. 
   A thermosetting resin in a B-stage solid at room temperatures can be used as a resin composing the resin  4 A. As B-stage thermosetting resin, epoxy resin, polyimide resin, phenol resin, bismaleimide resin, cyanate ester resin, bismaleimide-triazine resin, benzicyclobutene resin, polyimide resin, polybenzoxazol resin, butadiene resin, silicone resin, polycarbo-di-imide resin, polyurethane resin and soon can be used, and an electrostatic charge control agent can be added thereto as necessary. It is also recommendable to disperse fine particles of silica or the like contained in the resin particles  4 A at a prescribed ratio, thereby enabling to control the characteristics such as stiffness, coefficient of thermal expansion and the like especially in a multilayer wiring substrate, so that improvement in reliability of substrate can be realized. 
   After the visible image (pattern) is formed on the surface of the photoconductive drum  11 , it is electrostatically transferred onto the desired substrate  1  from the photoconductive drum  11  by the transfer printing machine  15  (step  34 ). The photoconductive drum  11  after the transfer is recovered by removing the resin particles  4 A left on the surface of the photoconductive drum  11  with a cleaning apparatus (not shown). 
   After the visible image is transferred onto the substrate  1 , the visible image is heated by means of the fixing apparatus  16  to soften the resin particles  4 A composing the visible image so that a resin layer  4 B is formed. Then, the resin layer  4 B is hardened by heat or light irradiation with the fixing apparatus  16  to fix the resin layer  4 B on the substrate  1  (step  35 ). Through the above process, the resin layer  4 B is formed on the substrate  1  as shown in  FIG. 3C . Here, the resin layer  4 B is formed in a manner such that the thickness of the resin layer  4 B is at most twice the average particle size of the resin particles  4 A. 
   After forming the resin layer  4 B on the substrate  1 , the forming process of the resin layer in steps  31  to  35  is repeated so that resin layers  4 B having the same pattern as that of the resin layer  4 B are continually piled on the resin layer  4 B one by one. When a resin layer  4 B is fixed on the resin layer  4 B here, the former is integrated with the latter. The forming process of the resin layer is repeated until the thickness of the integrated resin layer  4 B reaches a prescribed thickness, for instance, from 15 to 50 μm, thereby forming an insulating layer  4  composed of the integrated resin layer  4 B as shown in  FIG. 3D . 
   After forming the insulating layer  4  on the substrate, the forming process of the electronic circuit in steps  1  to  3  is repeated to form a multilayered substrate for the electronic circuit  5  shown in  FIG. 3E . 
   In the present embodiment, since the forming process of the resin layer is repeated a number of times in a manner such that the resin layers  4 B are layered to form the insulating layer  4  which has a prescribed thickness and is composed of the integrated resin layer  4 B, on the substrate  1 , it is possible to obtain the insulating layer  4  having a sufficient thickness and resolution, and few voids. It is thought that the reason for the generation of voids is that when the resin particles  4 A are softened, air existing between the resin particles  4 A remains in the insulating layer  4  without being discharged from the insulating layer  4 . On the other hand, when the insulating layer  4  is formed by printing using electrophotography, since the thickness of a resin layer capable of being formed in one printing depends on the average particle size of the resin particles  4 A as described above, if an insulating layer  4  of 20 μm in thickness is formed in two or more printings for instance, it is necessary to use resin particles  4  having the average particle size smaller than that of the resin particles used to form the insulating layer  4  in one printing. Considering the cases where the insulating layers are formed using resin particles having a small average particle size and a large average particle size respectively, air existing between the resin particles is less in the case of using resin particles having a smaller average particle size than in the case of using resin particles having a larger average particle size. Besides, since the softening is carried out at every printing, the void is discharged each time. Therefore, it is possible to obtain an insulating layer having fewer voids by forming the insulating layer in two or more printings than by forming the insulating layer in one printing. Besides, since the forming process of the resin layer is repeated until the thickness of the insulating layer reaches a prescribed thickness, an insulating layer  4  having sufficient thickness can be obtained. 
   In the present embodiment, since the resin layer  4 B, the thickness of which is at most twice the average particle size of the resin particles  4 A, can be formed in one forming process of the resin layer, it is possible to obtain an insulating layer  4  with high resolution. 
   EXAMPLE 
   An example will be explained below. In this example, resin layers having prescribed patterns are respectively prepared by electrophotography using resin particles different in average particle size, and resolution and void for the respective cases are studied. 
   In the present example, resin particles having average particle sizes of 7.9 μm, 11.7 μm, 21.2 μm, and 29.8 μm are prepared, resin layers having prescribed patterns are respectively formed by electrophotography using these resin particles, and resolution and void for the respective cases are studied. 
   The result of the above-described study will be described next.  FIG. 6  is a graph showing relations between the number of printings and the thickness of the resin layer. As shown in  FIG. 6 , the larger the average particle size of the resin particles used, the greater the thickness of the resin layer formed in one printing becomes. Accordingly, when the resin particles have an average particle size as large as possible, the prescribed thickness can be reached in fewer printings. However, when resin layers are formed using resin particles having average particle sizes of 21.2 μm, and 29.8 μm, the resin particles are scattered outside of the prescribed pattern. When resin layers are formed using resin particles having average particle sizes of 7.9 μm, 11.7 μm, and 21.2 μm, the number of voids inside the resin layer is small, but when a resin layer is formed using resin particles having an average particle size of 29.8 μm, numerous voids can be observed inside the resin layer. From these results, it is confirmed that by using resin particles having a smaller average particle size, better resolution can be obtained with fewer voids. 
   It should be noted that the present invention is not limited to the content of the description in the above-described embodiment, structures, materials, arrangements of respective members, and the like can be appropriately modified within the meaning and range of equivalency of the present invention.