Abstract:
A method of manufacturing a printed wiring board by a build-up technique improves the bonding force between a conductor circuit and a resin. After the surface of the first conductor pattern is roughened by oxidation, an insulating layer is formed to expose a viahole portion of the first conductor pattern. Then the resin insulating layer is roughened and the board is reduction processed before a plating operation is carried out.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a method of manufacturing a printed circuit board (referred to as PCB hereinafter), and more particularly to a method of manufacturing a PCB in which a viahole is formed by employing a so-called build-up technique for successively forming inner layer circuits and insulating layers. 
     2. Description of the Prior Art 
     According to the method disclosed in Japanese Laid-Open Patent Publ. No. 3-3298, after forming a first copper conductor pattern 1 on a board 2 (FIG. 1A), the surface 3 of the conductor pattern 1 is subjected to a roughening process (FIG. 1B) to increase the adhesive force between the conductor pattern 1 and a resin film 5 to be coated on the conductor pattern 1 as shown in FIG. 1C. 
     Then, the surface 6 of the resin insulating layer 5 is roughened to improve the bonding force with a plated layer 7 to be formed thereon and an exposed surface of the first conductor pattern 1 is made flat by a soft etching process as shown in FIG. 1D to improve an electrical connection with the second conductor pattern 7 which is formed by platings as shown in FIG. 1E such that a viahole 8 is formed on the exposed flat surface of the first conductor pattern 1. 
     In the roughening process for the first copper conductor pattern 1, black needle crystals of copper oxide are grown chemically to enlarge the surface area of the copper conductor pattern and improve its bonding force by the anchor effect due to pores formed on the surface of the conductor pattern 1. But such a roughening process generates a so-called pink ring or halo caused by the dissolution of copper oxide into an acidic solution used in the plating process of the second conductor pattern 7. The formation of the pink ring at the periphery of the viahole causes peeling of the resin insulating layer at the periphery of the viahole due to corrosion of copper oxide present on the adhesion surface of the resin insulating layer and the conductor pattern 1. To prevent the undesired pink ring phenomenon, the prior art recommends to subject the roughened surface of the first copper layer to a reduction process prior to forming the resin insulating layer 5. 
     However, such a reduction process likewise reduces the above-mentioned anchor effect. 
     Moreover, since the roughening process for the resin insulating layer uses a powerful oxidizing agent, the first conductor pattern which was already treated by the reduction process tends to be oxidized again and thus the recommended reduction process cannot effectively prevent the pink ring phenomenon. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of this invention to provide a method of manufacturing a PCB by employing a build-up technique in which the bonding force between the first conductor circuit and resin is improved while preventing the pink ring phenomenon. 
     According to the invention, a reduction process is applied to an exposed portion of the first conductor pattern only after forming an insulating layer. Preferably, a step of roughening the insulating layer is carried out after the reduction process. 
     According to this invention, the following benefits can be realized. 
     (1) For circuits or the like which are not connected to the upper layer by a viahole (in contrast to the situation for the first and the second layers), a roughening processing such as oxidation may be applied in order to improve their bonding force. 
     (2) In the portion to become the viahole, including the contact part with resin, cupric oxide is reduced to cuprous oxide, or even to metallic copper, so that the generation of a pink ring during plating, and the accompanying separation of the conductor patterns, can be prevented. 
     (3) An additional flattening process is not required in accomplishing the foregoing items (1) and (2). 
     (4) The PCB is hardly affected by any other oxidation processing which may be given when required. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other objects, features and advantages of this invention will become more apparent by reference to the following detailed description of the invention taken in conjunction with the accompanying drawings, wherein: 
     FIG. 1A shows a partial sectional view of the first stage of manufacture of a build-up PCB according to the prior art; 
     FIG. 1B shows a partial sectional view of the second stage of manufacture of a built-up PCB according to the prior art; 
     FIG. 1C shows a partial sectional view of the third stage of manufacture of a built-up PCB according to the prior art; 
     FIG. 1D shows a partial sectional view of the fourth stage of manufacture of a built-up PCB according to the prior art; 
     FIG. 1E shows a partial sectional view of the fifth stage of manufacture of a built-up PCB according to the prior art; 
     FIG. 2A shows a partial sectional view of an embodiment of the invention at the first stage of manufacture; 
     FIG. 2B shows a partial sectional view of an embodiment of the invention at the second stage of manufacture; 
     FIG. 2C shows a partial sectional view of an embodiment of the invention at the third stage of manufacture; 
     FIG. 2D shows a partial sectional view of an embodiment of the invention at the fourth stage of manufacture; 
     FIG. 2E shows a partial sectional view of an embodiment of the invention at the fifth stage of manufacture; 
     FIG. 2F shows a partial sectional view of an embodiment of the invention at the sixth stage of manufacture; 
     FIG. 2G shows a partial sectional view of an embodiment of the invention at the seventh stage of manufacture; 
     FIG. 2H shows a partial sectional view of an embodiment of the invention at the eighth stage of manufacture; 
     FIG. 3A shows a partial sectional view of another embodiment of the present invention at the first stage of manufacture; 
     FIG. 3B shows a partial sectional view of another embodiment of the present invention at the second stage of manufacture; 
     FIG. 3C shows a partial sectional view of another embodiment of the present invention at the third stage of manufacture; 
     FIG. 3D shows a partial sectional view of another embodiment of the present invention at the fourth stage of manufacture; 
     FIG. 3E shows a partial sectional view of another embodiment of the present invention at the fifth stage of manufacture; 
     FIG. 3F shows a partial sectional view of another embodiment of the present invention at the sixth stage of manufacture; 
     FIG. 3G shows a partial sectional view of another embodiment of the present invention at the seventh stage of manufacture; and 
     FIG. 3H shows a partial sectional view of another embodiment of the present invention at the eighth stage of manufacture. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIG. 2A to FIG. 2H, the first embodiment of the invention will be described for the case of a PCB comprising two layers of conductor patterns. 
     A mask pattern is formed using a dry film (for example &#34;LISTON&#34;® made by DuPont) on one surface of a 1.6 mm-thick glass epoxy base material having a cladding of 12 μm-thick copper foil, and a board 2 having a conductor pattern (first layer) 1 is formed by etching the base material using cupric chloride as the etchant (FIG. 2A). 
     Next, the board 2 is subjected to an oxidizing process or blackening process according to the following method. (Needless to say, the blackening process according to any other suitable method may also be used.) 
     
                       TABLE______________________________________Treatment Steps for Blackening Process                     Tempera-   TimeStep         Treatment    ture (°C.)                                (min.)______________________________________ 1  Cleaning     Alkaline cleaner                         60-70    4-6 2  Warm water   Warm water   40-50    1-3    washing 3  Water washing            Cold water   Room     6                         temperature 4  Microetching Mixed aqueous                         35       1-2            solution of            hydrogen            peroxide solution            and sulfuric acid 5  Water washing            Cold water   Room     1-2                         temperature 6  Diluted sulfuric            10% aqueous  Room     1-2    acid washing solution of  temperature            sulfuric acid 7  Water washing             Room     1-2                         temperature 8  Oxidation    Mixed aqueous                         70       5    treatment    solution of            hypochlorous acid            and sodium            hydroxide 9  Water washing            Water        Room     5                         temperature10  Drying       Hot air      80-90     8-10______________________________________ 
    
     The bonding force of the conductor pattern 1 is improved by the formation of black fine needle crystals 3 of copper oxide by the blackening process. Although the effect of the blackening depends on the processing conditions, in general the usual bonding force of less than 0.5 kg/cm 2  for the case of no blackening is improved to more than 1.0 kg/cm 2  by the blackening (FIG. 2B). 
     Next, the board 2 is coated with a photosensitive resin (for example, &#34;PROVIR&#34;® #52 made by Ciba-Geigy) which as an epoxy resin as the principal component to a thickness of 50 μm on the base material, and the resin 5 is partially dried (FIG. 2C). Following that, a desired pattern is formed on the photosensitive resin film 5 by carrying out an exposure of about 3000 mJ using a mask 9 (FIG. 2D). 
     The patterned board 2 is developed using a developer (mixture of microhexanone, methyl &#34;Cellosolve&#34; and gamma-butyrolactone), for example, &#34;DY-90&#34; also made by Ciba-Geigy, with a wetting time of 1 to 5 mins, to form a hole (with diameter of about 200 μm) to act as a viahole for connecting the first and the second conductor patterns. At that time, the blackened surface 3 is exposed in the area that will underlie the viahole (FIG. 2E). 
     Next, the resin insulating layer is subjected to an insulating layer roughening using one-to-one aqueous solution of 1N chromic acid and 1N sulfuric acid. As a result of this processing, there appears a roughened surface 6 of the resin insulating layer 5 having fine roughness and porosity (FIG. 2F). 
     In this process the exposed blackened surface is also subjected to oxidation, but it does not cause any problems since it is a blackened surface of copper oxide to begin with. 
     Next, the exposed blackened surface is reduction processes by dipping the board 2 into a 10 wt. % aqueous solution of dimethylamine borane. At that time, other circuits or the like which are already covered with the resin film will not be reduced by this step; only the exposed part, namely, the part which will underlie the bottom of the viahole 8 is subjected to the reduction process. A blackened surface 4 in which only the intended area is reduced, is produced in this processing (FIG. 2G). 
     The board 2 is next subjected to a chemical copper plating which involves reduction with formalin using EDTA as a complexing agent, and an electrolytic copper plating using copper sulfate as the principal component, to form a conductor pattern (second layer) 7 and to define the viahole 8. Since the reduction processing is done in the previous steps to the parts which are to be brought into contact with the plating, there will be no generation of pink ring, and no separation of these parts from one another even if these parts are attacked by hydrochloric acid or the like (FIG. 2H). 
     If the solution used for the roughening of the resin insulating layer does not too greatly affect copper and cuprous oxide, the reduction processing may be performed prior to the roughening of the resin insulating layer. 
     FIGS. 3A to 3H are partial sectional views showing a second embodiment of the invention. It will be appreciated that the method can likewise be applied to a plurality of layers that are already formed by another suitable method. Since the steps shown in FIGS. 3A to 3E are the same steps shown in FIGS. 2A to 2E, a description of those steps will not be repeated. 
     After the blackened surface 3 is exposed at the position planned for the viahole as shown in FIG. 3E, the exposed blackened surface is subjected to a reduction processing by dipping the board 2 into a 10 wt. % aqueous solution of dimethylamine borane. At that time, other circuits or the like are already covered with resin, so that they are not reduced by this step. Only the exposed part, namely, the part which is to underlie the viahole is reduced, and a reduced blackened surface 4 is formed (FIG. 3F). 
     Next, the resin insulating layer 5 is roughened using a one-to-one mixed aqueous solution of 2N potassium permanganate and 2N sodium hydroxide. As a result of this processing, the resin insulating layer 5 is roughened, and there appears a roughened surface 6 of the resin insulating layer having fine and durable porosity (FIG. 3G). 
     A conductor pattern (second layer) 7 defining a viahole 8 is formed by subjecting the board 2 to a chemical copper plating in which reduction by formalin using EDTA as the complexing agent is performed, with an electrolytic copper plating having copper sulfate as the principal component. No pink ring, nor any separation of layers occurs, since the parts to be brought into contact with the plating are subjected to the reduction processing in the previous step (FIG. 3H). 
     As described above, since the reduction is carried out for exposed portions of the circuits after forming a resin insulating layer, the present invention provides the following benefits. 
     (1) It is possible to prevent the generation of a pink ring which tends to be formed in the periphery of the photo viahole without deteriorating the bonding strength between the resin and the conductor pattern. 
     (2) At the same time, the reliability of the photo viahole itself will not be spoiled. 
     (3) No special mask or the like is required in accomplishing items (1) and (2). 
     (4) The process is simple and no additional manpower is called for. 
     Although the invention has been described with reference to special embodiments, this description is not meant to be construed in a limiting sense. Various modifications of the disclosed embodiments, as well as other embodiments of the invention, will become apparent to persons skilled in the art upon reference to the description of the invention. It is therefore contemplated that the appended claims will cover any modifications or embodiments as fall within the true scope of the invention.