Electronic techniques have been widely applied in various kinds of fields including not only the electric or communication fields of telephones, televisions, computers (including personal computers), lights and controlling devices utilized in various kinds of manufacturing industries, but also other fields including automobiles, office electronic supplies, cameras, toys, and the like. At the same time, the techniques have required high performance and high accuracy. Thus, in the fields of printed circuit boards, circuit patterns formed on the board have rapidly become more minute, as well as high density and multi-layered. As a result, the width of each circuit pattern and the distance therebetween have become narrower. Especially, in printed circuit boards of a through hole type, the holes have become smaller in diameter, and the number of holes has been increased. Further, the positions of holes have required high accuracy.
In drilling holes through a multilayered board, drilling small holes each having a diameter of 0.3-0.4 mm has been conventionally conducted. Recently, drilling much smaller holes each having a diameter of 0.1-0.3 mm has been practically conducted, and such demands as drilling smaller holes and increasing the number of holes with higher accuracy of drilling to position will be further required.
In drilling such small holes, as shown in FIG. 4, it has been known that placing an entry board 1 on a printed circuit board 5 enables easy introduction of a drill 8 which in turn improves accuracy of the drilling positions of a hole 9 drilled on the printed circuit board 5 and also enables drilling holes without causing burrs, or the like, around the holes. There have been several proposals concerning such entry boards.
Japanese Patent Publication 61-61921 (hereinafter referred to as JP 61-61921) discloses a method for drilling through a printed circuit board by using an entry board. The entry board comprises a base plate made of wood pulp and glass fiber and aluminum foils or the like covering both surfaces of the base plate. Japanese Patent Unexamined Publication SHO 62-214000 (hereinafter referred to as JP 62-214000) discloses a method for drilling a printed is circuit board by using an entry board which comprises a base plate made of aluminum foils and papers covering both surfaces of the base plate. Japanese Patent Unexamined Publication SHO 63-11207 (hereinafter referred to as JP 63-11207) discloses a method for drilling through a printed circuit board by placing an entry board made of metallic foil on the printed circuit board and by way of an adhesive layer formed on one surface of the metallic foil.
However, in the method disclosed in JP 61-61921, because hard materials such as JIS (Japanese Industrial Standard) 3003-H19 homogenized aluminum alloy as the aluminum foils are used as a surface layer of an entry board, a drill tends to slip on the aluminum foil, resulting in decreased drill positioning accuracy. Further, in such a case that adhesive layers are used to connect the aluminum foils to the base plate, the entry board becomes a five-layered structure, resulting in an increased manufacturing cost. In the disclosure of JP 62-214000, the paper thereof lacks heat-resistance ability and paper dust is generated during drilling, resulting in problems in the following procedures such as a plating procedure. Further, in the method disclosed in JP 63-11207, it is unavoidable to have the sticking of chips produced when drilling to the adhesive layers.
As mentioned above, in the conventional methods, there have still been problems in drilling small holes. Further, as the pattern formed on the printed circuit board reduces in size, the size of each hole to be drilled becomes smaller and the number of the holes increases, the rotation speed of the drill becomes higher and the diameter of the drill becomes smaller which results in rotational deflection of the drill. Furthermore, there have been more strict demands for improving accuracy of the drilling position and decreasing the allowable height of burrs. In spite of such demands, as the above mentioned conventional entry boards are still being used, accuracy of the drilling position has not been improved and drill breakages occur when it is attempted to improve the accuracy of the drilling position and to decrease the generation of burrs.