1. Field of the Invention
The present invention relates to a printed wiring board used in, for example, an operating panel of an electronic apparatus and particularly to a printed wiring board that can be used in a small electronic apparatus.
2. Description of the Related Art
FIG. 8 shows a conventional printed wiring board. A conductor pattern P1 is formed on a flexible insulative board 51. The conductor pattern P1 consists of a plurality of filed contact units 54, a plurality of wiring patterns 55 that extend so as to be in electrical contact with the fixed contact units 54, and connection terminals 56 for electrical connection between an external apparatus and the fixed contact units 54 and the wiring patterns 55. The connection terminals 56 are formed on an end portion 51c of the flexible insulative board 51 so as to be arranged parallel with each other.
The conductor pattern P1 is made of a conductive material in which Ag (silver) particles are dispersed in polyester resin. The Ag particles have a role of rendering the conductor pattern P1 conductive. Although the conductor pattern P1 has high conductivity because of the Ag particles that exhibit high conductivity, the Ag particles may cause a migration phenomenon in a high-humidity atmosphere.
To prevent a migration phenomenon by shielding the conductor pattern P1 from humidity, the wiring patterns 55 of the conductor pattern P1 is covered with a resist film 57 that is made of an insulative material such as poly(vinyl chloride) and the fixed contact units 54 and the connection terminals 56 are covered with conductive coatings 59 in which carbon power is dispersed in binder resin.
The conductor pattern P1 is formed by forming, by screen printing, patterns of conductive ink in which Ag powder is dispersed in organic solvent containing polyester resin and then volatilizing the organic solvent by drying the conductive ink.
The conductive coatings 59 covering the fixed contact units 54 and the connection terminals 56 are formed in the following manner. After the formation of the conductor pattern P1, patterns of conductive ink in which carbon particles are dispersed in organic solvent containing binder resin are formed by screen printing while being positioned so as to coextend with the patterns of the fixed contact units 54 and the connection terminals 56, respectively. Then, the conductive ink is dried to volatilize the organic solvent.
Movable contacts (not shown) are attached to the flexible insulative board 51 so as to able to come into electrical contact with the respective fixed contact units 54. The end portion 51c of the flexible insulative board 51, on which the connection terminals 56 are arranged parallel with each other, is inserted into the insertion mouth of a connector member that is incorporated in an electronic apparatus, whereby the conductive coatings 59 on the respective connection terminals 56 are brought into electrical contact with conductor portions that are provided inside the insertion mouth of the connector member. As a result, the conductor pattern P1 is electrically connected to other members of the electronic apparatus.
In recent years, connector members have been miniaturized as the size of electronic equipment has been decreased. As connector members are miniaturized, their insertion mouths are made narrower. Therefore, it is necessary to decrease a width dimension w1 of the end portion 51c of the flexible insulative board 51 that is to be inserted into the insertion mouth. To form the connection terminals 56 on the narrowed end portion 51c of the flexible insulative board 51, it is necessary that the connection terminals 56 be arranged parallel at smaller intervals.
However, the conventional method in which the connection terminals 56 are covered with the respective conductive coatings 59 to prevent a migration phenomenon in the connection terminals 56 has the following problems. In the manufacturing step of forming patterns of conductive coatings 59 so that they coextend with the patterns of the connection terminals 56, respectively, even a slight deviation of the pattern of a conductive coating 59 from the pattern of the associated connection terminal 56 may cause the conductive coating 59 to contact another, adjacent connection terminal 56 to short-circuit the two connection terminals 56. Therefore, the positioning of the patterns of the conductive coatings 59 with respect to the respective patterns of the connection terminals 56 is required to have higher accuracy, which results in a problem that the positioning work becomes more complicated.