This invention relates to a technological field of a flexible printed wiring board, more particularly to a technology of producing a double-face flexible printed wiring board having a complicated shape.
Flexible printed wiring boards each having a desired circuit pattern printed thereon have gained a wide application in the past and flexible printed wiring boards having various shapes corresponding to the shapes of various position at which the substrates are used have been required in recent years.
FIG. 4(a) shows an arrangement when T-shaped flexible printed wiring boards 152 are cut out from a rectangular master sheet 150. In this drawing, six flexible printed wiring boards 152 can be acquired.
However, when the flexible printed wiring boards 152 having special shapes are cut out, a large area of the master sheet 150 is discarded in vain.
In the conventional art, therefore, attempts have been made to take a flexible printed wiring board in a complicated shape apart into elemental pieces in simple shapes and then bond these pieces together to give a flexible printed wiring board.
FIG. 4(c) shows an example where a flexible printed wiring board 155 having the same shape as that of the flexible printed wiring board 152 described above is formed by bonding two kinds of elemental pieces 153 and 154 together.
When the elemental pieces 153 and 154 having simple shapes are cut out from the master sheet 150 as shown in FIG. 4(b), the master sheet 150 can be utilized effectively. In in the case of FIG. 4(b), eight elemental pieces 153 and 154 can be acquired respectively. In this case, eight T-shaped flexible printed wiring boards 155 can be acquired by bonding the elemental pieces 153 and 154 to each other. Consequently, a greater number of T-shaped flexible printed wiring boards can be acquired than the case when the T-shaped flexible printed wiring boards 152 are directly cut out.
To bond plural elemental pieces to each other to thereby form a flexible printed wiring board, it is necessary to connect these elemental pieces mechanically and electrically to each other.
Reference numerals 120 and 130 in FIG. 3(a) show the elemental pieces which include metal wirings 122 and 132 comprising patterned copper thin films are formed on each of the polyimide films 121 and 131. To bond the elemental pieces to one another, the elemental pieces 120 and 130 are first arranged in such a manner that the metal wirings 122 and 132 face one another.
Solder coating films 123 and 133 are deposited by plating to each of the metal wirings 122 and 132. The solder coating films 123 and 133 are brought into close contact with each other, are then pressed while being heated to melt the solder coating films 123 and 133 and are then cooled, providing a solder layer 104. The solder layer 104 so formed gives a bond portion 112 at which the metal wirings 122 and 132 are firmly connected to each other.
Because these two elemental pieces 120 and 130 are firmly fixed through the bond portion 112, one flexible printed wiring board 103 can be acquired (FIG. 3(b)).
However, the pitch of the metal wirings 122 and 132 has become smaller and smaller because a large number of metal wirings on a flexible printed wiring board is required in recent years.
When the solder coating films 123 and 133 are molten while being pressed, the solder is likely to scatter because of the pressure. The scattering solder may exist sporadically inside the flexible printed wiring board 103 and in the extreme case, the outflow of the molten solder forms a bridge 116 between adjacent bond portions 112 and invites short-circuit of the adjacent portions 112. As the pitch of the metal wirings 122 and 132 becomes smaller, the bridge 116 is more likely to develop. In consequence, the production yield of the flexible printed wiring board 103 using the elemental pieces 120 and 130 drops.
The present invention has been completed in order to solve the problems of the prior art described above, and is directed to provide a technology of producing specially-shaped double-face flexible printed wiring boards with a small pitch of metal wirings and a high production yield.
To accomplish the object described above, the present invention relates to a method of producing a flexible printed wiring board by laminating a plurality of elemental pieces each including a resin film, a metal wiring formed on said resin film and a low melting point metal coating film formed in at least a part of regions on the metal wiring, which method comprises the steps of arranging an adhesive resin that exhibits an adhesiveness upon heating between the metal wiring of one of the elemental pieces and the metal wiring of another elemental piece; pressing one elemental piece to another elemental piece while they are being heated; connecting the metal wiring of one elemental piece to the metal wiring of another elemental piece via the low melting point metal coating film that is so molten; and bonding one elemental piece and another elemental piece via the adhesive resin.
The present invention relates to a method of producing a flexible printed wiring board, wherein the adhesive resin is film-like.
The present invention relates to a method of producing a flexible printed wiring board wherein the low melting point metal coating film is formed on the surface of both of the metal wirings that are to be connected mutually.
The present invention relates to a method of producing a flexible printed wiring board, wherein either one or both of the surface of the metal wiring and the surface of the low melting point metal coating film are exposed on one of the surfaces of the flexible printed wiring board.
The present invention relates to a method of producing a flexible printed wiring board, wherein either one or both of the surface of the metal wiring and the surface of the low melting point metal coating film are exposed on both of the surfaces of the flexible printed wiring board.
The present invention relates to a flexible printed wiring board that comprises at least two elemental pieces each having a resin film, a metal wiring formed on said resin film and a low melting point metal coating film formed in at least a part of regions on the metal wiring, wherein the elemental pieces are bonded to one another via said adhesive resin, and the metal wirings of the elemental pieces are connected to one another via said low melting point metal coating film.