Metal working – Method of mechanical manufacture – Electrical device making
Reexamination Certificate
1999-03-15
2001-05-22
Arbes, Carl J. (Department: 3729)
Metal working
Method of mechanical manufacture
Electrical device making
C029S825000, C174S254000, C361S748000, C361S749000, C428S209000, C428S607000
Reexamination Certificate
active
06233821
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a process for manufacturing a metal clad film consisting of a metal foil laminated by an insulating resin layer without using an adhesive layer for a flexible printed wiring board.
Also, the present invention relates to a process for manufacturing a flexible printed wiring board using the metal clad film consisting of a designated printed circuit pattern formed by the metal foil and a cover film protecting the printed circuit.
PRIOR ART
Generally, single-side flexible printed wiring boards consist of a copper-clad film with a designated printed circuit pattern and a cover film for protecting the circuit pattern.
Previously known copper-clad films include a polyimide film adhered to a metal conductor such as a copper foil via an adhesive and a polyimide resin directly applied on or laminated to a metal foil such as a copper foil.
It is known that the circuit region formed of the metal foil is protected by:
(1) printing a solder resist ink or a polyimide varnish on the surface of the copper-clad film;
(2) laminating a polyimide film carrying an adhesive to the copper-clad film; or
(3) laminating a film-like polyamic acid which is a precursor of a polyimide resin to the copper-clad film by solvent activity or other means, then forming a polyimide film by imidating the polyamic acid.
However, these conventional techniques had the following problems.
(1) Technique of printing a solder resist ink or a polyimide varnish on the surface of the copper-clad film.
This is a technique in which a resist ink is printed using a silk screen in the same manner as for hard printed wiring boards. It has the disadvantage that the resist ink is less flexible because it is based on an epoxy resin and that the polyimide resist ink has low printability because its highly polar solvent absorbs water to hinder control of film thickness.
Another disadvantage of this technique is that fine lands or terminals can not be formed.
(2) Technique of laminating a polyimide film carrying an adhesive to the copper-clad film.
This is a technique in which future sites for lands or terminals are punched from a polyimide film carrying an adhesive layer using a mold or a die, then this film is contact-bonded under heat or otherwise bonded to a printed circuit board to form a flexible printed wiring board. It has the disadvantage that it has difficulty in forming fine lands or terminals even if the precision of the mold or die is improved and that the microcircuit may be contaminated with the adhesive squeezed out on bonding.
Flexible printed wiring boards on which a semiconductor chip is to be mounted should be flat. However, this technique invites partial contraction or curling due to the non-uniform thickness of the insulating protective film during printing it to damage its flatness and induce contraction between conductors.
Moreover, this technique can not satisfy the recent demands for dimensional stability in and between conductors to form a flexible printed circuit in which an insulating protective layer is buried between fine conductors forming circuit elements to ensure absolute isolation between adjacent conductors at terminals or the like.
(3) Technique of laminating a film-like polyamic acid to the copper-clad film by solvent activity or other means, then imidating the polyamic acid.
This technique is easy to practice on film-like materials, but requires some protection for working atmosphere because a solvent such as N-methyl-2-pyrrolidone is used as an active solvent.
If a thick tin plating or gold plating is applied to improve adhesion between the polyamic acid layer and the copper foil surface, the plating solution penetrates into the interface with the copper foil to lower electric characteristics such as insulating characteristics and adhesive force.
Moreover, the polyamic acid resin remains on the surface of the copper foil at access sites such as lands to impair solder wetability.
In view of the above problems of the prior art, an object of the present invention is to obtain a flexible printed wiring board with good flatness with the protect layer formed thereon.
Another object of the present invention is to improve dimensional accuracy and solderability at access sites such as lands in a flexible printed wiring board.
Still another object of the present invention is to obtain a flexible printed wiring board with good adhesion to conductors wherein any plating solution can not penetrate into the interface with conductors during plating processes.
Still another object of the present invention is to obtain a flexible printed wiring board with high heat resistance.
SUMMARY OF THE INVENTION
As a result of careful studies to overcome the above problems, we found that a flexible printed wiring board with high flatness which may overcome the above problems is obtained by using a metal clad film curled by thermally contracting an insulating resin layer as a protect layer applied on one surface of a conductor and then thermally contracting an insulating resin layer applied on the other surface of the conductor, and thus accomplished the present invention.
Accordingly, the present invention relates to a process for manufacturing a metal clad film used for a flexible printed wiring board, comprising the following steps of: forming an insulating resin layer on one surface of a foil-like conductor by applying a solution containing an insulating resin, and thermally contracting the insulating resin layer so as to form a convex surface, so that a metal clad film has a convex surface on the exposed side of the foil-like conductor.
Furthermore, the present invention relates to a process for manufacturing a flexible printed wiring board comprising the following steps of: forming an insulating resin layer on one surface of a metal foil by applying a solution containing an insulating resin, thermally contracting the insulating resin so that the other surface of the metal foil forms a convex surface of a curling surface to form an insulating resin layer, subjecting the foil-like conductor to a given treatment to form a given printed circuit pattern on the insulating resin layer, forming a protective resin layer on the printed circuit by applying the solution containing an insulating resin, and thermally contracting the protective layer so as to form a convex layer so that the flexible printed wiring board regains a flatness because contraction of the insulating resin layer and contraction of the protective resin layer cancel each other.
According to the present invention, a flexible printed wiring board with good flatness is obtained by using an intentionally curled metal clad film and thermally contracting a resin for forming a protective layer applied on the other surface of the conductor to absorb and correct the curling of the insulating resin layer and conductor of the metal clad film.
The present invention, which is not based on printing, can improve dimensional accuracy at access sites as described above, and also improve solderability at access sites by applying a resin solution to form the protective layer so that the resin at the access sites can readily be dissolved.
The present invention uses resin solutions both for the insulating resin layer and protective layer to provide high adhesion to conductors without causing penetration of the plating solution into the interface with conductors during plating processes.
The present invention forms the insulating resin layer and protective layer by heat treatment, which ensures that the reaction of resins can be fully completed to improve heat resistance. As a result, no blister occurs and flatness can be maintained during solder immersion.
In this case, it is also effective to form a given printed circuit pattern and/or the protective layer while the clad film is kept flat in the present invention.
Namely, the inventions obviate difficulty in the formation of printed circuit pattern and the coating process of a resin solution or other processes on forming the protective layer.
REFERENCES:
patent: 4293377 (1
Arai Akihiro
Arai Koji
Ichihara Osamu
Kudo Noriaki
Oaku Satoshi
Arbes Carl J.
Oliff & Berridg,e PLC
Sony Chemicals Corp.
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