Metal working – Method of mechanical manufacture – Electrical device making
Reexamination Certificate
1999-12-14
2004-03-16
Chang, Richard (Department: 3729)
Metal working
Method of mechanical manufacture
Electrical device making
C029S830000, C029S846000, C427S097100, C427S099300, C428S209000, C428S458000, C428S473500, C428S901000, C174S254000, C361S750000
Reexamination Certificate
active
06705007
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method for manufacturing a double-sided flexible printed board using a polyimide insulating layer.
2. Related Art of the Invention
The following boards are commonly used as conventional polyimide-based double-sided flexible printed boards:
(i) boards comprising copper foil, a thermoplastic polyimide adhesive, a non-thermoplastic polyimide film, and a thermoplastic polyimide adhesive and copper foil;
(ii) boards comprising copper foil, a thermoplastic polyimide adhesive layer, and another copper foil, where the (i) and (ii) boards are fabricated by high-temperature pressing, heat lamination, or the like into a layered structure and (iii) boards fashioned by techniques in which a non-thermoplastic polyimide is cast-molded on copper foil and then a copper layer is formed by sputtering followed by electrolytic plating on the non-thermoplastic polyimide yield a layered structure that comprises copper foil, a non-thermoplastic polyimide film, and a copper layer comprised of a copper sputtered thin film and an electrolytic copper plating film formed thereon.
The double-sided flexible printed boards in (i) are disadvantageous, however, in that they have inadequate heat resistance because of the thermoplastic nature of the adhesive. The double-sided flexible printed boards in (ii) are disadvantageous in that their heat resistance is inadequate because of the thermoplastic nature of the adhesive layer and that their dimensional stability is inadequate because shrinkage is apt to occur during heating. The double-sided flexible printed boards in (iii) are disadvantageous in that the electrolytic copper plating layer containing a copper sputtered thin film on the non-thermoplastic polyimide film has relatively low peel strength, unsatisfactory reliability, and inferior long-term aging characteristics under conditions of heating and humidification.
To overcome these shortcomings, it has recently been proposed to use double-sided flexible printed circuit boards (metal foil/polyimide/metal foil) fabricated by a method in which a highly wettable polyamic acid varnish is applied to each of two metal foil and dried to give two laminates (each having such a polyamic acid layer); each metal foil is patterned, and the polyamic acid layers are imidated while laminated under conditions of high temperature and pressure to form a polyimide insulating layer (Japanese Patent No. 2746643). Such double-sided flexible printed circuit boards have excellent heat resistance and dimensional stability, develop adequate adhesion between the polyimide layer and the metal foil on both sides, and possess proper long-term aging characteristics.
When, however, the polyamic acid layers are laminated each other under conditions of high temperature and pressure following metal foil patterning, there is a danger that the moisture generated during imidation will corrode the metal foil without evaporating from the polyimide layer. Another drawback is that such double-side patterning impairs handling and results in reduced manufacturing efficiency. Yet another drawback is the need to provide high positional accuracy for lamination.
SUMMARY OF THE INVENTION
An object of the present invention, which was perfected in order to overcome the above-described drawbacks of prior art, is to provide a double-sided flexible printed board that has excellent heat resistance and dimensional stability, possesses adequate long-term aging characteristics and adhesion between the polyimide layer and the metal layers on both sides thereof, is devoid of problems associated with the corrosion of metal layers by the moisture generated during imidation, is easy to manufacture due to its excellent handling, and is devoid of problems associated with positional accuracy.
The inventors perfected the present invention after making the following discovery with respect to a laminate configured such that a polyamic acid layer or other polyimide precursor layer is sandwiched between two metal layers when the polyimide precursor layer is imidated following the patterning of the metal layer on one side: (I) the moisture generated during imidation evaporates off through the patterned metal layer, making it possible to prevent the metal layer from being corroded by the moisture generated during imidation, (II) adequate long-term aging characteristics can be obtained because the polyimide insulating layer obtained by imidation has excellent heat resistance and dimensional stability, and adheres well to the metal layers on both sides, and (III) excellent handling is achieved because patterning is not performed on one side of the metal layers during imidation, so fabrication is facilitated, and there is no need to ensure excessively high positioning accuracy for lamination.
Specifically, the present invention provides a method for manufacturing a double-sided flexible printed board which comprises the following steps of:
(a) forming a polyimide precursor layer on a first metal-layer;
(b) forming a second metal layer on the polyimide precursor layer;
(c) patterning the second metal layer by a subtractive technique to form a second circuit layer or (c′) patterning the first metal layer by a subtractive technique to form a first circuit layer; and
(d) imidating the polyimide precursor layer to form a polyimide insulating layer.
The present invention also provides a method for manufacturing a double-sided flexible printed board which comprises the following steps of:
(A) forming a polyimide precursor layer on a metal layer;
(B) forming an upper circuit layer on the polyimide precursor layer by a semi-additive technique; and
(C) imidating the polyimide precursor layer to form a polyimide insulating layer.
Other objects and features of the present invention will be described or become apparent from the following disclosure.
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Kobunshi Ronbunshu, Polyamide Resin Handbook, 1990, 47 (6).
Nikkan Kogyo Shinbunsha, Polyamide Resin Handbook, 1998, p. 536.
Kurita Hideyuki
Watanabe Masanao
Chang Richard
Sony Chemicals Corp.
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