Stock material or miscellaneous articles – Structurally defined web or sheet – Discontinuous or differential coating – impregnation or bond
Reexamination Certificate
2002-09-12
2004-12-28
Stein, Stephen (Department: 1775)
Stock material or miscellaneous articles
Structurally defined web or sheet
Discontinuous or differential coating, impregnation or bond
C428S458000, C428S469000, C428S473500, C428S699000, C428S701000, C428S702000, C428S219000, C428S607000, C428S626000, C428S632000, C428S674000, C428S901000, C428S935000, C428S704000, C174S254000, C174S256000, C361S749000, C361S751000, C205S177000, C205S178000, C524S105000, C524S106000, C524S718000, C524S720000, C528S350000
Reexamination Certificate
active
06835442
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a flexible printed board comprising a copper foil and a polyimide resin layer formed thereon, with excellent adhesive strength between the copper foil and the polyimide resin layer, good flatness, and suitable for the implementation of fine wiring patterns.
BACKGROUND ART
Flexible printed boards having an insulating polyimide resin layer formed on a copper foil directly without using an auxiliary adhesive layer are manufactured by applying a polyamic acid varnish, obtained by addition polymerization of an aromatic dianhydride and an aromatic diamine in a solvent such as dimethylacetamide, onto a copper foil that has been subjected to a surface roughening pre-treatment in order to achieve a high adhesive strength, then drying the same to obtain a polyamic acid layer, further heating the polyamic acid layer to initiate the imidation, and thereby forming a polyimide resin layer on the copper foil.
In this context, copper foil refers mainly to rolled copper foils and electrodeposited copper foils, where electrodeposited copper foils are more widely used, because of the curling problems associated with rolled copper foils. In order to increase the adhesiveness between the electrodeposited copper foil and the polyimide resin layer, a roughening treatment is carried out on the surface of the foil manufactured by the electrolytic foil-making equipment (untreated copper foil), where the roughening treatment deposits minute copper particles on the foil. This roughening treatment is normally followed by a rust-inhibiting treatment.
Those printed wiring boards manufactured using the flexible printed boards as described above, that carry semiconductor devices and other kinds of electronic chip components, demand wiring layouts with ever higher fine-line densities, owing to the evolution of large-scale integration technologies. The roughening treatment of the electrodeposited copper foil, however, causes the loss of some etching properties of the copper foil, etching becomes problematic in high aspect-ratio boards, with undercut occurring during the etching, all of which hampers the achievement of the desired fine patterns.
Therefore, in flexible printed boards used in printed wiring boards, and in order to meet the demands imposed by fine pattern technologies, the extent of the electrodeposited copper foil roughening treatment is limited through low-profiling (decreasing roughness), with a view to minimize the occurrence of undercut during etching and to ensure insulation between patterns. For instance, Japanese Patent Application Laid-open No. H3-90938 discloses a low profiling method wherein the arithmetic mean roughness value of the electrodeposited copper foil Ra (as measured in the contact stylus test according to JIS B 0601) decreases to values from about 0.3 to about 0.8 &mgr;m.
However, low profiling the roughening of an electrodeposited copper foil lowers the adhesive strength between the electrodeposited copper foil and the insulating polyimide layer. For this reason, and because of the need to meet the highly demanding fine-line requirements of recent years, it is becoming increasingly difficult to maintain an intended adhesive strength, and there are also problems like the delamination (or peeling) of the wiring from the polyimide layer during processing.
The present invention solves the above problems in conventional technologies and provides a flexible printed board comprising a copper foil and a polyimide resin layer formed thereon, with excellent adhesive strength between the copper foil and the polyimide resin layer, good flatness, and suitable for the implementation of fine wiring patterns.
DISCLOSURE OF THE INVENTION
According to the present invention, it has been discovered that the etching characteristics of the copper foil can be improved and, at the same time, the adhesive strength between the electrodeposited copper foil and the polyimide resin layer can be enhanced, by the use of a low-curling electrodeposited copper foil with a zinc-based metallic layer deposited thereon, instead of carrying out a surface roughening of the electrodeposited copper foil.
In particular, the present invention provides a flexible printed board comprising an electrodeposited copper foil which has not been subjected to a surface roughening treatment, a zinc-based metallic layer deposited thereon in an amount of 0.25 to 0.40 mg/dm
2
, and a polyimide resin layer provided on the zinc-based metallic layer, that has been formed by imidation of a polyamic acid layer.
BEST MODE OF CARRYING OUT THE INVENTION
Here follows a detailed description of the present invention:
The flexible printed board of the present invention comprises an electrodeposited copper foil, a zinc (Zn)-based metallic layer deposited thereon and a polyimide resin layer provided over the latter.
The electrodeposited copper foils used in the present invention are formed by electrolytic plating and have a better curling behavior compared to rolled copper foils, helping thus to prevent the curling of the flexible printed board. The electrodeposited copper foils used in the present invention have not been subjected to any roughening treatment, and they have preferably an arithmetic mean roughness value Ra of less than 0.3 &mgr;m (as measured in the contact stylus test according to JIS B 0601).
The thickness of the electrodeposited copper foil will depend on the applications of the flexible printed board. From the viewpoint of achieving fine wiring patterns, it should preferably be as small as possible, usually from 3 to 35 &mgr;m, preferably from 3 to 18 &mgr;m and more preferably from 3 to 12 &mgr;m.
The zinc-based metallic layer provided on the electrodeposited copper foil increases the adhesive strength between the electrodeposited copper foil and the polyimide resin layer. This zinc-based metallic layer contains metallic zinc or zinc compounds (for example, zinc oxide, zinc hydroxide) and may be a layer formed through electroplating, electroless plating, vapor deposition, sputtering, etc., electroplating being preferred on account of the easier control of the layer thickness that it affords. Suitable electroplating baths are, for instance, zinc sulfate plating baths, zinc chloride plating baths, zinc cyanide plating baths, zinc pyrophosphate plating baths, etc. Among these, zinc sulfate may be preferably used owing to its advantage as regards processing costs. Suitable electroplating conditions for this electrolyte are disclosed in Japanese Patent Publication No. S61-33906, column 4 line 39 to column 5 line 2.
If the amount of zinc-based metallic layer formed (deposited amount) is too small, the adhesive strength of the polyimide resin layer diminishes after heat aging; if excessive, acids and other solutions employed during the processing of the board may invade the interface between the polyimide resin layer and the electrodeposited copper foil and thus lower the adhesive strength; a minimum of 0.25 to 0.40 mg/dm
2
of metallic zinc equivalent should be used, preferably 0.26 to 0.34 mg/dm
2
.
In the flexible printed board of the present invention, a further layer of chromium oxide (Cr
2
O
3
layer) is preferably disposed between the zinc-based metallic layer and the polyimide resin layer. This can contribute to enhance the adhesive strength between the electrodeposited copper foil and the polyimide resin layer and to prevent the occurrence of rusting on the electrodeposited copper foil. This chromium oxide layer contains chromium oxide and can be formed through a chromate process commonly used (Japanese Patent Publication No. S61-33906, column 5 lines 17 to 38).
If the amount of chromium oxide layer formed (deposited amount) is too high, the adhesive strength with the polyimide resin layer will decrease, therefore preferred deposition amounts range usually from 0.001 to 0.1 mg/dm
2
, preferably 0.005 to 0.08 mg/dm
2
, preferably still from 0.03 to 0.05 mg/dm
2
, based on a metal chromium.
The polyimide resin layer in the flexible printed board of the present invention is co
Fukuda Shin
Kudo Noriaki
Suzuki Akitoshi
Takabayashi Asaei
Oliff & Berridg,e PLC
Sony Chemicals Corp.
Stein Stephen
Xu Ling
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