Stock material or miscellaneous articles – Composite – Of metal
Reexamination Certificate
2000-12-07
2002-05-28
Nakarani, D. S. (Department: 1773)
Stock material or miscellaneous articles
Composite
Of metal
C428S209000, C428S473500, C428S901000, C528S183000, C528S187000, C528S220000, C528S229000, C528S341000, C528S353000
Reexamination Certificate
active
06395399
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention pertains to flexible printed substrates with insulating layers and cover layers formed from a polyamic acid varnish.
2. Description of the Related Art
Flexible printed substrates, wherein as insulating layers or cover layers polyimide layers are formed directly on metallic foil such as copper foil without using an adhesive, are made by applying a polyamic acid varnish, which is obtained by addition-polymerization of an aromatic diamine such as para-phenylene diamine and an aromatic acid dianhydride such as pyromellitic dianhydride in a solvent such as N-methyl-2-pyrrolidone, to copper foil and drying to make a polyamic acid layer (polyimide precursor layer), followed by imidating this layer by heating to 300 to 400° C. to form a polyimide layer.
However, there is a problem in that residual stress is generated in conventional flexible printed substrates during film formation with the polyamic acid varnish and there is a difference in the linear expansion coefficient between the metallic foil (for instance, copper foil) and the polyimide layer and, therefore, the substrate curls.
Therefore, attempts have also been made to produce flexible printed substrates that do not curl by forming a polyimide layer with a low linear expansion coefficient using varnish containing polyamic acid with a specific structure (Japanese Patent Application Laid-Open No. Sho 60-157286).
Nevertheless, when a polyimide layer with a low linear expansion coefficient is formed using varnish containing polyamic acid with a specific structure, there are problems in that it cannot be said that the adhesive strength between the polyimide layer and metallic foil, such as copper foil, is sufficient, and when the copper foil is patterned by etching, the substrate curls to the copper foil side and as a result, there is a marked drop in productivity during subsequent finishing processes.
SUMMARY OF THE INVENTION
A object of the present invention is to solve the above-mentioned problems with prior art. Specifically it is to provide a flexible printed substrate having a polyimide layer with good adhesive strength to the metallic foil and with which there is no curling before or after etching of the metallic foil.
The present inventors completed the present invention upon discovering that the above-mentioned object can be accomplished by using a polyimide layer with a linear expansion coefficient within a specific range and a softening point not more than the imidation temperature as the polyimide layer of a flexible printed substrate in which this polyimide layer is provided as the insulating layer or cover layer on metallic foil.
That is, the present invention provides a flexible printed substrate, comprising metallic foil and provided thereon a polyimide layer which is produced by forming a film of a polyamic acid varnish on the metallic foil, followed by imidating, wherein the polyimide layer has a linear expansion coefficient of 10×10
−6
to 30×10
−6
(1/K) and a softening point not more than the imidation temperature.
DETAILED DESCRIPTION OF THE INVENTION
The flexible printed substrate of the present invention has a structure in which a polyimide layer is provided on metallic foil, the polyimide layer being formed by forming a film of a polyamic acid varnish, followed by imidating.
The polyimide layer of the present invention must have a linear expansion coefficient of 10×10
−6
to 30×10
−6
(1/K). As a result, the range of the linear expansion coefficient of the polyimide layer can correspond with the range of the linear expansion coefficient of the metallic foil and curling of the flexible printed substrate can be easily prevented. Taking into consideration the fact that the copper foil is generally used as the metallic foil, the linear expansion coefficient of the polyimide layer is preferably set at 18×10
−6
to 23×10
−6
(1/K).
Furthermore, the softening temperature of the polyimide layer must be not more than the imidation temperature upon imidation of a film of a polyamic acid varnish (usually 300 to 400° C., preferably 330 to 370° C.). As a result, residual stress upon film formation of the polyamic acid varnish can be alleviated.
Determination of the “linear expansion coefficient” and “softening point” of the polyimide layer in the present invention can be performed using a thermomechanical analyzer (TMA, Seiko Denshi Co., Ltd.) For instance, TMA properties are determined within a temperature range of 30° C. to 400° C. under conditions of a temperature elevation speed of 5° C./minute in tensile mode under a 5 g load using a polyimide film with a thickness of 20 &mgr;m, width of 4 mm, and length of 20 mm as the sample, and the linear expansion coefficient can be found from the TMA curve that has been obtained. Moreover, it is possible to extend the almost straight part of the TMA curve on the low-temperature side to the high-temperature side and extend the tangent of the part of the curve, where at the part the changing rate of the TMA curve is increased due to softening, to the low-temperature side and find the point of intersection between these extensions as the “softening point” (in accordance with the method of finding needle temperature of JIS K7196).
Examples of a specific measure for obtaining a polyimide film having a linear expansion coefficient of 10×10
−6
to 30×10
−6
(1/K) and a softening point not more than the imidation temperature includes a measure in which imidazolyl-diamino azine is externally added to a polyamic acid varnish used for the formation of polyimide layers (that is, a mixture containing polyamic acid obtained by addition polymerization of aromatic diamine and aromatic acid dianhydride in a solvent); and a measure in which the the types and mixture ratios of the aromatic diamine component and aromatic acid dianhydride component constituting the polyamic acid are selected. These two measures can also be combined.
Those represented by general formula (1)
(where A is an imidazolyl group represented by formula (1a), (1b), or (1c)
R
1
is an alkylene group and m is 0 or 1 . R
2
is an alkyl group and n is 0, 1 or 2. R
3
and R
4
are alkylene groups and p and q are each 0 or 1 . B is an azine residue, diazine residue, or triazine residue.) are given as examples of imidazolyl-diamino azines that can be used in the present invention.
Furthermore, when m is 0 in the imidazolyl-diamino azine of formula (1), the alkylene group of R
1
is not present and the azine residue, diazine residue or traizine residue is directly bonded to the imidazole ring. Methylene, ethylene, propylene and the like can be given as examples of alkylene groups of R
1
when m is 1.
When n is 0, the alkyl group of R
2
is not present and the hydrogen atom is bonded with the imidazole ring. Methyl, ethyl, can be given as examples of alkyl groups of R
2
when n is 1 . Two R
2
groups are bonded to the imidazole ring when n is 2 and methyl, ethyl and the like can be given independently as examples of alkyl groups of each R
2
. Furthermore, depending on the case, R
2
may also be directly bonded with the nitrogen atoms of the imidazole ring.
When p is 0, the alkylene group of R
3
is not present and the amino group is bonded directly with the azine residue, diazine residue, or triazine residue. Methylene, ethylene and the like can be given as examples of the alkylene group of R
3
when p is 1.
When q is 0, the alkylene group of R
4
is not present and the amino group is directly bonded to the azine residue, diazine residue, or triazine residue. Methylene, ethylene and the like can be given as examples of the alkylene group of R
4
when q is 1.
B is an azine residue, diazine residue or triazine residue. Of these, diamines with triazine residues are preferred because they can be easily synthesized or easily procured industrially.
The following compounds where both p and q are 0 can be given as preferred specific examples of the imidazolyl-diamino
Kudo Noriaki
Nagashima Minoru
Nakarani D. S.
Oliff & Berridg,e PLC
Sony Chemicals Corp.
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