U.S. patent application number 13/431633 was filed with the patent office on 2012-09-27 for flexible laminate board, process for manufacture of the board, and flexible print wiring board.
Invention is credited to Katsuyuki MASUDA, Yoshitsugu MATSUURA, Kazuhito OBATA, Hirokazu SUZUKI, Masahiko SUZUKI, Masaki TAKEUCHI, Kenichi TOMIOKA, Sumio YOSHIDA.
Application Number | 20120244275 13/431633 |
Document ID | / |
Family ID | 37967615 |
Filed Date | 2012-09-27 |
United States Patent
Application |
20120244275 |
Kind Code |
A1 |
SUZUKI; Masahiko ; et
al. |
September 27, 2012 |
FLEXIBLE LAMINATE BOARD, PROCESS FOR MANUFACTURE OF THE BOARD, AND
FLEXIBLE PRINT WIRING BOARD
Abstract
A process for production of a flexible laminated sheet having
one or more laminated bodies each provided with a metal foil formed
on one side of a resin film. The process includes coating a varnish
containing a polyamic acid and a solvent onto the metal foil,
holding the coated film, drying in which at least a portion of the
solvent in the varnish is removed to form a layer composed of a
resin composition, and forming the resin in which the layer
composed of the resin composition is heated to form a resin film
containing a polyimide resin. The conditions for each step from the
coating up to the resin film-forming are adjusted based on a target
for the content of metal elements in the resin film.
Inventors: |
SUZUKI; Masahiko;
(Chikusei-shi, JP) ; OBATA; Kazuhito;
(Chikusei-shi, JP) ; MASUDA; Katsuyuki;
(Chikusei-shi, JP) ; TOMIOKA; Kenichi;
(Chikusei-shi,, JP) ; TAKEUCHI; Masaki;
(Chikusei-shi, JP) ; YOSHIDA; Sumio;
(Chikusei-shi, JP) ; SUZUKI; Hirokazu;
(Chikusei-shi, JP) ; MATSUURA; Yoshitsugu;
(Chikusei-shi, JP) |
Family ID: |
37967615 |
Appl. No.: |
13/431633 |
Filed: |
March 27, 2012 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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12091519 |
Nov 21, 2008 |
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PCT/JP2006/320851 |
Oct 19, 2006 |
|
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13431633 |
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Current U.S.
Class: |
427/99.4 |
Current CPC
Class: |
Y10T 428/261 20150115;
Y10T 428/27 20150115; H05K 2203/0759 20130101; H05K 2203/087
20130101; Y10T 428/24917 20150115; H05K 2201/0358 20130101; H05K
2203/163 20130101; H05K 2203/1157 20130101; H05K 3/022 20130101;
H05K 1/0346 20130101; H05K 2201/0154 20130101 |
Class at
Publication: |
427/99.4 |
International
Class: |
B05D 5/12 20060101
B05D005/12 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 25, 2005 |
JP |
P2005-310184 |
Dec 7, 2005 |
JP |
P2005-353800 |
Claims
1. A process for production of a flexible laminated sheet having at
least one laminated body, each laminated body being provided with a
metal foil formed on one side of a resin film, the process
comprising: a coating step in which a varnish containing a polyamic
acid and a solvent, for forming said resin film of each laminated
body, is coated onto the metal foil to form a coated film; a drying
step in which the solvent in the varnish is removed to a proportion
of 1-60 wt % of the total, to form a resin composition layer from
the varnish; and a resin film-forming step in which the resin
composition layer is heated to 250-550.degree. C. under a reducing
atmosphere to form said resin film, said resin film containing a
polyimide resin.
2. A production process according to claim 1, wherein the reducing
atmosphere is formed by a mixed gas composed of nitrogen gas and
hydrogen gas at between 0.1 vol % and 4 vol % of the total.
3. A production process according to claim 1, wherein the solvent
is removed by heating the varnish to 100-170.degree. C. in the
drying step.
4. A production process according to claim 1, wherein the metal
foil is a copper foil.
5. A production process according to claim 1, wherein a
concentration of the polyamide acid in the varnish is 8-40%, and
the viscosity of the varnish is 1-40 Pas.
6. A production process according to claim 1, wherein in said
drying step, the solvent in the varnish is removed to a proportion
of 25-45 wt %.
7. A production process according to claim 1, wherein in the
coating step, the varnish is coated in contact with the metal
foil.
8. A production process according to claim 1, wherein in the
flexible laminated sheet produced, a content of metal elements in
the resin film is no greater than 5 wt %.
9. A production process according to claim 1, wherein the resin
film-forming step is performed after the drying step.
10. A production process according to claim 1, wherein the drying
step is performed under a reduced pressure atmosphere or a reducing
atmosphere.
Description
[0001] This application is a Continuation application of
Application Ser. No. 12/091,519, having a filing date under 35 USC
371 of Nov. 21, 2008, the contents of which are incorporated herein
by reference in their entirety. Ser. No. 12/091,519 is a National
Stage Application, filed under 35 USC 371, of International (PCT)
Application No. PCT/JP2006/320851, filed Oct. 19, 2006.
TECHNICAL FIELD
[0002] The present invention relates to a flexible laminated sheet
that can be used for manufacture of flexible printed circuit
boards, and to a process for its production. The invention further
relates to a flexible printed circuit board.
BACKGROUND ART
[0003] A flexible printed circuit board is a flexible wiring board
with a conductor pattern formed on the surface of an insulating
resin film. Flexible printed circuit boards have become common in
recent years as means of achieving increased miniaturization and
higher density in electronic devices. Most flexible printed circuit
boards employ aromatic polyimides as resin films.
[0004] Flexible printed circuit boards employing aromatic
polyimides have conventionally been manufactured by a process in
which, generally, a copper foil is bonded to a polyimide film as
the insulating layer using an adhesive such as an epoxy resin or
acrylic resin. With flexible printed circuit boards obtained by
this process, the level of properties such as heat resistance,
chemical resistance, flame retardance, electrical characteristics
and adhesiveness depend on the properties of the adhesive used, and
therefore it has not been possible to satisfactorily exhibit the
excellent properties of aromatic polyimides.
[0005] Methods of heat sealing polyimide films to metal foils using
thermoplastic polyimides as adhesives have therefore been proposed
(Patent documents 1-3). There have also been proposed methods of
direct cast coating of a metal foil such as a copper foil with a
solution of a polyamic acid (polyimide precursor) having a thermal
expansion coefficient equivalent to that of the metal foil, and
removing the solvent to produce a high molecular weight product
(hereinafter referred to as direct coating methods) (Patent
documents 4, 5). There are also known methods of forming metal
layers by vapor deposition or sputtering onto polyimide films
(Patent document 6). [0006] [Patent document 1] Japanese Unexamined
Patent Publication HEI No. 3-104185 [0007] [Patent document 2]
Japanese Unexamined Patent Publication No. 2005-44880 [0008]
[Patent document 3] Japanese Unexamined Patent Publication No.
2005-96251 [0009] [Patent document 4] Japanese Unexamined Patent
Publication SHO No. 58-190093 [0010] [Patent document 5] Japanese
Unexamined Patent Publication SHO No. 63-69634 [0011] [Patent
document 6] Japanese Patent Publication No. 3447070
DISCLOSURE OF THE INVENTION
Problems to be Solved by the Invention
[0012] With flexible printed circuit boards obtained by
conventional production processes including those described in
Patent documents 1-6, however, it has been difficult to
sufficiently reduce the permittivities of the polyimide films.
Consequently, further improvement is desired in terms of the
characteristics, and especially dielectric characteristics, of the
flexible printed circuit boards.
[0013] It is therefore an object of the present invention to
provide a process for production of a flexible laminated sheet that
can be used to manufacture flexible printed circuit boards that
exhibit satisfactory dielectric characteristics, by comprising a
resin film containing a polyimide resin with sufficiently reduced
permittivity.
[0014] The flexible printed circuit boards obtained by conventional
production processes have not been adequately resistant to wire
breakage caused by peeling of wiring under repeated flexural stress
or thermal history. In other words, further improvements in terms
of flexible printed circuit board reliability are desired.
[0015] For example, the aforementioned flexible printed circuit
boards of the prior art which employ thermoplastic polyimides as
adhesives do not always exhibit adequate heat resistance by the
thermoplastic polyimides, and therefore the resistance to thermal
history has been insufficient. The high molding temperature
required for heat sealing has also led to problems of increasing
complexity of the production equipment. The flexible printed
circuit boards obtained by conventional direct coating methods have
also often been insufficient from the standpoint of
reliability.
[0016] On the other hand, sputtering production processes require
special equipment for sputtering, and the problem of production
step complexity arises when plating and high-temperature heat
treatment steps are necessary.
[0017] It is therefore another object of the invention to provide a
process for production of a flexible laminated sheet that allows
production of flexible printed circuit boards with sufficiently
high reliability using simple steps.
Means for Solving the Problems
[0018] The present inventors have studied the cause of the high
permittivity of polyimide films in flexible laminated sheets, from
the viewpoint of the elements in the films. As a result, it was
discovered that the polyimide films contain metal elements that
should be absent from the starting materials for polyimide films.
Upon further research, the present inventors found that the metal
elements in polyimide films are the same species as the component
elements of the adjacent metal foil, and their concentration
distribution in the thickness direction decreases from the metal
foil side of the polyimide film toward the opposite side.
[0019] The present inventors therefore surmised that the content of
the metal elements in the polyimide film increases because the
metal elements move from the adjacent metal foil into the polyimide
film of the flexible laminated sheet (a phenomenon known as
"migration"), thus causing an increase in permittivity. As a result
of yet further detailed investigation on flexible laminated sheet
production processes with the goal of preventing such migration,
the present invention was completed.
[0020] One aspect of the invention is a process for production of a
flexible laminated sheet having one or more laminated bodies each
provided with a metal foil formed on one side of a resin film, the
process comprising a coating step in which a varnish containing a
polyamic acid and a solvent is coated onto the metal foil to form a
coated film, a holding step in which the coated film formed on the
metal foil is held, a drying step in which at least a portion of
the solvent in the varnish is removed to form a layer composed of a
resin composition, and a resin film-forming step in which the layer
composed of the resin composition is heated to form a resin film
containing a polyimide resin, wherein the conditions for each step
after the coating step up to the resin film-forming step are
adjusted based on a target for the content of metal elements in the
resin film.
[0021] According to the invention it is possible to sufficiently
reduce the permittivity of the resin film containing the polyimide
resin which is formed on the flexible laminated sheet, so that a
flexible printed circuit board fabricated from the flexible
laminated sheet can exhibit satisfactory dielectric
characteristics. The present inventors believe the reasons for this
effect to be the following. Other factors, however, may be
involved.
[0022] For conventional fabrication of flexible laminated sheets
with polyimide films, the metal foil after coating of the varnish
containing the polyamic acid and solvent is usually stored for a
certain period of time (for example, 1-2 days) in air at room
temperature without active drying of the varnish, from the
viewpoint of allowing more flexibility in the production steps. In
most cases, this is followed by removal of the solvent in the
varnish to form a layer composed of the resin composition, but at
times the entire solvent is removed while at other times only a
portion of the solvent is removed. In addition, different
temperatures and atmospheres are used for the solvent removal. The
conditions for curing of the layer composed of the resin
composition (for example, the temperature and atmosphere) are
adjusted according to the type of polyimide. However, migration of
metal elements from the metal foil into the polyimide film is
believed to occur because of the acidic polyamic acid in the
varnish coated on the metal foil. Specifically, it is conjectured
that the polyamic acid dissolves the metal foil and promotes
migration of the metal elements in the metal foil into the varnish,
thus resulting in inclusion of metal elements into the polyimide
film obtained from the varnish.
[0023] However the conditions, including the temperature,
atmosphere and time for storage, the temperature and atmosphere for
removal of the solvent in the varnish, the degree of removal of the
varnish and the atmosphere for curing of the resin composition,
have not been considered from the viewpoint of the content of metal
elements in the polyimide film. The content of metal elements in
the polyimide film is therefore affected by the polyamic acid, such
that it has not been possible to control the permittivity of the
polyimide film sufficiently as desired.
[0024] According to the production process of the invention,
however, the various conditions for the step after coating of the
varnish on the metal foil and for the steps prior to formation of
the resin film, such as the holding step and/or drying step, are
adjusted based on a target for the content of metal elements in the
resin film. This allows the permittivity of the resin film to be
sufficiently controlled. A resin film with an adequately reduced
permittivity is thus obtained, permitting fabrication of a flexible
printed circuit board that exhibits satisfactory
characteristics.
[0025] According to the invention, the conditions for each of the
steps are preferably adjusted so that the content of metal elements
in the resin film is no greater than 5 wt %. Investigation by the
present inventors has suggested that a content of no greater than 5
wt % for metal elements in the resin film tends to result in a more
sufficient permittivity for practical use.
[0026] According to the invention, the layer composed of the resin
composition is preferably heated in a reducing atmosphere during
the resin film-forming step. This allows oxidation of the metal
foil to be more effectively prevented, for improved adhesiveness
between the metal foil and resin film in the flexible laminated
sheet.
[0027] Another aspect of the invention is a process for production
of a flexible laminated sheet having one or more laminated bodies
each provided with a metal foil formed on one side of a resin film,
the process comprising a coating step in which a varnish containing
a solvent and either a polyimide resin or its precursor is coated
onto the metal foil, a drying step in which the solvent in the
varnish is removed to a proportion of 1-60 wt % of the total, and a
resin film-forming step in which the resin composition layer is
heated to 250-550.degree. C. under a reducing atmosphere to form a
resin film containing a polyimide resin.
[0028] According to this production process of the invention, the
varnish is dried until the solvent proportion is within the
aforementioned specified range and is then heat treated at a
temperature within the aforementioned specified range to produce a
polyimide resin, and therefore a flexible laminated sheet that can
yield flexible printed circuit boards with sufficiently high
reliability can be produced by a simple process.
[0029] In the production processes described above, the reducing
atmosphere is preferably one formed by a mixed gas composed of
nitrogen gas with hydrogen gas at between 0.1 vol % and 4 vol % of
the total. This will more reliably prevent reduction in reliability
by oxidation of the metal foil, while also allowing fabrication of
a flexible laminated sheet via safer steps.
[0030] In the drying step, the varnish is preferably heated to
100-170.degree. C. for removal of the solvent. The metal foil is
preferably a copper foil.
[0031] The invention provides a flexible laminated sheet comprising
one or more laminated bodies each having a metal foil formed on one
side of a resin film, wherein the content of metal elements in the
resin film is no greater than 5 wt %. The metal foil-clad flexible
laminated sheet is obtained by the production processes of the
invention described above. The flexible laminated sheet can be used
to obtain flexible printed circuit boards exhibiting sufficiently
satisfactory dielectric characteristics.
[0032] A metal foil-clad flexible laminated sheet of the invention
is a flexible laminated sheet obtained by the production processes
of the invention described above. The flexible laminated sheet can
be used to obtain flexible printed circuit boards with sufficiently
high reliability.
[0033] The relative permittivity of the resin film in the flexible
laminated sheet of the invention is preferably no greater than 3.3
at 5 GHz. This can further increase the reliability of the flexible
printed circuit board. The resin film in the flexible laminated
sheet of the invention preferably has a relative permittivity of no
greater than 3.3 at 5 GHz, a thermal expansion coefficient of no
greater than 25 ppm/.degree. C. and a peel strength of the resin
film from the resin film is at least 1.2 kN/m. This can further
increase the reliability of the flexible printed circuit board.
[0034] The flexible printed circuit board of the invention has a
conductor pattern formed by removing a portion of the metal foil on
the flexible laminated sheet of the invention. Alternatively, the
flexible printed circuit board of the invention may be a one
obtainable by removing the metal foil and forming a conductor
pattern on the exposed resin film. Such flexible printed circuit
boards have sufficiently high reliability since they are produced
using a flexible laminated sheet of the invention as described
above.
Effect of the Invention
[0035] According to the invention there is provided a process for
production of a flexible laminated sheet that can be used to
fabricate flexible printed circuit boards that exhibit satisfactory
dielectric characteristics, by comprising a resin film containing a
polyimide resin with sufficiently reduced permittivity. Also, the
process for production of a flexible laminated sheet according to
the invention can be used to produce a flexible laminated sheet
that allows manufacturing of flexible printed circuit boards with
sufficiently high reliability by simple steps.
BRIEF DESCRIPTION OF THE DRAWINGS
[0036] FIG. 1 is a cross-sectional view of an embodiment of a
flexible laminated sheet according to the invention.
[0037] FIG. 2 is a cross-sectional view of an embodiment of a
flexible laminated sheet according to the invention.
[0038] FIG. 3 is a cross-sectional view of an embodiment of a
flexible laminated sheet according to the invention.
[0039] FIG. 4 is a cross-sectional view of an embodiment of a
flexible printed circuit board according to the invention.
[0040] FIG. 5 is a cross-sectional view of an embodiment of a
flexible printed circuit board according to the invention.
EXPLANATION OF SYMBOLS
[0041] 1a, 1b, 1c: Flexible laminated sheets, 2a, 2b: flexible
printed circuit boards, 10: laminated body, 11: resin film, 12:
metal foil, 15: adhesive layer, 20: conductive pattern.
BEST MODES FOR CARRYING OUT THE INVENTION
[0042] Preferred embodiments of the invention will now be described
in detail. However, the present invention is not limited to the
embodiments described below.
[0043] FIG. 1 is a cross-sectional view of an embodiment of a
flexible laminated sheet according to the invention. The flexible
laminated sheet la shown in FIG. 1 comprises a laminated body 10
formed by bonding a metal foil 12 onto one side of a resin film 11.
The thickness of the resin film 11 will normally be about 1-100
.mu.m.
[0044] The content of metal elements in the resin film 11 is
preferably no greater than 5 wt %. If the content exceeds 5 wt %,
the permittivity of the resin film 11 will tend to be too high for
practical use. In particular, a metal element content of greater
than 10 wt % will notably impair the electrical characteristics
including the dielectric characteristic of the flexible printed
circuit board. Therefore, from the viewpoint of more reliably
maintaining the electrical characteristics of the flexible printed
circuit board, the metal element content is preferably no greater
than 5 wt %. This tendency is more pronounced when the metal foil
12 is a copper foil. While a lower metal element content is
preferred, the lower limit will usually be about 2 wt %.
[0045] The metal element content of the resin film 11 can be
measured by X-ray Photoelectron Spectroscopy (XPS). The
measurements referred to throughout the present specification are
all preceded by removal of the metal foil 12 from the resin film
11. Then, argon etching is carried out from the side of the resin
film 11 in contact with the metal foil, to a prescribed depth in
the direction of thickness. Next, an ESCA5400 X-ray photoelectron
spectrometer (trade name of Ulvac-Phi, Inc.) is used for
measurement of the quantity of metal elements in the surface
exposed by etching. Argon etching is then continued to the next
prescribed depth, and the metal element quantity is measured in the
same manner. The procedure is repeated until the metal element
quantity reaches the detection limit. Finally, the metal element
quantity with respect to the total resin film 11 is calculated from
the metal element quantity at each depth, and the content is
calculated therefrom.
[0046] The relative permittivity of the resin film 11 at 5 GHz is
preferably no greater than 3.3. The relative permittivity is an
index of the insulating property of the resin film, and in cases
where the pitch between copper wirings is narrowed or the
interlayer thickness is reduced as means for achieving
miniaturization and high density of electronic devices, it is
preferred for the relative permittivity of the resin film to be a
small value. Electronic devices must be operated at high frequency,
and a low relative permittivity in the gigahertz band is especially
preferred.
[0047] The relative permittivity of the resin film 11 can be easily
measured by a method using a resonant cavity perturbation complex
permittivity evaluator (hereinafter referred to as "cavity
resonator"). When the relative permittivity is measured using a
cavity resonator, such as a "CP511" (trade name of Kantoh
Electronics Application and Development Inc.), measurement may be
performed with three test resin films having a thickness of 0.6 mm,
a width of 1.8 mm and a length of 80 mm, and the mean value
recorded as the relative permittivity. In cases where the thickness
of the test films is insufficient, a plurality of resin films may
be stacked to ensure that the prescribed thickness is obtained.
[0048] The coefficient of linear thermal expansion of the resin
film 11 is preferably no greater than 25 ppm/.degree. C. and more
preferably 10-25 ppm/.degree. C. The coefficient of linear thermal
expansion is an index of the heat-dependent elongation percentage
of the material, and when two or more different materials are
attached, a smaller difference in coefficients of linear thermal
expansion of the materials is preferred from the viewpoint of
reliability. The coefficient of linear thermal expansion of the
metal foil will generally be 10-25 ppm/.degree. C. (for example, 10
ppm/.degree. C. for stainless steel, 20 ppm/.degree. C. for copper
alloys or 22 ppm/.degree. C. for aluminum alloys). When resin films
with coefficients of linear thermal expansion exceeding 25
ppm/.degree. C. are attached to the metal foils, warping tends to
occur upon heating during the attachment or after the attachment,
and wiring breakage or molding defects become more likely, thus
reducing the reliability.
[0049] The coefficient of linear thermal expansion may be
conveniently measured by a TMA method. For measurement of the
coefficient of linear thermal expansion by TMA, for example, a test
piece with a thickness of 0.025 mm, a width of 13 mm and a length
of 15 mm may be raised to a temperature of from 50.degree. C. to
300.degree. C. at 10.degree. C./min using a "TMA2940" (trade name
of TA Instruments) under a load of 0.5 gf, and then cooled to room
temperature and again raised in temperature from 50.degree. C. to
350.degree. C. at 10.degree. C./min under a load of 0.5 gf, at
which time the coefficient of linear thermal expansion in the range
of 50.degree. C.-250.degree. C. may be calculated to determine the
coefficient of linear thermal expansion.
[0050] The peel strength of the metal foil from the resin film is
preferably at least 1.2 kN/m. The peel strength of the metal foil
from the resin film, i.e. the adhesive force, is related to the
reliability when wiring is formed by etching or the like. For
improved reliability, it is desirable that no peeling or wire
breakage occur under the stress of repeated bending or with thermal
history. The peel strength is the maximum value of stress at which
the metal foil of a test piece with a thickness of 0.025 mm and a
width of 10 mm peels from the main side of the resin film at an
angle of 90 degrees.
[0051] The present inventors measured metal element contents and
relative permittivities by the methods described above. However,
values measured by methods under different conditions including
different apparatuses and different test piece shapes can be
compared if compensation is made.
[0052] As the metal foil 12 there may be suitably used foils of
copper, aluminum, iron, gold, silver, nickel palladium, chromium
and molybdenum or their alloys. Copper foil is preferred among the
above. In order to increase the adhesive force with the resin film
11, the surface may be mechanically or chemically treated by
chemical roughening, corona discharge, sanding, plating or
treatment with aluminum alcoholate, aluminum chelate or silane
coupling agents.
[0053] The flexible laminated sheet 1a is obtained by a production
process comprising a coating step in which a varnish containing a
polyamic acid and a solvent is coated onto the metal foil 12 to
form a coated film, a holding step in which the coated film formed
on the metal foil is held, a drying step in which at least a
portion of the solvent in the varnish is removed to form a layer
composed of a resin composition (hereinafter referred to as "resin
composition layer"), and a resin film-forming step in which the
resin composition layer is heated to form a resin film 11
containing a polyimide resin, wherein the conditions for each step
after the coating step up to the resin film-forming step are
adjusted based on a target for the content of metal elements in the
resin film 11.
[0054] Alternatively, the flexible laminated sheet la may be
produced by a production process comprising a coating step in which
a varnish containing a solvent and either polyamic acid as a
polyimide resin precursor is coated onto a metal foil, a drying
step in which the solvent in the varnish is removed to a proportion
of 1-60 wt % of the total, and a resin film-forming step in which
the resin composition layer is heated to 250-550.degree. C. under a
reducing atmosphere to remove the solvent remaining in the resin
composition layer while forming a polyimide resin from the polyamic
acid, to form a resin film 12 containing the polyimide resin.
[0055] The varnish used in the coating step contains one or more
polyamic acids as polyimide resin precursors. The polyamic acid is
converted to a polyimide resin by heating primarily in the resin
film-forming step.
[0056] The concentration of the polyamic acid in the varnish is
preferably 8-40 wt %. The viscosity of the varnish is preferably
1-40 Pa-s and more preferably 10-40 Pas. If the viscosity of the
varnish is outside of this range, defects in appearance such as
cissing may result upon coating onto the metal foil, thus tending
to lower the film thickness precision.
[0057] In the coating step, two or more different varnishes may be
coated one after the other. In this case, a separate varnish may be
coated onto the coated film formed on the metal foil, and a varnish
separate from that on the lower layer may also be coated over the
resin composition layer after the drying step or over the resin
film after the resin film-forming step.
[0058] The polyimide resin is a polymer with imide groups on the
main chain, and for example, it contains polymer chains represented
by the following general formula (1).
##STR00001##
[0059] Polyamic acids contain amide and carboxyl groups and are
precursors of polyimide resins. The amide groups and carboxyl
groups of a polyamic acid react under heating to form imide groups,
thus resulting in conversion to a polyimide resin. For example, the
polyamic acid may be a polymer with a polymer chain represented by
the following general formula (2).
##STR00002##
[0060] In formulas (1) and (2), R.sup.1 represents a residue
obtained by removing an amino group from a diamine, or a residue
obtained by removing an isocyanato group from a diisocyanate, and
R.sup.2 represents a residue obtained by removing the carboxylic
acid derivative portion of an aromatic tetracarboxylic acid
derivative. The letter n represents an integer of 1 or greater.
[0061] The polyamic acid may be synthesized by reacting a
tetracarboxylic acid or its derivative with a diamine and/or
diisocyanate.
[0062] An aromatic amine is preferred as the diamine. As specific
examples of aromatic amines there may be mentioned p-, m- or
o-phenylenediamine, 2,4-diaminotoluene, 2,5-diaminotoluene,
2,4-diaminoxylene, diaminodurene, 1,5-diaminonaphthalene,
2,6-diaminonaphthalene, benzidine, 4,4'-diaminoterphenyl,
4,4'''-diaminoquaterphenyl, 4,4'-diaminodiphenylmethane,
1,2-bis(anilino)ethane, 4,4'-diaminodiphenyl ether,
diaminodiphenylsulfone, 2,2-bis(p-aminophenyl)propane,
2,2-bis(p-aminophenyl)hexafluoropropane, 2,6-diaminonaphthalene,
3,3-dimethylbenzidine, 3,3'-dimethyl-4,4'-diaminodiphenyl ether,
3,3'-dimethyl-4,4'-diamino diphenylmethane, diaminotoluene,
diaminobenzotrifluoride, 1,4-bis(p-aminophenoxy)benzene,
4,4'-bis(p-aminophenoxy)biphenyl,
2,2'-bis[4-(p-aminophenoxy)phenyl]propane, diaminoanthraquinone,
4,4'-bis(3-aminophenoxyphenyl)diphenylsulfone,
1,3-bis(anilino)hexafluoropropane,
1,4-bis(anilino)octafluorobutane,
1,5-bis(anilino)decafluoropentane,
1,7-bis(anilino)decafluorobutane,
2,2-bis[4-(p-aminophenoxy)phenyl]hexafluoropropane,
2,2-bis[4-(3-aminophenoxy)phenyl]hexafluoropropane,
2,2-bis[4-(2-aminophenoxy)phenyl]hexafluoropropane,
2,2-bis[4-(4-aminophenoxy)-3,5-dimethylphenyl]hexafluoropropane,
2,2-bis[4-(4-aminophenoxy)-3,5-ditrifluoromethylphenyl]hexafluoropropane,
p-bis(4-amino-2-trifluoromethylphenoxy)benzene,
4,4'-bis(4-amino-2-trifluoromethylphenoxy)biphenyl,
4,4'-bis(4-amino-3-trifluoromethylphenoxy)biphenyl,
4,4'-bis(4-amino-2-trifluoromethylphenoxy)diphenylsulfone,
4,4'-bis(3-amino-5-trifluoromethylphenoxy)diphenylsulfone and
2,2-bis[4-(4-amino-3-trifluoromethylphenoxy)phenyl]hexafluoropropane.
[0063] As diamines there may be mentioned siloxanediamines
represented by the following general formula (3). In formula (3),
R.sup.3 represents a monovalent organic group, R.sup.4 represents a
divalent organic group and n represents an integer of 1 or
greater.
##STR00003##
[0064] As diisocyanates there may be mentioned diisocyanates
obtained by reaction between diamines and phosgene. As specific
examples of isocyanates there may be mentioned aromatic
diisocyanates such as tolylene diisocyanate, diphenylmethane
diisocyanate, naphthalene diisocyanate, diphenylether diisocyanate
and phenylene-1,3-diisocyanate.
[0065] As tetracarboxylic acids for reaction with diamines, there
may be used ones having two pairs of two adjacent carboxyl groups.
As specific examples of tetracarboxylic acids there may be
mentioned pyromellitic acid, 2,3,3',4'-tetracarboxydiphenyl,
3,3',4,4'-tetracarboxydiphenyl, 3,3',4,4'-tetracarboxydiphenyl
ether, 2,3,3',4'-tetracarboxydiphenyl ether,
3,3',4,4'-tetracarboxybenzophenone,
2,3,3',4'-tetracarboxybenzophenone,
2,3,6,7-tetracarboxynaphthalene, 1,4,5,7-tetracarboxynaphthalene,
1,2,5,6-tetracarboxynaphthalene,
3,3',4,4'-tetracarboxydiphenylmethane,
2,2-bis(3,4-dicarboxyphenyl)propane,
2,2-bis(3,4-dicarboxyphenyl)hexafluoropropane,
3,3',4,4'-tetracarboxydiphenylsulfone,
3,4,9,10-tetracarboxyperylene,
2,2-bis[4-(3,4-dicarboxyphenoxy)phenyl]propane,
2,2-bis[4-(3,4-dicarboxyphenoxy)phenyl]hexafluoropropane,
butanetetracarboxylic acid and cyclopentanetetracarboxylic acid.
Esters, acid anhydrides and hydrochlorides of these tetracarboxylic
acids may also be reacted with diamines.
[0066] For reaction between the diamine and tetracarboxylic acid or
its derivative, the molar ratio of the diamine or diisocyanate with
respect to the tetracarboxylic acid or its derivative is preferably
0.95-1.05. If the ratio is outside of this range for the reaction,
the molecular weight of the polyamic acid and the polyimide resin
produced therefrom will be reduced tending to result in impaired
physical properties of the film which may include film brittleness
and a poorly maintained film shape.
[0067] The reaction is normally conducted in a solvent such as
N-methyl-2-pyrrolidone (NNP), N,N-dimethylformamide (DMF),
N,N-dimethylacetamide (DMAc), dimethylsulfoxide (DMSO), dimethyl
sulfate, sulfolane, .gamma.-butyrolactone, cresol, phenol, a
halogenated phenol, cyclohexane or dioxane. The reaction
temperature is preferably 0-200.degree. C.
[0068] During the reaction, a modifying compound with a reactive
functional group may be added to introduce a crosslinked or ladder
structure into the polyimide resin. Such modifying compounds
include, for example, compounds represented by the following
general formula (4). Modification with such compounds introduces a
pyrrolone ring or isoindoloquinazolinedione ring into the polyimide
resin.
##STR00004##
[0069] In formula (4), R.sup.5 represents a 2+x valent aromatic
organic group and Z represents --NH.sub.2, --CONH.sub.2,
--SO.sub.2NH.sub.2 or --OH, at the ortho position with respect to
the amino group. The letter x represents 1 or 2.
[0070] The modifying compound used may be a compound such as an
amine, diamine, dicarboxylic acid, tricarboxylic acid or
tetracarboxylic acid derivative that has a polymerizable
unsaturated bond. This will result in a crosslinked structured in
the polyimide resin. As such compounds there may be mentioned
maleic acid, nadic acid, tetrahydrophthalic acid, ethynylaniline
and the like.
[0071] The varnish may also contain, in addition to the polyamic
acid, a polyimide resin produced by partial reaction of the
polyamic acid. The varnish may also comprise additional
crosslinking components such as epoxy compounds, acrylic compounds,
diisocyanate compounds and phenol compounds, and additives such as
fillers, particles, coloring materials, leveling agents, coupling
agents and the like. However, if the amount of such additional
components is greater than the content of the polyimide resin or
its precursor, the properties of the resin film 11 may be
impaired.
[0072] The varnish may be coated on the metal foil using a roll
coater, comma coater, knife coater, doctor blade, flow coater,
sealed coater, die coater, lip coater or the like. In such cases,
the varnish is discharged from a film-forming slit and coated as
evenly as possible.
[0073] After the varnish has been coated onto the metal foil to
form a coated film, the coated film is held for a prescribed time
in that state on the metal foil (holding step). The conditions for
holding, such as the holding temperature, atmosphere and time, are
preferably set so that the content of metal elements in the resin
film 11 does not significantly increase the relative permittivity
of the resin film 11, and in consideration of production cost and
equipment investment. The temperature, atmosphere and time for the
holding step are even more preferably set in consideration of all
of the aforementioned matters in each of the steps up to the resin
film-forming step.
[0074] A higher holding temperature and longer holding time may
result in a higher metal element content. Also, a more oxidizing
atmosphere for the holding atmosphere may result in a higher metal
element content than a reducing atmosphere. However, the preferred
ranges for the conditions will probably differ depending on the
metal foil 12 material and the varnish composition. Thus, the
conditions are preferably set after determining the correlation
between the metal foil 12 material, the varnish composition and the
holding temperature, atmosphere and time, and the metal element
content in the resin film 12.
[0075] Adjustment of the conditions in the holding step, for
example when the temperature for holding is room temperature and
the atmosphere is air in order to minimize production cost and
equipment investment, will involve adjusting the holding time so
that the content of metal elements in the resin film 11 is within
the preferred range of no greater than 5 wt %. However, adjustment
of the conditions is not limited to this method.
[0076] After the coated film has been held for the prescribed time
in that state on the metal foil, it is heated at preferably
100-170.degree. C. (more preferably 100-160.degree. C.) to remove
the solvent in the varnish to a proportion of 1-60 wt % (more
preferably 25-45 wt %) of the total in order to form a resin
composition layer (drying step). Here, the heating may be
accomplished under a reduced pressure atmosphere or under a
reducing atmosphere as described hereunder. If the proportion of
solvent after the drying step is lower than 1 wt %, the resin
composition layer or resin film will shrink in subsequent steps,
often producing warping in the obtained flexible laminated body. If
the proportion of solvent after the drying step is higher than 60
wt %, the outer appearance may be impaired due to foaming and the
handling property may be reduced due to excess tack, when the layer
is heated to form a resin film.
[0077] The conditions for the drying step, such as the temperature,
atmosphere, pressure and time for drying and the residual amount of
solvent in the varnish, are preferably set so that the metal
element content in the resin film 11 does not significantly
increase the relative permittivity of the resin film 11, in
consideration of production cost and equipment investment, and also
in consideration of minimizing warping of the flexible laminated
body, impairment of the outer appearance of the resin film due to
foaming and reduced handling property due to excess tack. The
conditions for the drying step are even more preferably set in
consideration of all of the aforementioned matters in each of the
steps up to the resin film-forming step.
[0078] A higher drying temperature and longer drying time may
result in a higher metal element content. Also, a more oxidizing
atmosphere for the drying atmosphere may result in a higher metal
element content than a reducing atmosphere. However, the preferred
ranges for the conditions will in most cases differ depending on
the metal foil 12 material and the varnish composition. Thus, the
conditions are preferably set after determining the correlation
between the metal foil 12 material, the varnish composition and the
drying temperature, atmosphere, pressure and time, and the metal
element content in the resin film 12.
[0079] Adjustment of the conditions for the drying step may
include, in consideration of minimizing warping of the flexible
laminated body, impairment of the outer appearance of the resin
film due to foaming and reduced handling property due to excess
tack, for example, controlling the drying temperature to
100-170.degree. C. and the residual solvent in the varnish to a
range of 1-60 wt % of the total, and when the atmosphere is air,
controlling the temperature range and residual solvent so that the
metal element content in the resin film 11 is no greater than the
preferred limit of 5 wt %. However, adjustment of the conditions is
not limited to this method.
[0080] Subsequent heating of the resin composition layer forms a
resin film 11 containing a polyimide resin (resin film-forming
step).
[0081] The conditions for the resin film-forming step, such as the
temperature, atmosphere and time for formation of the resin film,
are preferably set so that the content of metal elements in the
resin film 11 does not significantly increase the relative
permittivity of the resin film 11, and in general consideration of
production cost and equipment investment and of the physical
properties of the flexible laminated body or flexible printed
circuit board. The conditions for the resin film-forming step are
even more preferably set in consideration of all of the
aforementioned matters in each of the steps up to the resin
film-forming step.
[0082] A lower resin film-forming temperature and longer time may
result in a higher metal element content. Also, a more oxidizing
atmosphere for the resin film-forming atmosphere may result in a
higher metal element content than a reducing atmosphere. However,
the preferred ranges for the conditions will in most cases differ
depending on the metal foil 12 material and the composition of the
varnish or resin composition layer. Thus, the conditions are
preferably set after determining the correlation between the metal
foil 12 material, the composition of the varnish or resin
composition layer and the resin film-forming temperature,
atmosphere and time, and the metal element content in the resin
film 12.
[0083] Adjustment of the conditions for the resin film-forming step
may include, in consideration of the properties of the flexible
laminated body or flexible printed circuit board, for example,
controlling the atmosphere so that the metal element content in the
resin film 11 is no greater than the preferred limit of 5 wt % when
the heating temperature is controlled to a range of 250-550.degree.
C. Specifically, the adjustment is preferably carried out in the
following manner, with the understanding that adjustment of the
conditions is not limited to this method.
[0084] Preferably, heating is carried out to 250-550.degree. C.
(more preferably 250-400.degree. C.) under a reducing atmosphere in
the resin film-forming step to form a polyimide resin-containing
resin film 12. The solvent remaining in the resin composition layer
is removed in the resin film-forming step. However, it is
sufficient if the solvent is essentially removed, and the flexible
laminated sheet or flexible printed circuit board may contain a
trace amount of residual solvent so long as the properties are not
impaired. When the varnish and resin composition layer contain a
polyamic acid, heating in the resin film-forming step produces a
polyimide resin from the polyamic acid.
[0085] A reducing atmosphere is an atmosphere formed from a mixed
gas of essentially an inert gas and a reducing gas. The mixed gas
preferably contains substantially no oxygen, and specifically the
mixed gas preferably has an oxygen concentration of no greater than
5 vol %. Adequate management of the oxygen concentration with an
oxygen densitometer in the resin film-forming step is important for
quality and safety.
[0086] The inert gas may be helium, neon, argon, nitrogen, or a
mixture thereof. Nitrogen gas is preferred among the above from the
viewpoint of convenient manageability. Hydrogen gas is preferred as
a reducing gas.
[0087] The reducing atmosphere is most preferably one formed by a
mixed gas composed of nitrogen gas and hydrogen gas at between 0.1
vol % and 4 vol % of the total. If the hydrogen gas concentration
is less than 0.1 vol % the effect of the invention will tend to be
reduced, and if it is greater than 4 vol % the lower explosive
limit for hydrogen gas may be exceeded. For further improved
reliability, the concentration of hydrogen gas in the mixed gas is
more preferably between 0.1 vol % and 1 vol %.
[0088] Formation of the resin film 11 by heating in a reducing
atmosphere prevents oxidation of the polyimide resin. It will also
allow control of migration of metal elements from the metal foil 12
into the resin film 11, so that a resin film with high reliability
can be obtained. Another advantage is that coloration of the resin
film is prevented and workability during the process is
improved.
[0089] Migration of metal elements during the resin film-forming
step is believed to occur upon exposure to an oxygen-containing
atmosphere at high temperatures of 200.degree. C. and above.
Exposure to an oxygen-containing atmosphere at high temperature
promotes oxidation of the metal foil 12 and may lead to instability
of the metals on the surface. Moreover, presumably because of the
high acidity of the varnish or resin composition, the metals on the
surface of the metal foil 12 more readily elute and metal migration
takes place. In order to inhibit this phenomenon, it is important
to maintain an oxygen-shielded state in the aforementioned reducing
atmosphere.
[0090] The flexible laminated sheet of the invention may have a
construction different from the flexible laminated sheet 1a shown
in FIG. 1, such as one obtained by laminating two laminated bodies
10 each comprising a metal foil 12 bonded on one side of a resin
film 11, as shown in FIG. 2 or 3.
[0091] The flexible laminated sheet 1b shown in FIG. 2 has a
construction wherein two laminated bodies 10 are laminated with
their resin films 11 bonded together. The flexible laminated sheet
1b is obtained, for example, by thermocompression bonding of two
laminated bodies 10. The method of thermocompression bonding may be
a pressing method or laminating method.
[0092] The flexible laminated sheet 1c shown in FIG. 3 comprises
two laminated bodies 10 and an adhesive layer 15 sandwiched between
their resin films 11. That is, the flexible laminated sheet 1c has
two laminated bodies 10 bonded with an adhesive. The flexible
laminated sheet 1c is obtained, for example, by thermocompression
bonding of two laminated bodies 10 having an adhesive sandwiched
between them. The adhesive is not particularly restricted so long
as it is capable of bonding the resin films. For example, a resin
composition containing an epoxy resin, acrylic resin,
polyamideimide resin, thermoplastic polyimide resin or the like may
be used as the adhesive.
[0093] A flexible printed circuit board may be obtained by a method
in which a portion of the metal foil on the flexible laminated
sheet is removed to form a conductor pattern. FIGS. 4 and 5 are
cross-sectional views of separate embodiments of a flexible printed
circuit board according to the invention.
[0094] The flexible printed circuit board 2a shown in FIG. 4 is
provided with a resin film 11 and a conductor pattern 20 formed on
one side of the resin film 11. The conductor pattern 20 is formed
by removing portions of the metal foil 12 on the flexible laminated
sheet 1a and performing patterning. Patterning of the metal foil 12
may be carried out by a method such as photolithography.
Alternatively, the metal foil 12 may be removed from the flexible
laminated sheet 1a and electric conductor material directly written
onto the exposed resin film 11 to form a conductive pattern, in
order to obtain the flexible printed circuit board 2a.
[0095] The flexible printed circuit board 2b shown in FIG. 5 is
provided with an insulating layer comprising two resin films 11,
and a conductive pattern 20 formed on both sides of the insulating
layer. The flexible printed circuit board 2b can be produced by the
same method as the flexible printed circuit board 2a using the
flexible laminated sheet 1b of FIG. 2.
[0096] The modes described above are only preferred modes of the
invention, and the invention is not limited to these preferred
modes. For example, the steps in which the conditions are adjusted
based on a target for the content of metal elements in the resin
film are not limited to the holding step, drying step and resin
film-forming step. When a holding step in which the resin
composition layer-formed metal foil is held in that state is
carried out between the coating step and resin film-forming step,
the temperature, atmosphere and time may be adjusted so that the
metal element content is kept within the prescribed numerical
range.
EXAMPLES
[0097] The present invention will now be explained in greater
detail by examples. However, the invention is not limited to these
examples.
[0098] The relative permittivities, metal element contents,
coefficients of linear thermal expansion and metal foil peel
strengths of the resin films in the examples and comparative
examples were measured according to the following methods.
[0099] Relative Permittivity
[0100] A rectangular test piece with a width of 2 mm and a length
of 80 mm was cut out from the resin film obtained by removing the
metal foil of the flexible laminated sheet by etching, and was
dried at 105.degree. C. for 30 minutes. A stack of 200 dried test
pieces was used to measure the relative permittivity at 5 GHz by a
cavity resonator method. Measurement of the relative permittivity
was accomplished using a "CP511" cavity resonator by Kantoh
Electronics Application and Development Inc. and a "E7350A" network
analyzer by Agilent Technologies.
[0101] Coefficient of Linear Thermal Expansion
[0102] A rectangular test piece with a width of 5 mm and a length
of 15 mm was cut out from the resin film obtained by removing the
metal foil of the flexible laminated sheet by etching. The test
piece was used for measurement of the coefficient of linear thermal
expansion in tensile mode. The measurement was performed using a
"TMA2940" by TA Instruments, and the coefficient of linear thermal
expansion was calculated from the elongation from 50.degree. C. to
250.degree. C.
[0103] Metal Element Content
[0104] A rectangular test piece with a width of 10 mm and a length
of 15 mm was cut out from the resin film obtained by removing the
metal foil of the flexible laminated sheet by etching. The metal
element content on the main side adjacent to the metal foil of the
test piece was measured with an XPS apparatus. Next, argon etching
was performed from the main side to a depth of 0.05 .mu.m
(approximately 10 minutes), and the metal element content on the
exposed side was measured. This was followed by argon etching for
approximately 10 minutes and measurement of the metal element
content 6 times in the same manner, for a total of 8 metal element
content measurements. The metal element content in the resin film
was then calculated from the measured metal element contents. The
XPS apparatus used was a Model ESCA5400 (trade name of Ulvac-Phi,
Inc.).
[0105] Metal Foil Peel Strength
[0106] The metal foil of the flexible laminated sheet was etched
with a 1 mm-wide mask to prepare a test piece with a 1 mm-wide
metal foil. The load at which the 1 mm-wide metal foil portion
peeled with a peel angle of 90 degrees and a peel rate of 50 mm/min
was measured, and the maximum load was recorded as the peel
strength.
Synthesis Example
[0107] After placing 867.8 g of p-phenylenediamine, 1606.9 g of
4,4'-diaminodiphenyl ether and 40 kg of N-methyl-2-pyrrolidone in a
60 L stainless steel reactor equipped with a thermocouple, stirrer
and nitrogen inlet port while circulating approximately 300 mL/min
of nitrogen, the mixture was stirred to dissolve the diamine
component. The solution was cooled to below 50.degree. C. with a
water jacket while slowly adding 4722.2 g of
3,3',4,4'-biphenyltetracarboxylic dianhydride to promote
polymerization reaction, thereby obtaining a viscous polyamic acid
varnish containing polyamic acid and N-methyl-2-pyrrolidone. In
order to improve the coated film workability, it was cooked for
80.degree. C. until the rotating viscosity of the varnish reached
10 Pas.
Coating Example 1
[0108] The polyamic acid varnish obtained in the synthesis example
was coated to a thickness of 50 .mu.m on the roughened surface of a
copper foil using a coating machine (comma coater). The copper foil
used was a rolled copper foil with a width of 540 mm and a
thickness of 12 .mu.m which had been roughened on one side
("BHY-02B-T", trade name of Nikko Materials Co., Ltd.). After
holding it in air at room temperature for a prescribed retention
time, a forced-draft drying furnace was used to remove the solvent
from the polyamic acid varnish coated on the copper foil, to a
residual solvent content of 20 wt %, thus forming a resin
composition layer containing the polyamic acid.
Coating Example 2
[0109] A resin composition layer was formed in the same manner as
Coating Example 1, except that the polyamic acid varnish obtained
in the synthesis example was coated to a thickness of 50 .mu.m and
the solvent was removed from the polyamic acid varnish coated on
the copper foil until the proportion of solvent was 50 wt %.
Coating Example 3
[0110] A resin composition layer was formed in the same manner as
Coating Example 1, except that the polyamic acid varnish obtained
in the synthesis example was coated to a thickness of 50 .mu.m and
the solvent was removed from the polyamic acid varnish coated on
the copper foil until the proportion of solvent was 70 wt %.
Examples 1-6, Comparative Examples 1-3
[0111] The resin composition layers formed by the methods of
Coating Examples 1-3 were continuously heated using a hot air
circulation oven under the heating conditions shown in Table 1 or
2, to fabricate flexible laminated sheets. The relative
permittivities, coefficients of linear thermal expansion and metal
element contents of the resin films and the peel strengths of the
metal foils in the fabricated flexible laminated sheets were
measured according to the methods described above. The results are
shown in Tables 1 and 2. In the tables, "0.5% hydrogen" means that
a nitrogen/hydrogen mixed gas containing 0.5 vol % hydrogen was
used. The same applies for "2% hydrogen" and "0.1% hydrogen".
TABLE-US-00001 TABLE 1 Example Example Example Example Example
Example 1 2 3 4 5 6 Resin Coating Coating Coating Coating Coating
Coating composition Example 1 Example 1 Example 1 Example 1 Example
1 Example 2 layer formation Heating Nitrogen/0.5% Nitrogen/0.5%
Nitrogen/0.5% Nitrogen/2% Nitrogen/0.1% Nitrogen/0.5% conditions
hydrogen hydrogen hydrogen hydrogen hydrogen hydrogen Atmosphere
250-400 250-300 250-550 250-400 250-400 250-400 Temperature
.degree. C. Relative 3.1 3.2 3.1 3.3 3.1 3.5 permittivity Metal
element 3.8 2.2 3.6 4.1 4.5 3.8 content (wt %) Linear thermal 23 25
22 23 22 22 expansion coefficient (ppm/.degree. C.) Copper foil 1.4
1.2 1.2 1.3 1.4 1.5 peel strength (kN/m)
TABLE-US-00002 TABLE 2 Comparative Comparative Comparative
Comparative Example 1 Example 2 Example 3 Example 4 Resin Coating
Coating Coating Coating composition Example 1 Example 1 Example 1
Example 3 layer formation Heating conditions Atmosphere Nitrogen
Air Nitrogen/ Nitrogen/ 0.5% 0.5% hydrogen hydrogen Temperature
250-400 250-400 200 250-400 .degree. C. Relative 3.5 3.6 3.5 Un-
permittivity measurable Metal element 8.0 10.0 8.6 Un- content (wt
%) measurable Linear thermal 24 27 33 Un- expansion measurable
coefficient (ppm/.degree. C.) Copper foil 1.5 0.8 0.9 Un- peel
strength measurable (kN/m)
[0112] As shown in Table 1, the flexible laminated sheets of the
Examples exhibited satisfactory properties in terms of relative
permittivity, coefficient of linear thermal expansion and copper
foil peel strength. In contrast, the flexible laminated sheets of
Comparative Examples 1-3 were inadequate in at least one of these
properties. In the case of Comparative Example 4, the obtained
copper foil-clad flexible laminated body generated significant
foaming at the resin film sections and could not produce a
copper-clad flexible board material with a satisfactory outer
appearance, and therefore evaluation of the relative permittivity
and other properties could not be performed.
* * * * *