Single component aqueous acrylic adhesive compositions for flexible printed circuits and laminates made therefrom

Described is a single component aqueous adhesive composition especially suitable for flexible printed circuits. The composition consists of water and a homogeneous film-forming acrylic polymer. The acrylic polymer is a polymerization product of (A) acrylonitrile or methacrylonitrile, or mixture thereof, (B) an alkyl acrylate or methacrylate having 1-12 carbon atoms in the alkyl group, or mixture thereof, (C) an oxirane-containing polymerizing ethylenic monomer and (D) a hydroxyl- or amide-containing acrylate or methacrylate. The adhesive composition is useful for bonding metal foils to polyimide films in the manufacture of flexible printed circuits.

The present invention relates to adhesive compositions for flexible printed 
circuits (FPC), and more particularly to water-based adhesive compositions 
comprising a homogeneous acrylic polymer made from (A) acrylonitrile or 
methacrylonitrile, or mixture thereof, (B) an alkyl acrylate or 
methacrylate having 1-12 carbon atoms in the alkyl group, or mixture 
thereof, (C) an oxirane-containing polymerizing ethylenic monomer, and (D) 
a hydroxyl-, and/or amide-containing acrylate or methacrylate. The 
invention also relates to adhesive-coated polyimide films, and laminates 
made from the films and metal foils. 
BACKGROUND OF THE INVENTION 
In recent years, with the advancement of electronic technology, reduction 
in size and in weight, and high reliance of instruments (e.g. those for 
communication and defense industries), appliances and the like, and 
simplification of packaging systems have been required. A printed circuit 
board having light and flexible plastic film as insulating base board has 
long been desired. 
The plastic films suitable for use in FPC include polyester films and 
polyimide films. The polyimide films exhibit excellent solderability and 
other physical properties and have become the main stream of FPC 
substrates. 
It is well-known in the art that many adhesives do not bond well to 
polyimide films. Conventional adhesives having commercially acceptable 
adhesion to bond the polyimide films to metal foils (such as copper, 
aluminum, and the like) are acrylonitrile-butadiene copolymer, butyral 
resin, epoxy resin, nylon/epoxy resin, acrylonitrile-butadiene/phenolic 
resin, acrylics/epoxy resin, etc. However, the use of flammable organic 
solvents such as toluene, methylethyl ketone, acetone, etc. is required in 
these adhesive compositions. This could cause several disadvantages. For 
example, evaporation of organic solvents during the coating process needs 
special precautions to avoid fire hazard and air pollution; or the solvent 
recovery system adds extra cost. 
Water-based adhesives will solve the aforementioned problems. However, to 
find an aqueous-type adhesive having balanced properties of adhesiveness, 
thermal stability, flexibility, solderability, dimensional stability, 
solvent resistance, flow property and electrical property remains to be a 
challenge. A latex of an acrylic terpolymer disclosed in U.S. Pat. No. 
3,900,662 represents an example. However, a crosslinking agent must be 
added in the latex. Moreover, the polyimide film on which the latex is to 
be coated requires surface pre-treatment such as one with an alkaline 
solution as taught in U.S. Pat. No. 3,728,150. This adds extra cost and 
increases the possibility of contamination. 
Japanese Patent Laid-Open No. 60,118,781 discloses that a water soluble 
epoxy compound and an emulsified epoxy resin are mixed with an acrylic 
latex to form an adhesive composition useful in FPC. However, the mixing 
procedure is time-consuming and will result in difficulties in quality 
control. 
SUMMARY OF THE INVENTION 
This invention provides a single-component water-based acrylic adhesive 
composition which possesses excellent adhesiveness to bond metal foils to 
polyimide films without pretreating polyimide films. This invention also 
provides a polyimide film coated with the said acrylic adhesive 
composition and laminates made therefrom.

DETAILED DESCRIPTION OF THE INVENTION 
The aqueous acrylic adhesive composition of this invention consists of 
water and a film-forming acrylic polymer. The acrylic polymer is a 
polymerization product of (A) acrylonitrile or methacrylonitrile, or 
mixture thereof,(B) an alkyl acrylate or methacrylate having 1-12 carbon 
atoms in the alkyl group, or mixture thereof (C) an oxirane-containing 
polymerizing ethylenic monomer and (D) a hydroxyl- or amide-containing 
acrylate or methacrylate, which has good film-forming properties and good 
balance between the flow property and the crosslinking temperature. The 
composition also provides FPC laminates with excellent properties such as 
excellent electrical property, thermal stability, solvent resistance, and 
solderability, and particularly it imparts high flexibility and 
adhesiveness between a metal foil and a polyimide film. A film coated with 
the composition of this invention can be used directly without any 
pretreatment. The composition of this invention can be prepared by 
emulsion polymerization. Therefore pollution control and solvent recovery 
are no longer required. 
The composition of this invention has a solid content of about 10-60% by 
weight. The remainder of the composition is water and optionally small 
amounts of diluents and solvents which can be introduced during 
polymerization of the acrylic polymer or introduced together with other 
constituents. 
The film-forming acrylic polymer suitable for use in the invention is a 
tetrapolymer which is made from (A) 15-60% by weight of the tetrapolymer 
of acrylonitrile or methacrylonitrile, or mixture thereof (B) 30-85% by 
weight of the tetrapolymer of an alkyl acrylate or methacrylate having 
1-12 carbon atoms in the alkyl group, or mixture thereof (C) 1-30% by 
weight of the tetrapolymer of an oxirane-containing polymerizing ethylenic 
monomers, and (D) up to 15% by weight of the tetrapolymer of a hydroxyl 
and/or amide-containing acrylate or methacrylate. Although the acrylic 
polymer preferably comprises component (D), the acrylic polymer containing 
only components (A), (B) and (C) is suitable for use in FPC. Such polymer 
is also contemplated as within the scope of the present invention. 
The alkyl acrylates and methacrylates suitable for use in the invention 
have 1-12, preferably 2-8, carbon atoms in the alkyl groups. For example, 
they may be ethyl acrylate and methacrylate, propyl acrylate and 
methacrylate, isopropyl acrylate and methacrylate, butyl acrylate and 
methacrylate, isobutyl acrylate and methacrylate, pentyl acrylate and 
methacrylate, hexyl acrylate and methacrylate, 2-ethylhexyl acrylate and 
methacrylate, nonyl acrylate and methacrylate, lauryl acrylate and 
methacrylate and the like. Ethyl acrylate, butyl acrylate and 2-ethylhexyl 
acrylate are most preferred. For certain uses, it may be desirable to add 
styrene. 
Examples of oxirane-containing polymerizing ethylenic monomers are glycidyl 
acrylate, glycidyl methacrylate, allyl glycidyl ether and the like. 
Glycidyl acrylate and methacrylate are preferred. 
The hydroxyl and/or amide-containing acrylate or methacrylate useful in the 
present composition is selected from the group consisting of 
2-hydroxylethyl acrylate, 2-hydroxylpropyl acrylate, acrylamide, 
2-hydroxylethyl methacrylate, 2-hydroxylpropyl methacrylate, 
N-methylolacrylamide, and methacrylamide. 2-Hydroxylethyl acrylate, 
acrylamide, N-methylolacrylamide and 2-hydroxylethyl methacrylate are 
preferred. 
Preferably, the acrylic polymer is of about 30-40% by weight of 
acrylonitrile or methacrylonitrile, 50-65% by weight of an alkyl acrylate 
having 2-8 carbon atoms in the alkyl group, 2-15% by weight of glycidyl 
methacrylate, and 0.5-10% by weight of 2-hydroxylethyl acrylate or 
acrylamide. 
The acrylic polymer can be prepared by solvent or emulsion polymerization. 
Emulsion polymerization is preferable because of low cost and non-use of 
organic solvents. 
The emulsion polymerization may be conducted in batch. However, 
semicontinuous emulsion polymerization is preferred. The monomer 
conversion is normally higher than 98% and the final nonvolatile content 
is usually 45-55% by weight of the composition. Particle size is 
determined by the level of primary surfactant in the initial reactor 
charge. Polymer Tg should be between 15.degree. C. and 45.degree. C. 
In preparing the acrylic polymer, components (A), (B), (C), and (D) are 
copolymerized in water through free radical induced polymerization, using 
a peroxy or azo compound as initiator, e.g. a persulfate, benzoyl 
peroxide, t-butylhydroperoxide, cumene hydroperoxide, 
azobis-isobutyronitrile, dimethyl azobis-isobutyrate, etc. If necessary, 
the initiator may contain alkali metal sulfate or ammonium persulfate with 
or without a modified reducing substance (e.g. sodium bisulfite or ferrous 
sulfate). A preferred initiator is ammonium persulfate or potassium 
persulfate. The initiator normally is used at a level of about 0.1% to 
3.0% by weight of the monomers. Polymerization generally is conducted at 
about 50.degree.-80.degree. C. for about 1-4 hours to form an emulsion or 
a dispersion in which the resulting polymer has a weight average molecular 
weight of at least 250,000, preferably from 400,000 to 800,000. 
The pH of the polymer thus obtained is about 3-7. A suitable amount of 
amine may be added to adjust the pH up to about 7-10. Typical amines that 
can be used are triethylamine, diethanol amine, ethanolamine, N-methyl 
ethanolamine, methyl diethanolamine, diethylene tetramine and the like. 
Other minor additives such as defoamers or thickeners can be incorporated 
into the composition as needed. 
The adhesive composition of the present invention can be applied to the 
polyimide film by conventional procedures such as spraying, dipping, 
brushing, roll coating, and the like. This adhesive composition can be 
applied in varying thickness, depending on the particular end use of 
coated films. Greater adhesive thickness can be achieved by a plurality of 
coatings. Typically, the acrylic adhesive has the thickness of at least 
0.1 mils, preferably about 0.5-3 mils. At present, there is no need for 
the adhesive coated on the polyimide film to be thicker than about 10 
mils. 
Polyimides suitable for use as films in this invention are disclosed in 
U.S. Patent No. 3,900,662, the disclosure of which is hereby incorporated 
by reference. The usual polyimide films are, for example, Kapton.RTM. 
(available in E. I. Du Pont Nemours Co., Delaware, U.S.A.) and Upilex.RTM. 
(Ube Ind., Ltd., Japan) films. The polyimide films coated with the 
adhesive composition of this invention can be used to form various types 
of laminate articles and can be used in combination with various external 
layers of metal foils such as copper, aluminum, nickel, and the like to 
yield useful printed circuit boards. Standard lamination procedures can be 
used to construct these laminates such as vacuum bag lamination, press 
lamination, roll lamination and the like. 
The article obtained by bonding a polyimide film to a metal foil with the 
adhesive composition of this invention is insoluble in organic solvents, 
such as methylethyl ketone, trichloroethylene, methylene chloride, 
acetone, methanol, toluene and the like, which property is essential in 
the production of printed circuits. The laminate is resistant to chemicals 
such as a 10% aqueous caustic soda solution, a 10% aqueous ammonium 
persulfate solution and the like, and is excellent in soldering heat 
resistance, flexibility, electrical insulation, and adhesiveness, and 
therefore it is an excellent base board for flexible printed circuits 
without impairing the characteristics of the base film. 
The adhesive composition of this invention is useful as an adhesive or as a 
film adhesive in the backing of a flexible printed circuit with a hard 
board for the purpose of reinforcement of its parts-loading position, in 
the intergral lamination of a flexible printed circuit with a hard printed 
circuit for the purpose of forming high density wiring of circuits and 
simplification of connection, and in the multilamination of a flexible 
printed circuit with one another. The adhesive property, flow property, 
and heat resistance of the adhesive composition satisfy the quality 
required for common flexible printed circuits. 
This invention is illustrated by but is not limited to the following 
specific examples in which the parts and percentages are expressed on the 
weight basis. 
EXAMPLE 1 
[Preparation of acrylic emulsion copolymer containing acrylonitrile (AN)/ 
2-ethylhexylacrylate (EHA)/ glycidyl methacrylate(GMA)/ 2-hydroxylethyl 
methacrylate (HEMA)] 
250 parts of deionized water, 2.0 parts SDS(sodium dodecyl sulfate), and 
0.1 parts sodium bisulfite were loaded into a reactor and heated under 
nitrogen with agitation to 60.degree. C. A mixture of 14 parts 
acrylonitrile, 30 parts 2-ethylhexyl acrylate, 1.5 parts glycidyl 
methacrylate, and 1.5 parts 2-hydroxylethyl methacrylate was then added to 
the reactor and emulsified for 10 minutes, whereupon 0.25 parts of 
potassium persulfate were added. The reaction mixture is allowed to react 
for another 20 minutes, before the subsequent monomer feeding started. 
The subsequent monomer mixture contained 70 parts of AN, 150 parts of EHA, 
7.5 parts of GMA, and 7.5 parts of HEMA. This mixture was added at a 
constant rate to the reactor so that the addition was completed within 3.0 
hours. The batch was then hold for one hour at the reaction temperature 
before cooling and filtration. The amount of coagulant is less than 0.01 
parts. 
The adhesive thus obtained was coated on an unpretreated polyimide film 
(Upilex 25S, a registered trademark of Ube Co.) of 25.mu. in thickness, by 
means of a two-roller coater in which a coating roll section, a drying 
zone, and a cover-lay laminator were connected in series. The resulting 
coated film was about 53.mu. thick. The coated film was then adhered to a 
copper foil having the thickness of 35.mu. by pressing them in a hot press 
at 180.degree. C. under a pressure of 300 psi for 10 minutes. The 
resulting flexible copper-clad laminate was excellent in bond strength, 
chemical resistance, and electrical properties, and outstanding in heat 
resistance, and could withstand 30-second immersion in a solder bath at 
260.degree. C. The results were shown in Table 1. 
EXAMPLE 2 
In a manner similar to Example 1, an adhesive was prepared by using the 
following components: 
______________________________________ 
Components Parts 
______________________________________ 
methacrylonitrile 20 
butylacrylate 70 
glycidyl methacrylate 
5 
N--methylolacrylamide 
5 
Emulsifier 6 
(Abex* 18S, 35% 
aqueous solution) 
D.I. Water 120 
______________________________________ 
(*A trade name of Alcolac Inc., USA) 
The resulting adhesive was coated on an unpretreated polymide film 
(Kapton.RTM., a registered trademark of Du Pont Company) of 50.mu. in 
thickness, until the coated film has the thickness of about 80.mu.. After 
drying at 120.degree. C. for 5 minutes, the coated film was adhered to a 
copper foil, 35.mu. in thickness, by pressing them in a hot press to 
prepare a flexible copper-clad laminate, which properties were shown in 
Table 1. 
EXAMPLE 3 
In a manner similar to Example 1, a latex was prepared by using the 
following components: 
______________________________________ 
Components Parts 
______________________________________ 
Acrylonitrile 40 
Ethylhexylacrylate 
70 
Styrene 5 
Glycidyl methacrylate 
8 
Acrylamide 10 
Emulsifier SDS 1.5 
D.I. Water 200 
______________________________________ 
To 200 parts of the resulting latex was added 0.1 parts sodium polyacrylate 
to adjust the viscosity to 220 cps. The varnish thus obtained was coated 
on a polyimide film (Upilex-25S), 25.mu. in thickness, until the coated 
film is about 55.mu. thick. After drying at 150.degree. C. for 5 minutes, 
the coated film was adhered to a copper foil, 35.mu. in thickness, in a 
hot press at 180.degree. C. under a pressure of 300 psi for 10 minutes to 
form a copper-clad laminate. The properties of the resulting laminate were 
shown in Table 1. 
EXAMPLE 4 
In the same manner as in Example 1, the adhesive varnish having a 
concentration of 45% was prepared by reacting the following components: 
______________________________________ 
Components Parts 
______________________________________ 
Acrylonitrile 50 
2-Ethylhexylacrylate 180 
Glycidyl methacrylate 
15 
2-hydroxylethyl methacrylate 
15 
Emulsifier (Abex 18S) 
12 
D.I. Water 300 
______________________________________ 
In the same manner as in Example 1, the adhesive varnish thus obtained was 
coated on a polyimide film, 125.mu. in thickness, and the coated film of 
153.mu. in thickness was bonded to a copper foil of 35.mu. in thickness, 
by application of heat and pressure by means of a hot press to obtain a 
flexible copper-clay laminate, the properties of which were shown in Table 
1. 
EXAMPLE 5 
Repeating the procedure of Example 1 and using the following components, an 
adhesive varnish was prepared: 
______________________________________ 
Components Parts 
______________________________________ 
Acrylonitrile 40 
Ethylacrylate 140 
Glycidyl methacrylate 
10 
Methacrylamide 10 
D.I. Water 200 
SDS 6 
______________________________________ 
In the same manner as in Example 1, the adhesive varnish thus obtained was 
coated on a polyimide film, 50.mu. in thickness, and the coated film of 
78.mu. in thickness was bonded to a copper foil of 70.mu. in thickness, by 
application of heat and pressure by means of a hot press to obtain a 
flexible copper-clad laminate, the properties of which were shown in Table 
1. 
COMATIVE EXAMPLE 1 
______________________________________ 
Components Parts 
______________________________________ 
Acrylonitrile 35 
Butylacrylate 60 
Glycidyl methacrylate 
5 
Methacrylic acid 5 
Emulsifier (SDS) 1.5 
D.I. Water 120 
______________________________________ 
In the same manner as in Example 1, the adhesive varnish obtained from the 
above recip was coated on a polyimide film, 50.mu. in thickness, and the 
coated film of 78.mu. in thickness was bonded to a copper foil, 35.mu. in 
thickness, by application of heat and pressure (180.degree. C., 350 psi) 
by means of a hot press for 40 minutes to obtain a flexible copper-clad 
laminate. The peel strength was poor as shown in Table 1. 
EXAMPLE 6 
______________________________________ 
Components Parts 
______________________________________ 
Acrylonitrile 35 
Butylacrylate 60 
Glycidyl methacrylate 
5 
Emulsifier (SDS) 1.5 
D.I. Water 120 
______________________________________ 
The resulting latex made from the above recipe had about 8% of coagulant. 
The filtrated adhesive varnish was coated on a polyimide film. The 
copper-clad laminate was made according to the same manner as in example 
1. The properties of the laminate were shown in Table 1. 
EXAMPLE 7 
By use of the laminate of the present invention, a flexible printed circuit 
with a circuit land area was prepared by the etching method. 
The adhesive of this invention was coated on a polyimide film of 25.mu. in 
thickness, and dried at 120.degree. C. for 5 minutes, and thereafter the 
areas corresponding to the land were punched off from the film. The film 
thus obtained was placed over a printed circuit in a registered position, 
and the resulting composite was heated and pressed for 60 minutes by means 
of a hot press operating at 180.degree. C. and under 350 psi. The 
resulting printed circuit board had the copper foil circuit portion 
completely embedded in a cover-lay coated with the adhesive, and the 
adhesive would not exude to the exposed portion of the circuit land, 
indicating satisfactory covering. The printed circuit would neither fail 
at the boundry of the cover-lay when subjected to the attack of chemicals 
as a flex during soldering, nor suffer from rupture of adhesive bonding 
when subjected to flow soldering at the temperature of 260.degree. C. This 
indicates that the circuit has excellent heat resistance. 
TABLE 1 
__________________________________________________________________________ 
Testing 
Example 
Example 
Example 
Example 
Example 
Example 
Comparative 
Item of Test 
Method 1 2 3 4 5 6 Example 1 
__________________________________________________________________________ 
Peel strength 
IPC-650- 
12 11 12 11 13 9 3 
(lbs/in) 
2.4.9A 
Dielectric 
ASTM-D150 
4.0 3.5 3.7 3.6 3.4 4.0 3.7 
Constant 
(f = 1 MHZ) 
Dissipation 
ASTM-D150 
0.030 
0.020 
0.026 
0.020 
0.022 
0.030 
0.022 
Factor (f = 1 MHZ) 
Dielectric 
ASTM-D149 
3.0 3.9 3.3 3.5 3.8 3.0 3.7 
Strength 
(KV/mil) 
Volume ASTM-D257 
4 .times. 10.sup.15 
5 .times. 10.sup.15 
5 .times. 10.sup.15 
6 .times. 10.sup.15 
5 .times. 10.sup.15 
3 .times. 10.sup.15 
4 .times. 10.sup.15 
Resistivity 
(.OMEGA.-cm) 
Surface 
ASTM-D257 
4 .times. 10.sup.15 
2 .times. 10.sup.15 
3 .times. 10.sup.15 
4 .times. 10.sup.15 
2 .times. 10.sup.15 
3 .times. 10.sup.15 
2 .times. 10.sup.15 
Resistivity 
(.OMEGA.) 
Chemical 
IPC-650- 
No No No No NO NO Swollen 
Resistance 
2.3.2A Change 
Change 
Change 
Change 
Change 
Change 
Soldering 
Solder No NO No NO No NO Delamination 
Resistance 
float 30 sec 
Change 
Change 
Change 
Change 
Change 
Change 
@ 260.degree. C. 
__________________________________________________________________________