Flame-proof flexible printed circuit board

A flexible printed circuit board having excellent flame proofness and a very little danger of causing a fire can be produced by subjecting a substrate or an insulating film consisting of a flexible resin film to surface treatment to provide a surface tension of 36 dyne/cm or more and then forming a resin film containing a flame retardant on the treated surface of said substrate or insulating film.

BACKGROUND OF THE INVENTION 
The present invention relates to a flexible printed circuit board having 
excellent flame proofness and a very little danger of causing a fire. 
In general, flexible printed circuit boards such as the printed circuit of 
a speedometer for automobiles have heretofore been produced by fastening a 
metal foil such as a copper foil constituting an electric circuit onto a 
flexible substrate consisting of a resin film, for example, a polyester 
resin film with an adhesive. Further, for the insulation of the electric 
circuit, the metal foil is coated with a flexible film of a resin such as 
a polyester resin. The flexible printed circuit boards permit the 
production of a remarkably lighter printed circuit board and a remarkable 
reduction in cost as compared with printed circuit boards using a phenol 
resin laminated board as a substrate which have previously often been 
employed. Therefore, it is expected that the flexible printed circuit 
boards will have various uses. In the flexible printed circuit boards, 
however, an inflammable flexible film such as a polyester film is widely 
used as a substrate and as an insulating film from the viewpoints of cost 
and processability. Therefore, heat is generated when an excess current 
flows on a copper foil constituting an electric circuit. In this case, a 
fuse portion formed in the circuit for safety by narrowing the copper foil 
burns out and the substrate, the insulating film or the adhesive catches 
fire. A flame spreads at a fairly high speed and a fire is caused from 
this portion. Thus, the flexible printed circuit boards are a possible 
cause of causing a fire. This is the reason why the flexible printed 
circuit boards have not yet been put to practical use widely. 
Therefore, an object of the present invention is to obviate the defect of 
prior art flexible printed circuit boards. 
Another object of the invention is to provide a flexible printed circuit 
board having excellent flame proofness and a very little danger of causing 
a fire.

DETAILED DESCRIPTION OF THE INVENTION 
According to the present invention, there is provided a flame-proof 
flexible printed circuit board comprising a copper foil constituting an 
electric circuit and a flexible resin film fastened onto one surface or 
both surfaces of said copper foil with an adhesive wherein said flexible 
resin film is subjected to surface treatment to provide a surface tension 
of 36 dyne/cm or more and a resin film containing a flame retardant is 
then formed on the treated surface of said flexible resin film. 
The following examples illustrate the present invention in more detail. 
EXAMPLE 1 
This example illustrates the formation of a flexible printed circuit board 
as shown in FIG. 1 by coating a fire proofing film 5 consisting of a flame 
proofing mixture of a resin and a flame retardant onto the surfaces of a 
substrate 1 and an insulating film 4 each consisting of a flexible resin 
film. 
Although polyester, polyvinyl chloride, polypropylene, polyethylene, 
polyamide and polyimide resins may be used as the flexible resin film used 
in the present invention, a polyester resin was used in this example. 
Also, as the resin used in the flame proofing mixture, an epoxy resin, a 
polyurethane resin or a polyester resin, which were comparatively 
excellent in adhesive property for the substrate 1 or the insulating film 
4, was used. Further, as the flame retardant to be mixed with said resins, 
a halogenated organo phosphoric acid ester, a halogenated aromatic acid 
anhydride or a halogenated aromatic compound was used. It goes without 
saying that the flame proofing film contains such constituent materials as 
a curing agent, a diluting solvent, etc. required for the constitution of 
the flame proofing film. 
The process for producing the flexible printed circuit board will be 
explained below. A polyester resin film (Diafoil NF manufactured by 
Mitsubishi Diafoil Co., Ltd. in Japan) having a thickness of 75.mu. as a 
substrate 1 is first subjected to surface treatment by corona discharge to 
improve the adhesion between the substrate 1 and the flame proofing film 
5. It has been found that it is preferable that the surface tension of the 
corona discharge-treated polyester surface is 50 dyne/cm or more. In the 
case of the other treatments, for example, ultraviolet ray irradiation 
treatment, however, it has been found that satisfactory adhesion can be 
secured if the surface tension is 36 dyne/cm or more. If the surface 
tension exceeds 60 dyne/cm, it is necessary to place a releasing paper 
between polyester resin films as there is the possibility that blocking 
occurs if one polyester resin film is put on top of another. Therefore, a 
polyester resin film the surface tension of which had been adjusted to 50 
to 60 dyne/cm by corona discharge treatment was used in this example. Said 
flame proofing mixture was then spray-coated onto the surface of the 
substrate 1 at an appointed viscosity, an appointed air pressure and an 
appointed amount exhausted. The resulting coating was allowed to stand at 
room temperature for 5 to 10 minutes to smooth the flame proofing film 5, 
and was then cured by heating in a thermostat under appointed curing 
conditions. A sheet-form hot melt type adhesive 3 (Vyron 30P manufactured 
by Toyo Spinning Co., Ltd. in Japan) having a thickness of 20 to 30.mu. 
was inserted between the substrate 1 thus coated with the flame proofing 
film 5 and a copper foil (an electrolytic copper foil manufactured by 
Fukuda Kinzoku Hakufun Co., Ltd. in Japan) 2 having a thickness of 35.mu. 
for consituting an electric circuit and pressed by an oil pressure press 
to bond the substrate 1 with the copper foil 2 at an elevated temperature. 
Here, the curing conditions were temperature 160.degree. C., pressure 6 
kg/cm.sup.2 and time 5 seconds. The whole was then dripped in a copper 
etching solution consisting mainly of ferric chloride to remove the 
portion of the copper foil other than the portion necessary as an electric 
circuit, washed with water and then dried. 
A flame proofing film 5 was then further formed on the surface of the 
insulating film 4. In the same manner as in the above-mentioned formation 
of the flame proofing film 5 on the substrate 1, the flame proofing film 5 
was formed on an insulating film 4 consisting of a polyester resin film 
(Diafoil NF #50 manufactured by Mitsubishi Diafoil Co., Ltd. in Japan) 
having a thickness of 50.mu.. Further, said adhesive 3 was inserted 
between the insulating film 4 and the copper foil 2, and the whole was 
heated and pressend under the same conditions as those described above to 
bond the insulating film 4 with the copper foil 2. Thus, the flexible 
printed circuit board as shown in FIG. 1 was completed. 
Next, the flexible printed circuit boards obtained in this example will be 
explained with reference to the experimental results as shown in Tables 1 
to 3. As a resin used in the flame proofing mixture constituting the flame 
proofing film 5, an epoxy resin was used in the samples of Table 1, a 
polyurethane resin was used in the samples of Table 2, and a polyester 
resin was used in the samples of Table 3. Also, as a flame retardant, 
tris(2,3-dibromopropyl)phosphate, tetrabromophthalic anhydride and 
tetrabromobisphenol A were selected from halogenated organo phosphoric 
acid esters, halogenated aromatic acid anhydrides and halogenated aromatic 
compounds, respectively. The blending ratios of these flame retardents to 
each of the above-mentioned resins were varied, and flame proofness and 
the adhesion between the flexible film in the substrate 1 or the 
insulating film 4 and the flame proofing film 5 were estimated. As a 
result, a blending ratio satisfying the requirements of flame proofness 
and adhesion was obtained in all combinations of a resin and a flame 
retardent constituting the flame proofing film 5 as shown in Tables 1 to 
3. According to the present inventors' experiments, it was found that 
samples of Experiments Nos. 5, 6, 9, 10, 14 and 15 in Table 1, Experiments 
Nos. 21, 26 and 30 in Table 2, and Experiments Nos. 41, 46 and 51 in Table 
3 showed particularly good flame proofness and adhesion. If only flame 
proofness is required, useful samples are not limited to these ones. The 
spray coating conditions for a flame proofing mixture in the experiments 
in Tables 1 to 3 were the viscosity of the flame proofing mixture 
18.times.2 sec/Ford cup #4, 20.degree. C., the pressure of air 4.0.+-.0.2 
kg/cm.sup.2, an amount exhausted 35.+-.2 mg/min. The thickness of the 
flame proofing film after curing was 5 to 15.mu.. In the experiments in 
Tables 1 to 3, three each of the resins and flame retardants constituting 
the flame proofing film 5 were selected and used. However, it was found 
that a good quality was able to be obtained by replacing the resins and 
flame retardants by the other resins and flame retardants, for example, an 
acrylic resin as a resin and halogenated aliphatic compounds and inorganic 
or organic antimony compounds as a flame retardant or by using a mixture 
of two or more of the resins and/or a mixture of two or more of the flame 
retardants. Further, with regard to the thickness of the fire proofing 
film and coating method, the present invention is not limited to the 
above-mentioned example. Also, even if a fire proofing film is formed on 
only any one of the flexible film of the substrate 1 and the flexible film 
of the insulating film 4, the effect of securing fire proofness can be 
still obtained. Further, the surface treatment of the flexible film of the 
substrate 1 or the flexible film of the insulating film 4 at their surface 
to be coated with a fire proofing film 5 for improving adhesion is not 
limited to corona discharge treatment although corona discharge treatment 
is low in cost. The other methods such as chemical treatment can also 
produce satisfactory effects. 
TABLE 1 
__________________________________________________________________________ 
(Example 1) 
Fire proofness, combus- 
Flame retardant 
tion test according to 
Amount 
MVSS No. 302 Adhesion 
Experi- blended Burning 
peeling test 
ment (parts 
Burning rate 
distance 
by adhesive 
No. Resin Name by wt.) 
(cm/min) 
(mm) tape 
__________________________________________________________________________ 
1 -- -- 17-20 Continued 
Good 
to burn 
2 40 5-10 5-10 Good 
3 Epoxy resin 
Tris(2,3- 
80 0.5-1 5-7 Good 
100 parts 
dibromopropyl) 
Immediately 
&lt;5 Good 
4 by weight 
phosphate 
120 fire died out 
5 Curing 160 Immediately 
" Good 
agent fire died out 
6 3 parts 200 Immediately 
" Slightly bad 
by weight fire died out 
7 40 3-5 5-10 Good 
8 80 0.5-0.7 
5-8 Good 
Tetra- 
9 bromophthalic 
120 Immediately 
&lt;5 Good 
anhydride fire died out 
10 160 Immediately 
" Good 
fire died out 
11 200 Immediately 
" Slightly bad 
fire died out 
12 40 8-10 12-15 Good 
13 80 1-2 5-10 Good 
Tetrabromo- Immediately 
&lt;5 Good 
14 bisphenol A 
120 fire died out 
15 160 Immediately 
" Good 
fire died out 
16 200 Immediately 
" Slightly bad 
fire died out 
__________________________________________________________________________ 
(Notes) 
Epoxy resin: Epikote 828 20 parts by weight (Manufactured by Shell 
Chemical Co.) Epikote 871 80 parts by weight (Manufactured by Shell 
Chemical Co.) 
Curing agent: EMI (Manufactured by Shikoku Kasei K.K. in Japan) 
Curing conditions: 180.degree. C., 15 minutes 
TABLE 2 
__________________________________________________________________________ 
Fire proofness, combus- 
Flame retardant 
tion test according to 
Amount 
MVSS No. 302 Adhesion, 
Experi- blended Burning 
peeling test 
ment (parts 
Burning rate 
distance 
by adhesive 
No. Resin Name by wt.) 
(cm/min.) 
(mm) tape 
__________________________________________________________________________ 
17 -- -- 20-25 Continued 
Good 
to burn 
18 40 8-15 10-13 Good 
19 Tris(2,3- 
80 5-10 5-9 Good 
20 Polyure- 
dibromo- 
120 0-7 &lt;5 Good 
thane resin 
propyl) 
21 100 parts 
phosphate 
160 Immediately 
" Good 
by weight fire died out 
22 200 Immediately 
" Slightly bad 
fire died out 
Curing 
23 agent 40 5-10 8-15 Good 
24 25 parts 80 3-6 5-11 Good 
by weight 
Tetrabromo- 
25 phthalic 
120 0.5-2 5-7 Good 
anhydride Immediately 
&lt;5 Good 
26 160 fire died out 
27 200 Immediately 
" Slightly bad 
fire died out 
28 40 8-10 5-10 Good 
29 80 0-5 &lt;5 Good 
Tetrabromo- 
30 bisphenol A 
120 Immediately 
" Good 
fire died out 
31 160 Immediately 
" Slightly bad 
fire died out 
32 200 Immediately 
" " 
fire died out 
__________________________________________________________________________ 
(Notes) 
Polyurethane resin: Retan No. 3000 (Manufactured by Kansai Paint Co., Ltd 
in Japan) 
Curing agent: Curing agent for Retan No. 3000 (Manufactured by Kansai 
Paint Co., Ltd. in Japan) 
Curing conditions: 80.degree. C., 15 minutes 
TABLE 3 
__________________________________________________________________________ 
Fire proofness, combus- 
Flame retardant 
tion test according to 
Amount 
MVSS No. 302 Adhesion, 
Experi- blended Burning 
peeling test 
ment (parts 
Burning rate 
distance 
by adhesive 
No. Resin Name by wt.) 
(cm/min) 
(mm) tape 
__________________________________________________________________________ 
33 -- -- 18-22 Continued 
Good 
to burn 
34 40 10-15 15-19 Good 
35 80 5-15 10-15 Good 
Polyester 
Tris(2,3- 
36 resin dibromo- 
120 1-5 &lt;5 Good 
100 parts 
propyl) Immediately 
" Good 
37 by weight 
phosphate 
160 fire died out 
38 Reaction 200 Immediately 
" Slightly bad 
accelerator fire died out 
39 1 part 40 8-15 10-15 Good 
40 by weight 80 5-10 5-10 Good 
Tetrabromo- 
41 phthalic 
120 2-3 5-8 Good 
anhydride 
42 160 Immediately 
&lt;5 Good 
fire died out 
43 200 Immediately 
" Slightly bad 
fire died out 
44 40 6-18 15-20 Good 
45 Tetrabromo- 
80 6-10 5-8 Good 
46 bisphenol A 
120 2-4 5-6 Good 
47 160 Immediately 
fire died out 
&lt;5 Good 
48 200 Immediately 
" Slightly bad 
fire died out 
__________________________________________________________________________ 
(Notes) 
Polyester resin: Nitoron V280 (manufactured by Nitto Denki Kogyo K.K. in 
Japan) 
Reaction accelerator: Catalyst No. 6 for Nitron V 280 (manufactured by 
Nitto Denki Kogyo K.K. in Japan) 
Curing conditions: 130.degree. C. 6 minutes 
EXAMPLE 2 
In this example, a fire proofing film 5 was formed on a substrate 1 of a 
similar flexible printed circuit board to that of Example 1 as shown in 
FIG. 1. However, the insulating film 4 in Example 1 was replaced by a 
flame retardant-containing ink film 6 as shown in FIG. 2. 
As the resin constituting the fire proofing ink film 6 used in this 
example, an ultraviolet ray-setting type epoxy resin was used to shorten a 
curing time remarkably and reduce the cost. As the flame retardant a blend 
of, a halogenated organic phosphoric acid ester and a halogenated aromatic 
compound was used. Also, it is needless to say that the constituent 
materials required for the constitution of the fire proofing ink such as a 
sensitizer for effecting ultraviolet ray cure, a pigment for forming a 
colored coating film are contained. 
The process for producing the fire-proof flexible printed circuit board 
will be explained below. The steps of forming a fire proofing film 5 on a 
substrate 1 in FIG. 3, fastening a copper foil 2 thereonto with an 
adhesive 3, and then carrying out etching treatment are the same as the 
steps in Example 1. Therefore, the explanation of these steps is omitted. 
On the thus formed circuit board, an ultraviolet ray-setting type ink 
containing said flame retardent having an adjusted viscosity of 2600 to 
3700 cps/25.degree. C. was printed through a screen made of a 225 mesh 
Tetoron (a polyester fiber) plate, allowed to stand at room temperature 
for 2 to 3 minutes to smooth the printed surface, and then cured by the 
use of an ultraviolet ray irradiation apparatus to form a film 6. Curing 
conditions were supplied by a mercury-vapor lamp of 80 W/cm.times.2, an 
irradiation distance of 5 cm and a conveyor speed of 3 m/min. The 
thickness of the film 7 after curing was 10 to 30.mu.. 
Next, this example will be explained below in reference to the experiment 
results as shown in Table 4. The experiment results in Table 4 show fire 
proofness and adhesion between the copper foil 2 or the substrate 1 and 
the flame proofing ink film 6. In this example, as the resin for the 
ultraviolet ray-setting type ink, an epoxy acrylate resin was used. As the 
flame retardant, tris(2,3-dibromopropyl) phosphate and tetrabromobenzene 
were selected from halogenated organic phosphoric acid esters and 
halogenated aromatic compounds, respectively. A blending ratio of these 
flame retardants to the respective resins for the ink was varied and the 
characteristics of the resulting fire-proof flexible printed circuit 
boards were evaluated. 
As a result, as shown in Table 4, a blending ratio satisfying fire 
proofness and adhesion was able to be obtained in all the flame 
retardants. Thus, in the present inventors' experiments, the samples of 
Experiments Nos. 50, 54 and 55 in Table 4 showed particularly good fire 
proofness and adhesion. Also, the sample of Experiment No. 49 wherein no 
flame retardant had been added to the ink film showed barely satisfactory 
fire proofness since the fire proofing film 5 had been formed on the 
surface of the substrate 1. It was found that, even if no fire proofing 
film 5 was formed on the surface of the substrate 1 on the contrary, 
barely satisfactory fire proofness was able to be ensured by adding a 
flame retardant to the ink film. 
In this example, an ultraviolet ray-setting type ink was used as the fire 
proofing ink, but the fire proofing inks used are not limited to this 
type. Generally hot setting type or a cold setting type may also be used. 
As the flame retardant, halogenated aliphatic compounds, halogenated 
aromatic acid anhydrides and inorganic and organic antimony compounds may 
also be used. Also, these resins and flame retardants may respectively be 
used in admixture of two or more thereof. The thickness of a coating film 
and whether or not pretreatment is carried out prior to the coating step 
are not limited to those as shown in this example. 
TABLE 4 
__________________________________________________________________________ 
Fire proofness, combus- 
Flame retardant 
tion test according to 
Amount 
MVSS No. 302 
Adhesion, 
Experi- 
Resin for blended Burning 
peeling test 
ment 
printing (parts 
Burning rate 
distance 
by adhesive 
No. ink Name by wt.) 
(cm/min) 
(mm) tape 
__________________________________________________________________________ 
49 -- 0 10-15 5-10 
Very good 
Immediately 
50 10 fire died out 
&lt;5 Very good 
Tris(2,3- 
51 dibromopropyl) 
20 Immediately 
" Good 
Epoxy phosphate fire died out 
52 resin 30 Immediately 
" Good 
100 parts 40 fire died out 
53 by weight Immediately 
" Good 
fire died out 
54 10 Immediately 
&lt;5 Very Good 
fire died out 
Tetrabromo- 
55 benzene 20 Immediately 
" Very good 
fire died out 
56 30 Immediately 
" Good 
fire died out 
57 40 Immediately 
" Good 
fire died out 
__________________________________________________________________________ 
(Notes) 
Resin for printing ink: UVR 40010 (manufactured by Asahi Kagaku Kenkyusho 
in Japan) 
The same fire proofing film 5 as that in the sample of Experiment No. 10 
in Example 1 was formed on the surface of the substrate 1. It was found 
that almost similar fire proofness to that of the sample of Experiment No 
49 was able to be obtained even in the samples of Experiments Nos. 50 to 
57 wherein the fire proofing film 5 was not formed on the surface of the 
substrate 1. 
EXAMPLE 3 
In said Example 2, etching treatment was carried out to remove the part of 
a copper foil other than that required as an electric circuit, but the 
etching treatment was replaced by die stamping treatment in this example. 
The composition of the fire proofing ink, the printing method and printing 
conditions in this example were the same as those in said Example 2, and 
therefore the explanation of them is omitted. As for the production 
process used, a substrate 1 consisting of a polyester film was subjected 
to surface treatment in the same manner as in said Example 2, and a fire 
proofing film 5 was then spray-coated and cured by heating. An adhesive 3 
was inserted between the substrate 1 and a copper foil 2. The steps up to 
this time were the same as those in Example 1. The copper foil 2 
constituting an electric circuit was cut by a stamping die previously 
heated at 80.degree. C. at a surface pressure of 500 kg/cm.sup.2 in one 
second and the unnecessary part of the copper foil and the adhesive was 
then removed to obtain the desired circuit. Further, the copper foil 2 was 
pressure-bonded with the substrate 1 under heating to fix the copper foil 
2 constituting an electric circuit. The bonding conditions in this case 
were the same as those in Example 1, that is, temperature 160.degree. C., 
pressure 6 kg/cm.sup.2, and time 5 seconds. Thereafter, the surface 
tension of the substrate 1 was adjusted to 36 dyne/cm or more, and 
particularly 44 to 48 dyne/cm by ultraviolet ray irradiation treatment in 
order to improve adhesion again. On the thus formed board, a fire proofing 
ink having the same constitution as that of the fire proofing ink used in 
said Example 2 was printed and cured in the same manner and under the same 
conditions as in Example 2 to form a fire proofing ink film 6. 
The experimental results of the fire-proof flexible printed circuit board 
obtained in this example were similar to the results of Table 4 in said 
Example 2. 
EXAMPLE 4 
This example is shown in FIG. 4. In this example, the fire proofing 
treatment of an adhesive 7 for bonding a substrate 1 and a copper foil 2 
or an insulating film 4 was carried out in place of the formation of a 
fire proofing film 5 on the surface of an insulating film 4 in Example 1. 
As the adhesive resin constituting the fire-proof adhesive 7 used in this 
example, a polyester resin was used. As the flame retardant, a halogenated 
organic phosphoric acid ester or a halogenated aromatic compound was used. 
It is needless to say that a plasticizer, a diluting solvent, etc. 
required for the constitution of the adhesive 7 are contained in the 
adhesive. The production process will be explained below. The steps up to 
the formation of a fire proofing film 5 on a substrate 1 in FIG. 4 are all 
the same as those in Example 1. Therefore, the explanation thereof is 
omitted. In order to bond the thus formed board with a copper foil 2, said 
adhesive 7 which had been adjusted to a viscosity of 1500 to 2000 
cps/25.degree. C. with a mixed solvent was coated onto the polyester film 
surface of the substrate 1 to be bonded with the copper foil 2 by roll 
coating. The resulting coating was then dried at 100.degree. C. in a 
thermostat for 5 to 10 minutes to vaporize the solvent. The thickness of 
the adhesive film was 20 to 30.mu. in this case. The substrate 1 was then 
bonded with the copper foil 2 constituting an electric circuit by an oil 
pressure press. The bonding conditions in this case were temperature 
120.degree. C., pressure 2 kg/cm.sup.2 and time 5 seconds. The unnecessary 
portion of the copper foil was removed by dipping in a copper etching 
solution consisting mainly of ferric chloride and the remainder was then 
washed with water and dried in the same manner as in Example 1. Further, 
the adhesive 7 was coated onto an insulating film 4 by roll coating in the 
same manner as in said substrate 1. The solvent was vaporized and the 
insulating film 4 thus coated with the fire-proof adhesive 7 was then 
coated onto the copper foil 2 on the substrate 1 and contact bonding was 
carried out in the same manner as described above. 
Then, this example will be explained below in detail with reference to the 
experimental results in Table 5. Fire proofness and adhesion between the 
copper foil 2 constituting an electric circuit and the substrate 1 or the 
insulating film 4 (that is, the adhesive force of the adhesive 7) are 
shown in Table 5. In this example, a polyester resin was used as the 
adhesive resin, and as the flame retardant, tris(2,3-dibromopropyl) 
phosphate and hexabromobenzene were selected from halogenated organic 
phosphoric acid esters and halogenated aromatic compounds, respectively. A 
blending ratio of these flame retardants to the adhesive was varied and 
fire proofness and adhesion were evaluated. As a result, a blending ratio 
satisfying both fire proofness and adhesion was able to be obtained in the 
case of all the flame retardants. Thus, it was found by the present 
inventors' experiments that the samples of Experiments Nos. 59, 63 and 64 
in Table 5 showed particularly good fire proofness and adhesion. Also, the 
sample of Experiment No. 58 wherein the adhesive 7 contained no flame 
retardant showed barely satisfactory fire proofness since the fire 
proofing film 5 was formed on the surface of the substrate 1. It was found 
that, on the contrary, barely satisfactory fire proofness was also able to 
be ensured by adding a flame retardant to the adhesive 7 even if the fire 
proofing film 5 was not formed on the surface of the substrate 1. 
Also, the adhesive 7 to be fire proofed in this example is not limited to 
polyester resins but the other resins such as polyurethane resins may be 
used. The flame retardant used in this example may also be a halogenated 
aliphatic compound, a halogenated aromatic anhydride or an inorganic or 
organic antimony compound. Further, these resins and flame retardants may 
respectively be used in admixture of two or more thereof. 
TABLE 5 
__________________________________________________________________________ 
Fire proofness, combus- 
Flame retardant 
tion test according to 
Amount 
MVSS No. 302 
Experi- blended Burning 
Adhesion, 
ment 
Adhesive (parts 
Burning rate 
distance 
180.degree. T- 
No. resin Name by wt.) 
(cm/min) 
(mm) peeling test 
__________________________________________________________________________ 
58 -- -- 10-15 5-10 Very good 
5 Immediately 
&lt;5 Very good 
59 fire died out 
Tris(2,3- 
60 dibromo 
10 Immediately 
" Good 
Polyester 
propyl) fire died out 
61 resin phosphate 
15 Immediately 
" Passable 
100 parts fire died out 
62 by weight 20 Immediately 
" Passable 
fire died out 
63 5 Immediately 
&lt;5 Very good 
fire died out 
Hexabromo- 
64 benzene fire died out 
" Good 
65 15 Immediately 
" Good 
fire died out 
66 20 Immediately 
" Passable 
fire died out 
__________________________________________________________________________ 
(Notes) 
Adhesive resin: Starfix SSO3 (manufactured by Fuji Photo Film Co., Ltd. i 
Japan) 
The same fire proofing film 5 as that used in the sample of Experiment No 
10 in Table 1 in Example 1 was formed on the surface of the substrate 1 i 
FIG. 4. However, it was found that even the samples of Experiments Nos. 5 
to 66 wherein the fire proofing film 5 was not formed on the surface of 
the substrate 1 gave almost similar fire proofness to that of the sample 
as shown in Experiment No. 58. 
EXAMPLE 5 
This example as shown in FIG. 5 is the same as said Example 4 in that a 
flame retardant is added to an adhesive 7, but is different from the 
Example 4 in that an ink film 8 is formed in place of an insulating film 
4. 
As the adhesive resin constituting the fire-proof adhesive 7 used in this 
example, a polyester resin was used as in the said Example 4. As the flame 
retardant, a halogenated organic phosphoric acid ester or a halogenated 
aromatic compound was used. Also, it is needless to say that a 
plasticizer, a diluting solvent, etc. which are required for the 
constitution of the adhesive 7 are contained in this example. As the resin 
constituting the ink, an ultraviolet ray-setting type epoxy resin (UVR 
4001G manufactured by Asahi Kagaku Kenkyusho in Japan) was used to shorten 
curing time greatly and try to reduce the cost as in the ink used in the 
said Example 2. Also, it is needless to say that the constituent materials 
such as a sensitizer for effecting ultraviolet-ray cure required for the 
constitution of the ink 6, a pigment for obtaining a colored coating film, 
etc. are contained in the ink. 
The production process will be explained below. Here, the steps up to the 
formation of a fire proofing film 5 on a substrate 1 are all the same as 
in Example 1. Also, the steps of subjecting a copper foil 2 to contact 
bonding with a fire-proof adhesive 7 under heating and removing the 
portion other than that required as an electric circuit are all the same 
as in Example 4. Therefore, the explanation of the steps up to this time 
is omitted. On the thus formed circuit board, said ultraviolet ray-setting 
type ink which had been adjusted to a viscosity of 2000 to 2500 
cps/25.degree. C. was printed through a screen made of a 250 mesh Tetoron 
(a polyester fiber), allowed to stand at room temperature for 2 to 3 
minutes to smooth the printed surface, and then cured by an ultraviolet 
ray irradiation apparatus. The curing conditions were supplied by a 
mercury-vapor lamp 80 W/cm.times.2, the irradiation distance 20 cm and the 
irradiation time 8 seconds. The thickness of the film after cure was 10 to 
30.mu.. 
The experimental results obtained in this example was almost similar to 
those as shown in Table 5 in Example 4. The detailed explanation thereof 
is omitted here since it is almost the same as the explanation in Example 
4. 
As described above, the fire-proof flexible printed circuit board according 
to the present invention can be obtained by subjecting a resin flexible 
film to surface treatment and then forming a fire proofing film thereon. 
Even when the flexible printed circuit board is used in a bent state in an 
automobile, etc., the fire proofing film is not peeled off from the 
flexible film and the fire proofing of the flexible printed circuit board 
as the desired object can be accomplished. Therefore, it is expected that 
the fire-proof flexible printed circuit board according to the present 
invention has various uses.