Adhesive composition and coverlay film therewith

A coverlay film, which is used for protection of a flexible printed circuit board, is provided by the invention as being imparted with improved adhesiveness, heat-resistance against molten solder alloy, punching workability and other properties. The inventive coverlay film is characterized by the novel and unique formulation of the adhesive to form an adhesive layer on one surface of a substrate plastic, e.g., polyimide, film. The adhesive is formulated with an epoxy resin, carboxylated nitrile rubber, curing agent for the epoxy resin, curing accelerator and fine inorganic powder each of a specified type and in a specified weight proportion.

BACKGROUND OF THE INVENTION 
The present invention relates to a novel adhesive composition and a 
coverlay film prepared therewith for protecting a flexible printed circuit 
board having excellent adhesiveness and heat resistance to withstand 
soldering. A coverlay film consists of a heat resistant plastic film or 
sheet as a substrate and a layer of an adhesive in a semi-cured or 
solvent-free state on one surface of the substrate. 
Along with the trend in recent years in the electronic instruments toward 
further and further decreased weight, thickness and size still with more 
and more increased and upgraded performance, the demand for flexible 
printed circuit boards is rapidly growing with expansion of the 
application fields in which they are used. The growing use of flexible 
printed circuit boards is necessarily accompanied by more and more 
frequent use of coverlay films for protection of flexible printed circuit 
boards and performance thereof is eagerly desired to be improved to comply 
with any high-grade applications. Particular problems of recent frequent 
issues relative to the coverlay films include the adhesiveness to flexible 
printed circuit boards, high heat resistance to withstand soldering, 
electric insulation with low moisture absorption, flexibility, 
weatherability under adverse ambient conditions and the like. Namely, it 
is desired that a coverlay film should be imparted simultaneously with 
improved characteristics in all of these properties. 
As is mentioned above, a coverlay film is a laminate consisting of a heat 
resistant plastic film or sheet and a layer of an adhesive in a semi-cured 
or solvent-free state usually covered with a sheet of surface-release 
paper or plastic film for temporary protection from inadvertent sticking. 
Various kinds of adhesives are conventionally used in the prior art for 
the purpose including blends of NBR and a phenolic resin, NBR and an 
epoxy-containing phenolic resin, NBR and an epoxy resin, an epoxy resin 
and a polyester resin and an epoxy resin and acrylic resin as well as 
acrylic resins as such. These conventional adhesives have their own 
advantages and disadvantages and none of them can satisfy all of the 
requirements simultaneously. For example, the adhesives compounded with an 
NBR are generally poor in the thermal stability and their thermal 
degradation is accelerated by compounding with an inorganic flame 
retardant. Epoxy resin-based adhesives in general exhibit relatively low 
peeling resistance while brominated epoxy resin-based ones as well as 
polyester-based and acrylic adhesives are poor in the heat resistance. 
Furthermore, the coverlay films prepared by using the above described 
conventional adhesives are disadvantageous in respect of the low 
workability because the layer of the adhesive can be cured at a high 
temperature of 170.degree. to 180.degree. C. taking as long as 30 to 60 
minutes after the coverlay film freed from the sheet of release paper is 
applied to the surface of a flexible circuit board. 
SUMMARY OF THE INVENTION 
The present invention accordingly has an object to provide a novel and 
improved adhesive composition and a coverlay film for protection of a 
flexible circuit board having improved performance in respect of the 
adhesiveness, heat resistance against soldering, especially, even in a 
moistened state, punching workability and curability by overcoming the 
above described disadvantages in the prior art coverlay films. 
Thus, the adhesive composition of the invention is a uniform blend which 
comprises: 
(a) 100 parts by weight of an epoxy resin; 
(b) from 40 to 150 parts by weight of a nitrile rubber containing carboxyl 
groups; 
(c) from 1 to 50 parts by weight of a curing agent; 
(d) from 0.1 to 5 parts by weight of a curing accelerator selected from the 
group consisting of imidazole compounds, tertiary amine tetraphenyl 
borates, zinc borofluoride, tin borofluoride and nickel borofluoride; and 
(e) from 5 to 70 parts by weight of a finely divided inorganic powder 
selected from the group consisting of metal hydroxides, inorganic oxides, 
metal carbonates, metal silicates and boron nitride. 
The coverlay film of the present invention for the protection of a flexible 
circuit board accordingly comprises: 
(A) a heat-resistant plastic film as a substrate; and 
(B) a layer of an adhesive composition in a semi-cured or solvent-free 
state on one surface of the heat-resistant plastic film, the adhesive 
composition comprising, as a blend: 
(a) 100 parts by weight of an epoxy resin; 
(b) from 40 to 150 parts by weight of a nitrile rubber containing carboxyl 
groups; 
(c) from 1 to 50 parts by weight of a curing agent; 
(d) from 0.1 to 5 parts by weight of a curing accelerator selected from the 
group consisting of imidazole compounds, tertiary amine tetraphenyl 
borates, zinc borofluoride, tin borofluoride and nickel borofluoride; and 
(e) from 5 to 70 parts by weight of a finely divided inorganic powder 
selected from the group consisting of metal hydroxides, inorganic oxides, 
metal carbonates, metal silicates and boron nitride.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
As the heat-resistant plastic film as the substrate in the inventive 
coverlay film which is coated on one surface with the inventive adhesive 
composition, films of various plastics can be used without particular 
limitations including polyimides, polyphenylene sulfides, poly(parabanic 
acids), heat-resistant polyesters, poly(ether sulfones), polylether ether 
ketones) and the like, of which polyimides are preferred. The films should 
have a thickness in the range from 0.010 to 0.20 mm or, preferably, from 
0.013 to 0.125 mm. The plastic films should have heat resistance not to 
cause substantial degradation by heating in the course of the soldering 
works using molten solder alloys at a temperature of 200.degree. C. or 
higher. 
The most characteristic feature of the inventive coverlay film consists in 
the unique formulation of the adhesive composition for forming a layer on 
one surface of the above mentioned heat-resistant plastic film. The 
adhesive composition is formulated with five kinds of essential 
ingredients, of which the epoxy resins as the component (a) can be any one 
of known epoxy resins having at least two epoxy groups per molecule 
without particular limitations including the glycidyl ether type ones such 
as bisphenol A type epoxy resins, novolac resins and the like, alicyclic 
epoxy resins, aromatic epoxy resins, halogenated epoxy resins and the 
like. These epoxy resins can be used either singly or as a combination of 
two kinds or more according to need. Among the above named various types 
of epoxy resins, halogenated epoxy resins are preferable and, in 
particular, brominated epoxy resins are more preferable in respect of the 
excellent flame retardancy of the adhesive composition compounded 
therewith. 
The above mentioned brominated epoxy resin is not particularly limitative 
provided that the molecule contains epoxy groups and bromine atoms 
including bisphenol-type and novolactype ones. Several commercial products 
of brominated epoxy resins are available including Epikotes 5045 
containing 19% by weight of bromine, 5046 containing 21% by weight of 
bromine, 5048 containing 25% by weight of bromine, 5049 containing 26% by 
weight of bromine and 5050 containing 49% by weight of bromine (each a 
product by Yuka Shell Epoxy Co.) and BREN-S containing 35% by weight of 
bromine (a product by Nippon Kayaku Co.). These brominated epoxy resins 
can be used either singly or as a combination of two kinds or more 
according to need. Preferably, the bromine content of the brominated epoxy 
resin should be in the range from 21 to 51% by weight. 
The nitrile rubber having carboxyl groups as the component (b) is a 
copolymeric rubber of acrylonitrile and butadiene carboxylated at the 
molecular chain ends. Commercial products of nitrile rubber suitable for 
the purpose include Hycars CTBN and CTBNX each produced by Goodrich Co., 
Nipols 1072J, 1072B, DN 612, DN 631 and DN 601 each produced by Nippon 
Zeon Co. and the like. These carboxyl-containing nitrile rubbers can be 
used either singly or as a combination of two kinds or more according to 
need. Preferably, the carboxyl-containing nitrile rubber should contain 
from 2 to 8% by weight of the carboxyl groups. 
The amount of the carboxyl-containing nitrile rubber as the component (b) 
in the adhesive composition is in the range from 40 to 150 parts by weight 
or, preferably, from 40 to 100 parts by weight per 100 parts by weight of 
the epoxy resin as the component (a). When the amount thereof is too 
small, the adhesive layer would have an unduly low peeling resistance 
while, when the amount thereof is too large, the adhesive would be 
thermally unstable to cause degradation at high temperatures. 
The curing agent as the component (c) is a curing agent for the epoxy resin 
as the component (a). Various kinds of known epoxy-curing agents can be 
used without particular limitations including amine compounds such as 
diethylene triamine, triethylene tetramine, m-xylylene diamine, 
4,4'-diaminodiphenyl methane, 4,4'-diaminodiphenyl sulfone and the like, 
acid anhydrides such as phthalic anhydride, hexahydrophthalic anhydride, 
tetrahydrophthalic anhydride, trimellitic anhydride and the like, 
dicyandiamide, boron trifluoride-amine complex compounds and the like. 
These epoxy-curing agents can be used either singly or as a combination of 
two kinds or more according to need. 
The amount of the above named epoxy-curing agent as the component (c) in 
the adhesive composition is in the range from 1 to 50 parts by weight or, 
preferably, from 3 to 30 parts by weight per 100 parts by weight of the 
epoxy resin as the component (a) although the exact amount thereof should 
be selected depending on the types of the epoxy resin and the curing 
agent. When the amount thereof is too small, the adhesive cannot be fully 
cured by heating so that the heat resistance of the coverlay film would be 
decreased. When the amount thereof is too large, on the other hand, the 
curing reaction of the adhesive may proceed even at room temperature so 
that the storability of the coverlay film with an adhesive layer would be 
decreased in addition to the disadvantages of the decreased heat 
resistance against soldering, decrease in the peeling resistance and 
decrease in the flowability of the adhesive composition. 
The curing accelerator as the component (d) in the adhesive composition 
serves to promote the curing reaction of the epoxy resin with the curing 
agent. Various compounds can be used for the purpose including imidazole 
compounds such as 2-alkyl-4-methyl imidazoles, 2-alkyl-4-ethyl imidazoles, 
1-(2-cyanoethyl)-2-alkyl imidazoles, 2-phenyl imidazole and the like, 
tertiary amine tetraphenyl borates, such as triethyl ammonium tetraphenyl 
borate of the formula [(C.sub.2 H.sub.5).sub.3 NH][B(C.sub.6 
H.sub.5).sub.4 ] and the like, zinc borofluoride, tin borofluoride, nickel 
borofluoride and the like. These compounds can be used either singly or as 
a combination of two kinds or more according to need. Among the above 
compounds, tertiary amine tetraphenyl borates and the borofluorides of 
zinc, tin or nickel are preferable and the borofluorides of zinc, tin and 
nickel are more preferable. 
The amount of the curing accelerator as the component (d) in the adhesive 
composition is in the range from 0.1 to 5 parts by weight or, preferably, 
from 0.2 to 3 parts by weight per 100 parts by weight of the epoxy resin 
as the component (a). When the amount thereof is too small, curing of the 
adhesive takes an unduly long time and the workability of the coverlay 
film is poor with too high flowability of the adhesive and poor heat 
resistance against soldering. When the amount thereof is too large, on the 
other hand, the coverlay film would have decreased storability due to 
curing of the adhesive composition even at room temperature along with a 
decrease in the workability due to the poor flowability of the adhesive. 
The finely divided inorganic powder as the component (e) in the adhesive 
composition serves to impart the coverlay film having the adhesive layer 
with good punching workability along with improved heat resistance 
imparted to the adhesive. Suitable inorganic powders include metal 
hydroxides such as aluminum hydroxide, magnesium hydroxide and the like, 
inorganic oxides such as zinc oxide, magnesium oxide, antimony trioxide, 
silicon dioxide and the like, metal carbonates such as calcium carbonate, 
magnesium carbonate and the like, metal silicates such as aluminum 
silicate, magnesium silicate, calcium silicate and the like, boron nitride 
and so on. These inorganic powders can be used either singly or as a 
combination of two kinds or more according to need. The particles of the 
inorganic powder should have a particle diameter not exceeding 10 .mu.m 
or, preferably, not exceeding 5 .mu.m in order to comply with the recent 
trend in the flexible printed circuit boards toward finer and finer 
patterning of the circuit down to a few tens of micrometers of fineness or 
even finer. 
It is sometimes advantageous that the finely divided inorganic powder as 
the component (e) is subjected to a hydrophobilizing treatment prior to 
compounding with the other ingredients with an object to improve the 
compatibility of the particles with the resinous matrix and to decrease 
the moisture absorptivity. Examples of the agents used in the 
hydrophobilizing treatment of the particle surface include silicone 
fluids, organochlorosilanes such as dimethyl dichlorosilane and the like, 
silane coupling agents such as alkyl triethoxy silanes, methyl triethoxy 
silane and the like, and so on. 
The amount of the finely divided inorganic powder as the component (e) in 
the adhesive composition should be in the range from 5 to 70 parts by 
weight or, preferably, from 10 to 50 parts by weight per 100 parts by 
weight of the epoxy resin as the component (a). When the amount thereof is 
too small, the coverlay film would have poor heat resistance and punching 
workability. When the amount thereof is too large, on the other hand, the 
peeling resistance of the adhesive layer from a flexible printed circuit 
board would be decreased. 
The adhesive composition to be applied to the surface of the heat-resistant 
plastic film as the substrate can be prepared by uniformly blending the 
above described components (a) to (e) with other optional ingredients 
conventionally used in adhesive compositions including organic solvents, 
exemplified by methyl ethyl ketone, dioxane, dimethyl formamide, toluene, 
tetrahydrofuran and the like, to control the applicability of the 
composition in coating. The thus prepared adhesive composition is applied 
to the surface of the heat-resistant plastic film by using a suitable 
coating machine. The amount of coating should preferably be such that the 
adhesive layer after drying of the solvent may have a thickness in the 
range from 10 to 60 .mu.m. The adhesive layer is then brought into a 
solvent-free state, if necessary, by heating, for example, at a 
temperature of 80.degree. to 150.degree. C. for 1 to 30 minutes. 
The coverlay film consisting of a heat-resistant plastic film and an 
adhesive layer formed thereon is usually overlaid with a sheet of release 
paper or film on the surface of the adhesive layer for temporary 
protection by using a roller laminator and the like and wound up in a roll 
to facilitate handling and transportation. Examples of suitable release 
paper or film include films of polyethylene, polypropylene, TPX resin and 
the like, polyester films laminated with a silicone-based surface release 
agent, sheets of paper coated with a film of a polyolefin, e.g., 
polyethylene and polypropylene, polyvinylidene chloride and the like, and 
so on though not particularly limitative thereto. 
In the following, examples and comparative examples are given to illustrate 
the inventive coverlay films in more detail but not to limit the scope of 
the invention in any way. In the following description, the terms of 
"parts" and "%" always refer to "parts by weight" and "% by weight", 
respectively, relative to the solid matter. 
EXAMPLES AND COMATIVE EXAMPLES 
In each of the 22 experiments, of which Experiments No. 1 to No. 12 were 
for the invention and No. 13 to No. 22 were for comparative purpose, the 
ingredients shown in Table 1 each taken in an amount shown in the table in 
parts were uniformly dissolved or dispersed in methyl ethyl ketone by 
using a ball mill to give an adhesive composition containing 30% of the 
solid matter. The ingredients indicated in the table with abridgements are 
characterized as follows. 
Epoxy resins 
A: Epikote 828, bisphenol A-type, epoxy equivalent 190 g/equivalent, a 
product by Yuka Shell Epoxy Co. 
B: Epikote 1001, bisphenol A-type, epoxy equivalent 475 g/equivalent, a 
product by Yuka Shell Epoxy Co. 
C: Epikote 5045, brominated epoxy resin, epoxy equivalent 475 g/equivalent, 
bromine content 19%, a product by Yuka Shell Epoxy Co. 
D: Epikote 5048, brominated epoxy resin, epoxy equivalent 673 g/equivalent, 
bromine content 25%, a product by Yuka Shell Epoxy Co. 
E: Epikote 5050, brominated epoxy resin, epoxy equivalent 390 g/equivalent, 
bromine content 49%, a product by Yuka Shell Epoxy Co. 
Carboxylated nitrile rubbers 
A: Nipol 1072J, a product by Nippon Zeon Co. containing sodium ions and 6 
to 7% of carboxyl groups 
B: Nipol 1072, a product by Nippon Zeon Co. containing calcium ions and 6 
to 7% of carboxyl groups 
Curing agent 
4,4'-Diaminodiphenylsulfone 
Curing accelerators 
A: 1-(2-Cyanoethyl)-2-ethyl-4-methyl imidazole 
B: Triethyl ammonium tetraphenyl borate 
C: Zinc borofluoride 
D: Tin borofluoride 
Inorganic powders 
A: Antimony trioxide, having an average particle diameter of 0.02 .mu.m 
B: Silica, having an average particle diameter of 0.02 .mu.m 
C: Aluminum hydroxide, having an average particle diameter of 1 .mu.m 
D: Zinc oxide, having an average particle diameter of 0.3 .mu.m 
A film of polyimide resin (Kapton 100H, a product by Du Pont Co.) having a 
thickness of 25 .mu.m was coated on one surface with the above prepared 
adhesive composition in such a coating amount that the adhesive layer 
after evaporation of the solvent had a thickness of 30 .mu.m and then 
heated in an oven first at 80.degree. C. for 2 minutes and then at 
120.degree. C. for 5 minutes so as to completely evaporate the solvent and 
bring the adhesive composition into the so-called B-stage. The thus 
adhesive-coated surface of the polyimide film was overlaid with a sheet of 
silicone-coated release paper and laminated therewith by press-bonding in 
a roller laminator at 50.degree. C. under a linear pressure of 5 kg/cm at 
a line velocity of 2 meters/minute to prepare a release paper-overlaid 
coverlay film. 
The coverlay film freed from the release paper was laminated on the 
adhesive layer with a 35 .mu.m thick electrolytic copper foil on the 
glossy surface by pressing at 160.degree. C. for 10 minutes under a 
pressure of 50 kg/cm.sup.2 to prepare a laminate which was subjected to 
the evaluation tests to give the results shown in Table 2 according to the 
testing procedures given below. 
1. Peeling resistance 
According to the procedure specified in JIS C 6481, the specimen cut in a 
width of 10 mm was subjected to peeling of the copper foil at room 
temperature in a 90.degree. direction at a pulling velocity of 50 
mm/minute. 
2. Heat resistance against molten solder alloy 
A piece of the specimen was laid on the surface of a bath of molten solder 
alloy with the copper foil facing down at varied temperatures for 30 
seconds and the highest temperature, at which no blistering or other 
defects could be detected in the specimen, was recorded. The sample 
specimens were tested either after seasoning for 24 hours in an atmosphere 
of 60% relative humidity at 20.degree. C. or after moistening treatment by 
keeping for 1 hour in an atmosphere of 90% relative humidity at 40.degree. 
C. 
3. Flame retardancy 
The flame retardancy of the sample specimens was tested according to the 
procedure specified in UL-94 Standard and graded in four ratings of V-0, 
V-1, V-2 and HB, V-0 being for the best or highest flame retardancy. 
4. Punching workability 
Two coverlay films of 24 cm by 30 cm dimensions with the release paper were 
laid one on the other each with the release paper facing down and punched 
on a die to make 500 circular holes of 3 mm diameter and the condition of 
the peripheries of the thus formed 1000 holes was visually examined. The 
results of this visual test were quite satisfactory to give completely 
punched holes of good-conditioned peripheries in all of the experiments 
excepting Experiments No. 17 and No. 20 for comparative purpose, in which 
more than 5% of the holes were incompletely punched. 
5. Press-out of adhesive 
A coverlay film freed from the release paper and provided with a hole of 3 
mm diameter by punching was press-bonded with a copper foil at 160.degree. 
C. for 10 minutes under a pressure of 50 kg/cm.sup.2 with the adhesive 
layer contacting the glossy surface of a copper foil and the distance of 
the adhesive layer oozing out of the periphery into the hole was 
determined by using a cathetometer. 
As to the results obtained in the above described testing procedures, it is 
generally accepted that a coverlay film suitable for practical use should 
give the testing results including 50 to 200 .mu.m of the adhesive 
press-out, at least 1.0 kg/cm of the peeling resistance and at least 
300.degree. C. and at least 250.degree. C. of the heat resistance against 
molten solder alloy as seasoned under normal conditions and moistened 
under high-humidity and high-temperature conditions, respectively. 
TABLE 1 
__________________________________________________________________________ 
Experiment No. 
1 2 3 4 5 6 7 8 
__________________________________________________________________________ 
Epoxy resin 
A A A A A A C C 
(parts) (40) 
(40) 
(40) 
(40) 
(40) 
(40) 
(100) 
(30) 
B B B B B B E 
(60) 
(60) 
(60) 
(60) 
(60) 
(60) (70) 
Carboxylated nitrile 
A A B A B B A A 
rubber (parts) 
(80) 
(100) 
(80) 
(100) 
(100) 
(80) 
(100) 
(100) 
Curing agent, parts 
15 15 15 15 15 15 15 15 
Curing accelerator 
A A B B C D A A 
(parts) (1.0) 
(0.5) 
(0.2) 
(0.5) 
(3.0) 
(1.0) 
(0.5) 
(0.5) 
Inorganic powder 
C B B C B C A B 
(parts) (40) 
(10) 
(10) 
(50) 
(10) 
(40) 
(20) 
(10) 
D D D 
(5) (5) 
(5) 
__________________________________________________________________________ 
Experiment No. 
9 10 11 12 13 14 15 
__________________________________________________________________________ 
Epoxy resin 
D E D D A A A 
(parts) (10) 
(100) 
(10) 
(10) 
(40) 
(40) 
(40) 
E E E B B B 
(90) (90) 
(90) 
(60) 
(60) 
(60) 
Carboxylated nitrile 
B A B B A A A 
rubber (parts) 
(80) 
(100) 
(70) 
(80) 
(100) 
(100) 
(30) 
Curing agent, parts 
10 15 15 15 15 15 15 
Curing accelerator 
B B C D C B 
(parts) (0.5) 
(0.2) 
(1.3) 
(1.0) 
(6.0) (0.5) 
Inorganic powder 
C B C C B B C 
(parts) (50) 
(10) 
(50) 
(40) 
(10) 
(10) 
(50) 
D D 
(2.5) (2.0) 
__________________________________________________________________________ 
Experiment No. 
16 17 18 19 20 21 22 
__________________________________________________________________________ 
Epoxy resin 
A A D D C C D 
(parts) (40) 
(40) 
(10) 
(10) 
(100) 
(100) 
(10) 
B B E E E 
(60) 
(60) 
(90) 
(90) (90) 
Carboxylated nitrile 
A A B B A A B 
rubber (parts) 
(100) 
(100) 
(70) 
(70) 
(100) 
(30) 
(80) 
Curing agent, parts 
15 15 15 15 15 15 15 
Curing accelerator 
B B C A A D 
(parts) (0.5) 
(0.5) 
(1.3) (0.5) 
(0.5) 
(7.0) 
Inorganic powder 
C C C A C 
(parts) (90) (80) 
(50) (20) 
(40) 
D D 
(5) 
(2) 
__________________________________________________________________________ 
TABLE 2 
__________________________________________________________________________ 
Experiment No. 
1 2 3 4 5 6 7 8 
__________________________________________________________________________ 
Adhesive press-out, .mu.m 
190 
200 
170 
160 
100 
120 
200 
200 
Peeling reistance, 
1.5 
1.7 
1.4 
1.6 
1.7 
1.4 
1.7 
1.6 
kg/cm 
Heat resistance on molten solder 
normal, .degree.C. 
310 
310 
310 
320 
320 
320 
300 
300 
moistened, .degree.C. 
250 
250 
250 
250 
250 
260 
250 
250 
Flame retardancy V-0 
V-0 
__________________________________________________________________________ 
Experiment No. 
9 10 11 12 13 14 15 
__________________________________________________________________________ 
Adhesive press-out, .mu.m 
170 
180 
140 
130 10 
1400 
130 
Peeling resistance, 
1.5 
1.6 
1.5 
1.4 0.8 
1.1 0.5 
kg/cm 
Heat resistance on molten solder 
normal, .degree.C. 
310 
300 
310 
310 310 
240 300 
moistened, .degree.C. 
250 
250 
250 
250 250 
200 250 
Flame retardancy 
V-0 
V-0 
V-0 
V-0 
__________________________________________________________________________ 
Experiment No. 
16 17 18 19 20 21 22 
__________________________________________________________________________ 
Adhesive press-out, .mu.m 
100 
200 
150 
1300 
220 
190 0 
Peeling resistance, 
0.4 
1.6 
0.4 
1.5 1.5 
0.6 0.7 
kg/cm 
Heat resistance on molten solder 
normal, .degree.C. 
320 
280 
300 
250 280 
300 310 
moistened, .degree.C. 
260 
220 
250 
200 220 
250 250 
Flame retardancy V-0 
V-0 V-1 
V-0 V-0 
__________________________________________________________________________