UV curable compositions for making improved solder mask coatings

A composition including a mixture of UV curable urethane acrylate and a urethane diacrylate compounds provides an aqueous alkaline developable, flexible solder mask having excellent electrical insulation resistance under hydrolytic testing conditions at elevated temperatures.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to solder mask coatings having improved performance 
characteristics, and, more particularly it is concerned with aqueous 
alkaline developable, UV curable composition for making flexible solder 
mask coatings having Class III (IPC) electrical insulation resistance 
under extreme testing conditions. 
2. Description of the Prior Art 
A number of U.S. patents disclose photopolymerizable prepolymers and 
compositions for use as solder masks in printed circuit boards, including 
U.S. Pat. Nos. 3,660,088; 3,753,720; 3,883,352; 3,887,450; 4,199,163; 
4,361,640; 4,422,914; 4,436,806; 4,458,007; 4,481,281; 4,506,004; and 
4,508,916. However, none of these UV curable compounds and compositions 
are entirely satisfactory for use as a commercial photoimagable solder 
mask. 
A feature of a preferred photoimagable solder mask coating is an ability to 
be developed with an aqueous, slightly alkaline solution, thereby avoiding 
the use of organic solvents in the developing step. The desired solder 
mask coatings also should exhibit flexibility, heat and chemical 
resistance, surface hardness, abrasion resistance, adhesion to the 
underlying metal of the printed circuit board, and a high cure depth at 
low dosages. Particularly sought after are solder masks which exhibit 
Class III (IPC) electrical insulation resistance and a rapid curing rate. 
These stringent requirements imply that the structure of the UV curable 
compound and its compositions must be carefully designed in order that the 
solder mask can achieve commercial acceptability. 
For example, in Rendulic, U.S. Pat. No. 4,436,806, there is described a 
method and apparatus for making a printed circuit board by imaging a 
liquid polymer which has been coated onto the board by roller coating. In 
this process, imaging is carried out while the polymer is wet, the photo 
tool being positioned in a close relationship with the coated board. The 
liquid polymer compositions disclosed by Rendulic for use in this process 
are urethane acrylates having a polyether or polyester extending chain 
unit. These compositions have a very low viscosity which is desirable for 
forming coatings with a roller coater. However, these compositions have a 
relatively low degree of ethylenic unsaturation per weight unit so that 
they require a relatively high energy level for curing. 
In contrast to the imaging technique of the above-mentioned patent, 
Sullivan, in U.S. Pat. No. 4,506,004, describes a contact method of 
imaging a liquid solder mask coating on a printed circuit board. In this 
method, the UV curable solder mask coating is screen printed to a given 
thickness, prehardened and imaged by exposure with suitable UV energy. The 
desired characteristics of good electrical performance and excellent 
printing resolution are the attributes of the Sullivan system. The 
requirements of a suitable photopolymerizable compound for use in this 
process include an ability to form a smooth, flexible coating which can 
impart chemical and heat resistance to the solder mask. Furthermore, the 
coating must exhibit excellent adhesion to the board, an ability to be 
cured to a desired depth with a minimum of UV energy, and excellent 
electrical insulation resistance. 
Accordingly, it is an object of this invention to provide a UV curable 
composition for making a solder mask having advantageous performance 
characteristics. 
Another object of this invention is to provide a UV curable composition, 
which includes a mixture of UV curable compounds, for use as a solder 
mask, and, particularly, a composition which can be developed rapidly in 
an aqueous, slightly alkaline solution. 
A particularly object herein is to provide a flexible solder mask coating 
which exhibits a Class III (IPC) electrical insulation resistance of 
greater than 5.times.10.sup.8 ohm-cm. after 7 days cyclic exposure, at 
25.degree.-65.degree. C. and 90% relative humidity. 
Still another object herein is to provide a solder mask coating which can 
be rapidly cured, preferably at an energy level of less than 0.5 joules 
per sq. cm., to provide a cure depth of at least 18 mils at this energy 
level. 
Another object of this invention is to provide a UV curable composition 
which can be coated, suitably by screen printing techniques, to form a 
smooth, uniform, glossy, flexible, coating up to 3 mil thickness. 
Yet another object of the invention is to provide a solder mask composition 
which is thermally stable without gelatin for at least six months. 
A further object herein is to provide a flexible solder mask coating having 
substantially 100% solution to the underlying metal of the printed circuit 
board, particularly on copper and tin-lead. 
Another object herein is to provide a solder mask product which exhibits 
good surface hardness, abrasion, heat and organic solvent resistance. 
Still another object is to provide a solder mask coating having excellent 
heat resistance so that it does not blister after 20 seconds at 
285.degree. C. 
A specific object herein is to provide a solder mask composition which is 
particularly adapted for use in the method and apparatus of making printed 
circuit boards described in U.S. Pat. No. 4,506,004. 
Another specific object herein is to provide a mixture of UV curable 
acrylate compounds which will meet all of the aforesaid objects in an 
advantageous manner. 
These and other objects and features of the invention will be made apparent 
from the following description of the invention. 
SUMMARY OF THE INVENTION 
This invention provides a UV curable composition for making a solder mask 
having advantageous performance characteristics. The composition of the 
invention includes a mixture of UV curable compounds, defined hereinafter, 
from which aqueous, alkaline developable, flexible solder mask coatings 
can be made, having excellent electrical insulation resistance. The 
coatings also are smooth, uniform and glossy up to 3 mil thickness, 
exhibit substantially 100% adhesion to the underlying metal of the printed 
circuit board, exhibit good surface hardness, and abrasion, heat and 
organic solvent resistance. 
The composition mixture comprises 
(a) UV curable compounds including: 
(i) 10-50% by weight, preferably 15-35% by weight, of a urethane acrylate, 
described in detail in co-pending application Ser. No. 735,411, filed May 
17, 1985, by the same named inventors as herein, and assigned to the same 
assignee as herein; (ii) 5-40% by weight, preferably 10-30% by weight, or 
an aromatic urethane diacrylate; (iii) 2-30% by weight, preferably 5-20% 
by weight, of an aliphatic or cycloaliphatic urethane diacrylate, or 
mixture thereof. 
(b)) 4-60% by weight of one or more reactive monomer diluent and 
(c) 0.5-10% by weight of a photoinitiator. 
The composition of the invention also may include one or more of the 
following: a thermal cross-linking agent, a pigment or dye, a flame 
retardant, a rheology modifier, a thermal stabilizer, and a pigment 
dispersion aid. 
The urethane acrylate compound is obtained by reacting (1) at least one 
diisocyanate compound selected from the group consisting of aliphatic and 
cycloaliphatic diisocyanates, e.g. dicyclohexylmethylene diisocyanate, 
isophorone diioscyanate, and hexamethylene diisocyanate, (2) one mole of a 
hydroxyalkyl acrylate; e.g. hydroxypropyl acrylate, (3) an alkylene polyol 
having 3-6 hydroxyl groups, and 3-6 carbon atoms, e.g. glycerol; and (4) 
at least one mole of a dicarboxylic acid anhydride, saturated or 
unsaturated, e.g. maleic anhydride. 
The urethane acrylate compound is characterized by (a) terminal ethylenic 
unsaturation; (b) at least one terminal carboxylic acid group; and (c) 
alkylene connecting groups intermediate said terminal groups, one of which 
preferably is substituted with at least one hydroxy group. 
The urethane diacrylate compounds of the composition of the invention are 
obtained by reacting (1) a diisocyanate compound selected from aliphatic, 
cycloaliphatic and aromatic diisocyanates, e.g. dicyclohexyl methylene 
diisocyanate, isophorone diisocyanate, hexamethylene diisocyanate and 
toluene diisocyanate, with (2) two moles of a hydroxyalkyl acrylate, e.g. 
hydroxypropyl acrylate. 
The urethane acrylate component of the mixture of UV curable compounds 
provides the desired flexibility characteristics, aqueous alkaline 
developability, adhesion, surface hardness, and high cure depth at a low 
energy level; and the combination of aromatic and aliphatic or 
cycloaliphatic urethane diacrylates provides Class III electrical 
insulation resistance while retaining flexibility.

DETAILED DESCRIPTION OF THE INVENTION 
The UV curable, urethane acrylate compound can be obtained by a three- 
step, two-part reaction sequence, the first step of which involves 
reacting a suitable diisocyanate compound with one mole of a hydroxyalkyl 
acrylate to form the corresponding isocyanate-capped acrylate. Suitable 
diisocyanates for use in this step include aliphatic and cycloaliphatic 
diisocyanates, e.g. dicyclohexylmethylene diisocyanate, isophorone 
diisocyanate, hexamethylene diisocyanates, and trimethylhexamethylene 
diisocyanate. Aromatic diisocyanates are not preferred for making the 
urethane acrylate compound because they form non-flexible coatings. 
The hydroxyalkyl acrylate reactant may be selected from such compounds as 
hydroxymethyl acrylate, hydroxyethyl acrylate, hydroxypropyl acrylate, 
hydroxybutyl acrylate and the like, although other acrylates known in the 
art may be used as well. For purposes of this invention the term 
"acrylate" includes the corresponding "methacrylate" derivatives. 
The second step in the reaction sequence is esterification of a suitable 
alkylene polyol reactant with a dicarboxylic acid anhydride to form an 
ester of the dicarboxylic acid. Suitable alkylene polyols have at least 
three, and up to six, hydroxyl groups in the molecule, and from three to 
six carbon atoms, and include such compounds as glycerol, 
trimethylolpropane, 1,2,6-hexanetriol, pentaerithrylol, and caprolactone 
polyol. Glycerol is preferred. 
To improve heat resistance of the solder mask, part of the polyol, e.g. 
about 20 mole % thereof, may be substituted for by a suitable 
sulfur-containing diol, e.g. dihydroxydiphenylsulfone or the corresponding 
sulfide. 
Suitable dicarboxylic acid anhydride reactants for this step include such 
acid anhydrides as maleic anhydride, succinic anhydride, glutaric 
anhydride, adipic anhydride, phthalic anhydride and the like. Maleic 
anhydride is preferred. 
In carrying out this reaction with an alkylene triol, e.g. glycerol, one 
mole of the acid anhydride is used, thus forming the monoester 
intermediate, while leaving two hydroxyl groups available for subsequent 
reaction. One hydroxyl group then will be substituted on the alkylene 
group in the final product. The corresponding diesters also may be 
prepared by using two moles of the acid anhydride, in which case two 
terminal carboxylic acid groups will be present in the final compound. 
The third and final step in the process is the condensation of the 
isocyanate-capped acrylate with the ester of the dicarboxylic acid 
anhydride. The final reaction product is characterized by terminal 
ethylenic unsaturation at one end, and at least one terminal carboxylic 
acid group at the other end, with the organic diisocyanate moiety attached 
to the unsaturated group, and a alkylene group adjacent said terminal acid 
group. Preferably, a hydroxy group is present as a substituent on the 
alkylene group. Two terminal carboxylic acid groups also may be included 
in the final compound. 
The desired urethane acrylate may be prepared by a three-step, one-pot 
synthesis wherein the four reactants are added sequentially in the same 
reaction vessel. In this method, the diisocyanate is charged and the 
hydroxyalkyl acrylate is added slowly. After an initial reaction between 
these reactants, the alkylene polyol is added, followed by the 
dicarboxylic acid anhydride. 
The reaction three-step, two part sequence is given below. 
METHOD OF PREING URETHANE ACRYLATE COMPOUND 
##STR1## 
where 
x is 3-6, 
n is 1-5, 
(x-n) is a positive integer, 
R.sub.3 is alkylene, e.g. C.sub.3 -C.sub.6, and 
R.sub.4 is aliphatic or cycloaliphatic, saturated or unsaturated, or 
aromatic; 
##STR2## 
where 
y is 0-4, provided, however, that if y is 0, then n is at least 2. 
As an illustration of this synthesis, the reaction of dicyclohexylmethylene 
diisocyanate, one mole of hydroxypropyl acrylate, glycerol and one mole of 
maleic anhydride proceeds as follows: 
##STR3## 
With 2 moles of maleic anhydride, the reaction sequence proceeds as 
follows: 
##STR4## 
The UV curable, urethane diacrylate compounds are prepared by reacting a 
suitable diisocyanate with 2 moles of a hydroxylalkyl acrylate to form the 
corresponding isocyanate diacrylate. Suitable diisocyanates include 
aliphatic, cycloaliphatic and aromatic diisocyanates, e.g. 
dicyclohexylmethylene diisocyanate, isophorone diisocyanate, hexamethylene 
diisocyanates, trimethylhexamethylene diisocyanate and toluene 
diisocyanate. The hydroxylalkyl acrylate reactant may be selected from 
such compounds as hydroxymethyl acrylate, hydroxyethyl acrylate, 
hydroxypropyl acrylate, hydroxybutyl acrylate, and the like, although 
others known in the art may be used as well. 
The UV curable, aqueous alkaline developable composition for forming a 
soldering mask comprises: 
(i) 10-50% by weight of said of urethane acrylate, 
(ii) 5-40% by weight of said aromatic urethane diacrylate, 
(iii) 2-30% by weight of said aliphatic or cycloaliphatic urethane, or 
mixtures thereof, 
(iv) 4-60% by weight of one or more reactive monomer diluents, and, 
(v) 0.5-10% by weight of a photoinitiator. 
The reactive diluent monomers are included in the composition of the 
invention to reduce its viscosity and further increase its curing rate. 
Suitable reactive diluent monomers for use therein include ethylenically 
unsaturated monomers that are compatible and copolymerizable with the 
substituted urethane acrylate and urethane diacrylates of the invention. 
Such ethylenically unsaturated monomers include mono-, di- and 
tri-acrylates as, for example, hydroxyalkyl acrylates, such as e.g. 
hydroxyethyl acrylate; and acrylate esters, e.g. methyl methacrylate, 
ethyl acrylate, 2-ethylhexyl acrylate, cyclohexyl acrylate, 
isobutoxymethyl methacrylate, t-butyl acrylate, methyl acrylate, butyl 
acrylate, 2-(N-ethylcarbamyl)ethyl methacrylate; aryloxyalkyl acrylates, 
e.g. phenoxyethyl acrylate; bis-phenol-A diacrylate, ethylene glycol 
diacrylate, polyethylene glycol diacrylate, glycerol diacrylate and 
methacrylate, bis-phenol A diacrylate, tetrapropylene glycol diacrylate, 
and the like. Suitable triacyclates include glycerol triacrylate, 
ethoxylated trimethylol propane triacrylate, and the like. 
Other reactive compounds can be included in the composition of the 
invention to increase the cross-linking density of the coating. Such 
reactive compounds include, but are not limited to, pentaerythritol 
3-mercaptopropionate, 1,4-butylene dimethacrylate or acrylate, 
1,1,6-6-tetrahydroperfluorohexanediol diacrylate, ethylene dimethacrylate, 
glycerol diacrylate or methacrylate, glycerol trimethacrylate, diallyl 
phthalate and 1,3,5-tri(2-methacryloxyethyl)-s-triazine. 
The UV curable composition of this invention also contains a photoinitiator 
which generates free radicals owing to actinic light. Suitable examples of 
such photoinitiators include substituted and unsubstituted polynuclear 
quinones, such as 2-ethylanthraquinone, 2-t-butylantraquinone, 
octamethylanthraquinone, 1,2-benzanthraquinone, 2,3-diphenylanthraquinone, 
thioxanthone, e.g. chloro and isopropyl derivates, and the like; 
ketoaldonyl compounds such, as diacetyl, benzyl and the like; 
.alpha.-ketoaldonyl alcohols and ethers, such as benzoin, pivalone, and 
the like; .alpha.-hydrocarbon-substituted aromatic acyloins such as 
.alpha.-phenylbenzoin, .alpha.,.alpha.-diethoxyacetophenone, and the like; 
and aromatic ketones such as benzophenone, 
4,4'-bisdialkylaminobenzophenone, and the like. These photoinitiators may 
be used alone or as a combination of two or more of them. Examples of 
combinations include 2,4,5-triarylimidazole dimer and 
2-mercaptobenzoquinazole, leucocrystal violet, 
tris-(4-diethylamino-2-methylphenyl)methane, or the like, and compounds 
which may not have photoinitiating properties alone but which nevertheless 
can constitute a good photoinitiating system, in combination with the 
above-mentioned materials. Such compounds include, for example, tertiary 
amines, such as triethanolamine and the like, which are used in 
combination with benzophenone. These photoinitiators and/or photoinitiator 
systems preferably are present in an amount of about 0.5 to 10% by weight 
of the composition. 
The composition preferably may also include other components of a 
commercial composition including cross-linker, e.g. Cymel 300; a flame 
retardant, such as a mixture of decabromidiphenyloxide and antimony 
trioxide or pentoxide; a rheology modifier; a pigment dispersion aid; and 
a cure rate enhancer, such as triphenylphosphite. 
EXAMPLE 1 
3-Step, One-Pot Synthesis of Urethane Acrylate (1) Desmondur 
W/HPA/Glycerol/Maleic Anhydride (1) 
__________________________________________________________________________ 
##STR5## (1) 
Parts Equivalents 
__________________________________________________________________________ 
A. 4,4'-dicyclohexylmethylene diisocyanate (Desmondur 
262 2 
B. Dibutyltin dilaurate 1.7 2000 ppm 
C. Hydroxylpropyl acrylate (HPA) 130 1 
D. Phenoxyethyl acrylate 250 1.3 
E. Glycerol 92.1 1 
F. Maleic anhydride 98 1 
G. Hydroquinone methyl ether 1.7 2000 ppm 
__________________________________________________________________________ 
A resin kettle equipped with a mechanical stirrer, thermometer, drying tube 
and pressure equalizing dropping funnel was charged with (A), (B) and (D). 
The mixture then was stirred while (C) was added slowly over 20 min. The 
temperature was maintained at below 55.degree. C. Upon completion of the 
addition, the mixture was maintained at 55.degree.-60.degree. C. until the 
NCO number was 6.8.+-.0.3, as determined by titration. (E) then was added 
slowly over 30 min. and the temperature was maintained at below 55.degree. 
C. Heating was continued at 60.degree. C. until IR showed the absence of 
NCO absorption at 2275 cm.sup.-1. (F) then was added to the reaction 
mixture at 70.degree.-75.degree. C. over 20 min., followed by (G). Heating 
was continued at 75.degree. C. until the maleic anhydride was completely 
reacted, as indicated by the absence of peaks at 1845 and 1975 cm.sup.-1. 
The product was a viscous liquid having a Brookfield viscosity of 64,000 
cps at 25.degree. C., and an acid group contents of 1.48 meq/gm. 
EXAMPLE 2 
3-Step, One-Pot Synthesis of Urethane Acrylate (2) Desmondur 
W/HPA/Glycerol/(Maleic Anhydride).sub.2 (2) 
__________________________________________________________________________ 
##STR6## (2) 
Parts Equivalents 
__________________________________________________________________________ 
A. 4,4'-dicyclohexylmethylene diisocyanate 
262 2 
B. Dibutyltin dilaurate 1.9 2000 ppm 
C. Hydroxylpropyl acrylate 130 1 
D. Phenoxyethyl acrylate 291.5 1.52 
E. Glycerol 92.1 1 
F. Maleic anhydride 196 2 
G. Hydroquinone methyl ether 
1.9 2000 ppm 
__________________________________________________________________________ 
A resin kettle equipped with a mechanical stirrer, thermometer, drying tube 
and pressure equalizing dropping funnel was charged with (A), (B) and (D). 
The mixture then was stirred while (C) was added slowly over 20 min. The 
temperature was maintained at below 55.degree. C. Upon completion of the 
addition, the mixture was maintained at 55.degree.-60.degree. C. until the 
NCO number was 10.7.+-.0.3, as determined by titration. (E) then was added 
slowly over 30 min. and the temperature was maintained at below 55.degree. 
C. Heating was continued at 60.degree. C. until IR showed the absence of 
NCO absorption at 2275 cm.sup.-1. (F) then was added to the reaction 
mixture at 70-75.degree. C. over 20 min., followed by (G). Heating was 
continued at 75.degree. C. until the maleic anhydride was completely 
reacted, as indicated by the absence of peaks at 1845 and 1975 cm.sup.-1. 
The product was a viscous liquid having a Brookfield viscosity of 320,000 
cps at 25.degree. C., and an acid group contents of 2.60 meq/gm. 
EXAMPLE 3 
3-Step, 2-Pot Synthesis of Urethane Acrylate of Ex. 1 
______________________________________ 
Parts Equivalents 
______________________________________ 
A. 4,4'-dicyclohexylmethylene 
262 2 
diisocyanate 
B. Dibutyltin dilaurate 
1.7 2000 ppm 
C. Hydroxylpropyl acrylate 
130 1 
D. Phenoxyethyl acrylate 
168 
E. Glycerol 92.1 
F. Maleic anhydride 98 
G. Phenoxyethyl acrylate 
82 
H. Hydroquinone methyl ether 
1.7 2000 ppm 
______________________________________ 
A resin kettle, equipped with a mechanical stirrer, thermometer, drying 
tube and pressure equalizing dropping funnel was charged with (A), (B) and 
(D). The mixture was stirred while (C) was added slowly over 20 min. The 
temperature was maintained at below 55.degree. C. Upon completion of the 
addition, the mixture was maintained at 55.degree.-60.degree. C. until the 
NCO number was 10.7.+-.0.3 as determined by titration. In another resin 
kettle equipped with a mechanical stirrer, a thermometer and drying tube 
was charged with (E), (F) and (G). The mixture was heated with stirring at 
90.degree. C. until the maleic anhydride was completely reacted, as 
indicated by the absence of peaks at 1845 and 1975 cm.sup.-1. This product 
was then added to the intermediate prepared in the first kettle, described 
above, over 45 min. at a temperature below 55.degree. C. The reaction 
temperature was maintained at 60.degree. C. until an infrared spectrum 
showed the absence of NCO absorption peak at 2275 cm.sup.-1. (H) then was 
added and the mixture stirred until (H) completely dissolved. The product 
was a viscous liquid having a viscosity of 72,000 cps at 25.degree. C., 
and acid groups contents of 1.23 meq/gm. 
EXAMPLE 4 
Preparation of Aromatic Urethane Diacrylate 
##STR7## 
Toluene diisocyante (1 mole) and hydroxypropyl acrylate (2 moles) were 
reacted at 55.degree. C. for 6 hrs. to give the product, which was a 
viscous liquid having a Brookfield viscosity of 68,000 cps at 50.degree. 
C. 
EXAMPLE 5 
Aliphatic Urethane Diacrylate 
Aliphatic urethane diacrylates were available (1) as Photomer-6008 from 
Diamond Shamrock Corp. or (2) as SR-9503 from Sartomar Co. (100% reactive 
materials). 
EXAMPLE 6 
The following composition was prepared and used in forming a solder mask 
composition for a printed circuit board. 
______________________________________ 
COMPOSITION 
Component Parts by Wt. 
______________________________________ 
Urethane acrylate of Ex. 1 
23 
Aromatic Urethane diacrylate of Ex. 4 
17.5 
Aliphatic Urethane diacrylate Ex. 4 
9 
Tone-100 (Union Carbide) and other 
20 
reactive diluent monomers 
CAB-O-SIL thixotropic agent 
4 
Cymel 301 (Am. Cyan.) - thermal cross-linker 
8 
IRG 651 (Ciba-Geigy) photoinitiator 
2 
CNF 853 - green pigment 0.3 
Colloid 640 (Colloid Chem.) - leveling agent 
0.8 
MTBHQ - mono-tert.-butyl hydroquinone- 
0.2 
stabilizer 
Triphenylphosphite-cure depth accelerator 
1 
Decabromodiphenyl oxide and antimony trioxide - 
8 
flame retardant 
______________________________________ 
This composition has a Brookfield viscosity of 15,000 cps at 100 rpm at 
23.degree. C., and is stable without gelation for more than 1 year. 
SOLDER MASK 
A copper-clad epoxy fiber glass printed circuit board was cleaned by 
scrubbing to remove corrosion and foreign material and coated by screen 
printing with the above composition to a thickness of about 3 mils. Then 
another 3 mil thick coating was applied onto a photo tool in accordance 
with the method given in U.S. Pat. No. 4,506,004, Example 2. The two 
coatings then can be given a flash curing if desired, with about 0.2-0.4 
joules per cm.sup.2 of energy, using a mercury vapor lamp, to effect 
partial hardening. The two coatings then were mated to form a composite 
coating of 6 mils thickness, flipped over and given a main exposure with 
0.5 joules per cm.sup.2 for 30 seconds. The unexposed coating then was 
developed by removing it (negative working) in a 1% sodium carbonate 
solution (pH 11) at room temperature for 4 min., baked at 150.degree. C. 
for 1 hr., or by conveyorized IR heating and finally cured with 2.5 joules 
per cm.sup.2 of UV energy. 
The cured solder mask was a smooth, uniform, glossy, and flexible coating 
having 100% adhesion under a cross-hatch tape test both before and after 
application of solder; resistant to organic solvents for more than 15 
minutes in an immersion test; exhibited excellent legend ink adhesion 
characteristics; excellent heat resistance as measured by a 20-second dip 
in solder at a temperature of 285.degree. C. without blistering; surface 
pencil hardness of greater than H, and very good abrasion resistance; 
excellent adhesion to the printed circuit board, and an electrical 
insulation resistance which exceeds Class III (IPC) requirements of 
5.times.10.sup.8 ohm-cm. after 7 days at 25.degree.-65.degree. C. cycling 
at 90% R.H. 
EXAMPLE 7 
The procedure of Ex. 6 was repeated using the urethane acrylate of Ex. 2, 
which has dicarboxylic acid terminal groups, to provide another solder 
mask with advantageous properties. 
EXAMPLE 8 
The procedures of Exs. 1-7 were repeated using a polyol mixture of 20 mole 
% of dihydroxydiphenylsulfone and 80 mole % of glycerol as the polyol in 
the synthesis of the urethane acrylates. The solder masks using this 
polyol showed improved heat resistance as compared to those using glycerol 
alone. 
EXAMPLE 9 
The procedures of Exs. 1-7 were repeated using 1,2, 6-trihydroxyhexane in 
place of glycerol with similar results. 
While the invention has been described with particular reference to certain 
embodiments thereof, certain changes and modifications may be made which 
are within the skill of the art. It is intended to be bound only by the 
appended claims, in which: