SOLDER MASK COATINGS

This invention relates to solder mask coatings having improved performance characteristics, and, more parti¬ cularly, it is concerned with aqueous alkaline developable, UV curable urethane acrylate compounds and compositions useful for making such coatings.

Sullivan, in U.S. Patent 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 given as the attributes of the Sullivan system. The requirements of a suitable photo- polymerizable 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. Further- more, the coating must exhibit excellent adhesion to the board, and an ability to be cured to a desired depth with a minimum of UV energy.

An advantageous feature of a 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 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. These stringent requirements imply that the structure of the prepolymer and its formulation must be carefully designed before the solder mask can achieve commercial acceptability.

This invention provides a UV curable, aqueous alkaline developable urethane acrylate compound for formulation into a composition useful in making a solder mask coating. The urethane acrylate compound of the invention is obtained by reacting (i) at least one diisocyanate compound selected from the group consisting of aliphatic and cycloaliphatic diisocyanates, e.g. dicyclohexylmethylene diisocyanate, isophorone diisocyanate, and hexamethylene diisocyanate, (ii) one mole of a hydroxyalkyl acrylate; e.g. hydroxypropyl acrylate, (iii) a polyol having at least 3 hydroxyl groups, e.g. glyce ol; and (iv) at least one mole of a dicarboxylic acid anhydride, saturated or unsaturated, e.g. maleic anhydride.

The urethane acrylate compound of the invention is characterized by the following: (a) terminal ethylenic unsaturation; (b) at least one terminal carboxylic acid group; and (c) alkylene connecting groups intermediate said terminal groups, one of which is preferably substituted with at least one hydroxy group. Further, this invention provides a UV curable, aqueous alkaline developable urethane acrylate solder mask composi¬ tion comprising:

(a) 40-70% by weight of (i) the described urethane acrylate compound, or (ii) a mixture of 20-99 by weight of the urethane acrylate and 1-80% by weight of a urethane diacrylate;

(b) 20-50% by weight of one or more reactive diluent monomers;

(c) 0.5-10% by weight of a photoinitiator; and

(d) 0-8% by weight of triphenylphosphite. The composition also may include about 1-5% by weight of one or more of the following: a cross-linking agent, a pigment or dye, a rheology modifier, and a thermal stabi- lizer; and 1-15% by weight of a filler.

A feature of the invention is the provision of a printed circuit board having a cured solder mask coating thereon which can be screen printed to form a smooth, uniform, glossy, flexible coating. This solder mask coating can be cured at a low energy level to provide a high cure depth and excellent surface hardness, and abrasion, heat and organic solvent resistance. The coating can be developed rapidly in an aqueous, slightly alkaline medium at room temperature. Another feature of the invention is the provision of a UV curable urethane acrylate compound having a high degree of terminal ethylenic unsaturation per unit weight, which enables curing to take place at low energy levels. The compound has at least one terminal carboxylic group so that the cured coating can be developed rapidly in- an aqueous, slightly alkaline medium. The compound is further character¬ ized by the presence of an alkylene connecting group inter¬ mediate the terminal groups, which is preferably substituted with a hydroxyl group, which increases the aqueous solubil- ity of the compound.

The UV curable, aqueous alkaline developable 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 hydroxy- alkylacrylate to form the corresponding isocyanate-capped acrylate. Suitable diisocyanates for use in this step including aliphatic and cycloaliphatic diisocyanates, e.g. dicyclohexylmethylene diisocyanate, isophorone diisocyanate, hexamethylene diisocyanates, and trimeth lhexamethylene

diisocyanate. Aromatic diisocyanates are not preferred 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 esterifi- cation of a suitable polyol reactant with a dicarboxylic acid anhydride to form an ester of the dicarboxylic acid. Suitable polyols have at least three, and up to six, hydroxyl groups in the molecule, and include such compounds as glycerol, trimethylolpropane, 1,2,6-hexanetriol, caprol- actone polyol, and sugars, such as fructose. Glycerol is preferred.

To improve heat resistance of the solder mask, part of the polyol, e.g. about 20% mole thereof, may include a suitable sulfur-containing diol, e.g. dihydroxydiphenyl- sulfone or the corresponding sulfide.

Suitable dicarboxylic acid anhydrides 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, for example, with a triol, one mole of the acid anhydride is used, thus forming the monoester intermediate, and 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.

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The third and final step in the process is the conden¬ sation of the isocyanate-capped acrylate with the ester of the dicarboxylic acid anhydride. The final reaction 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 hydroxyl group is present as a substituent on the alkylene group. Two termi- nal 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 polyol is added, followed by the dicarboxylic acid anhydride.

The reaction sequence is given below.

METHOD OF PREPARING URETHANE ACRYLATE COMPOUND

OF INVENTION ^'

0

II a) 0CN-R ₁ -NC0 + H0R ₂ -0CCH=CH ₂ ■» diisocyanate hydroxyalkyl acrylate

0 0 II H CH ₂ =CHC-0-R ₂ 0C-NHR ₁ NC0 (I) isocyanate-capped alkyl acrylate where R, is aliphatic or cycloaliphatic, and

R« is alkylene;

-6-

0

0 dicarboxylic acid anhydride

0

Rj(OH x-n-(0-C-R4,-C00H n (II) polyacid ester

where x is 3-6, n is 1-5,

(x-n) is a positive integer,

R-, is alkylene, and

R, is aliphatic or cycloaliphatic, saturated or unsaturated, or aromatic;

where y is 0-4, provided, however, that if y is 0, then n is at least 2. As an illustration of the invention, the reaction of dicyclohexylmethylene diisocyanate, one mole of hydroxy- propyl acrylate, glycerol and one mole of maleic anhydride is as follows: .

( ID

(III)

With 2 moles of maleic anhydride the reaction sequence is as follows:

0 b) »-CH ₂ 0CCH=CHC00H

CH ₂ 0H

CH 0CCCH=CHC00H 2 M

(IV)

-8-

0 0

* II

-NHCOCHCH ₂ OCCH=CHCOOH

(V)

This invention also provides a UV curable, aqueous alkaline developable urethane acrylate composition for forming a soldering mask comprising: a) 40-70% by weight of (i) a urethane acrylate compound which is obtained by reacting substan¬ tially equal molar quantities of an aliphatic or cycloaliphatic diisocyanate and a hydroxyalkyl acrylate, with a polyol having at least three hydroxyl groups, and at least one mole of a dicarboxylic acid anhydride per mole of said polyol; or (ii) a mixture of 20-99% by weight of the urethane acrylate and 1-80% by weight of a urethane diacrylate; b) 20-50% by weight of a reactive monomer diluent; c) 0.5-10% of a photoinitiator; and d) 0-8% by weight of a triphenylphosphite.

The reactive diluent monomers are added to the urethane acrylates of the invention to reduce its viscosity in the composition, and increase the curing rate. Suitable reac¬ tive diluent monomers for use herein include ethylenically unsaturated monomers that are compatible and copolymerizable with the substituted urethane acrylates of the invention. Such ethylenically unsaturated monomers include mono-, di-

and tri-acrylates as, for example, hydroxyalkyl acrylates, such as e.g. hydroxyeth l 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 di- acrylate, tetrapropylene glycol diacrylate, and the like. Suitable triacrylates include glycerol triacrylate, ethoxylated trimethylol triacrylate, and the like.

Other reactive compounds can be included in the compo¬ sition of the invention to increase the cross-linking density of the coating. Such reactive compounds include, but are not limited to, pentaerythritol- 3-mercaptopro- pionate, 1,4-butylene dime hacr late or acrylate, 1,1,6,- 6-tetrahydroperfluorohexanediol diacrylate, ethylene di¬ methacrylate, glycerol diacrylate or methacrylate, glycerol trimethacrylate, diallyl phthalate and 1,3,5-tri(2-meth- acryloxyethyl)-s-triazine.

The UV curable composition also contains a photo- initiator which generates free radicals owing to actinic light. Suitable examples of such photoinitiators include substituted and unsubstituted polynuclear quinones, such as 2-eth lanthraquinone, 2-t-butylanthraquinone, octamethyl- anthraquinone, 1,2-benzanthraquinone, 2,3-diphenylanthra- quinone, thioxanthone, _e.g. chloro and isopropyl derivates, and the like; ketoaldonyl compounds such, as diacetyl, benzyl and the like; α-ketoaldonyl alcohols and ethers, such as benzoin, pivalone, and the like; α-hydrocarbon-substi¬ tuted aromatic acyloins such as α- phen lbenzoin, α,α- 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 combi¬ nations include 2,4,5-triarylimidazole dimer and 2-mercapto- benzoquinazole, leucocrystal violet, tris(4-diethylamino- 2-meth lphenyl)methane, or the like, and compounds which may not have photoinitiating properties alone but which never¬ theless 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 photo- initiator systems preferably are present in an amount of about 0.5 to 10% by weight of the composition.

The urethane diacrylate compound is obtained by react- ing (i) at least one diioscyanate compound selected from the group consisting of aliphatic, cycloaliphatic and diisocya¬ nates, e.g. dicyclohexyl m ^' ethylene diisocyanate, isophorone diisocyanate, hexamethylene diisocyanate and toluene diiso¬ cyanate, with (ii) two moles of a hydroxyalkyl acrylate, e.g. hydroxypropyl acrylate.

The urethane acrylate component of the mixture of UV curable compounds provides flexibility characteristics , aqueous alkaline developability, adhesion, surface hardness, and high cure depth at a low energy level; while the urethane diacrylate provides enhanced electrical insulation resistance.

The triphenylphosphite is present in an effective amount, suitably about 0.1 to 8% by weight of the composi¬ tion, preferably about 0.4 to 5%, and, optimally, about 3%. In combination with a photoinitiator, the triphenylphosphite additive in the composition of the invention provides a cure

2 depth of at least 18 mils at an energy level of 0.2 J/cm .

The composition with triphenylphosphite is further characterized by stability toward gelation for long periods

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of time, and the retention of the other desirable properties of a commercial solder mask composition.

Exam le 1

3-Step, One-Pot Synthesis of Urethane Acrylate (1) Desmondur W/HPA/Glycerol/Maleic Anhydride (1)

(1)

Parts Equivalents

A. 4,4' -dicyclohexylmethylene diisocyanate (Desmondur W) 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°C. Upon completion of the addition, the mixture was maintained at 55°-60°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°C. Heating was continued at 60°C. until IR showed the absence of NCO

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absorption at 2275 cm . (F) then was added to the reaction mixture at 70-75°C. over 20 min., followed by (G) . Heating was continued at 75°C. until the maleic anhydride was completely reacted, as indicated by the absence of peaks at 1845 and 1975 cm ^"1 .

The product was a viscous liquid having a Brookfield viscosity of 64,000 cps at 25°C. , and an acid group contents of 1.48 meq/gm.

Exam le 2

3-Step, One-Pot Synthesis of Urethane Acrylate (2) Desmondur W/HPA/Glycerol/(Maleic Anhydride) ₂ (2)

H ₂ C=CH-

(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

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was stirred while (C) was added slowly over 20 min. The temperature was maintained at below 55°C. Upon completion of the addition, the mixture was maintained at 55°-60°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°C. Heating was continued at 60°C. until IR showed the absence of NCO absorption at 2275 cm ^" . (F) then was added to the reaction mixture at 70-75°C. over 20 min., followed by (G) . Heating was continued at 75°C. until the maleic anhydride was completely reacted, as indicated by the absence of peaks at 1845 and 1975 cm ^"1 .

The product was a viscous liquid having a Brookfield viscosity of 320,000 cps at 25°C. , and an acid group con- tents of 2.60 meq/gm.

Example 3

3-Step, 2-Pot Synthesis of Urethane Acrylate of Ex. 1

Parts Equivalents

A. 4,4' -dicyclohexylmethylene diisocyanate 262 2

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 temper¬ ature was maintained at below 55°C. Upon completion of the addition, the mixture was maintained at 55°-60°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°C. until the maleic anhydride was completely reacted, as indicated by the absence of peaks at 1845 and 1975 cm ^" . This product was then added to the intermediate prepared in the first kettle, described above, over 45 min. at a temperature below 55°C. The reaction temperature was maintained at 60°C. until an infrared spectrum showed the absence of NCO absorp- tion peak at 2275 cm ^" . (H) then was added and the mixture stirred until (H) completely dissolved. The product was a viscous liquid having a viscosityof 72,000 cps at 25°C. , and acid group contents of 1.23 meq/g .

Example 4

The following composition was prepared using the urethane acrylate of Example 1, and then used in forming a solder mask for a printed circuit board. COMPOSITION

Component Parts by Wt .

Urethane acrylate of Ex. 1 60 . 0

Tone-100 (Union Carbide) reactive diluent monomer 25 .5 Pentaerythritol 3-mercaptopropionate- cross-linker 3 . 0

CAB-O-SIL thixotropic agent 4

Cymel 301 (Am. Cyan.) - cross-linker 5

IRG 651 (Ciba-Geigy) photoinitiator 2 CNF 853 - green pigment 1

Colloid 640 (Colloid Chem.) leveling agent 0. 8

MTBHQ - mono-tert.-butyl hydroquinone-stabilizer 0.2

This composition is thixotropic with a Brookfield viscosity of 208,000 cps at 25°C at 0.5 rpm. and 39,200 cps at 100 rpm. , and is stable without gelations for more than 6 months.

Example 5

Preparation of Urethane Diacrylate

Toluene diisocyante (1 mole) and hydroxypropyl acrylate (2 moles) are reacted at 550°C. for 6 hrs. to give the product, which was a viscous liquid having a Brookfield viscosity of 100,000 cps at 25°C.

Example 6

The following composition was prepared using a UV curable mixture of equal weight quantities of the urethane acrylate of Example 1, and the urethane diacrylate of Example 5. The composition was used in forming a solder mask composition for a printed circuit board.

COMPOSITION Component Parts by Wt.

Urethane acrylate of Ex. 1 • . 30..0

Urethane diacrylate of Ex. 5 30.0 Tone-100 (Union Carbide) reactive diluent monomer 25J5 Pentaerythritol 3-mercaptopropionate- cross-linker 3.0

CAB-O-SIL thixotropic agent 4 Cymel 301 (Am. Cyan.) - cross-linker 5

IRG 651 (Ciba-Geigy) photoinitiator 2

CNF 853 - green pigment 1

Colloid 640 (Colloid Chem.) leveling agent 0.8 MTBHQ - mono-tert.-butyl hydroquinone-stabilizer 0.2

This composition is thixotropic with a Brookfield viscosity of 208,000 cps at 25°C at 0.5 rpm. and 39,200 cps at 100 rpm. , and is stable without gelation for more than 6 months.

Example 7

SOLDER MASK

A copper-clad epoxy fiber glass printed circuit board was cleaned by scrubbing to remove corrosion and foreign material, coated by screen printing with the above composi¬ tion of Example 1 to a thickness of about 3 mils. Then a similar 3 mil thick coating was applied on a photo tool in accordance with the method given in U.S. 4,506,004, Example

2. The two coatings then were given a flash curing with

2 about 0.2-0.4 joules per cm of energy using a mercury vapor lamp, to effect partial hardening. The two coatings were then mated to form a composite of 6 mils thickness, flipped

2 over and given a main exposure with 0.5 joules per cm 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. , and finally

2 cured with 2.5 joules per cm of UV energy.

The cured solder mask exhibited the following perfor¬ mance characteristics: a smooth, uniform, glossy, and flexible coating having 100% adhesion under a cross-hatch tape test both before and after application of solder, resistance to organic solvents of greater than 15 minutes by immersion test; excellent legend ink adhesion character¬ istics; excellent heat resistance as measured by a 20-second dip in solder at a temperature of 260°C. without blistering; very good surface hardness and abrasion resistance; and excellent adhesion to the printed circuit board.

Example 8

The procedure of making a solder mask was repeated using the urethane acrylate of Ex. 2, which has dicarboxylic acid terminal groups, to provide another solder mask with advantageous properties.

Example 9

The procedures of the above examples were repeated using a polyol mixture of 20 mole % of dihydroxydiphenyl- sulfone 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.