Background Art This invention is directed to a novel composition useful in preparation of solder mask coatings. More specifically, the invention relates to a liquid photopolymerizable composition which is capable of application by means of screen printing to a substrate, following which the composition may be converted to a dry, highly resistant coating without necessity for evaporation of solvents and/or thermal curing.

Solder mask coatings are typically used on quality printed circuit boards where molten solder, by a process known as wave soldering, is used to insure good interconnections between the components and the circuitry. In such cases, a solder mask must be extremely resistant to heat, impervious to a wide range of solvents, strongly adherent to a wide variety of metallic and non-metallic substrates, and chemically resistant to the conventional rosin-based fluxes. Furthermore, since solder masks are usually left on the circuitry as a protective coating, same must also have long term durability and excellent electrical insulating properties.

Conventional solder mask or resist coatings of the prior art typically contain a significant amount of solvent, which must be removed from the coating, and also typically are based on systems which require thermal curing. Such systems consume a great deal of energy and time, and the solvent vapors contained therein are typically pollutants. For these reasons, over the last several years there has been increased interest in utilizing solder masks which are curable by ultraviolet radiation as opposed to the foregoing systems, the incentive for same being obvious. Recently, U.S. Patents 4,003,877 and 4,064,287 have issued which disclose a photopolymerizable composition which is asserted to be useful as a solder

mask. The photopolymerizable materials disclosed therei are screen-printable, a conventional technique for th application of solder resists or masks to a substrate, an contain from 35 to 70 percent by weight of an aryloxyalky acrylate monomer or prepolymer, from about 15 to about 4 percent by weight of a photopolymerizable diluen containing at least one terminal ethylenic group, and free-radical generating system activatable by actini radiation. I have now found a composition, suitable for us as a solder mask, which is based on the foregoin aryloxyalkyl acrylates together with hereinafter define diacrylate diluents and styrene and its monofunctiona analogs.

Disclosure of Invention

In accordance with the invention there i provided a photopolymerizable composition suitable for us as a solder mask, the composition comprising from about 1 to about 85 percent by weight of an aryloxyalkyl acrylate from about 15 to about 85 percent by weight of diacrylate diluent, and an effective amount of monofunctional monomer selected from the group consistin of styrene and its monofunctional analogs.

This composition provides increased wave solde resistance over prior art solder masks.

Detailed Description The aryloxyalkyl acrylate monomers an prepolymers useful- in - my invention are the reactio products . of epoxy compounds or omega-hydroxy ethers an unsaturated carboxylic acids. The carboxylic acids may b mono- or dicarboxylic acids containing from 3 to 18 carbo atoms. Alpha, beta-unsaturated carboxylic acids such a acrylic and methacrylic acids are preferred.

Monomers and prepolymers of the above type ar commercially available under the tradenames of "Epocryl"

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(Shell Chemical Company), "Derakane" (Dow Chemical Company), "Nupol" (Freeman Chemical Company), and SR-348 and SR-349 (Sartomer Resins Company).

The use of aryloxyalkyl acrylate monomers and prepolymers in dry-film photoresists suitable for the manufacture of etched parts has been disclosed in U.S. Patent No. 3,661,576.

The aryloxyalkyl acrylates can be described as having the following general formula:

wherein X may be methylene, an alkyl-substituted methylene group, a dialkyl-substituted methylene group, a carbonyl group, a sulfide, a sulfoxide, a sulfone, an amine, an alkyl-substituted amine, an ethylene ether, a propylene ether, a 2-hydroxypropylene ether, wherein each alkyl group may have from 1 to 8 carbon atoms, or any combina¬ tion thereof; Y may be ethyl, propyl, 2-hydroxypropyl, or other lower alkyl or hydroxyalkyl groups having up to 8 carbon atoms; A may be an unsaturated acryloxy group having from 3 to 18 carbons, preferably acryloxy or methacryloxy; and n may be 9 through 20, m may be 0 or 1, and p may be 0 or 1.

In addition, the aromatic rings shown in the formula may be ring-substituted with 1 to 4 additional substituents such as chlorine or bromine.

I have ascertained that from 15 to 85, and preferably from 15 to 40, percent by weight of the aryloxyalkyl acrylates can be included in my photopolymerizable composition.

In addition to the foregoing, a diacrylate diluent must be included in the composition, examples of

such diluents including tetraethylene glycol diacrylate tetraethylene glycol dimethacrylate, 1,3-butylene glyco diacrylate, 1,3-butylene glycol dimethacrylate, diethylen glycol diacrylate, diethylene glycol dimethacrylate triethylene glycol diacrylate, triethylene glyco dimethacrylate, polyethylene glycol dimethacrylate pentaerythritol triacrylate, trimethylolpropan triacrylate, 1,6-hexane diol diacrylate, polyethylen glycol diacrylate, and divinyl benzene. Of th foregoing, the 1,3-butylene glycol diacrylate, triethylen glycol diacrylate and tetraethylene glycol diacrylate ar preferred diluents, with the latter being the mos preferred. From about 15 to about 85, and preferably fro about 40 to about 70, percent by weight of the compositio can be made up of the diacrylate diluent.

U.S. Patent No. 4,003,877 teaches that mono functional monomers may be included as polymerizabl diluents, but same are taught to be generally undesirabl because of volatility, and because the cured .coatin resulting therefrom exhibits a lack of durability Surprisingly, I have ascertained that the inclusion of a effective amount of certain monofunctional monomers ar essential to a production of a reproduceable coatin having the required flexibility, adhesion, resistance t molten solder and solvent resistance. These monofunc tional monomers include styrene and its monofunctiona analogs, examples of same being vinyl toluene, -methy styrene, and N-vinyl-2-pyrrollidone, with vinyl toluen being preferred. T e specific concentration level of th foregoing monofunctional monomers in the compositio depend on the concentration of the diacrylate diluen component thereof when compared to the aryloxyalky acrylate component. For example, when the ratio of th diacrylate diluent to the aryloxyalkyl acrylate is about

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25 to 75, I have ascertained that from about 1 percent to about 50 percent by weight of the monofunctional monomer can be utilized to impart increased wave solder resistance to the composition. When the concentration of the diacrylate diluent is increased to provide a ratio to the aryloxyalkyl acrylate of 50 to 50, or further to 75 to 25, the concentration of monofunctional monomer providing the increased wave solder resistance is from about 1 to about 30 percent by weight of the composition. Elimination of the monofunctional monomers from the composition has provided products having reduced wave solder resistance.

In addition, a photoinitiator system must be used in the composition, same being preferably those which are activatable by actinic light and thermally inactive at 185°C and below.

The concentration thereof in the composition varies widely depending on the particular photoinitiator selected. The optimum amount, however, can be readily determined by simple experimentation. Preferred initi¬ ators are the acyloin ethers such as the benzoin ethers, particular benzoin isobutyl ether, alkyl-substituted anthraquinones such as 2-tert-butylanthraquinone, and , -diethoxyacetophenone. The most preferred initiator is 2,2-dimethoxy-2-phenylacetophenone, commercially available as Irgacure 651 from Ciba Geigy.

Other initiators which may be used include the substituted or unsubstituted polynuclear quinones, such as 9,10-anthraquinone; 1-chloroanthraquinone; 1-chloroanthra- quinone; 2-methylanthraquinone; 2-ethylanthraquinone; octa ethylanthraquinone; 1,4-naphthoquinone;

9,10-phenanthraquinone; 2,1-benzanthraquinone;

2,3-benzanthraquinone; 2-methyl-l,4-naphthoquinone; 2,3-dichloronaphthoquinone; 1,4-dimethylanthraquinone; 2,3-dimethylanthraquinone; 2-phenylanthraquinone;

2,3-diphenylanthraquinone; sodium salt of anthraquinone alpha-sulfonic acid; 3-chloro-2-methylanthraquinone;

retenequinone; 7,8,9,10-tetrahydronaphthacenequinon l,2,3,4-tetrahydrobenz(a)anthracene-7,12-dione.

The following photoinitiators, described in U.S Patent No. 2,760,863, some of which may be thermal active at temperatures as low as 85°C, are also useful vicinal ketaldonyl compounds, such as diacetyl and benzil alphaketaldonyl alcohols, such as benzoin and pivaloi alphahydrocarbon-substituted aromatic acyloins; alph ethylbenzoin; alpha-allylbenzoin; and alpha phenylbenzoin.

Silver persulfate is also useful as free-radical generating initiator activatable by actini radiation. Certain aromatic ketones, e.g., benzophenon and 4,4'-bis-dialkylaminobenzophenones, are also useful. In addition to the components of the photopoly merizable composition of my invention enumerated above additional materials are typically added in order t obtain the properties necessary for using the compositio as a screen imaging composition. Such materials ca include thermal polymerizable inhibitors, which typicall act as antioxidants and stabilizers and include p-methoxy phenol, hydroquinone and alkyl and aryl-substituted hydro quinones and quinones, tert-butyl catechol, pyrogallol copper resinate, napthyl-amines, beta-naphthol, cuprou chloride, 2,6-di-tert-butyl p-cresol, 2,2-methylenebis (4-ethyl-6-t-butylphenol) , phenothiazine, pyridine, nitro benzene, dinitrobenzene, p-toluquinone, chloranil, ary phosphites, and aryl alkyl phosphites.

To optimize my composition for use as a scree printing coating, it is essential that same have appro priate viscosity and thixotropic properties. While usefu compositions can be obtained by the combination of th main components listed above, it is nevertheles generally desirable to add thixotropic agents, levelin agents, and defoamers to achieve a viscosity of from abou 5,000 to 200,000 centipoise and a thixotropic index o from about 1.00 to 6.00.

The thixotropic agents which may be used are well known to those skilled in the art. Examples of these materials are Bentone {a trademark of National Lead Company for an organic base salt of a clay mineral, e.g., montmorillonite) and other silicate-type materials. Other thixotropic agents are the aluminum, calcium and zinc salts of fatty acids, such as lauric or stearic acid, e.g.. Zinc Soap No. 26 (trademark of the Witco Chemical Co., Inc.); and fumed silicas such as Cab-o-Sil and Aerosil (trademarks of the Cabot Corporation and Degussa Corporation, respectively).

The leveling agents and defoamers which may be used include Modaflow and Multiflow, trademarks of the Monsanto Company for resin modifiers. Other leveling and flowout agents include aluminum stearate, calcium stearate, sucrose benzoate, and high molecular weight nonionic surface active agents.

Additional ingredients may also be added to the coatings of my invention, which can include plasticizers, pigments or colorants, fillers, and adhesion promoters.

The skilled artisan may readily ascertain the desirable amount of such materials.

Commonly used fillers are barium sulfate, talc, calcium carbonate, zinc oxide and silicas and silicates. Suitable plasticizers, used to increase the flexibility of the film, include the polyfunctional esters such as dioctylphthalate, tricresyl phosphate, polyethylene glycol acetate, and pentaerythritol tetramercaptopropionate.

In practicing the process of the invention, any of the well known screen printing techniques may be employed. The photopolymerizable material is applied by pouring a measured quantity on the screen. The squeegee is drawn uniformly and with even pressure to scrape the ink across the entire surface of the screen, thereby transferring the ink to the substrate below. Film thick¬ nesses ranging from 0.006 mm to 0.130 mm are applied by this procedure. Film thickness can be measured by such

means as micrometer or beta-ray backscattering.

After the print is completed, the screen lifted; then the wet substrate is removed and pass through the ultraviolet radiation source. Such sourc include carbon arcs, mercury vapor lamps, fluoresce lamps with ultraviolet radiation-emitting phosphors, arg glow lamps, electronic flash units and photographic flo lamps. Of these, the medium pressure mercury vapor lam are the most suitable. The period of exposure, as will readily understood by those skilled in the art, will dependent on the film thickness, the light intensity, t distance from the light source to the coating, and t temperature of the operation. A typical exposure ti using a 200 watt per linear inch medium pressure mercu vapor lamp at a distance of 4 inches is about 5 second After the exposure, the coating is completely cured a may be passed directly to the next processing step modify the uncoated portion of the substrate.

The photopolymerizable composition of my inve tion will now be more specifically described by the use the following non-limiting examples, wherein all parts a by weight unless otherwise specified.

In the following examples, the substrate co sisted of a 0.062 inch thick laminate (epoxy glass) havi printed wiring (copper) on one side. Typically, supplie of such substrates are Westinghouse, NVF Corp. and Gener Electric.

Prior to applying a coating of my compositio the surface of the printed circuit board was cleaned usi a soft cloth and a cleaner designed to remove excess oxid from the copper surface thereof, in this case a cleane commercially available as Shipley 11.

The solder mask was applied by screen printi the desired pattern over the printed circuitry. Th examples herein utilize a 280 mesh stainless steel scree which applied 1.0 mil thickness of coating. ^"

The solder mask coating was cured using a singl

lamp Ashdee 200 watt per inch medium pressure ultraviolet radiation machine. Three passes through the machine at 8 feet per minute per path were utilized. This cure amounts to a power density integral of between 0.5 and 0.6 watts per square centimeter of exposed surface area. The cured board was then allowed to age a minimum of 24 hours prior to testing.

Sample boards were tested for a number of parameters, among which were solder wave resistance, adhesion, and solvent resistance. The first step was to pass the board over a wave of liquid tin/lead (60-40) solder at 500°F. The machine utilized for such tests was a Hollis Wave Reflow machine. Immediately prior to reflow, the surface to be reflowed, i.e. , that onto which the solder mask has been applied, is coated (brushed) with a flux to aid in the reflowing procedure. The flux utilized was Alpha 711-35. Following fluxing, the fluxed board was mounted in a processing rack which was then fastened to the conveyor chain on the reflow machine. The conveyor moves the board over an infrared preheater section and then over the solder wave, with approximately 3 to 5 seconds dwell time in the wave being obtained.

Once the board has been soldered, it is removed from the rack and examined. Failure of the solder mask to withstand this wave test can be exhibited by blistering or delamination of the solder mask coating, by severe discoloration, and by softening to the point where the solder has wicked under the solder mask.

The second step in the evaluation was to remove the flux, which should be done within several minutes of reflow if possible. Removal is carried out by immersing the board in a vapor degreaser containing solvent, which was 1,1,1-trichloroethane in the examples. Immersions in the solvent vapors were undertaken for 2 minutes. Upon removal from the degreaser, the board was examined for attack, same being exhibited by swelling, lifting, or severe softening of the solder mask coating.

In addition to examination for solvent attack, the surfac was observed as to how well same cleaned up during th degreasing step, the surface normally being clean, glossy and free from scum or residues if an acceptable coating i present.

For comparison purposes, a number ranging from to 5 was assigned to the coating during evaluation, wit the higher the number the better the coating performance. Visual observation was utilized to ascertain the numerica rating assigned to an example.

In particular, 2 parameters are observed. Th first was the hold, or how well the coating contained th solder while going over the wave. For example, if th openings do not enlarge or undercut, the hold is ascer tained to be good. The second parameter was delamination, which was used to determine how well the coating remaine adhered to the substrate during the wave operation. I general, two forms of delamination may be observed. Th first is a slight lifting, but without breakage of th film. Same can be noticeable by light colored spots. Th second type of delamination is more severe, that bein where the coating actually ruptures and breaks apart. Th numerical designations given for the formulations whic follow are as follows: 1. gross coating degradation - unacceptable

2. medium amounts (50) percent of area) of coating degradation - unacceptable

3. minor amounts (10 percent of coating area) of coating degradation - unacceptable 4. minute amounts or isolated areas of coating degradation - marginally acceptable 5. no coating degradation - acceptable.

Example 1 To a suitable container was added 21.7 parts o tetraethylene glycol diacrylate and 65.9 parts of diacrylated bisphenol A resin. The mixture was brought to

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60°C, following which Irgacure 651, tradename for a photoinitiator from the Ciba Geigy Company, in the amount of 4 parts, were added, using gentle stirring until solvation occurred, at approximately 10 minutes. A blue pigment dispersion, in the amount of 2 parts, was added and mixed in completely. Then, Aerosil 380, tradename for a fumed silica from the Degussa Company, in the amount of 8.0 parts, was added and gently stirred until a uniform mixture was obtained. Following cooling to room temperature, milling of the mixture was undertaken on a conventional 3 roll mill, using a 0.020 inch orifice setting between the slowest and medium speed roll, a 0.005 inch orifice between the medium and fastest speed rolls.

In accordance with the aforementioned valuation of the coating, same was given a 1.0 value relative to the wave solder test. hen 4.2 percent vinyl toluene was added to the foregoing mixture, the wave test indicated a 4.5 value, and similar results were obtained at levels of vinyl toluene of approximately 50.0 percent, while keeping the ratio of the tetraethylene glycol diacrylate to the acrylated bisphenol A of 25 to 75.

When the ratio of tetraethylene glycol diacry¬ late to the diacrylated bisphenol A is reduced to 50 parts per 50 parts, similar results were obtained through a level of vinyl toluene of approximately 30 percent by weight.