Conformal Coating Systems

This invention concerns polymerizable compositions which have their primary utility in the area of coatings and sealants. The invention has particular utility in the area of conformal coatings for printed circuit boards and electronic components and will be described in connection with such utility, although other uses are contemplated.

Printed circuit boards and electronic components may be coated with a protective film to avoid or at least minimize degradation in electrical performance due to contamination. The most damaging, and usually the most prevalent contaminant generally is considered to be moisture or humidity. Excessive moisture or humidity will drastically lower insulation resistance between conductors, accelerate high-voltage breakdown and dendritic growth, and corrode conductors. Other contaminants which may damage printed circuit boards include various chemicals which may be residues of the manufacturing process such as fluxes, organic solvents, release agents, metal particles and marking inks, and contaminants which inadvertently may be deposited by human handling such as body greases, fingerprints, cosmetics and food stains. Ambient operating conditions may also contribute a variety of contaminants such as salt spray, dirt and dust, oil, fuel, acid, corrosive vapor and fungus. Although the list of possible contaminants is almost endless, in all but the most severe cases, their destructive action effectively can be eliminated by provision of a good conformal coating.

In addition to providing protection from contaminants, conformal coatings also provide a certain degree of protection to mechanical shock, vibration and tampering.

Various conformal coating systems are known in the art and are available commercially. Each has its advantages and disadvantages. One such prior art conformal coating system is based on acrylics. Acrylics are excellent coating systems from a production standpoint because they readily may be applied by spraying or brushing. Acrylic coatings have desirable electrical and physical properties and are fungus resistant. Additional advantages of acrylic coatings include long life, low or no exotherm during cure, and little or no shrinkage during cure. However, acrylic coatings typically are formed by solvent evaporation and reach optimum physical characteristics during cure typically in a matter of minutes. Conventional acrylic coatings are soluble in chlorinated solvents such as tricholorethane or methylene chloride.

Another prior art conformal coating system is based on polyurethanes. Polyurethanes are available as either single or two-component systems. Polyurethane coatings offer excellent humidity and chemical resistance and good dielectric properties. Single-component urethanes are relatively easy to apply and exhibit relatively long working pot life. However, single-component polyurethanes typically require a curing time of three to ten days at room temperature to reach optimum physical characteristics. Two-component polyurethanes typically achieve optimum cure at elevated temperatures within one to three hours, but exhibit relatively short working pot life. Also, surface preparation of substrate boards prior to application of polyurethane based coatings is important, since even minute quantities of moisture on a substrate board could produce blistering under humid conditions. Blisters, in turn, may lead to electrical failures and mandate costly rework. Polyurethane coatings are insoluble in most common solvents, which is a drawback to rework. Thus, replacement of a component on a polyurethane coated board requires a corrosive stripper to remove effectively all traces of the polyurethane film. However, extreme caution must be exercised when such a stripper is used, because the stripper also may corrode metallic surfaces on the board.

Epoxy resin systems also have been employed by the prior art for conformal coating printed circuit boards. Epoxy resins are available as two component systems only. Epoxy resin coatings provide good humidity resistance and high abrasive and chemical resistance. However, epoxy resins are virtually impossible to remove chemically for rework because any stripper that will attack the coating also will attack the epoxy coating of potted electronic components and the epoxy-glass of the printed circuit board as well. Thus, the only effective way to repair an epoxy resin coated board is to burn through the epoxy coating with a hot knife or soldering iron. However, burning introduces a cosmetic defect which is unacceptable to many consumers. Moreover, epoxy resins shrink somewhat during cure. Accordingly, a buffer material must be placed around fragile electronic components to prevent fracturing from shrinkage. Curing of epoxy systems can be accomplished in one to three hours at elevated temperature, or four to seven days at room temperature. Epoxy resins exhibit a relatively short working pot life which is an additional disadvantage.

Silicone resins also have been employed by the prior art for conformal coating circuit boards. Silicone resin coatings provide high humidity and corrosion resistance along with high temperature resistance which makes silicone resins preferred for coating printed circuit assemblies that contain high heat-dissipating components such as power resistors. However, silicone resins are relatively thick making them somewhate difficult to apply. Moreover, silicone resins require a relatively long cure, and repairability is difficult since silicone resins once cured are essentially insoluble, and cannot be vaporized with the heat of a soldering iron. Accordingly, the only effective way to repair a silicone resin coated circuit board is to mechanically remove the coating.

The prior art also has employed polyimides for conformal coating circuit boards. Polyimide coatings provide high-temperature, moisture and chemical resistance over extended periods of time. However, polyimide coatings require high temperature cure (one to three hours at 200.degree. to 250.degree. C.) which could damage heat sensitive electronic components, and this requirement severely limits the use of polyimide coating systems on most printed circuit board assemblies. Also, since polyimides are high-temperature, moisture and chemical resistant, the only effective way to repair a polyimide coated board is to mechanically remove the coating.

Diallyl phthalate varnishes also have been proposed by the prior art for conformal coating circuit boards and provide excellent high temperature and chemical resistance. However, diallyl phthalate varnishes require high temperature cure (approximately 150.degree. C.) which severely limits their use. And, diallyl phthalate coatings also typically must be mechanically removed to permit repair.

The preceeding discussion of background in the art of conformal coating circuit boards is taken largely from Printed Circuits Handbook, Clyde F. Coombs, Jr., Editor, McGraw Hill Book Company, Second Edition (1979).

As thus is apparent from the above discussion, none of the currently available conformal coating systems are completely satisfactory since all prior art conformal coating systems at best are a tradeoff of preferred application characteristics (i.e., processing speed, ease of application, pot life and cure conditions), preferred physical characteristics, (i.e., electrical properties, temperature, chemical and moisture resistance), and repairability.

It is thus a primary object of the present invention to provide an improved conformal coating system, i.e. method and materials which overcomes the aforesaid and other disadvantages of the prior art.

Other objects of the present invention are to provide an improved composition for conformal coating printed circuit board assemblies and the like which compositions demonstrate extended pot life and ease of application, rapid dry-to-touch cure, good electrical properties, superior adhesion and abrasion resistance, excellent high temperature, chemical and moisture resistance, and ease of repairability. Yet other objects will in part appear obvious and will in part appear hereafter.

The invention accordingly comprises the processes involving the several steps and relative order of one or more such steps with respect to each other, and the devices, materials and compositions possessing the features, properties and relations of elements which are exemplified in the following disclosure and scope of application of which will be indicated in the claims.

Generally, the foregoing and other objects of the invention are achieved by the provision of an ultraviolet (hereinafter "UV") curable polymerizable coating system having a secondary cross-linking curing mechanism built in. More specifically, in accordance with the invention the polymerizable system is a one component coating system comprising at least one urethane-acrylate or methacrylate, a mono or polyfunctional acrylate or methacrylate reactive diluent having at least one allylic reactive bond, a polymerization initiator of the photoinitiator type and a metal drier. A non-allylic reactive diluent is preferably also included. A particular feature and advantage of the instant invention is that the UV radiation cure produces extremely rapid dry-to-the-touch cure of all exposed areas of the coating thereby permitting substantially immediate handling of the coating products and maintenance of the shape of the coating which might otherwise sag and creep, while secondary curing which occurs by crosslinking through the allylic bond of the mono or polyfunctional reactive acrylate or methacrylate diluent provides substantially complete cure of unexposed (shadow) areas of the coating under conditions of ambient temperature and humidity.

For a fuller understanding of the nature and objects of the present invention, reference should be made to the following detailed description of the invention taken in connection with the accompanying examples.

The terms "liquid", and "solid" or "dry-to-the-touch" are used herein with reference to physical properties of the materials, are to be understood as to be referring to such properties as they exist under conditions as may be specified. For example, the term "liquid" refers to a physical state wherein a material is readily flowable while the terms "solid" and "dry-to-the-touch" are to be understood as referring respectively to physical states wherein a material is resistant to change in shape and is without free surface moisture or surface tackiness.

The urethane-acrylates or methacrylates employed in the coating system of the instant invention preferably comprise polyester-urethane-acrylates formed by reacting a polyester diol with a diisocyanate, and reacting the resulting product with a polymerizable acrylic or methacrylic acid ester. The preferred polyester diol is manufactured by the Indolex Company under the trade name Inolex 1400-120. This polyester diol is formed by reaction of neopentyl glycol and 1,6-hexanediol with adipic acid. Other polyester diols useful are formed by the reaction of neopentyl glycol and a diol of more than 3 carbon atoms, e.g., 1-4-butanediol with adipic acid. The preferred diisocyanate is toluene diisocyanate (TDI) although other relatively low molecular weight diisocyanates of the general formula:

wherein R is a C.sub.2-20 alkylene, alkenylene or cycloalkylene radical or a C.sub.6-40 arylene, alkarylene or aralkylene are useful.

The reaction ratio of the equivalent weights of the polyester diol to the diisocyanate should generally be in the range of about 1.0 of polyester diol to about 1.7 to about 2.2 of diisocyanate. The preferred reaction ratio is 1.0 equivalents of the polyester diol for every 1.9 equivalents of diisocyanate. This preferred reaction ratio is particularly important to the properties of the final composition. The 1:1.9 ratio yields a composition with a chemical structure having a high degree of flexibility and stability. The copolymer formed is generally of a random copolymer structure. This copolymer product is then reacted with an hydroxyl-containing acrylate or methacrylate ester monomer, producing acrylate end-capping. The useful range of equivalents of the ester monomers is about 0.9 to about 3.0, the preferred being in the range of 1.6 to 2.0, and the most preferred being 1.8 equivalents.

The polymerizable acrylate and methacrylate ester monomers used to cap the polyester diisocyanate reaction product may be mono- or difunctional. Mono-functional monomers are preferred. Those monofunctional monomers found most effective are selected from the group consisting of hydroxyalkyl acrylates and methacrylates, amino alkyl acrylates and methacrylates. The most preferred polymerizable ester monomers are hydroxyethyl methacrylate and hydroxypropyl methacrylate. Additional monofunctional polymerizable ester monomers deemed useful are represented by the following formula: ##STR1## wherein X is --O-- or ##STR2## R.sup.3 is hydrogen or lower alkyl of 1 to 7 carbon atoms; R.sup.1 is selected from the group consisting of hydrogen, chlorine and methyl and ethyl radicals; and R.sup.2 is a divalent organic radical selected from the group consisting of lower alkylene of 1-8 carbon atoms, phenylene and naphthylene.

Suitable hydroxy- or amine-containing materials are exemplified by, but not limited to, such materials as hydroxyethyl acrylate, hydroxyethyl methacrylate, aminoethyl methacrylate, 3-hydroxypropyl methacrylate, aminoethyl methacrylate, hydroxyhexyl acrylate, t-butylaminoethyl methacrylate, hydroxyoctyl methacrylate, and the monoacrylate or monomethacrylate esters of bisphenol-A, the fully hydrogenated derivative of bisphenol-A, cyclohexyl diol, polyethyleneglycol methacrylate, and the like.

Preferably but not necessarily the end-capping reaction will be accomplished in the absence of diluents. In the event diluents are employed they preferably will comprise mono or polyfunctional reactive acrylate or methacrylate compounds.

The mono or polyfunctional reactive acrylate or methacrylate reactive diluents employed in the coating system of the present invention comprise allylic-bond containing acrylates or methacrylates of the formula: ##STR3## in which R.sup.4 is a radical selected from the group consisting of hydrogen and lower alkyl of 1 to 4 carbon atoms, and R.sup.5 is an organic radical containing at least one allylic bond and does not contain any group which may adversely affect the coating system for purposes of disclosed herein. Preferably R.sup.5 will consist of an allylic-bond containing aliphatic or substituted aliphatic carbon ring structure of which dicyclopentyloxyethyl is presently preferred. Other allylic-bond containing carbon ring structures such as dicyclopentenyl which is given as exemplary may also be advantageously employed.

Other non-allylic reactive diluents may also be included in the coating composition. Such diluents are preferably acrylate or methacrylate compounds of which hydroxyethyl methacrylate (HEMA) is the most preferred compound.

The urethane acrylate or methacrylate typically comprises about 20 to about 50% by weight of the reactive mixture, preferably will cpmprise about 20-30% by weight, while the allylic group containing acrylate or methacrylate and the non-allylic reactive diluent typically comprise, respectively about 10 to about 50% and about 60 to about 20% by weight of the reactive mixture; most preferably each will comprise about 35% by weight of the reactive mixture.

Ultraviolet ("uv") activated polymerization initiators are incorporated into the coating system. Many uv activated polymerization initiators are known in the art and may be advantageously employed in the invention. For example, the uv activated initiators may be selected from metal carbonyls of the formula M.sub.x (CO).sub.y wherein M is a metal atom, x is 1 or 2, and y is an integer determined by the total valence of the metal atoms, generally 4 to 10. The preferred uv activated initiators are selected from (a) C.sub.1-16 straight or branched chain alkyl diones; and (b) carbonyl compounds of the general formula R.sup.5 (CO)R.sup.6 wherein R.sup.5 is a C.sub.1-10 alkyl, aryl, aralkyl or alkaryl group, and R.sup.6 is R.sup.5 or hydrogen. In addition, R.sup.5 or R.sup.6 can contain any substituents which do not adversely affect the compound in serving its function. For example, R.sup.5 or R.sup.6 can be alpha-substituted with an alkyl, aryl, alkaryl alkoxy or aryloxy radical, or with an amino or a mono- or dialkylamino derivative thereof, each of the above substituents containing up to about six carbon atoms. In addition, R.sup.5 and R.sup.6 taken together with carbonyl group form an aromatic or heterocyclic ketone containing up to about 16 carbon atoms.

The polymerization initiators are usually employed in amounts of about 1% to about 10% by weight of the urethane acrylate or methacrylate compound of the coating system.

It is useful, but not required to incorporate an adhesion promoter into the coating system. The adhesion promoter may be chosen from any of those commonly known to the person reasonably skilled in the art. Two types of promoters are preferred. The first type consists of mono- and dicarboxylic acids which are capable of copolymerizing with the urethane acrylate or methacrylate compound. The preferred species of these classes are methacrylic and acrylic acid. Such acids are used in proportions of 1-20% by weight of the coating system, preferably in proportion of 1-10%. The second preferred type of adhesion promoter is of the well-known silane type, present in the composition in proportions of 1-10% by weight of the coating system.

It is optional, but recommended, that chelators and inhibitors also be added to coating system for optimum performance. Chelators and inhibitors are effective in amounts of about 0.1 to about 1% by weight of the coating system. Ethylenediamine tetra-acetic acid and its sodium salt (Na.sub.4 EDTA), 1,1-ethylenebis-nitril methylidyne dipyridine and the class of beta-diketones are generally the most effective and are preferred.

The inhibitor concentration left over in the monomers from manufacture is often high enough for good stability. However, to insure maximum shelf life, the proportions mentioned above (about 0.1 to about 1% by weight of the coating system) are recommended. Of those inhibitors which have been found of adequate utility is the group consisting of hydroquinones, benzoquinones, naphthoquinones, phenanthraquinones, anthraquinones, and substituted compounds of any of the foregoing. Additionally, various phenols can be employed as inhibitors, the preferred one being 1,6-di-tert-butyl-4-methyl phenol.

The amount of thickness, viscosity, or thixotropy desired can be varied in accordance with the particular application required. Thickeners, plasticizers, and various other agents common to the art can be employed in any reasonable manner to produce the desired characteristics.

It is also optional, but recommended, that surfactants be present in the adhesive composition for optimum performance. Selection of suitable surfactants is a matter of simple experimentation. Obviously the surfactant must be soluble in the polymer composition and should be non-reactive with the prepolymer composition. The surfactants may be anionic materials such as the petroleum sulfonates having the formula:

wherein n is more than 20 and Me is a metal of valence x. Such materials are sold by, among others, Witco Chemical Corp., New York, N.Y., under the trademark "Alconate" 80, and by the Penreco Division of Penzoil, Butler, Pa., under the trademark "Petrobase."

Another suitable anionic type comprises the sodium alkyl or alkylaryl sulfonates having the formula: ##STR4## Such materials are sold by E. I. duPont de Nemours & Co., Wilmington, Del., under the trademarks "Alkanol" 189-S and "DW" and the trademark "Duponol," and by Union Carbide Corp., New York, N.Y., under the trademark "Tergitol" (numerical series).

Still another useful anionic class is the sulfonated ethoxylated types sold as the "Alipal" series by GAF Corp., New York, N.Y., and as "Neodol" 25-35 by Shell Chemical Co., Houston, Tex.

Non-ionic surfactants such as Union Carbide L-5320 may also be usefully employed in the inventive compositions.

Surfactant concentration will depend in any given case upon the particular surfactant and monomer composition being used. Optimization will be a matter of routine experimentation within the skill of the art. Ordinarily, for anionic surfactants, however, a minimum concentration of about 0.10 percent surfactant by weight of the coating composition will be needed to achieve an acceptable degree of emulsifiability, and a concentration of at least about 0.5 percent usually will be preferred. The maximum surfactant concentration will usually be about ten percent since above this level the surfactant may begin to interfere with the properties of the coating composition by adversely affecting, for example, its cure rate, stability or the cured products. As a practical matter, an upper concentration limit of about five percent, is usually satisfactory. For non-ionic surfactants the optimal concentration may be lower, typically around 0.1% or less.

The coating system of the present invention is designed primarily as a uv curing product; however, due to the configuration of many circuit boards, there are areas of the board that are in the shadow of other components such that they cannot be cured by uv light. To overcome this deficiency, a second curing mechanism has been built into the coating system. As noted supra, the reactive diluents employed in accordance with the present invention comprise acrylate or methacrylate compounds containing one or more allylic bonds. These allylic bonds are available to undergo significant crosslinking in the presence of appropriate metal driers. Useful metal-driers include salts of metals with a valence of two or greater and unsaturated organic acids. Amongst suitable compounds are mentioned the linoleates, naphthenates and resinates of cobalt, manganese, cerium, lead, chromium, iron, nickel, uranium and zinc. The metal driers are usually employed in concentrations of less than between about 0.01 and five percent by weight of the coating system. Cross-linking of the allylic bonds typically will provide cure of unexposed areas of the coating in twenty-four hours.

It is frequently desirable to add low levels, such as up to about 500 parts per million by weight, of a free-radical or uv stabilizer, many of which are known in the art, to prevent spurious polymerization of the composition prior to the time of its intended use. Suitable free-radical stabilizers are hydroquinone, p-benzoquinone, butylate of hydroxytoluene, and butylate of hydroxyanisole.

The invention will now be illustrated by the following description of a specific preferred embodiment thereof, given by way of example only.

A nitrogen-swept, four-necked UV shielded resin kettle equipped with a stainless steel stirrer, nitrogen inlet tube, thermometer, condensor, and entrance ports, was heated to approximately 40.degree. C., 165 grams of MOBAY TDI toluene diisocyanate added, and 460 grams of 1,6-hexanediol/neopentyl glycol adipate (Inolex 1400-120 polyester, Inolex Company, Philadelphia, PA) was slowly added over 20 minutes. At the completion of the polyester addition, heating was continued with stirring (40.degree. to 45.degree. C. temperature) for one hour, followed by two hours heating with stirring at 100.degree. C. Upon completion of the three hour reaction period, the bath temperature was lowered to 50.degree. C., and 230 grams of hydroxyethylmethacrylate was added. The reaction mixture was then heated, maintained at 50.degree. C. and stirred for two hours at that temperature.

The reaction mixture was permitted to return to room temperature. To 170 g of the product resin were added the following ingredients, with stirring: 275 g dicyclopentenyloxyethyl methacrylate; 29 g acrylic acid; 5.5 g glycidoxypropyltrimethoxysilane; 20 g Ciba-Geigy Igacure 651 dimethoxy-2-phenylacetophenone; 0.5 g Union Carbide L-5320 silicone surfactant and 1.5 g 3M Company Fluorad FC 430 leveling agent. The mixture was stirred until all ingredients were dissolved. The resulting solution contained about 34% of polyester-urethane-methacrylate resin (PEUMA).

A mixture containing 66.4% of the above resin solution, 33.2% of hydroxyethylmethacrylate and 0.3% of a 6% solution of Cobalt Naphthanate was used in the following working example.

Spray the resulting blend onto the top side surface (i.e. component side surface) of a printed circuit board. Cure of the coating was effected by exposing the boards to UV radiation (3650 angstroms, 60,000 microwatts per square centimeter). Those areas of the coating exposed to UV radiation were solid and dry to the touch within 10-45 seconds. Areas in the shadow of components, i.e. not receiving direct UV radiation, however, were observed to be wet to the touch, but found to be fully cured after standing at ambient temperature and humidity conditions for 5 days. Circuit boards were sliced through shadow areas after 5 days and the coating was found to be fully cured to the board.

Coated boards were placed on a rack in a stainless steel tank, and 500 grams of water added to the bottom of the tank. The tank was closed, sealed, and heated to a temperature of 115.degree. C. After heating for 40 hours, the tank was allowed to cool. The tank was then opened, and the boards removed and examined. No visible blistering or other evidence of coating failure was observed.

The procedure of Example I was repeated except an equal amount of Alcolac SIPOMER DCPM dicyclopentenyl methacrylate was employed in place of the dicyclopentenyloxyethyl methacrylate. Similar results to Example I were obtained.

The procedure of Example I was repeated except an equal amount of dicyclobutadiene methacrylate was employed in place of the dicyclopentenyloxyethyl methacrylate. Similar results to Example I were obtained.

As will be appreciated, the foregoing invention provides novel and improved coating systems for conformal coating printing circuit boards and the like. The coating systems may be applied to circuit boards assemblies and the like by any convenient manner, for instance a spraying, brushing, dripping, rolling, dipping, etc. Moreover, the coating systems cure through dual mechanisms including uv cure which permits fast fixture cure thereby achieving almost immediate dry-to-the touch curing. However, unlike conventional uv cured products, the coating systems of the present invention do not require full area irradiation to achieve full cure due to their built-in secondary cure mechanism. Moreover, the cured coatings have excellent adherence to plastics, metal, glass and wood, good abrasion resistance, and are hydrolytically stable and resistant to thermal cycling. The coatings also are repairable, i.e., can be removed by selective solvents such as methyl ethyl ketone, and then replaced by brush or spray, and uv cured.

The invention has been described particularly with applications to conformal coating circuit board assemblies. However, one skilled in the art would appreciate that the coating systems may be applied to other electrical electronic components such as transformers or the like. Moreover, the coating composition is not limited to the use in the electronics field but may be employed in any industrial area where conformal protective coating is desired.