GB2318796A - Coating compositions - Google Patents

Abstract

Photopolymerizable material, suitable for use in the production of photopolymerizable coatings or resists on, for example, printed circuit boards, is an ethylenically unsaturated polymerizable reaction product of an epoxidized compound of a condensate of a phenolic compound and an aromatic aldehyde and a mixture of (i) a major proportion of an ethylenically unsaturated carboxylic acid and (ii) a minor proportion of a saturated aliphatic or aromatic carboxylic acid; further modified by reaction with a polybasic-carboxylic acid anhydride.

Description

COATING COMPOSITIONS This invention is concerned with improvements in and relating to coating compositions and more particularly is concerned with coating compositions for the production of photopolymerizable coatings upon substrates.

In order to provide for the attachment of electrical components to a printed circuit board of the type comprising a patterned layer of an electroconductive metal (generally copper) on an electrically non-conductive substrate (generally a plastics impregnated substrate) a patterned layer of a solder resist is applied to the board so as to expose parts of the patterned metal layer and the board coated in this manner is contacted with molten solder so that the solder adheres to the patterned metal layer at the exposed portions thereof. Generally, before the board is contacted with the solder, the electrical components are placed on the other side of the board (although, of course, not in the case of surface-mounted boards) with conducting elements extending therefrom passing through holes in the board into the exposed portions of the patterned metal layer.

A radiation curable vehicle for coating and printing inks is described in US-A-4 100 045. This vehicle comprises the reaction product of a polyepoxide, a saturated dicarboxylic acid, an ethylenically unsaturated monocarboxylic acid and optionally, a saturated monocarboxylic acid. the particular polyepoxide described are glycidyl polyethanes of polyhydric alcohols or polyhdric phenols having an epoxy equivalent weight of from 150 to 2000.

It has now been found, that a patterned solder resist may be formed from a photopolymerizable composition comprising a particular modified epoxy resin, as hereinafter defined, by exposing a layer of such a composition to actinic radiation through a suitable transparent substrate bearing an opaque image, and subsequently, in a "development" step, removing unexposed composition with a solvent therefor, namely an aqueous alkaline solution. These compositions yield a film of superior hardness upon curing., as compared with that obtained using the compositions described in US A-4 100 045.

Thus, according to the present invention there is provided an ethylenically unsaturated polymerizable material obtainable as a reaction product of an epoxidized compound of a condensate of a phenolic compound and an aromatic aldehyde and a mixture of (i) a major proportion of an ethylenically unsaturated carboxylic acid and (ii) a minor proportion of a saturated aliphatic or aromatic carboxylic acid, further modified by reaction with a polybasic-carboxylic acid or anhydride thereof.

The invention also provides a coating composition, for the production of a photopolymerizable coating, comprising a modified reaction product as described above together with a photopolymerization initiator for the said polymerizable reaction product and a liquid carrier.

Preferably, the composition also contains an inert inorganic filler, the said filler and the said reaction product suitably being present in a ratio of from 5 to 25 parts by weight of filler to from 95 to 75 parts by weight of the modified reaction product.

The invention also provides a method of forming a photopolymerizable coating upon a substrate which comprises applying a coating of a coating composition as defined above to the substrate and allowing it to dry by evaporation of the liquid carrier.

The polybasic-carboxylic acid anhydride may preferably be a dicarboxylic acid anhydride.

The phenolic compound starting material may be any appropriate derivative of phenol or of a substituted phenol, e.g. a cresol derivative.

The aromatic aldehyde may for example be an aromatic aldehyde having a phenolic hydroxy group.

The modified reaction product of the epoxidized compound of a condensate of a phenolic compound and an aromatic aldehyde and the ethylenically unsaturated carboxylic acid, generally acrylic acid or methacrylic acid, as well as the saturated acid is hereinafter simply referred to as the "modified epoxy". The modified epoxy should preferably be solid or semi-solid at ambient temperature, e.g. should preferably have a ring and ball softening point (determined according to British Standard Specification No. 4692 of 1971) of at least 30"C, preferably at least 60"C.

The epoxidised compound is first reacted with a mixture of a major proportion of the ethylenically unsaturated carboxylic acid and a minor proportion of a saturated aliphatic or aromatic carboxylic acid to form the modified epoxy. Suitably the said mixture comprises from 65 to 90 mole% of unsaturated acid, preferably 75 to 85 mole% thereof, the balance being the saturated acid. Suitable unsaturated acids include monocarboxylic acids such as acrylic and methacrylic acids. Suitable saturated aliphatic acids include aliphatic monocarboxylic acids containing from 2 to 22 carbon atoms, preferably 4 to 16 carbon atoms, such as ethanoic, pentanoic, iso-butyric and stearic acids. Suitable saturated aromatic acids are acids containing from 7 to 9 carbon atoms, such as benzoic, methylbenzoic and ethylbenzoic acids. The use of such saturated acids has been found to improve the toughness and/or flexibility of the acid coating finally formed from the modified epoxy acrylates. The incorporation of a suitable saturated acid can improve both the flexibility of the coating and the drying window within which the solder resist can be processed. A further benefit which may be obtained is an increase of the exposure speed of these systems by incorporation of suitable saturated acids. The reason for this is not fully understood. The epoxidised compound will generally be reacted with the mixture of acids in approximately stoichiometric amounts of carboxylic acid groups to epoxide groups. Thus, the ratio of carboxylic acid groups to epoxy groups is suitably from 0.95:1 to 1 '1.

The materials according to the present invention also encompass those which are chain extended by forming the epoxidised compound by reacting an epoxy resin with a polyfunctional material having two or more groups capable of reacting with the epoxy resin. This can further enhance hardening and other performance parameters. Suitable epoxy resins for effecting this chain extension process include bisphenol epoxy, triphenolmethane-type epoxies, alicyclic epoxies and any other epoxy compound containing two or more groups per molecule. Suitable polyfunctional materials to effect chain extension include those with reactive hydroxy groups, amine groups, amide groups, carboxylic acid groups, imidazoles, imides, anhydrides and mercaptans.

The modified epoxy is subsequently further modified by reaction with a polybasic carboxylic acid anhydride, suitably to give a product having an acid value of from 30 to 130 mgKOH/g, preferably from 35 to 80 mgKOH/g. A wide variety of anhydrides may be employed to the purpose of modification and examples of these include succinic, didodecylsuccinic itaconic, citraconic, maleic, phthalic, hexahydrophthalic, tetrahydrophthalic, methylnadic and trimellitic anhydrides.

The photopolymerization initiator used in the compositions of the invention serves to induce polymerization of the epoxy acrylate when the composition, after application to a substrate, is subjected to actinic radiation. A wide variety of such photopolymerization initiators are known in the art, such as benzoin ethers and anthraquinone derivatives. Preferred initiators for use in the compositions of the invention are phenyl ketone initiators such as benzophenone, acetophone or Mischlers ketone or mixtures thereof.

The initiator is suitably present in the composition in an amount of from 1 to 10% by weight, based on the weight of the epoxy acrylate.

The compositions of the invention also contain a liquid carrier. This may, for example, be an organic solvent for the modified epoxy acrylate, with the modified epoxy acrylate dissolved therein, and examples of such solvents include lower carboxylic acid ester of lower alcohols (e.g. isopropyl acetate), lower dialkyl ethers (such as diethyl ether), ketones (such as acetone or methyl ethyl ketone), or, preferably, hydroxyalkyl ethers such as glycol monoethyl ether and ethylene glycol monobutyl ether.

Alternatively the carrier may comprise water with the modified epoxy acrylate dispersed as a solid therein or an organic solvent solution of the modified epoxy resin emulsified therein. Such compositions are described, for example, in EP-A-0536272 and EP-A-0573426.

The amount of carrier present in a coating composition of the invention as applied to a substrate (e.g. a printed circuit board) will, to some extent, depend upon the nature of the method by which the composition is to be applied to the substrate. Thus, where the composition is to be applied to a substrate by, for example, a screen printing process, it may contain up to 30% by weight of carrier whereas if it is to be applied to the substrate by a curtain coating process it may contain up to 50% by weight of carrier. The compositions of the invention may conveniently be formulated containing a lower amount of carrier than is required in the actual application process, the additional carrier required being added to the composition to dilute it prior to the application process.

The filler, which will be in powdered or finely divided form, serves to improve the resistance of the composition, when used as a solder resist, to heat or thermal shock such as is experienced when the cured composition is brought into contact with molten solder. The filler should, when the coating composition is employed in the production of a solder resist, not be one which undergoes thermal decomposition when heated by contact with molten solder and examples of suitable fillers include blanc fixed, aluminium hydrate, china clay, calcium carbonate (coated or uncoated) and micronised talc, or mixtures thereof. The weight ratio of filler to modified epoxy acrylate is from 5 - 25 : 95 - 75, preferably from 10 - 20 : 90 - 80, more preferably from 15 - 20: 85 - 80.

In general, in order to provide a generally tack-free coating from the composition (as is described below), it is preferable, when using less solid modified epoxy acrylates (i.e.

those having lower softening temperatures), to employ higher levels of inorganic fillers, but, of course, within the broad range noted above.

The coating compositions of the invention also suitably contain a colorant, for example an organic pigment such as a chlorinated phthalocyanine pigment, in order that the application of the composition to a substrate provides a visible image. Suitably such colorants will be present in amounts of up to 5% by weight, based on the weight of epoxy acrylate, filler and initiator, preferably from 0.5 - 2% by weight thereof. The coating compositions of the invention may also contain antifoaming agents, such as silicone oils, in order to improve their application properties and such antifoaming agents may be present in amounts similar to those given above for the colouring agents.

Whilst the compositions of the invention contain the modified epoxy acrylate as principal photopolymerizable ingredient, other photopolymerizable materials may be present and examples of such include esters of mono- or polyhydric alcohols with ehtylenically unsaturated carboxylic acids such as acrylic or methacrylic acid, and liquid epoxy acrylates. Such other photopolymerizable materials are not, however, essential and when used it is preferred that they be employed in minor amounts as compared with the modified epoxy acrylate, e.g. in amounts of less than 25%, preferably less than 10%, of the weight of the modified epoxy acrylate.

A coating composition of the invention is used to form a photopolymerizable coating upon a substrate by applying it to the substrate by a convenient method, such as screen printing, curtain coating, roller coating or spray coating, and then allowing the applied coating to dry to a tack-free condition (i.e. to a condition such that it does not adhere to a surface with which it may come into contact) and generally this drying will be accelerated by heating the applied coating.

The resultant coating may be polymerized by exposure to actinic radiation, for example from a source such as a mercury vapour lamp.

Another means of improving final film hardness is to utilise a two-pack arrangement wherein the material of the present invention is provided in one pack and a thermosettable epoxy resin is provided in a second pack. The contents of both packs are mixed prior to application. Following radiation -induced curing, a final hardening is induced by thermal curing. To assist the latter step, preferably one or both packs contain a thermally activatable catalyst.

As indicated above, a polymerizable coating obtained from a coating composition of the invention is particularly suitable for use in the production of a solder resist in a process for the manufacture of a printed circuit board. It should, however, be noted that such a polymerizable coating may also be used as to form a patterned etch or plating resist since the coating is acid and alkali resistant (it being possible to later wholly remove the coating by treatment with a hot dilute aqueous alkaline solution such as 5% aqueous sodium hydroxide solution, although such removal may not be necessary when the coating is used in the definition of inner layers in a multilayer construction since it is chemically compatible with adhesives used to bond multilayer constructions).

A further embodiment of the invention provides a method of forming a pattern of solder upon a layer of an electrically conductive metal supported on an electrically nonconductive substrate by providing the metal layer with a patterned resist coating, whereby portions of the metal layer are coated with the solder resist coating and other portions of the metal layer are not so coated, and contacting the metal layer provided with the solder resist coating with molten solder whereby solder adheres to the portions of the metal layer not coated with the solder resist coating, in which the solder resist coating is formed by polymerizing, by exposure to actinic radiation, a photopolymerizable coating obtained by applying a coating composition in accordance with the invention to the metal layer and allowing it to dry.

One such process comprises the steps of: (a) providing a circuit board having a patterned layer of a conductive metal (hereinafter simply referred to as copper) with a coating of a coating composition of the invention, at least over the copper layer, for example by screen printing process, by a curtain coating process or by electrostatic spray deposition, in all cases the coating of the composition will extend over the whole of the surface of the board; (b) allowing the coated composition to dry to a tack-free, i.e. by evaporation of volatile organic solvent therefrom; (c) exposing the coated board to actinic radiation through a positive for the desired solder pattern (i.e. a transparency, generally a photographic transparency, having light transmitting portions corresponding to the non-solderable portion of the desired solder pattern and non-transmitting portions corresponding to the solderable portions of the desired solder pattern) to cure the exposed portions of the coating, i.e. to photopolymerize the photomerizable material therein; (d) removing the non-exposed portions of the coating by means of aqueous alkaline solution (e.g. sodium carbonate solution); and (e) contacting the board having a patterned coating image with molten solder, e.g.

in the form of a so-called "standing wave" of solder, to apply solder to the board in the desired pattern.

This process makes it possible to provide circuit boards with a pattemed solder resist of high definition and accuracy since, in general, exposure through a positive to actinic radiation provides for good accuracy and definition which is of importance as the overall size of circuit boards, and hence the size of individual parts of the solder resist pattem, decrease, a tendency which has been noted recently. It is to be noted that in the above process, step (e) follows directly from step (d), that is there is no requirement for a heat-curing step after radiation curing. However, if desired such a heat-curing step may be employed.

In this case the composition applied to the board preferably also contains a thermal curing agent which will assist thermal curing of the modified epoxy acrylate. Suitable thermal curing agents include polyamines, polyamino resins, dicyandiamide, melamine derivatives and blocked isocyanurate materials. Bisphenol A or F resins and cresol or phenol-based epoxy novolaks can also be used as thermal curing agents. Where the composition applied to the board contains a thermal curing agent it is generally more convenient to put the system up as a two-part system, one part containing the modified epoxy acrylate and the other part containing the curing agent therefor. Suitable twopart systems, employing aqueous carriers are disclosed in WO 93/06350.

The circuit board having a patterned layer of copper thereon used as starting material in step (a) of the process described above may be produced in a number of ways, either by the so-called "subtractive" method or by the so-called "additive" method.

In the subtractive method a laminate comprising a layer of copper on a non-conducting substrate is first provided with a positive patterned image of an acid-resisting coating and the exposed copper is then etched away with a suitable acidic etchant, e.g. acid cupric chloride, the remaining copper then being exposed by removal of the acidresisting coating. A coating of acid-resistant material may, as is well-known, be applied by a patterned coating method, such as a screen printing method, or by coating the copper with a layer of a photosensitive composition and subsequently exposing this to light through a positive or negative image of the desired copper layer, depending on whether the resist coating is a so-called positive or negative working resist and subsequently removing the developable (i.e. solvent-soluble) portions of the image with a suitable solvent.

The compositions of the invention are, themselves, perfectly suitable for use as negative working resists, i.e. resists the exposed portions of which are cured to give an insoluble coating. Thus, the circuit boards having a patterned layer of copper on the surface thereof may be provided by firstly coating a copper-clad substrate with a coating composition of the invention, allowing the coating to dry, exposing the dried coating to actinic radiation through a positive of the desired copper layer pattern to cure the portion of the coating exposed to radiation, removing the uncured portion of the coating with a solvent therefor and subsequently etching the coated board. The cured coating may then be removed by washing with a solvent therefor, e.g. aqueous sodium hydroxide solution. The board may then be provided with a patterned solder coating as described above.

In the additive method for the preparation of the board having a patterned layer of copper, a non-conductive substrate is first coated with an activating material for a socalled electroless copper plating solution, the board is then provided with a negative patterned image of a resist coating, and the board is then immersed in an electroless copper plating solution to form a layer of copper on the exposed portions of the board, i.e. those not covered with the resist. Here again the composition of the invention may be used to form the resist layer, for example by coating the activated board with a layer of the coating composition, allowing it to dry and exposing it to actinic radiation through an appropriate positive of the circuit pattern so that the exposed portion of the coating is photo hardened, the non-exposed portion subsequently being removed with a suitable solvent.

Whilst the compositions of the invention are photosensitive, they are not effectively sensitive to subdued light or light having a wavelength above 420 nanometers. Thus the initial application of the coating composition to a substrate can be carried out in the light (although of course not light having a high amount of actinic radiation).

In order that the invention may be well understood the following examples are given by way of illustration only. In the examples all parts are by weight unless otherwise stated.

EXAMPLE 1 36.97 parts of a polyfunctional epoxy resin EPPN 502H (Nippon Kayaku) was dissolved in 30 parts of propylene glycol methyl ether acetate. The mixture was then heated to 1300C and 5.82 parts of Decanoic acid and 0.1 parts of triphenyl phosphine was added. The reaction mixture was then held for the acid value to drop below 0.5mgKOH/g and cooled to 115"C. Then 0.1 parts hydroquinone, 12.91 parts acrylic acid and 0.1 parts triphenyl phosphine was added and the mixture held until the reaction was complete. The material was then cooled to 900C and 14 parts of tetrahydro phthalic anhydride was added to give an acid value of 75mgKOH/g. This was called resin 1.

EXAMPLE 2 38.64 parts of XD9053, a polyfunctional resin available from the Dow Chemical CO, was reacted with 15.38 parts of acrylic acid and 1.58 parts of benzoic acid in the presence of 0.1 parts of hydroquinone, 0.2 parts of triphenyl phosphine and 35 parts of propylene glycol methylether acetate. After the reaction was complete 9.1 parts of maleic anhydride was added and the material held for an acid value of 80mgKOH/g.

This was called resin 2.

EXAMPLE 3 37.60 parts of Tactix 742, a polyfunctional epoxy resin available from the Dow Chemical CO, was dissolved in 30 parts of ethoxy ethyl proprionate. 15.79 parts of acrylic acid, 2.31 parts of Decanoic acid, 0.1 parts of hydroquinone and 0.2 parts of triphenyl phosphine were added and the mixture was held for 16 hours at 100"C. 14 parts of phthalic anhydride was added and held for an acid value of between 7580mgKOH/g. This was called resin 3.

EXAMPLE4 39.0 parts of EPPN 501H (polyfunctional epoxy resin ex Nippon Kayaku) was reacted with 14.1 parts of acrylic acid and 5.6 parts of stearic acid in the presence of 0.1 parts hydroquinone, 0.2 parts of triphenyl phosphine and 30 parts of propylene glycol methylether acetate. After completion of the reaction, 11 parts of trimellitic anhydride was added to give a material with an acid value of about 9OmgKOH/g. This was called resin 4.

EXAMPLE 5 36.18 parts of a polyfunctional epoxy resin EPPN 502H (Nippon Kayaku) was charged to a reaction vessel and heated to 1350C with stirring. 1.48 parts of Bisphenol A and 0.01 parts of ethyl triphenyl phosphonium bromide was added and the epoxy value and B & R mpt measured until the epoxy value was < 250mgKOH/g and the B & R mpt > 80 C.

30.67 parts of propylene glycol diacetate was added and when the solution was fully homogenous 10.66 parts of acrylic acid and 6.70 parts of stearic acid. The reaction mixture was then held at 1 100C, with 0.10 parts of hydroquinone and 0.20 parts of triphenyl phosphine, for 12 hours until the reaction is complete. A further 14.00 parts of tetrahydro phthalic anhydride was added to produce a carboxylated photosensitive propolymer. This was called resin 5.

EXAMPLE 6 36.89 parts of a polyflinctional epoxy resin Tactic 742 (Dow Chemicals was charged to a reaction vessel and heated to 145"C with stirring 1.8 parts of Pyromelittic diimide was added and the epoxy value and B & R mpt measured until the epoxy value was < 260mgKOH/g and the B & R mpt > 70 C. 30.0 parts of propylene glycol methylether acetate was added and when the solution was fully homogenous 10.30 parts of acrylic acid and 6.71 parts of myristic acid. The reaction mixture was then held at 1200C, with 0.10 parts of hydroquinone and 0.20 parts of triphenyl phosphine, for 9 hours until the reaction is complete. A further 14.00 parts of tetrahydro phthalic anhydride was added to produce a carboxylated photosensitive prepolymer. This was called Resin 6.

EXAMPLE 7 37.77 parts of a polyfunctional epoxy resin EPPN 501H (Nippon Kayaku) was charged to a reaction vessel and heated to 140"C with stirring. 2.03 parts of Adipic acid was added and the epoxy value and B & R mpt measured until the epoxy value was < 260mgKOEVg and the B & R mpt > 750C. 30.00 parts of propylene glycol methylether acetate was added and when the solution was fully homogenous 11.11 parts of acrylic acid and 4.79 parts of lauric acid. The reaction mixture was then held at 115"C, with 0.10 parts of hydroquinone and 0.20 parts of triphenyl phosphine, for 10 hours until the reaction is complete. A further 14.00 parts of tetrahydro phthalic anhydride was added to produce a carboxylated photosensitive prepolymer. This was called Resin 7.

The resins 1-7 can then be incorporated into the following ink to produce material that can be used as a photodefinable solder resist.

Material Parts Resin 60.00 Irgacure 907 5.70 Quantacure ITX 1.60 BYK 035 1.10 Barium Sulphate 19.70 Aerosil R374 3.10 Pigment Concentrate 1.57 Propylene glycol methylether acetate 7.23 A thermal curing agent for the above ink was produced to the following formulation: Material Parts Epoxy novolac resin (mpt) approx 85"C) 30.00 Propylene glycol methylether acetate 25.00 Tris (2-hydroxy ethyl) isocyanurate triacrylate 15.00 Talc 30.00 The ink was mixed with the thermal curing agent in a 2:1 ratio and screen printed through a 32T mesh and dried in the oven for 30 minutes at 85"C. The resist was then exposed to 350mJ/cm2 through suitable artwork and the pattern developed using 0.6% w/w sodium carbonate solution at 40"C. The resultant film had excellent hardness and had good resistance to solder after a postbake at 150"C and exposure to a further UV cure of 2000mJ/cm2.

The incorporation of a suitable saturated acid can improve either the flexibility and/or the hardness, given an improvement in the drying window in which the solder resist can be dried without having stability problems, and lastly gives an improvement in the exposure speed to Lw radiation.

In the light of this disclosure, modifications of the described examples, as well as other examples, all within the scope of the present invention as defined by the appended claims will now become apparent to persons skilled in this art.

Claims (14)

CLAIMS:

1. An ethylenically unsaturated polymerisable material obtainable as a reaction product of an epoxidized compound of a condensate of a phenolic compound and an aromatic aldehyde and a mixture of (i) a major proportion of an ethylenically unsaturated carboxylic acid and (ii) a minor proportion of a saturated aliphatic or aromatic carboxylic acid; further modified by reaction with a polybasic-carboxylic acid anhydride.

2. A material according to claim 1, wherein the polybasic-carboxylic acid anhydride is a dicarboxylic acid anhydride.

3. A material according to either preceding claim, wherein the phenolic compound is a derivative of phenol or a substituted phenol.

4. A material according to any preceding claim, wherein the aromatic aldehyde has a phenolic hydroxy group.

5. A material according to any preceding claim, wherein the epoxidised compound is obtainable as the reaction product of an epoxy resin having two or more epoxy groups with a polyfunctional material having two or more groups capable of reacting with the epoxy resin.

6. A material according to claim 5, wherein the epoxy resin is selected from bisphenol epoxy, triphenolmethane-type epoxies, alicyclic epoxies and any other epoxy containing two or more epoxy groups per molecule.

7. A material according to any preceding claim, wherein the ethylenically unsaturated carboxylic acid forms from 65 to 90 mole% of the mixture with the saturated aliphatic or aromatic carboxylic acid.

8. A material according to any preceding claim, wherein the ratio of carboxylic acid groups to epoxy groups in the reaction product of polyepoxide and the said mixture is from 0.95:1 to 1:1.

9. A material according to any preceding claim, wherein acid value of the dicarboxylic acid or anhydride thereof is from 30 to 130 mgKOH/g.

10. A coating composition, for the production of a photopolymerizable coating, comprising an ethylencially unsaturated material according to any preceding claim, together with a photopolymerization initiator therefor and a liquid carrier.

11. A composition according to claim 10, further comprising an inert organic filler.

12. A two-pack presentation comprising a first component containing at least a material according to any preceding claim and a second pack containing at least a thermosettable compound having at least two epoxy groups per molecule.

13. A two-pack presentation according to claim 12, wherein at least one of the first and second packs contains a thermally activatable catalyst.

14. A method of forming a photopolymerizable coating upon a substrate which method comprises applying a coating of a coating composition according to claim 5 or claim 6, to the substrate and allowing it to dry by evaporation of the liquid carrier.