GB2071668A

GB2071668A - Resinous protective coating

Info

Publication number: GB2071668A
Application number: GB8001216A
Authority: GB
Original assignee: Kollmorgen Technologies Corp
Current assignee: Kollmorgen Technologies Corp
Priority date: 1979-01-12
Filing date: 1980-01-14
Publication date: 1981-09-23
Also published as: JPH027492A; JPH023431B2; GB2071668B; IT1218426B; IT8047586A0; JPS56111292A; JPH0345557B2

Abstract

The coating is useful in the production of printed circuit patterns and for the manufacture of masking layers, including solder masks, adhering to metallic or insulating surfaces of both, e.g. in the manufacture of printed circuit boards with circuit patterns having a resolution of 0,25 mm lines and spaces thereon or less. The protective coating comprises resins especially epoxy resins, dissolved in solvents, wherein the coating, upon being subjected to heat, goes from a liquid or soft "gel-like" state into a solid phase without "bleeding". The protective coating is resistant to copper electroplating baths, adheres to insulating substrates, adhesive coated base materials as well as metallic substrates and is capable of withstanding the thermal shock of, e.g., dip soldering.

Description

SPECIFICATION High resolution screen printable resists The present invention relates in general to a resinous protective coating capable of being applied to preselected portions of an insulating or metallic substrate. More particularly, the present invention relates to a resinous composition that adheres well to metallic and plastic surfaces and adheres especially well to copper, a surface which is difficult to bond to. The present invention also related to a resinous protective coating for use in producing patterns on circuit boards, the protective coating including a highly functional solid resin having a melting point preferably between 60 and 2000 C.

The temporary or permanent protective coatings used as resists, solder masks, etc. in the prior arts methods of producing circuit boards have had a number of deficiencies which are set forth subsequently herein. For high density circuits, it has been customary to use dry film photoresists which are extremely costly. Screen printed resists or masks, while economical, have not been able to achieve the high resolution necessary for high density circuit boards. For example, the problems associated with the use of prior art solder masks are detailed hereinbelow as an illustration of the type of problems associated with the use of protective coating compositions of the prior art as temporary or permanent resists or the like.

One problem in the prior art is caused by the poor adhesion of plastics to copper substrates. It is well known, for example, that copper oxidizes even when covered with a coating layer and that, consequently, organic coatings do not adhere as well to the loosely adherent copper oxidized films. In fact, the organic coating separates after a period of time from the copper substrate due to diffusion of the oxygen through the coating on the copper substrate and subsequent oxidation of the copper substrate. Attempts to solve this problem have included precoating the copper substrate with an adherent copper oxide, e.g., a hot alkaline hypochloride solution, brass or zinc plating.

Virtually all printed circuit board assemblies, even in small quantity production, are wave or dip soldered. Heretofore, in producing circuit boards which have a high circuit density per unit area, difficulty has been experienced due to the fact that the holes in such boards (1) tend to have an extremely small diameter, e.g., 0,35 to 1 mm; and (2) tend to be extremely closely spaced at least in some portions of the circuitry. In conventional practice, a plated through hole board is produced with a circuit on one or more exposed surfaces. Before soldering, a registered solder mask is printed on the circuit pattern(s) to leave holes and lands or pads (i.e., small areas on the surface surrounding the holes) as well as fingers (i.e., terminal or contact areas of the circuit pattern) exposed.Subsequently, the components are fastened to the circuit as by dipping in a solder bath to apply solder to the component leads and on the exposed areas, e.g., on the exposed lands and in the metallized holes. The solder mask protects the major portion of the circuit from the solder and thus guards against short circuiting by confining solder to only those areas where it is desired.

In such conventional circuits, the lands or pads are exposed while conductor lines making up the conductor pattern or patterns are protected by the solder mask.

To maintain fine printing tolerances in such boards, extremely thin prints are used. Thus, even when great precautions are taken in printing the solder mask on high density circuit boards of the type described, there is a good possibility of the masks breaking down in part, thereby causing the solder to bridge from one land to another, or from one conductor line to another. When thicker prints are used to avoid mask breakdown, prior art solder masks tend to block the holes, thereby preventing proper soldering.

In the prior art, various thermosetting resinous compositions have been used to provide protective coatings such as plating resists, temporary or permanent resists and solder masks. The thermosetting resins used in these compositions had low melting points and were liquid at room temperature. These resinous protective coatings were incapable of providing lines and spaces having a resolution lower than 0,6 mm without substantially bleeding over onto the exposed holes and lands surrounding the holes when a conductive pattern was being applied by screen printing. These resins flowed at temperatures from about room temperature up to about 1 690 C during curing.Consequently, with the prior art thermosetting resinous compositions screen printed onto an insulating substrate or onto a metallic pattern already on the substrate, as the substrate was heated to evaporate the solvent in the resinous composition, the resinous composition would further liquefy. The printed resinous composition pattern would become extremely liquid and "bleed" or spread to cause poorly defined, fuzzy pattern edges thereon. When such "bleeding" or spreading occurred, solder would not coat pattern edges which border lands and holes during the subsequent dip/wave solder operation.

Only after "bleeding" or spreading out of the printed pattern of the resinous composition would the resinous composition become polymerized and solidified (thermoset). This deficiency was attributable to the fact that in the curing step the resins would liquefy before they hardened. During this liquefaction phase, the resinous composition further spread out to create a poorly defined pattern edge which borders areas where soldering is desired. Liquefaction of the resins during cure resulted in sloppy mask edges, e.g., the original print was sharp, but during cure the edges of the printed mask spread.

Edges which are sharp and have not spread are quite important in high density boards, and not overly important in low density boards.

Prior art masking compositions were not very chemically resistant to adhesion promotion solutions such as chromic acid and highly alkaline, electroless copper baths used in the "fully additive" technique. Moreover, the prior art masking compositions were not smooth and produced rough coatings whose surfaces would be receptive to the production of adherent extraneous copper deposits when exposed to, e.g., electroless copper plating baths, thus forming unwanted spots of copper on the resist pattern.

The expression "fully additive technique" as used herein is meant to define a technique wherein the conductors are at least in part formed by electroless metal deposition. The following example of one embodiment of said technology is given for illustrative purposes only: A suitable insulating substratum is prepared having a distance between hole centers of about 2,5 mm or less, the substratum and walls of the holes are sensitized and seeded using known seeding and sensitizing agents such as stannous chloride, palladium chloride, activation, a permanent protective coating or resist is screened to produce a permanent background resist leaving the desired circuit pattern exposed. The resist is heat cured and copper is electrolessly deposited on the exposed areas of the substratum and in the holes to form the conductor pattern.

Another problem unique to printed circuit boards is that the hardeners or curing agents used in prior art solder mask compositions stain gold deposits such as exposed gold plated tabs or fingers.

It is an object of the present invention to provide temporary and permanent protective coating compositions for use in the manufacture of printed circuit boards.

A further object of the invention is to provide high resolution screen printable resists.

Another object of the invention is to provide an improved method of improving the resolution of, conductive patterns on circuit boards.

An object of this invention is to provide an improved resist for screen printing of patterns on circuit boards.

It is also an object of this invention to provide a protective coating composition useful in producing printed circuit patterns including lines and spaces having a resolution of at least 0,25 mm and as low as 0,1 mm.

An object of this invention is to provide a thermoset resin based protective coating the solid resin having a melting point between about 60 and 2000 C.

Another object of this invention is to provide a protective coating comprised of a blend of resins and solvents such that, during heat cure, the coating passes directly from a gel-like stage into a solid phase without liquefying.

Still another object of this invention is to provide a registered mask which, upon heat cure, retains its sharp edges and does not bleed or spread out over holes and lands surrounding the holes.

An object of this invention is to provide a thermoset resin based protective coating capable of adhering to adhesive coated base materials as well as metallic substrates, particularly copper substrates, and of withstanding the thermal shock associated with dip soldering.

A further object of this invention is to provide a smooth, glossy protective coating which is not conducive to the formation of adherent, indiscriminate copper deposits when used in the additive method for producing circuit boards.

Another object of this invention is to provide an epoxy resin based electroplating resist.

Another object of this invention is to provide a temporary resist useful in the print and etch process of printed circuit board manufacture.

Still another object of this invention is to provide a protective coating which maintains an initial gel-like state until cure-hardening and remains adherent to metallic substrates when heated for an extended period of time at a temperature of about 1 600 C.

It is, therefore, an object of this invention to provide a method of improving adhesion of organic coatings to copper substrates.

Another object of this invention is to provide a protective coating which does not stain gold deposits.

Additional objects and advantages of the invention will be set forth in part in the description, or may be realized by practice of the invention, the objects and advantages being realized and attained by means of the methods, processes, instrumentalities and combinations particularly pointed out in the appended claims, These and further objects of the present invention are achieved by a protective coating composition comprising a thermosetting resin or resin composition and a curing agent dissolved in a solvent, said thermosetting resin or composition being a solid at a temperature of 600 C or below, and preferably at room temperature, and having a functionality greater than three; said coating composition being in an initial state as a high viscosity solution or a soft gel-like state and, upon being cured, transforming from said initial state directly and substantially or completely without liquefying into a final state as a solid.

In a preferred embodiment the thermosetting resin is an epoxide resin having an epoxide functionality of less than 10, and preferably less than 7.

In its embodiment suitable for forming a permanent masking layer, said composition comprises a thermosetting epoxy resin dissolved in a solvent; and a primary curing agent comprised of an alicyclic amine which is corrosive to copper surfaces, said curing agent being present in an amount sufficient to effect substantially complete cure of said coating composition on heating, said coating composition being in an initial state as a high viscosity solution or gel-like structure and, upon curing, being transformed directly into a gel and into the final solid state without bleeding; and a secondary curing agent comprised of an aromatic amine, said secondary curing agent being present in an amount sufficient to provide said cured coating composition with resistance to cleaning agents, e.g., for flux removal after soldering; and a flow agent; and at least one modifying agent selected from the group consisting of a screen printing aid, a thickening agent and a toughener.

The coating composition of this invention may comprise the thickening agent in an amount sufficient to produce a soft, gel-like state of said composition: and an organic acid in an amount sufficient to reduce or eliminate the basicity of an alicyclic amine present in said composition as a curing agent, thus retaining the thixotropic character of said composition.

For illustrative purposes only and not intending the invention to be so limited, the application of the permanent protective coating composition of this invention is subsequently described herein in conjunction with a typical method (employing the additive technique) in the manufacture of printed circuit boards. The surface of an insulating substrate on which a printed circuit pattern is to be formed is coated with a permanent resinous protective coating or mask according to this invention. The mask has the characteristics of being able to resist attack by acids and alkali to which printed circuit boards will ordinarily be subjected in processing. The mask has a smooth glossy surface.Holes defining cross-overs have been preformed or may be formed in the insulating substrate by any method which does not damage the mask or substrate materials surrounding the holes, such as by drilling, piercing or punching methods. A catalytic insulating blank such as the one described in US Patents 3,600,330 and 3,672,986 may be used as the insulating substrate. The board is contacted with a strong oxidizing solution such as chromic-sulfuric acid to adhesion promote the exposed or unmasked areas of the blank.

Then the board is contacted with an electroless metal deposition solution to metallize the walls of the holes and the conductor pattern. In the next step, the insulating blank having circuit patterns electrolessly deposited thereon is printed with a permanent solder mask according to the present invention leaving the hole walls and, if desired, the lands surrounding the holes as well as finger areas exposed. The permanent solder mask compositions of this invention are capable, upon curing, of forming a smooth, glossy surface which not only is resistant to the thermal shock of soldering, but also is resistant to flux removing solvents.The permanent solder mask compositions of this invention adhere well to the metallic substrate (circuit pattern) even when subjected to temperatures of about 1 600C for about 16 hours.

In order to illustrate another embodiment of the present invention, the application of a temporary protective coating composition of the invention is subsequently described herein in conjunction with a typical method (employing the electroplating technique) in the manufacture of printed circuit boards.

Holes are drilled in a copper foil clad insulating substrate before the substrate is sensitized and seeded with known seeding and sensitizing agents. Copper is electrolessly deposited onto the copper foil and into the holes. The electrolessly deposited copper on the surface of the substrate may be removed by lightly sanding the surface. A temporary background resist according to the present invention is screen printed onto the substrate in a pattern before copper is electroplated onto the pattern. The pattern is then solder plated before the temporary background resist is removed with a solvent as subsequently described herein. Unwanted background copper may then be etched away.

A direct polyvinyl alcohol emulsion on a stainless steel wire screen is a common printing screen stencil employed for circuit printing because it is a tough and resistant stencil which meets the demands of relatively large production and is not difficult to prepare. Any practical screen emulsion, however, including transfer type screen process film, may be employed, depending upon the type of protective coating to be printed. By "transfer type films" are meant films which are first processed on a base and then transferred to the screen fabric on the printing screen. When a resinous protective coating is printed onto a substrate, any screen emulsion that will not be dissolved by the protective coating may be used.Examples of suitable screen fabrics include stainless steel wire screens, polyester fibre screens, and nickel coated polyester screens, preferably sized between about 1 10 and 325 (USA Sieve Size) mesh.

According to the present invention, a thermosetting resin in the protective coating having a high melting point and a high functionality permits the solvent in the protective coating to evaporate and the curing operation to occur without melting and liquefying the resin in the coating, thus avoiding spreading of the coating composition during the curing operation. It is believed that the viscosity of the protective coating composition increases as the thermosetting resin therein cures into a solid. The high functionality of the thermosetting resin also creates rapid crosslinking so that the printed mask pattern does not bleed or spread between lines or onto the lands and holes as maximum curing temperatures of the mask are reached. Consequently, the mask retains sharp edges.

It has been discovered that a thermosetting resin based screening mask capable of (1 ) producing printed circuit patterns having a resolution of at least 0,25 mm lines and spaces on a substrate; (2) withstanding a copper plating bath; (3) adhering to adhesive coated base materials as well as to metallic substrates during extended heat treatment; and (4) withstanding the thermal shock of soldering and subsequent flux removal, is provided by a protective coating composition which comprises a highly functional thermosetting epoxy resin dissolved in a solvent and a preselected alicylic hardener as the primary curing agent. In the embodiment for use as a solder mask, the primary curing agent is corrosive to metals such as copper.The thermosetting epoxy resin is a solid at room temperature and does not melt at temperatures up to its curing temperature, typically between about 60 and 2000 C.

A simple test was devised to determine whether a particular solid thermosetting resin meets the criteria of the present invention. A chunk or piece of a sample thermosetting resin approximately 5 to 10 g in weight is placed in an oven and heated to a temperature between 100 and 1 600 C. If the sample thermosetting resin melts or spreads during cure, it is not suitable for the present invention. On the other hand, if the thermosetting resin becomes and remains tacky during cure, it is suitable for the present invention.

By highly functional thermosetting resins are meant those thermosetting resins having more than 3 and less than 10 functional groups, preferably between about 4 and about 7 functional groups.

Suitable thermosetting resins include epoxy resins, melamines, ureas (amines), phenolic, polyamideimide and polyimide, alkyd, polyurethane and blends thereof having an average functionality of between about 3 and about 7 and a melting point between about 60 and 2000 C.

Suitable amino resins as defined hereinabove include melamine formaldehydes and urea formaldehydes. A suitable urea formaldehyde is Uformite F#240MRTM and a suitable melamine formaldehyde is Uformite MM55RTM. Other suitable melamine resins as defined above include RESIMENE 812RTM (a colorless resin powder), MELMAC 1077RTM (a melamine resin with a cellulosefiller), MELMAC 404RTM (a translucent melamine resin), and MELMAC 483RTM (a phenol modified melamine resin).

Suitable phenolic resins as defined hereinabove include oil soluble heat reactive phenolic resins (prepolymers) such as CH-1 2-82RTM and CK-1 6#34RTM, both having respective Durran Softening points of 1800--2100 F and 1 900#2200 F; prepolymers of phenol formaldehydes such as RESINOX p~goRTM and 3700RTM and SP 801 4RTM and SP 6600RTM, having respective melting points of 1 600 F and 1 500 F and SP 8014 having a specific gravity of 1,25.

A suitable polyamide-imide resin is KERIMID 501 RTM and a suitable polymide resin is KERIMID 601 RTM.

Suitable alkyd resins include ethylene glycol maleate polyesters such as PlaskonRTM and DurezflTM.

Suitable polyurethane resins include prepolymers combined with polyols such as Solithane 113 and Polycin U56.

The thermosetting epoxy resin of the present invention preferably comprises 35 to 75% of the dry mask, the balance being curing agents and modifier(s).

Suitable epoxy resins include functional epoxy Novolac or bisphenol A type epoxy resins that exist normally as a solid at room temperature (having molecular weights of between about 350 and 1 5.000) and have a melting point between about 600 C and 2000 C. For illustrative purposes, the subsequent discussion of the thermosetting resin based protective coatings of this invention will be directed to epoxy resin based protective coatings, but this is not intended to limit the scope of the invention thereto.

Preferred epoxy resins having a functionality above 3 include the following commercially available epoxy resins listed in Table I hereinbelow: TABLE I Durran Average Epoxy Softening Epoxy Resin Functionality Equivalent Weight Point EPON 1031 4 220 1780F ECN 163 4 200 1780F End1273 5 225 730C ECN 1280 4.1 230 70-800C ECN 1299 6 235 990C DEN 439 3.8 200 380C Other solid epoxy resins which are believed to be highly functional thermosetting resins within the scope of this invention include APOGEN 1013RTM, EPl-REZ 521 RTM having an average epoxy equivalent weight of 200 and a Durran softening point between 700 C and 800 C, EPI-REZ 5291 RTM having an average epoxy equivalent weight of 310 and a Durran softening point between 900 C and 950 C, and EPOTURF 37-171 RTM having an average epoxy equivalent weight of 150 and a Durran softening point between 1720 Fand 1800 F.

Other less functional epoxy resins may be optionally included in the coating composition to reduce the brittleness of the cured coating composition. Such epoxy resins include copolymers of epichlorohydrin (1-chloro-2,3-epoxy propane) which have melting points within the range of 700 C to 1910 C and molecular weights of about 350 to 15.000. Although epichlorohydrin is the source of most important organic epoxides employed in the formation of the epoxy resins, other epoxides such as, e.g., 1 ,2,3,4-diepoxy butane may be used. Moreover, while it is preferred to use epichlorohydrin in the preparation of the resinous polymeric epoxide of the present invention, other epihalohydrins such as epibromohydrin also may be used advantageously.Similarly, epoxy resins derived from phenols other than bisphenol A may be used including, e.g., the reaction product of epichlorohydrin with resorcinol, with phenols derived from cashew nut oils, with hydroquinone, with 1,5-dihydroxy napthalene or with 2,25,5-tetrabis-(4-hydroxyphenyl) hexane. Phenolic intermediates of the resol type, hydrazines and sulfonamides, such as, e.g., 2,4-toluene disulfonamide may also be used for reaction with an organic epoxide to produce epoxy resins suitable for use. Aliphatic epoxy resins are suitable, including, e.g., the reaction product of epichlorohydrin with glycerol, with ethylene glycol or with pentaerythritol.

The less functional modifier epxoy resins comprise up to about 30 weight %, preferably up to about 1 5 weight %, and when used typically comprise more than about 10 weight % of the protective coating composition of this invention.

The highly functional epoxy resin is dissolved with suitable solvent to prepare a solution between about 50% and 90% by weight highly functional epoxy resin, and preferably about 70% by weight highly functional epoxy resin. Suitable solvents include glycol ethers and esters such as diethylene glycol ethyl ether, ethylene glycol, methyl ethers, acetates of the glycol ethers, secondary butyl acetate, normal butyl acetate, primary amyl acetate.

The preselected primary hardener or curing agent suitable for the highly functional epoxy resin the protective coating composition destined to metallic surfaces, e.g., for use as solder masks, is an alicyclic amine which is corrosive to copper surfaces. The corrosive nature of such as amine to copper surfaces has in the past suggested that its use be avoided. Contrary to prior teachings, applicant has discovered that excellent adhesion of the protective coating compositions of this invention to copper substrates may be achieved with the use of alicyclic polyamine curing agents which are ordinarily considered corrosive to copper substrates.

Preferred alicyclic polyamine curing agents include methane diamine, 1 ,3-diamino-cyclohexane, isophorone diamine, triethylenediamine, and hexamethylenetetramine. The use of these preselected alicyclic polyamine curing agents in the protective coating compositions of this invention improved their heat resistance and adhesion to metallic surfaces.

Gelling agents may be added without impairing properties of the coating composition to improve antisag properties and screen printing techniques. When racking, storing and/or curing of the protective coating on the substrate in a substantially vertical orientation is contemplated, without the addition of the thickening or gelling agent, sagging of the printed image would occur. The thickening or gelling agent prevents sagging of the printed image during storage (particularly vertical storage) and/or heat curing. The addition of the thickening or gelling agent to the protectiva coating composition of this invention operates to improve the screen printing capabilities of the protective coating composition by thickening it to a soft "gel-like" state.

This soft "gel-like" state can be described as being a non-Newtonian fluid which, when freestanding, does not flow. When a force is applied such as by a screen printing squeegee, the flow occurs, and when such applied force is removed, the composition returns to its non-flowing, non-sagging state.

It has been found that, when the alicyclic polyamines are employed by themselves as a curing agent in the protective coating compositions of this invention, resistance to flux removal processes may suffer. Examples of such flux removal processes are methylene chloride vapor degreasing and hot water scrubbing or brushing using machines.

In order to regain resistance to flux removal processes such as in methylene chloride vapor degreasing, it has been found that an aromatic amine should be included as a secondary curing agent in the curing agent composition. Preferred aromatic amines include tris-2-ethyl hexoate salt of tris(dimethyl-aminomethyl)phenol, diamino diphenyl sulfone, benzyl dimethyl amine, metaphenylene diamine, and methylene dianiline. This is because the aromatic amine cured epoxy resin system gives more rigid, heat and solvent resistant structures, as is well known.

Since the alicyclic polyamines are a strongbase, the desired gel structure or thixotropic character necessary for smearless screen printing processes and which prevents sagging during storage in a vertical orientation also is lost about 5 minutes after the alicyclic polyamine is added to an epoxy resin solder mask composition, particularly when silica aerogels are used as thickeners.

To reduce or eliminate the basicity of the alicyclic amine in the protective coating composition of this invention, while retaining the thixotropic character of such protective coating composition, a number of preselected organic acids should also be included in the curing agent composition. These organic acids may be included in the resin system, the curing agent, or in the combination of the resin system and curing agent.Preferred organic acids include (a) carboxy terminated butadieneacrylonitrile polymers and carboxy terminated butadiene polymers; (b) fatty acids such as linoleic acid, oleic acid and the like: an example of such a fatty acid is a dimer or trimer fatty acid with two or three carboxylic groups per molecule produced by the polymerization of C18 fatty acids such as Empol 1 040RTM, a substantially trimer fatty acid having an acid value of 175 to 1 92, a saponification value of 192-200 and being 20% dimer acid and 80% trimer acid;Dimac SRTM, a dimer acid having an acid number of 180--190, a saponification value of 192-198 and containing 8~10% monomer, 65~69% dimer, the balance, 26-30%, being higher polymers; and dibasic acids such as adipic acid, glutaric acid, azelaic acid, sebasic acid and suberic acid.

Carboxyl terminated liquid acrylonitrile rubbers with or without pendant carboxyl groups from an amine salt with the basic alicyclic polymers. The reaction to the salt proceeds smoothly and quickly in a manner of minutes without external energy and with a slight, but noticeable exotherm occurring. The main benefit is maintaining gel. Other important benefits of using carboxyl rubbers are: 1) they provide toughening by reacting with the highly functional epoxy resin, and 2) they eliminate or retard carbonate formation. Alicyclic polyamines such as methane diamine will, by themselves, on prolonged standing in air, absorb carbon dioxide from the air to form a useless white carbonate salt.

In preparing a permanent protective coating according to the present invention, an amount of curing agent is employed which is sufficient to completely cure the highly functional epoxy resin in the protective coating. This may be calculated according to the amine equivalent weight (A.E.W.) of the curing agent and the epoxy equivalent weight (E.E.W.) of the epoxy resin according to the formula: A.E.W.

weight of hardener = -#-- x weight of epoxy resin (curing agent) E.E.W.

Various modifiers are added to the epoxy resin-solvent solution to improve flow, screenability, to increase toughness and optionally to impact color, antioxidation and antisag properties.

Flow promoters (or flow control agents) prevent fish eyes" from forming in a screening mask by lowering surface tension to provide a smooth, continuous surface. As indicated in N.l. Gaynes et al, Formulation of Organic Coatings (D. van Vostrand Co., 1967), pp. 184-1 85, 294-295 and 300-301, resin films tend at times to crater or pinhole. A flow promoter helps to provide a smooth, homogeneous film, without the unsightly craters and "orange peel" effect normally developed with the use of these resins. Examples of suitable flow promoters include alkyl acrylate polymers and silicones.

Preferred flow promoters are MODAFLOWRTM, RAYBO 1 5RTM, and DC 840RTM. MODAFLOWRTM is a high molecular weight polymer believed to be a blend of isobutylacrylate and ethyl acrylate polymers or a copolymer thereof. It enhances the surface levelling properties of the composition such that the composition when applied to the insulating or conductive bases flows out to a smooth level without forming ripples or bubbles, so that on drying, it produces a smooth, glossy surface. The flow promoters comprise up to about 6 weight %, preferably up to about 4 weight %, and when used, typically comprise more than about 1 weight % of the protective coating composition of this invention.

Screening aid act as a lubricant, facilitating screen printing of the highly functional epoxy resin containing coating compositions of this invention to produce smooth coatings. Suitable screening aids include epoxy resins which are liquid at room temperature. Preferred epoxy resins are DER 331 RTM, DER 332RTM, EPON 820RTM, EPOTUF 37~151 RTM, EPOTUF 37-1 34RTm, EPOTUF 37-1 35RTM, EPOTUF 37~250RTM, EPI-REZ 508 and 51 0RTM, and ARALDITE 6005RTM and ARALDITE 601 0RTM, The screening aids comprise up to about 60 weight %, preferably up to about 40 weight % and when used, typically comprise more than about 1 weight %, preferably more than about 2 weight % of the protective coating composition of this invention.

Suitable tougheners include the liquid acrylonitrile butadiene copolymer rubbers and solid epoxy resins with high epoxy equivalent weights. The rubber tougheners precipitate out during curing as, e.g., rubber-rich microglobules uniformly dispersed throughout the cured polymer. The microglobules stop crack propagation from occurring in the cured polymer. Preferred rubber tougheners include the following liquid, monomeric, reactive rubbers: CTBRTM, CTBNRTM, CTBNXRTM and ATBNRYM. The solid epoxy resin tougheners toughen the protective coating composition by flexibilizing the cross-linked rigid structure. Preferred epoxy resin tougheners include the following epoxy resins listed in Table II which are solid at room temperature with epoxy equivalent weights greater than 350.

TABLE II Toughener Softening Resin E.E.W. Point DER 661 RTM 475-575 70-800C DER 667 1000-2000 113-1230C Epon 1001RTM 450-550 65--740 C Araldite 7097RTM 1 650-2000 113--1230 C The tougheners comprise up to about 10 weight %, preferably up to about 7 weight % and when used, typically comprise more than about 1 weight % of the protective coating composition of this invention.

Suitable color agents (pigments) include the following: Cyan Green B-15-3100RTM, Cyan Green Y-1 5#3040RTM, Titanium dioxide (Rutile) OR-600RTM,-lrgazin Yellow 2GLTRTM, Monastral Red Tr#79DflTM and Blue BT-41 7RTM, Antioxidants can optionally be included in the protective coating composition of this invention as a modifying agent when continuous use of the cured coating at temperatures above about 1000 C is contemplated. The antioxidants prevent excessive air oxidation of the coating composition in such usage. Excessive air oxidation would otherwise cause the coating composition to become discolored.

Brittleness and loss of adhesion to a substrate to which it is coated are also avoided.

Suitable antioxidants include thioesters such as dialkyl thiodipropionate, dilauryl thiodipropionate, distearyl thiopropionate, and dimyristyl thiodipropionate; phosphites, such as tris(nonylphenol)phosphite and alkaryl phosphite; and phenolics such as fatty acid modified substituted phenolics, phosphited hindered phenolics, and high molecular weight hindered phenolics (e.g., butylated hydroxy toluene); and mixtures thereof.

Preferred antioxidants include a combination of a thioester with a hindered phenolic in a ratio by weight of 9 to 1 hindered phenolic to thioester. The antioxidants comprise more than about 1 weight %, preferably more than about 3 weight %, less than about 10 weight %, preferably less than about 7 weight % of the protective coating composition of this invention.

Suitable thickening or gelling agents include a fumed silica having particles of submicroscopic size with a total surface area of 200 to 400 m2 per gram, such as Cab#OSilRTM, organic modified montmorillonite clays such as Bentone 27RTM, a tri-alkyiaryl ammonium smectite and Betone 38RTM, a tetraalkyl ammoinium smectate, an amine treated bentonite, a colloidal silica made according to US patents 2 574 902 and 2 577 485 such as LUDOXRTM, a silica aerogel having particles of small microscopic size with a total surface area of 280 m2/g such as SANTOCEL ZRTM.

The thickening or gelling agent comprises up to about 5 weight %, and when employed typically comprises more than about 1 weight %, preferably more than about 2 weight % of the epoxy resin solids content of the protective coating composition of this invention. Upon curing of the protective coating composition containing the thickening or gelling agent, the "gel-like" composition remains in its soft "gel-like" state until it is fully cured and solidified in situ without bleed, flow or sag occurring.

Fumed silica may also be added to the primary curing agent to provide it with body and pourability with less splashing. The advantage of such admixture is the minimization of potential weighing errors which may occur when the primary curing agent composition is added to the protective coating composition of this invention.

The apparent viscosity of the epoxy resin screen printable mask is between about 10.000 centipoise and about 200.000 centipoise, preferably between about 15.000 and about 100.000 centipoise. For purposes of control, the apparent viscosity of the gels of this invention is measured with the Brookfield Viscometer at 10 rpm being a No. 7 spindle.

Suitable coreactive curing agents for the highly functional epoxy resin in the protective coating composition may be another resin, such as, e.g., a phenolic type resin, a polyamide resin, or a melamineformaldehyde resin; or it may be, e.g., a dibasic acid. Suitable curing agents are amines, e.g., methylene dianiline, diethylene triamine or metaphenylene diamine; or an amide such as, e.g., dicyandiamide. A particularly preferred curing agent is the ethylhexoic acid salt of a tertiary amine or 50% by weight of the above described salts in combination with another amine. Such preferred curing agent provides sufficient latency, about 8 hours, but provides fast cure.

In preparing a permanent protective coating according to the present invention, the amount of curing agent employed may be calculated according to the formula on page 6, tine 20: A.E.W.

Weight of hardener = ----- x weight of epoxy resin (curing agent) E.E.W.

Where the curing agent does not have a readily known amine equivalent weight such as when a catalytic curing agent like dicyandiamide is used, the amount of curing agent to effect substantially complete cure is determined empirically by trial and error.

When a temporary protective coating according to the invention is desired, the amount of curing agent which is employed should be sufficient to only partially cure the protective coating. It has been found that, when less than the amount of curing agent required to effect substantially complete cure of the highly functional epoxy resin in the protective coating is used, the protective coating is only temporary. The amount of curing agent to effect partial cure of the highly functional epoxy resin in the coating is more than about 10 weight %, should be more than about 20 weight %, typically is more than about 30 weight %, and suitably is more than 40 weight % of the amount of curing agent required to effect substantially complete cure of the highly functional epoxy resin in the protective coating.

Moreover, the amount of curing agent to effect partial cure of the highly functional epoxy resin in the coating is less than about 80 weight %, typically is less than about 70 weight %, should be less than about 60 weight % and preferably is less than 50 weight % of the amount of curing agent required to effect substantially complete cure of the highly functional epoxy resin of the protective coating.

Consequently, the protective coating so formed when cured is temporary and may be subsequently removed with hot alkaline cleaners or solvents.

Use of the amount of curing agent delineated hereinabove, which amounts are all less than the amount of curing agent used in forming the permanent protective coating of this invention, allows a temporary coating to be formed. In the curing of electroplating resists, the temperatures which are typically used are lower than the melting point of the highly functional epoxy resin used in the protective coating compositions of this invention. Even in the absence of a curing agent, there would be some cure of the protective coatings of this invention. Such partial cure would produce a removable coating having the attributes of the permanent coating of this invention, i.e., high resolution without bleeding allowing closer spacing of lines on a printed circuit board.

Use of the temporary and permanent protective coatings of the present invention eliminates spreading out or bleeding upon cure of the coatings and, therefore, eliminates the problems concomitant with the use of known resinous masking compositions which bleed during the curing process. Also important is the fact that the use of these temporary and permanent protective coatings leads to the achievement of high resolution at the edges of the coatings, a result not possible with conventional masking compositions.

Consequently, improved high density printed circuit boards may be manufactured by various methods of producing circuit boards which involve the use of the improved temporary and permanent protective coatings of this invention.

Other objects and advantages of the invention will be set forth in part herein and in part will be obvious herefrom or may be learned by practice with the invention, the same being realized and attained by means of the instrumentalities and combinations pointed out in the appended claims.

The following examples illustrate at least one of the best modes of the protective coating compositions and methods of the present invention as presently understood.

Seven typical mask compositions are given hereinbelow: EXAMPLE 1 Formulation 1:An Additive Permanent Plating Resist Material Amount by Weight (g) 70% epoxy novolac with 5,4 functionality 643 and a melting point of 990 C in diethylene glycol ethyl ether solvent epoxy novolac phenol formaldehyde resin 20 with an epoxy equivalent weight of 172-179 and a viscosity of 1400-2000 centipoise at 250 C (liquid epoxy screening aid) 25% copper phthalocyanine pigment dispersed 40 in bisphenol A epoxy resin with an epoxide equivalent weight of 180--190 and a viscosity of 11000-14000 centipoise at 260 C as green colorant MODAFLOWRTM (flow promoter) 12 carboxy terminated acrylonitrile rubber 25 having an average molecular weight of 3500, a carboxyl content of 2,3% by weight, a functionality of 1,85 and 18% bound acrylonitrile by weight (liquid rubber -toughener) diethylene glycol ethyl ether 40 2-ethyl hexoic acid salt of tris-(dimethyl- 50 aminomethyl)phenol (curing agent) Formulation 2: A Permanent Mask Resist Material Amount by Weight (g) tetrafunctional bisphenol A epoxy 100 resin with a softening point of 800 C epoxy novolac phenol formaldehyde resin 85 with an epoxy equivalent weight of 172~179 and a viscosity of 1400-2000 centipoise at 250 C (liquid epoxy screening aid) MODAFLOWRTM (flow promoter) 3,4 butyl cellosolve acetate solvent 20,0 2-ethyl hexoic acid salt of tris-(dimethyl- 23,8 aminomethyl)phenol (curing agent) Formulation 3:A Permanent Mask Resist Material Amount by Weight (g) difunctional bisphenol A epoxy resin 75 having a Durran softening point of about 750 C and an epoxy equivalent weight of 475-575 in diethylene glycol ethyl ether solvent (epoxy coating modifier) Material Amount by Weight (g) tetrafunctional bisphenol A epoxy resin 25 with a softening point of 800 C 25% copper phthalocyanine pigment dispersed 2,5 in bisphenol A epoxy resin with an epoxide equivalent weight of 180-190 and a viscosity of 11000~14000 centipoise at 250 C, as green colorant MODAFLOWRTM (flow promoter) 4,5 carboxy terminated acrylonitrile rubber 1 1,3 having an average molecular weight of 3500, a carboxyl content of 2,3% by weight, a functionality of 1,85 and 18% bound acrylonitrile by weight (liquid rubber toughener) butyl cellosolve acetate solvent 35,0 2-ethyl hexoic acid salt of tris-(dimethyl- 26,3 aminomethyl)phenol (curing agent) Formulation 4: A Permanent Mask Resist Material Amount by Weight (g) difunctional bisphenol A epoxy resin 48,9 having a Durran softening point of about 750 C and an epoxy equivalent weight of 475-575 in diethylene glycol ethyl ether solvent (epoxy coating modifier) 70% epoxy novolac with 5,4 functionality 227,2 and a melting point of 990 C in diethylene glycol ethyl ether solvent 25% copper phthalocyanine pigment dispersed 34,1 in bisphenol A epoxy resin with an epoxide equivalent weight of 180-190 and a viscosity of 11000-14000 centipoise at 250 C as green colorant MODAFLOWRTM (flow promoter) 13,1 diethylene glycol ethyl ether solvent 33,0 2-ethyl hexoic acid salt of tris-(dimethyl- 26,4 aminomethyl)phenol (curing agent) methylene dianiline (curing agent) 11,3 Formulation 5: An Electroplating Resist Material Amount by Weight (g) difunctional bisphenol A epoxy resin 50 having a Durran softening point of about 750 C and an epoxy equivalent weight of 475-575 in diethylene glycol ethyl ether solvent (epoxy coating modifier) Material Amount by Weight (g) 70% epoxy novolac with 5,4 functionality 60 and a melting point of 990 C in diethylene glycol ethyl ether solvent epoxy novolac phenol formaldehyde resin 10 with an epoxy equivalent weight of 172-1 79 and a viscosity of 1 400-2000 centipoise at 250 C (liquid epoxy screening aid) 25% copper phthalocyanine pigment dispersed 2 in bisphenol A epoxy resin with an epoxide equivalent weight of 180-190 and a viscosity of 11 000-14000 centipoise at 250 C as green colorant MODAFLOWRTM (flow promoter) 4 diethylene glycol ethyl ether solvent 20 2-ethyl hexoic acid salt of tris-(dimethyl- 2 aminomethyl)phenol (curing agent) methylene dianiline (curing agent) 1 Formulation 6. A Permanent Masking Resist Material Amount by Weight (g) 70% difunctional bisphenol A epoxy resin 1 692 having a Durran softening point of about 750 C and an epoxy equivalent weight of 475-575 in diethylene glycol ethyl ether solvent (a toughening epoxy modifier) 70% tetrafunctional epoxy resin whose 1 427 epoxide equivalent weight is 200-240 and Durran softening point is 70--800 C in di ethylene glycol ethyl ether (a base epoxy resin) MODAFLOWRTM (flow promoter) 96 25% copper phthalocyanine pigment dis- 83 persed in bisphenol A epoxy resin with an epoxide equivalent weight of 180-1 90 and a viscosity of 11 000-14000 centipoise at 250 C as a green colorant a fumed silica having an average particle 159 size of 0,015 microns and a total measured surface area of 200 m2/g (thickening/anti sag agent) Benzotriazole (metal deactivator) 218 (dissolved in 140 g of diethylene glycol ethyl ether solvent) 2-ethyl hexoic acid salt of tris-(dimethyl aminomethyl)phenol (curing agent) 143 Formulation 7:A Permanent Masking Resist Material Amount by Weight (g) 70% epoxy novolac with 5,4 functionality 1 427 and a melting point of 990 C in diethylene glycol ethyl ether solvent 70% difunctional bisphenol A epoxy resin 1 692 having a Durran softening point of about 750 C and an epoxy equivalent weight of 475-575 in diethylene glycol ethyl ether solvent (a toughening epoxy modifier) MODAFLOWRTM (flow promoter) 96 25% copper phthalocyanine pigment dispersed 83 in bisphenol A epoxy resin with an epqxide equivalent weight of 180-190 and a viscosity of 11000-14000 centipoise at 250 C as green colorant 'diethylene glycol ethyl ether solvent 140 trialkylaryl ammonium smectite (anti- 150 sag/thickening agent) non-volatile liquid hindered phenol 11 (high molecular weight) (antioxidant) 20% by weight dilauryl thiodipropionate 54 in butylcellosolve acetate (antioxidant) tertiary amine (curing agent) 133 In the formulations described hereinabove, MODAFLOWRTM is a high molecular weight polymer believed to be a blend of isobutylacrylate and ethyl acrylate polymers or a copolymer thereof which is commercially available from the Monsanto Chemical Co., St. Louis, Missouri. MODAFLOWRTM enhances the surface leveling properties of the composition such that the composition, when applied to the insulating or conductive bases, flows out to a smooth level without forming ripples or bubbles, so that on drying it produces a smooth, glossy surface.

The following examples illustrate at least one of the best modes of the protective coating compositions for use as solder masks and methods of the present invention as presently understood.

Five typical mask compositions are given hereinbelow: Formulation 1: Solder Mask Composition A. Formulation Makeup Material Amount by Weight (g) 50% by weight difunctional bisphenol A 537,0 epoxy resin having a Durran softening point of about 750 C and an epoxy equivalent weight of 475-575 in diethylene glycol ethyl ether solvent (epoxy coating modifier) 70% by weight epoxidized novolac diglycidil 83,4 ether bisphenol A resin in diethylene glycol ethyl ether solvent with a 4,4 average epoxide functionality and a melting point of 990 C diethylene glycol ethyl ether (solvent) 29,1 Material Amount of Weight (g) 25% by weight copper phthalocyanine pigment 18,3 (dispersed in bisphenol A epoxy resin with an epoxide equivalent weight of 180--190 and a viscosity of 11000-14000 at 250C as green colorant fumed silica having an average particle 33 3 size of 0,015 microns and a total measured surface area of 200 m2/g (thickening agent) polyisobutyl acrylate (flow promoter) 4,2 liquid epoxy phenol formaldehyde resin having 18,2 an epoxy equivalent weight of 172-179 and a viscosity of 1400-2000 centipoise at 250 C (screening aid) synthetic silica, dry white powder having 14,5 a surface area of 150 m2/g and-an average particle size of 0,021 microns (flattening agent) B.Primary Curing Agent Composition to be Admixed with A Material Amount by Weight (g) methane diamine 61,0 2-ethyl hexoic acid salt of tris-(dimethyl- 7,9 aminomethyl)phenol carboxy terminated acrylonitrile butadiene 16,5 and pendant carboxyl groups having an average molecular weight of 3500, a carboxyl content of 2,37% by weight, a functionality of 1,85 and having 1 8% by weight bound acrylonitrile C. Modifying Additives to be Added Optinally to A and B for Flame and/or Oxidation Resistance Material Amount by Weight (g) tetrabromobisphenol-A (a brominated 181,0 phenol) (flame resistant agent) a non-volatile hindered phenol 2,2 (oxidation resistance) 20% by weight dilauryl thiodipropionate 11,1 in ethylene glycol butyl ether acetate (oxidation resistance) Formulation 2: Solder Mask Composition A.Formulation Makeup Same as in Formulation 1 Hereinabove B. Primary Curing Agent to be Admixed with A Material Amount by Weight (g) 50% by weight triethylenediamine in 127,0 methyl cellosolve Material Amount by Weight (g) carboxyl terminated acrylonitrile butadiene 16,5 and pendant carboxy groups having an average molecular weight of 3500, a carboxyl content of 2,37% by weight, a functionality of 1,85 and having 18% by weight bound acrylonitrile 2-ethyl hexoic acid salt of tris-(dimethyl- 4,6 aminomethyl)phenol (secondary curing agent) a fumed silica having an average particle 10,0 size of 0,01 5 microns and a total measured surface area of 200 m2/g (thickening agent) Formulation 3: Solder Mask A and B are same as in Formulation 2 except that 65 g of N-(2-aminoethyl)-piperedine are used instead of 127 g of 50% by weight triethylenediamine in methyl cellosolve.

Formulation 4: Solder Mask A and B are the same as in Formulation 2 except that 60 g of bis(P-aminocyclohexyl)methane are used instead of 127 g of 50% by weight triethylenediamine in methyl cellosolve.

Formulation 5: Solder Mask A. Formulation Makeup Same as in Formulation 1 Hereinabove B. Primary Curing Agent Composition to be Admixed with A Material Amount by Weight (g) methane diamine 47 dimer acid (primarily composed of C38 13 dibasic acid having a molecular weight of approximately 565) 2-ethyl hexoic acid salt of tris-(di- 6 methylaminomethyl)phenol (secondary curing agent)

Claims

1. A protective coating composition for use in the manufacture of printed circuit boards, said composition comprising a thermosetting resin or resin composition and a curing agent dissolved in a solvent, said thermosetting resin or resin composition being a solid at a temperature of 600 C or below and having a functionality of three or greater; said coating composition being in an initial state as a high viscosity solution or a soft gel-like matter and, upon being cured, transforming from said initial state directly, and substantially or completely without liquefying, into a final state as a solid.

2. The composition as claimed in claim 1 wherein the thermosetting resin is a solid at room temperature.

3. The composition of one or more of claims 1 or 2 wherein said thermosetting resin is selected from urea, melamine, phenolic, polyamide-imide, polyimide, alkyd, polyurethane and epoxy resins and blends thereof.

4. The composition of claim 3 wherein the said thermosetting resin is an epoxy resin having a functionality of less than 7.

5. The composition of one or more of claims 1 to 4 wherein said thermosetting resin composition comprises an epoxy resin having a melting point of between 60 and 2000 C.

6. The composition of one or more of claims 1 to 5 wherein said composition further comprises an anti-oxidant.

7. The composition as claimed in claim 6 wherein said anti-oxidant is selected from thioesters, phosphites, phenols and mixtures thereof.

8. The composition of one or more of claims 1 to 7 further comprises a primary curing agent; and/or a flow agent; and/or at least one modifying agent selected from the group consisting of screen printing aids, thickening agents and tougheners.

9. The composition as claimed in one or more of claims 1 to 8 for use as temporary masking layer removable after curing wherein the primary curing agent is present in an amount insufficient to effect substantially complete cure of said coating composition.

10. The composition as claimed in at least one of the preceding claims 1 to 8 for use in a permanent masking layer wherein the primary curing agent is present in an amount sufficient to effect substantially complete cure of said coating composition.

11. The composition as claimed in claim 10 wherein said primary curing agent is comprised of an amine which is corrosive to copper surfaces.

12. The composition as claimed in claim 11 wherein said curing agent is an alicyclic amine or a reaction product of said amine.

13. The composition as claimed in one or more of claims 8 to 12 wherein said primary curing agent is an alicyclic polyamine selected from methane diamine, 1 3-diamino-cyclohexane, isophorone diamine, triethylenediamine, and hexamethylenetetraamine.

14. The composition as claimed in one or more of claims 8 to 13 further comprising a secondary curing agent comprised of an aromatic amine, said secondary curing agent being present in an amount sufficient to provide said cured coating composition with resistance to cleaning agents, e.g., for flux removal after soldering.

1 5. The composition of claim 14 wherein said aromatic amine is selected from tri-2-ethyl hexoate sale of tri-(di-methylaminomethyl)phenol; diamine diphenyl sulfone; benzyl dimethyl amine; metyphenylene diamine; and methylene dianiline.

1 6. The composition as claimed in one or more of claims 8 to 1 5 wherein said thickening agent is present in an amount sufficient to produce a soft gel-like state of said coating composition; and further comprising an organic acid in an amount sufficient to reduce or eliminate the basicity of the alicyclic amine present as said curing agent thus retaining the thixotropic character of said composition.

17. The composition as claimed in claim 16 wherein said thickening agent is fumed silica.

18. The composition of claim 17 wherein the fumed silica is comprised of particles of submicroscopic size with a total surface area between 200 and 400 m2/g.

19. The composition of at least one of claims 16 to 18 wherein said organic acid is selected from the group consisting of carboxyl terminated liquid acrylonitrile rubbers with or without pendant carboxyl groups, fatty acids and di-basic acids.

20. The composition as claimed in claim 19 wherein said fatty acids are selected from the group consisting of linoleic and oleic acid, and said dibasic acids are selected from the group consisting of adipic acid, glutaric acid, azelaic acid, sebasic acid and suberic acid.

21. The composition as claimed in claim 16 wherein said organic acid is an acid having at least two carboxyl groups.

22. The composition as claimed in claim 16 wherein the alicyclic amine hardener is added to the composition as its salt with said organic acid.

23. The composition as claimed in claim 22 wherein the salt is the reaction product of a carboxylic terminated acrylonitrile rubber and an alicylic amine.

24. The composition of one or more of claims 8 to 19 wherein said screen printing aid is a liquid resin being capable of reacting with said thermosetting resin thus on curing to produce a copolymer.

25. The composition of claim 24 wherein said screen printing aid comprises an epoxy resin which is liquid at room temperature, said liquid epoxy resin having an epoxy equivalent weight of 225 or below.

26. The composition as claimed in one or more of claims 8 to 25 wherein said flow agent comprises an alkyl acrylate polymer or a silicone.

27. The composition of one or more of claims 8 to 26 wherein said toughener comprises a liquid acrylonitrile butadiene copolymer or a solid epoxy resin having an epoxy equivalent weight of above 350.

28. The composition of at least one of claims 8 to 27 wherein the viscosity of said composition is between 10,000 and 200,000 centipoise.

29. A method for producing printed circuit boards including the steps of forming a pattern by printing a resinous mask of a composition as claimed in at least one of claims 1 to 28 and curing said mask by heating.

30. A method for producing printed circuit boards which includes the steps of forming a conductor pattern prior to printing a resinous solder mask of a composition as claimed in one or more of claims 1 to 8 and 10 to 30; and then heating the solder mask thus curing it.

31. The method of claim 29 wherein said solder mask is registered with respect to the conductor pattern.

32. A method for producing printed circuit boards on a substrate provided with a metal layer on at least one surface, which includes the steps of forming a pattern by printing a temporary resist masking layer onto its surface, employing a mask composition as claimed in one or more of claims 1 to 9 and 11 to 29, heating said masking layer to cure the said resist composition, removing the metal layer from the unmasked areas by etching and removing the resinous masking layer.

33. The method of one of claims 29 to 32 wherein the resinous masking layer is cured at a temperature between 60 and 2000 C.