NO159729B - PROCEDURE FOR MANUFACTURING A PATTERN METAL PATTERN ON A LAYER ELECTRIC CONDUCTIVE METAL BEARED BY A NON-CONDUCTIVE SUBSTRATE. - Google Patents

Description

The present invention relates to a method for producing a pattern of solder on a layer of electrically conductive metal which is supported by a non-conductive substrate by equipping the metal layer with a pattern of solder protective coating.

To provide connection between the electrical components of a printed circuit board of the type that consists of a patterned layer of electroconductive metal (usually copper) on an electrically non-conductive substrate (usually a plastic-impregnated substrate), a patterned layer of solder protector is applied and the coated plate is then brought into contact with the molten solder so that the solder bonds to the patterned metal layer on the exposed parts. Typically, the electrical components are placed on one side of the plate so that the lead elements extending from them are passed through holes in the plate into the exposed part of the patterned metal layer before the plate is brought into contact with the solder metal.

It has now been found according to the invention that a patterned solder protector can be produced from a photopolymerizable preparation containing a solid or semi-solid reaction product of a polyepoxide and an ethylenically unsaturated carboxylic acid, a photopolymerization initiator for this and an inert inorganic filler, by exposing a layer of the preparation to chemical radiation through a suitable transparent substrate with an opaque image.

According to this, the present invention relates to a method of the nature mentioned at the outset where parts of the metal layer are coated with a solder protective coating and other parts of the metal layer are not coated in this way, and to bring the metal layer applied with the solder protective coating into contact with molten solder so that the solder is bonded to the parts of the metal layer that are not coated with a solder protective coating, where the solder protective coating is produced by polymerization through exposure to chemical irradiation of a photopolymerizable coating provided by applying a coating agent comprising: a) an ethylenically unsaturated, polymerizable reaction product of an aromatic polyepoxide and an ethylenically unsaturated carboxylic acid; b) an inert inorganic filler; c) a photopolymerization initiator for said polymerizable reaction product; d) a volatile, organic solvent for said polymerizable reaction product, as well as e) possibly one or more other photopolymerizable components in an amount of less than 25 wt-Æ, calculated

on the weight of the photopolymerizable reaction product,

and this method is characterized by the fact that:

as component a) a solid or semi-solid product is used; and as component b) filler and polymerizable reaction product are used in a weight ratio of from 20 to 65 parts by weight of filler to from 80 to 35 parts by weight of polymerizable reaction product.

A main component of the coating agent used according to the invention is the reaction product of a polyepoxide and an ethylenically unsaturated carboxylic acid, usually acrylic acid or methacrylic acid, and which will hereinafter be referred to as "epoxy acrylate". The epoxy acrylate should be solid or semi-solid at room temperature, i.e. it should have a ring-and-ball softening point (determined according to British Standard Specification No. 4692 of 1971) of at least 5°C, and preferably at least 30°C. Epoxy acrylate is a which results from the reaction between a polyepoxide and an ethylenically unsaturated carboxylic acid or reactive derivatives thereof. The polyepoxide should be an aromatic polyepoxide and any polyepoxide can be used provided that the reaction product with the ethylenically unsaturated acid is a solid or semi-solid substance at room temperature.

Aromatic polyepoxides are polyepoxides containing phenyl groups (polyphenylpolyepoxides), such as polyepoxides from the reaction between bisphenols, especially polynuclear bisphenols such as bisphenol A, with epichlorohydrin or epoxidized phenyl novolacs, the former usually being preferred. Aromatic polyepoxides are well-known materials and are described e.g. in "Chemistry of Organic Film Formers", Soloman D.H., 2nd Edition, Krieger Publishing, 1977, at pages 188, 189 and 192. Suitable epoxy acrylates can be prepared by reacting a polyepoxide from the reaction between bisphenol A and epichlorohydrin and having a molecular weight of from 400 to 1500, preferably from 400 to 850, with a C3-C6-a,<p->ethylenically unsaturated monocarboxylic acid.

The other main component in the agent used according to the invention is an inert, inorganic filler. The filler, which can be in powdered or finely divided form, serves to increase the resistance of the preparation, when used as a solder protector, against heat or thermal shocks that occur when the hardened preparation is brought into contact with molten solder. The filler should, when the coating preparation is used for the manufacture of a solder protector, not undergo thermal decomposition when heated by contact with molten solder, and examples of suitable fillers include barite white, aluminum hydrate, kaolin, calcium carbonate (coated or uncoated) and micronized talc or mixtures thereof. The weight ratio between filler and epoxy acrylate is, as mentioned, from 20-65 : 80-35, preferably from 25-55 : 75-45 and more specifically from 30-45 : 70-55.

In order to obtain a tack-free coating from the agent (which is described below), it is usually preferred when using less solid epoxy acrylates (i.e. those with a lower softening temperature) to use larger amounts of inorganic filler, but of course within the broad ranges indicated above. The photopolymerization initiator used in the agents according to the invention serves to induce polymerization from epoxy acrylate when the agent, after application to the substrate, is subjected to chemical radiation. A wide variety of such photopolymerization initiators are known, such as benzoin ethers and anthraquinone derivatives. Preferred initiators for use in the agents used according to the invention are phenylketone initiators such as benzophenone, acetophenone or Mischler's ketone or mixtures thereof.

The initiator is preferably present in the agent in an amount of from 1 to 20 wt-#, preferably from 5 to 15 wt-#, based on the weight of the epoxy acylate.

The agent used according to the invention also contains a volatile, organic solvent for epoxy acrylate, in which the epoxy acrylate is dissolved, examples of such solvents are lower carboxylic acid esters 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 those sold under the trade names "Cellosolve" or "Butyl Cellosolve" (ethylene glycol monoethyl ether and ethylene glycol monobutyl ether).

The amount of organic solvent present in the coating agent applied to the substrate (e.g. a printed circuit board) will, to some extent, depend on the method used when applying the agent to the substrate. When the agent e.g. if applied to the substrate by a film printing method, it may contain up to 5 wt-# of volatile organic solvents, while if applied to the substrate by a textile printing method, it may contain up to 75 wt-£ of volatile organic solvents. The agents can conveniently be prepared so that they contain a smaller amount of solvent than is required for the practical application, and additional solvent can be added to the agent to dilute it before application. In any case, it should contain sufficient volatile organic solvent to dissolve the epoxy acrylate, and prior to dilution as described above, the composition suitably contains up to 35% by weight of volatile organic solvent.

The coating agents also appropriately contain a dye, e.g. an organic pigment such as chlorinated phthalocyanine pigment, so that the application of the agent to the substrate produces a visible image. Suitable such dyes will be present in amounts of up to 5 wt.-£, based on the weight of epoxy acrylate, filler and initiator, preferably from 0.5-2 wt.-5é. The coating agents can also contain antifoaming agents, such as silicone oil to thereby improve the application properties. Such antifoaming agents may be present in amounts corresponding to those indicated above for the dyes.

While the agents used contain epoxy acrylate as the main photopolymerizable component, other photopolymerizable materials may be present and examples of such include esters of mono- or polyhydric alcohols with ethylenically unsaturated carboxylic acids such as acrylic or methacrylic acids and liquid epoxy acrylates. However, such other photopolymerizable materials are not essential and, when used, it is preferred that they be used in smaller amounts compared to the epoxy acrylate, as mentioned, in amounts of less than 25%, preferably less than 10% by weight, of the solid or semi-solid epoxy acrylate.

A coating agent used in the method according to the invention is used to produce a photopolymerizable coating when applied to the substrate by a suitable method such as film printing, textile printing or roller coating, and the applied coating is therefore allowed to dry to a non-adhesive state (i.e. to a state such that it does not stick to surfaces it may come into contact with) and usually this drying can be accelerated by heating the applied coating.

The resulting coating can be polymerized by exposing it to actinic radiation, e.g. from a mercury vapor lamp.

As indicated above, a polymerizable coating from a coating agent as described is particularly suitable for use in the manufacture of a solder protector for a method of manufacturing a printed circuit board.

A method of producing a pattern of solder on a layer of electrically conductive metal placed on an electrically non-conductive substrate comprises providing the metal layer with a patterned protective coating, such that parts of the metal layer are coated with the solder protective coating and other parts of the metal layer are not is coated in this way, and to bring the metal layer to which the solder protective layer is applied in contact with molten solder, so that the solder binds to the parts of the metal layer that are not coated with the solder protective coating, where the solder protective coating is produced by polymerization, by exposure to chemical irradiation, of a photopolymerizable coating provided by applying the coating preparation according to the invention to the metal layer, after which it is allowed to dry.

Such a procedure consists of the following steps:

(a) producing a circuit board with a patterned layer of conductive metal (hereinafter simply referred to as copper) with a coating as described at least over the copper layer, e.g. by film printing or by textile printing, where the coating will extend over the entire surface of the card; (b) allow the applied agent to dry to a non-tacky state condition, e.g. by evaporation of volatile organic solvent; (c) exposing a coated card to chemical irradiation through a positive desired solder pattern (e.g., a transparent, typically a photographic transparent, having light-transmitting portions corresponding to the non-solderable portion of the desired solder pattern and non-transmissive portions corresponding to the solderable portions of the desired solder pattern) for to harden the exposed parts of the agent, e.g. to photopolymerize the photopolymerizable

the material of the remedy;

(d) removing the unexposed parts of the coating using a solvent, e.g. a ketone such as a cyclohexanone or a halogenated organic solvent

such as methylene chloride or trichlorethylene; and

(e) bringing the card with a patterned coating into contact with molten solder e.g. in the form of a so-called "standing wave" of solder, to apply the solder to the card in the desired pattern.

The method of the invention makes it possible to provide a circuit board with a patterned solder shield with high definition and accuracy because exposure through a positive to chemical irradiation usually provides good accuracy and definition, which is important because the overall size of the circuit boards and therefore the size of the individual parts of the solder metal protection pattern are reduced, which has been recorded in recent times.

The circuit board with a patterned coating of applied copper can 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-conductive substrate with a positive, patterned image of an acid-resistant coating is first provided and the exposed copper is then etched away with a suitable acid (e.g. hydrochloric acid), and the the remaining copper is then exposed by removing the acid-resistant coating. A coating of acid-resistant material can, as is known, be applied by a pattern coating method such as film printing or by coating the copper with a layer of photosensitive preparation, after which this is exposed to light through a positive or negative image of the desired copper layer, depending on whether the protective coating is a so-called positive or negative working protection, after which the developable (e.g. solvent-soluble) parts of the image are removed with a suitable solvent.

The agents used according to the invention are themselves completely usable as negative working protections, i.e. protections where exposed parts are hardened so that an insoluble coating is obtained. Thus, circuit boards having a patterned layer of copper on the surface can be provided by first coating a copper clad substrate with the coating agent as described, drying the coating and exposing the dried coating to chemical radiation through a positive of the desired copper layer pattern to harden the part of the coating exposed to radiation, after which uncured portions of the coating are removed with a solvent, followed by etching of the coated card. The hardened coating can then be removed by washing with a solvent, e.g. N-methyl-pyrrolidone. The card can then be given a patterned solder coating as described above.

Dried but uncured coatings of the agent are resistant to the acid etchants used to remove copper and consequently the circuit board with an applied patterned image of copper can be produced by printing (e.g. by a film printing method) a patterned coating of the agent on a copper coated non-conductive substrate, followed by drying of the coating, after which the exposed copper is etched from the card. To obtain a patterned solder coating on the resulting board, it is only necessary to expose the board to chemical irradiation through a positive of the solder pattern (as described in step (c) above) without applying an additional coating of the agent, and then proceed as described in steps (d) and (e) above. If, however, a further coating of the agent is desired, this can be applied to the card before exposure to chemical irradiation in the manner described in step (c) above.

In the additive method for producing the card with a patterned layer of copper, a non-conductive substrate is first coated with an activating material for so-called electroless pre-copper solution, the card is then equipped with a negative patterned image of the protective coating, and the card is then immersed in an electroless pre-copper solution for to produce a copper layer on exposed parts of the card, i.e. those not coated with a protective coating. Here again, the agent as described can be used to produce the protective layer, e.g. by coating the activated card with a layer of the coating agent which is then allowed to dry, after which it is exposed to chemical radiation through a suitable positive of the circuit pattern so that the exposed part of the coating is photocured while the unexposed part is then removed with a suitable solvent.

Although the agent as described is photosensitive, it is not effectively sensitive to weak light or light having a wavelength above 420 nanometers. Thus, the first application of the agent to the substrate can be carried out in light (although of course this does not have to be light with a high degree of actinic radiation).

In order for the invention to be understood more clearly, the following examples are given as illustrations. In these examples, all parts and percentages are by weight unless otherwise stated.

Manufacturing example A

Production of epoxy acrylate

86.3 parts bisphenol A/epichlorohydrin polyepoxy solid "Epikote 1001" was placed in a reaction vessel equipped with a stirrer and heated so that the polyepoxy melted. Then 13.7 parts glacial acrylic acid (mixed with 0.25#, calculated on

basis of the total weight of polyepoxide and acid, of triphenyl-phosphine and 0.25É, based on the total weight of polyepoxide and acid, of a phenolic polymerization inhibitor) added to the reaction vessel•

The reaction mixture was stirred at 120-130°C until the acid number in the mixture had fallen to 10 mg KOH/g (97 hours at 125°C). The product was then removed from the vessel and cooled to room temperature and was a solid with a ring-and-ball softening point of 64-67.5°C.

Manufacturing example B

The procedure from production example 1 was followed, and an epoxy equivalent of commercially available epoxy novolac resin "Dow Epoxy Novolak DEN 438" was reacted with an equivalent of glacial acrylic acid to an acid value of 8.5 mg KOH/g. The product was a viscous, semi-solid substance with a ring-and-ball softening point of approx. 10°C.

EXAMPLE 1

60 parts of the product from example A were dissolved in 1 40 parts "Butyl Cellosolve" so that a printing ink base I was obtained.

A protective ink was prepared as follows:

The ink was thinned with 55é "Butyl Cellosolve" and applied by film printing using a 77 mesh/cm film over the entire surface of a pure copper coated epoxy laminate where copper was already present in the form of a circuit pattern. The coated laminate was placed in an infrared dryer for 5 minutes (at 120"C) to render the coating tack-free. When dry, a positive of the required solder pattern was placed over the coating and the resulting combination was exposed to ultra-violet irradiation to cure the coating by passing it under two 80 W/cm medium pressure mercury vapor lamps at a speed of 160 cm/minute. Once the coating was cured, the positive was removed and the solder pattern was developed by washing with trichlorethylene. Finally, the board was coated with flux ( using "Superspeed 17" flux) and dried.Flux treated boards were then passed over a standing wave of molten solder at about 260°C.

An excellent result was obtained and the soldering points were precisely placed and the coating resisted soldering well.

EXAMPLE 2

70 parts "Beckopox VEM 37/1" (a commercially available solid epoxy acrylate with a ring-and-ball softening point of 58-63° C) was dissolved in 30 parts "Butyl Cellosolve" to a printing ink base II.

A protective ink was prepared from the following:

The ink was thinned with 15# "Butyl Cellosolve" and applied to a clean, copper clad phenolic laminate by film printing through a 77 mesh/cm patterned film to provide an image of the required circuit pattern. This coating was dried in an oven at 120°C for 4 minutes. The card was placed in an acid etching bath (17 parts concentrated hydrochloric acid, 3 parts 100 volume hydrogen peroxide flash, 80 parts tap water) at 40°C to remove unwanted copper, and the card was rinsed and dried. The coating resisted the acid etching and produced very clean groove edges.

The resulting card with copper in the circuit pattern already coated with the agent was placed, together with a positive of the desired solder pattern in an exposure frame and exposed to ultraviolet irradiation as described in Example 1. The exposed coating was developed after removal of the positive by washing with methylene chloride. The board was first treated as described in example 1 and passed over a standing wave of molten solder at 260°C. The solder points were precisely located and the coating resisted the solder metal.

EXAMPLE 3

65 parts of the product from example A was dissolved in 35 parts "Cellosolve" to a basis III.

A protective ink was prepared from the following:

The printing ink was diluted with 10% "Cellosolve" and then used as described in example 1 with largely the same results.

EXAMPLE 4

60 parts "Beckopox VEM 37/1" were dissolved in 40 parts "Butyl Cellosolve" to a printing ink base IV.

A protective ink was prepared from the following:

The ink was diluted with 10% "Butyl Cellosolve" and printed using a 77 mesh/cm film over the entire surface of a clean, copper coated phenolic laminate. The coating was dried in an LR dryer (at 120°C) and then placed in an exposure frame with a negative of the desired circuit pattern. This was exposed to ultraviolet radiation as described in Example 1, and unexposed areas of the coating were developed after removal from the frame by washing with methylene chloride.

The card was then placed in an acid etching bath (as described in Example 2) at 40°C which removed the exposed copper. After rinsing and drying, the card was coated with a copper circuit coated with a hardened layer of the agent. The coating resisted etching and produced very clean groove edges. The card was immersed in a bath of N-methylpyrrolidone and the hardened coating removed.

The card was then cleaned and again film printed using a 77 mesh/cm film with a coating of the printing ink over the entire surface of the card. The coated board was dried as above and placed in contact with a positive of the desired solder pattern in an exposure frame where it was exposed to ultraviolet radiation as described above. The unexposed areas of the coating were developed by washing with methylene chloride.

The developed card was then treated with flux as described in example 1 and passed over a standing wave of molten solder at 260°C. The resulting board had accurate trace lines and placement of the solder points. The coating resisted soldering with no signs of degradation.

EXAMPLE 5

A printing ink was prepared from the following:

The ink was used as described in example 1 except that the flux-treated board was soldered by immersion in a bath of molten tin/lead solder which was held at 260°C for 10 seconds. The hardened coating showed good resistance to the solder metal.

EXAMPLES 6 AND 7

Printing ink was produced with the following composition:

Ever of the printing inks were used as described in example 5 and showed good solder resistance. ••Liquid acrylate 1 was a triacrylate of an oxypropylated trimethylolpropane.

"••Liquid acrylate 2 was the obtained reaction product according to the method from production example A by

reacting 0.35 epoxy equivalents of a list solution of bisphenol A/epichlorohydrin polyepoxide in butyl glycidyl ether with 0.33 equivalents glacial acrylic acid.

Claims (2)

Process for producing a pattern of solder on a layer of electrically conductive metal supported by a non-conductive substrate by providing the metal layer with a pattern of solder protective coating such that parts of the metal layer are coated with the solder protective coating and other parts of the metal layer are not coated in this way, and to bring the metal layer to which the solder protective coating is applied in contact with molten solder so that the solder is bonded to the parts of the metal layer that are not coated with the solder protective coating, where the solder protective coating is produced by polymerization through exposure to chemical irradiation of a photopolymerizable coating provided by applying a coating agent comprising: a) an ethylenically unsaturated, polymerizable reaction product of an aromatic polyepoxide and an ethylenically unsaturated carboxylic acid; b) an inert inorganic filler; c) a photopolymerization initiator for said polymerizable reaction product; d) a volatile, organic solvent for said polymerizable reaction product, as well as e) possibly one or more other photopolymerizable components in an amount of less than 25% by weight, calculated on the weight of the photopolymerizable reaction product, characterized in that it is used as component a) a solid or semi-solid product; and as component b) filler and polymerizable reaction product are used in a weight ratio of from 20 to 65 parts by weight of filler to from 80 to 35 parts by weight of polymerizable reaction product. 2. Method according to claim 1, characterized in that a weight ratio is used between inert, inorganic filler and polymerizable reaction product of 25-55 : 75-45, and preferably 30-45 : 70-55.