GB2283617A - Blocking through-holes in laminar substrates - Google Patents

Abstract

In order to block through-holes in laminar substrates, such as printed circuit boards, to prevent accumulation of contaminants in the through-holes, through-holes are blocked by applying a photocurable blocking material to one face of the substrate, and exposing the opposite face of the substrate to radiation to which parts of the substrate which are to be kept clean are opaque, to cure blocking material in the through-holes only. Excess uncured material may then be easily removed eg. by washing with a suitable aqueous solution.

Description

BLOCKING THROUGH-HOLES IN LAMINAR SUBSTRATES The present invention relates to improvements in and relating to blocking through-holes in laminar substrates. The invention is particularly, but not exclusively, concerned with the use of photocurable compositions to block through-holes in printed circuit boards and the like.

Printed circuit boards are typically made from boards of phenolic laminate or other suitable laminate, coated with a thin layer of copper. They may be provided with through-holes (also called via-holes, or vias) drilled through the copper and the laminate, for various purposes. For example, double-sided printed circuit boards have through-holes, to connect together the layers of multi-layer circuits, to conduct heat away from inner layers of multi-layer circuits and/or to act as anchor points for components which are attached by wire leads.

The through-holes may be made conductive by coating with copper. The through-holes may typically be from 0. 1 to 2. Omm in diameter.

Such through-holes can catch and hold contaminants such as ionic residues, flux, solder or dust during manufacture of the printed circuit board, which contaminants may affect the board' s final performance.

Therefore, attempts have been made in the past to seal the through-holes.

In one known process, a layer of dry film is placed across the through hole, which is known as "tenting".

This process has problems in application, for example on removal of the dry film, which requires an expensive stripping solution.

In another process, a thermally curing material is printed into the through-holes and cured by heating.

This process requires excess material to be scrubbed from the surface of the board before further processing, for example from parts of the copper which are to be etched or solder-coated.

Solvent borne photocuring inks have also been used, but the volume shrinkage caused by the solvent evaporation leads to shrinkage which can give inefficient through-hole blocking. 100% solids photocuring inks have also been used, which have low volume shrinkage.

They are applied by squeegeeing the ink onto the board and into the through-holes and photocuring the ink.

Again, excess cured ink must be scrubbed from the surface of the board.

The present invention sets out to provide a method of blocking through-holes in laminar substrates which gives good through-hole blocking but in which the cleaning of excess blocking material from parts of the substrate which are to be kept clean is simplified.

The present inventors have discovered that, in accordance with the present invention, through-holes in a laminar substrate may be blocked by applying a photocurable blocking material to the through-holes on one side and then curing the blocking material in the through-holes by irradiation with electromagnetic radiation from the other side of the substrate, the parts of the substrate which are to be kept free of cured blocking material being opaque to the electromagnetic radiation used.

The present invention therefore provides a method of blocking through-holes in a laminar substrate, comprising: applying a photocurable blocking material to one face of the substrate, so that blocking material enters through-holes of the substrate; and curing blocking material which is in through-holes of the substrate by exposing the opposite face of the substrate to electromagnetic radiation, to which radiation parts of the substrate which are to be kept free of cured blocking material are opaque.

The invention is particularly applicable to printed circuit board material, in which it is often desired to keep the copper layer free from cured blocking material, the copper coating being opaque to a wide range of electromagnetic radiation.

By through-holes are meant holes which extend continuously from one face of the laminar substrate to the other. According to the invention, it is preferred that the through-holes are blocked completely across their cross-section and filled from one face substantially to the other face of the substrate in order to completely seal the areas where contaminant may build up, and to avoid leaving trapped air which can expand if the substrate is heated, rupturing the cured blocking material.

The blocking material is preferably fluid, to allow it to be readily applied to the through-holes. The blocking material preferably comprises a thixotropic material, or one of very low cold flow, so that it does not flow out of the through-holes before curing. Such materials may be used to block large through-holes (of diameter 1. Omm to 2. Omm for example) as the blocking material meniscus in the hole is less likely to break.

The blocking material may be applied by screen printing, for example through a suitably prepared coarse mesh screen or through a suitably perforated aluminium or steel template. The blocking material may also be applied using a roller coater or by flooding the substrate with the material and using a squeegee drawn across the surface of the substrate to force the blocking material into the through-holes while clearing most of the excess material from the surface of the substrate.

Preferably, the blocking material is applied to the substrate as a patterned image, substantially restricted to all or some of the through-holes, to minimise wastage of blocking material.

In the method of the invention, excess blocking material may remain after curing on the opaque parts of the laminar substrate on the face opposite the face which is exposed to electromagnetic radiation.

Very good specificity of the areas exposed to the electromagnetic radiation may be achieved using the invention; cured blocking material is not present on parts of the board where it is not required, for example copper pads at the mouths of the through-holes in printed circuit boards, and does not have to be scraped off or otherwise removed.

The present invention allows this to be achieved without the use of artwork to shield the substrate from the radiation where curing is not required. Artwork may, however, be used if it is desired to prevent blocking material from being cured on parts of the substrate which are transparent to the irradiating radiation.

The blocking material may comprise a composition that can be hardened further after photocuring by thermal curing, to improve its heat resistance and adhesion.

Photocurable compositions generally comprise a photocurable material, that is a material capable of curing or polymerising upon exposure to electromagnetic radiation. Typically, they comprise an ethylenically unsaturated or polyethylenically unsaturated material.

Where, as is most common, curing is designed to be effected by exposure to ultra-violet light, the composition will also generally comprise a photosensitiser or photoinitiator, for initiating curing or polymerisation of the photosensitive material.

One general class of photocurable system comprises a curable reactive material (generally oligomeric or polymeric in nature) together with an initiator component which on exposure to ultra-violet radiation reacts with the curable components to cross link it or cure it. A particular embodiment of this sort of photocurable system comprises polyvinyl alcohol as curable component together with a diazo initiator.

However, this particular system does not have very good heat resistance and would not be used to block holes permanently in printed circuit boards, as it would be unlikely to be able to resist the soldering temperatures.

Another general class of photocurable system comprises an ethylenically unsaturated curable material (again generally oligomeric or polymeric in nature) together with a photoinitiator which, on exposure to the appropriate radiation, gives rise to free radicals which initiate polymerisation of the double bonds in the curable component. An example of photocurable materials which may be used in the second general class of photocurable systems include multi functional acrylate oligomers such as pentaerythritol triacrylate, trimethylol-propane triacrylate and ethylene glycol polyacrylate. Other photosensitive materials are those which may be obtained by reacting multi functional isocyanate compounds with ethylenically unsaturated compounds containing a group containing an active hydrogen atom such as a hydroxyl group or carboxylic acid group. Examples of suitable isocyantes include hexamethylene di-isocyanate, tolylene di-isocyanate or isophorone di-isocyanate, or dimers or trimers formed therefrom. Suitable unsaturated compounds containing active hydrogen include, for example, hydroxy-ethyl acrylate, hydroxy ethyl methacrylate, acrylic acid and methacrylic acid.

A further class of UV sensitive curable materials are those formed by the reaction of poly-epoxy compounds (so-callc epoxy resins) with ethylenically unsaturated acids sucr as acrylic acid or methacrylic acid; which reaction products may simply be termed "epoxy acrylates". The epoxy compound may be a simple glycidyl ether such as ethylene glycol diglycidyl ether or phenyl glycidyl ether; or a bis-phenol A/epichlorohydrin adduct such as those sold under the trade name "Epikote".

Preferably, the hole blocking material comprises no volatile diluents, so that it does not require drying and so that the volume-change on curing is as little as possible.

The blocking material is preferably one which, in its uncured state, is readily removable by washing in aqueous solutions. This permits the material to be "developed" by removal of uncured portions of the blocking material after exposure to electromagnetic radiation. Preferably, the blocking material comprises carboxylic acid groups so that uncured blocking material may be stripped from the board by using an aqueous alkaline solution, for example a dilute solution (for example 0. 2 to 1.0% w/w) of sodium carbonate. Such a dilute solution will not remove the cured blocking material.

Examples of such resins include epoxy acrylate resin, epoxy cresol novolak acrylate resin, epoxy novolak acrylate resin, acrylated acrylic resin, polyester acrylate resin or urethane acrylate resin.

A particularly effective way to render epoxy acrylate materials soluble or developable by aqueous alkaline solutions is to react the epoxy acrylate or acrylated acrylic resin with one or more dicarboxylic acid anhydrides (serving to introduce free carboxy groups into the final epoxy acrylate). Suitable dicarboxylic acid anhydrides for this purpose include succinic, itaconic, maleic and phthalic anhydrides.

Suitable carboxylated epoxy novolak acrylate resins or carboxylated epoxy cresol novolak acrylate resins include the reaction products of phenolic novolak derived epoxy resins (for example those sold under the trade names Quatrex 2410 [Dow Chemicals] or NPPN 638 [Nan Ya Plastics]) or epoxy cresol novolak resins (including those sold under the trade names Quatrex 3410 [Dow Chemicals] or NPCN 703 [Nan va Plastics]) with acrylic or methacrylic acid which have been carboxylated using a suitable diacid anhydride such as maleic, tetrahydrophthalic, hexahydrophthalic, itaconic or trimellitic anhydride. Carboxylated acrylated acrylics include the Sarboxy resins produced by Sartomer Inc.

Alternatively, the blocking material may comprise a non-carboxylated photocurable resin mixed with a component having carboxylic acid functionality, which component renders the formulation alkali soluble.

Photocurable compositions which may be thermally cured after photocuring include compositions based upon resins having unreacted epoxy groups, of which several must be present in each molecule. Typical examples include the reaction products of bis-phenol A, phenol novolaks or cresol novolaks with epichlorohydrin. Epoxy phenol novolaks and epoxy phenol novolaks are particularly suited for high temperature resistance materials.

Catalysts for the thermal curing reaction may be added to the composition, for example dicyandiamide, imidazole compounds and amine compounds.

Reactive liquid diluents may be mixed with the photocurable component of the blocking material, to make them more mobile and suitable for application. Such diluents become part of the cured blocking material on exposure to radiation, having, for example, acrylic functionality. Reactive diluents which are suitable for this invention include trimethylol propane triacrylate, pentaerythritol tetraacrylate, di-trimethyl propane tetraacrylate, glyceryl propane triacrylate or dipentaerythritol pentaacrylate. They may be used separately or in mixtures to obtain suitable flow characteristics in the finished ink.

The blocking material may further comprise known conventional additives such as photoinitiators, inorganic fillers, colourants, anti-foaming agents and flow aids.

A wide variety of photoinitiators are known for use in photocuring systems and examples of these include anthroquinones such 2-ethyl-anthroquinone, 2-methyl-anthroquinone and 1-chloro-anthroquinone; thioxanthones such as 2, 4-dimethyl-thioxanthones, 2,4-diethyl-thioxanthones and 2-chloro-thioxanthones; acetophenones, benzophenones, and benzil photoiniators.

Suitable initiators include those sold under the trade names Irgacures 651 and 907 (Ciba-Geigy). These photoinitiators can be alone or in admixture and may also be used together with photopolymerization accelerators such as benzoic acid type accelerators or tertiary amine accelerators.

Inorganic fillers are added to the formulation to increase the mechanical strength of the cured plug, to alter blocking material rheology and to minimise shrinkage of the blocking on curing. Suitable fillers include talc, barium sulphate, silica, mica or china clay.

Colourants are not essential to the present invention, but may be added if required to match the colour of the blocking material to any material coated over it. For example phthalocyanin pigments may be used.

The method of the present invention finds application particularly in a number of processes in the manufacture of printed circuit boards and the like. In particular, the cured blocking material may provide support for so-called "resists", that is compositions that are applied in patterned form to a substrate to facilitate and define the area of operation of subsequent processes such as plating, etching, the application of solder etc.

Resist compositions can be applied to the substrate in patterned form by suitable printing processes (for example, screen printing) and then cured onto the substrate.

Alternatively, a "photoresist" is employed, that is, a photocurable composition is applied over the surface of the substrate and imagewise exposed to appropriate radiation through a patterned mark or artwork to cure portions of the resist exposed to the radiation, and later "developed" (by removal of uncured portions of the material) to form a patterned image of cured material nn the substrate.

It is particularly desired that cured blocking material should be substantially absent from the areas to which etch or solder resist is to be applied, to avoid affecting the performance of the resist. In particular, the copper coating of printed circuit boards should be kept substantially free of cured blocking material.

Small deposits of cured blocking material on such areas may be tolerated, as long as they do not affect the performance of the resists.

A generally clean surface extending over the tops of the through-holes is required to ensure good adhesion of the resist over the through-holes.

Accordingly, the present invention further provides a method of etching, plating or applying solder to a substrate by the steps of: a) blocking through-holes in the substrate as set out above; b) forming a patterned image of a cured resist material upon the substrate; and c) etching, plating or applying solder to portions of the substrate not protected by the resist.

Preferably, the cured resist material and the cured through-hole blocking material are removed, preferably by the use of an aqueous solution. For example, where the resist and/or hole blocking material comprise components having carboxylic acid groups, the cured material may be removed by using concentrated caustic soda solution.

Alternatively, the through-hole blocking material may be left in the through-holes in a substrate, to provide a permanent protection against the accumulation of dirt etc.

Optionally, the cured blocking material may be subjected to a further thermal curing step, to improve heat resistance and adhesion.

Where permanent through-hole blocking is required, epoxy novolak and epoxy cresol novolak acrylate carboxylates are preferred components of the through-hole blocking material, as they have sufficient heat stability to withstand the subsequent soldering processes that the substrate will be subjected to.

Anti foaming agents such as those sold under the trade names C-100 (Basildon Chemicals), Defoamer 052, 065 or 070 (Byk-Chemie) or Efka-20 (Efka Chemicals) may be used. Flow aids such as the flow aid sold under the trade name Modaflow (Monsanto Chemicals) may be used.

In order that the invention may be well understood the following examples are given by way of illustration only.

Example 1 A photocurable, alkali soluble resin was prepared by dissolving 1 equivalent of an epoxy cresol novolak resin (Quatrex 3410, Dow Chemicals) in trimethylol propane triacrylate. This was acrylated using 1 equivalent of acrylic acid and then carboxylated with sufficient maleic anhydride to give a resin solution with an acid value of 57 mgROH, and a reactive diluent content of 50% w/w.

The resin solution was used to produce a blocking material with the following formulation (expressed in parts by weight), the components being dispersed using a high shear stirrer: Photocurable resin solution (as above) 47 Hydroxyethyl methacrylate 19 Adhesion Promoter (PM2, Nippon KIYAKU) 1 Photoinitiator (ethyl anthroquinone) 1. 5 Silicon antifoam (C-100, Basildon Chemicals) 0.5 Talc 24 Silica (Syloid ED30, Grace) 7 This blocking material was coated onto a plated printed circuit board having through-holes using a 21 thread/centimetre polyester printing screen blocked with a screen emulsion which had been photoimaged and developed so that only the areas in the screen which coincided with through-holes on the printed circuit board were left open. The material blocking the through-holes was then W cured by shining UV light onto the opposite site of the PCB from the printed side. The printed circuit board was then passed through a spray developing machine containing 0. 6% w/w sodium carbonate solution at 38 C, to wash unwanted blocking material from the copper pads at the end of the through-holes, leaving cured 'plugs' in the through-holes.

A photoimageable two pack solder resist (XV501 TSM, Coates Electrographics) was then curtain coated to yield a printed circuit board with plugged and sealed through-holes which were resistant to dip solder and wave soldering for 10 seconds in molten solder at 260'C. Through-holes of between 0.2mm and 1.2mm diameter were successfully plugged in this fashion.

Example 2 A solution of UV curable polyester acrylate was prepared by reacting neopentyl glycol (0.56 equivalents), trimethylol propane (0. 54 equivalents) and terephthalic acid (0.56 equivalents) to give a solid polyester resin with a ball and ring melting point of 105it and a hydroxy value of 140mgROH/g. This resin was dissolved in solvent and acrylated using acrylic acid by condensation reaction. The acrylated polyester was then carboxylated using maleic anydride and the solvent vacuum stripped from the resin while di-trimethylol propane tetraacrylate was fed into the reaction vessel.

This yielded a resin solution which was 50% w/w polyester acrylate, 50% w/w reactive diluent, with an acid value of 35 mgKOH/g.

The resin solution was then used to produce a blocking material formulation using a Diaf high shear stirring unit, having the following constituents (expressed in parts by weight): W curable poyester acrylate solution 50 Hydroxyethyl methacrylate 15 Adhesion promoter (Lubrizol 2063, Lubrizol Ltd) 2 Photoiniti ator (Irgacure 907, Ciba-Geigy) 7 Silicone antifoam (BYK-065, BYK Chemie) 0. 5 Barium Sulphate 21. 5 Silica (Cab-O-Sil M5, Cabot Chemicals) 3 Photoinitiator (thioxanthone ITX, Ward-Blenkinsop) 1 The blocking material produced was applied to an unetched printed circuit board by printing it through an aluminium foil stencil, punched so that the material was only forced into through-holes which were required to be plugged.

The circuit board was then illuminated from the opposite side to cure the material in the through-holes. Excess, uncured material was washed off using warm 0. 6% w/w aqueous sodium carbonate solution. The board was then coated on either side with a 12 micron (dry film thickness) coating of Aqualine H20/750 primary etch resist (Coates Electrographics Ltd). The resist was photoimaged and developed to give a pattern of the desired wiring pattern and the exposed copper etched away using acid cupric chloride etchant. The etch resist and cured blocking material were then stripped using 5% w/w caustic soda solution at 55 C, to yield a fully etched board with all through-holes which had been blocked with blocking material showing no sign of etchant attack (ie these copper lined through-holes had been successfully protected from the etchant by the blocking material) or blocking material residues.

Example 3 A photocurable alkali soluble resin was prepared as in Example 1. The resin solution was used in conjunction with a thermally curing element in the composition defined below, specified in parts by weight.

Photocurable resin (as Example 1) 40 Hydroxyethyl methacrylate 19 Adhesion promoter (PM2 Nippon Kiyaku) 1 Photoinitiator (ethyl anthroquinone) 1. 5 Silicon anti foam (C-100, Basildon Chemicals) 0. 5 Talc 23 Silica (Syloid ED30, Grace) 7 Di cyandi amide 0. 5 Hardener resin, Phenolic Novolak (Hoechst) 0.5 30% solution of Quatrex 2410 (Dow 7 Chemicals) in trimethylol propane triacrylate The blocking material was applied to an etched printed circuit board by printing it through a polyester mesh, suitably blocked to allow the material onto the parts of the printed circuit board (PCB) required. The material was forced into the required through-holes by this means.

Photocuring and development was affected as with previous examples. The circuit board was then subjected to a normal solder resist process during which the PCB is baked at 140-C. During this part of the process the thermal curing resin in the through-hole blocking material cured and promoted adhesion in the holes giving greater resistance to removal during soldering.

Claims (8)

Claims

1. A method of blocking through-holes in a laminar substrate, comprising: applying a photocurable blocking material to one face of the substrate, so that blocking material enters through-holes of the substrate; and curing blocking material which is in through-holes of the substrate by exposing the opposite face of the substrate to electromagnetic radiation, to which radiation parts of the substrate which are to be kept free of cured blocking material are opaque.

2. A method according to Claim 1, when used to block through-holes in printed circuit board material.

3. A method according to Claim 1 or 2, wherein the blocking material comprises an ultra-violet photocurable composition.

4. A method according to Claim 3, wherein the blocking material comprises a carboxylated epoxy acrylate resin, carboxylated epoxy cresol novolak acrylate resin, carboxylated epoxy novolak acrylate resin, or carboxylated acrylated acrylic resin.

5. A method according to any of the preceding claims, wherein the blocking material is, in its uncured form, soluble in dilute alkaline solution, uncured blocking material being removed after exposure to radiation by washing in a dilute alkaline solution.

6. A method according to any of the preceding claims, further comprising thermally curing the blocking material after photocuring.

7. A method of etching, plating or applying solder to a substrate by the steps of: a) plugging through-holes in the substrate by a method according to any of the preceding claims; b) forming a patterned image of a cured resist material upon the substrate; and c) etching, plating or applying solder to portions of the substrate not protected by the resist.

8. A method of blocking through-holes in a laminar substrate, substantially as herein described with reference to Example 1, Example 2 or Example 3.