WO1991003770A1

WO1991003770A1 - Resists

Info

Publication number: WO1991003770A1
Application number: PCT/GB1990/001401
Authority: WO
Inventors: Nicholas Eric Ivory
Original assignee: Coates Brothers Plc
Priority date: 1989-09-11
Filing date: 1990-09-11
Publication date: 1991-03-21
Also published as: GB8920540D0

Abstract

In a process for the production of a patterned resist layer upon a substrate by the steps of: (i) forming a layer of radiation-sensitive material upon the substrate; (ii) imagewise exposing the layer of radiation-sensitive material to radiation to thereby cure portions of the layer exposed to the radiation; and (iii) removing uncured (unexposed) portions of the layer to form a patterned layer of cured material upon the substrate; the layer of radiation-sensitive material is formed upon the substrate by applying a layer of a powdered solid radiation-sensitive material to the substrate, heating the powdered material to cause it to melt or soften to form a continuous layer of radiation-sensitive material upon the substrate, and allowing the continuous layer to solidify.

Description

Resists

This invention is concerned with improvements in and relating to the production of patterned resists on substrates, especially in the production of printed circuit boards.

Three classes of patterned resists are commonly used in the manufacture of printed circuit boards; namely etch resists, plating resists and solder resists. Etch resists are used to partially mask a layer of metal (typically copper) formed on an insulating substrate (typically a phenolic resin-based laminate), so that, on application of an appropriate etchant, unmasked portions of the metal layer are removed, to form a patterned layer of metal on the substrate. Plating resists are employed to form a patterned mask upon a substrate which is subsequently plated with a conductive metal (typically copper) on those portions not covered by the mask, again to form a patterned metal layer on the substrate. Patterned solder resists are used on boards already having a patterned metal layer to define the areas (those not covered by the resist) to be coated with solder upon application of molten solder to the resist-covered board. One commonly used method for the formation of patterned resists (be they etch, plating or solder resists) is a photo-imaging technique. In this technique, a complete layer of radiation sensitive material (a resist precursor) is formed upon a substrate and is then imagewise exposed to radiation (commonly UV light) to cure portions of the layer exposed to the radiation. The uncured (unexposed) portions of the layer are then removed, in a so-called "development" step, by means of an appropriate solvent.

The layer of resist precursor may be formed upon the substrate in a variety of ways, for example by transferring a preformed layer of resist, formed on a suitable transfer support, to the substrate, or by applying a liquid coating to the substrate. In the latter case the coating may or may not contain volatile organic solvent or diluent. The advantage of the presence of such a solvent is that, by evaporation of the solvent, it is possible to form a dry (tack-free) coating on the board but, of course, there are the usual disadvantage attendant upon the use of volatile organic solvents or diluents. If the coating composition contains no volatile solvent or diluents, as in the case of compositions containing liquid photopolymerizable solvents or diluents, the disadvantages of the use of volatile solvents or diluents are, to a large extent, overcome. However, in this latter case, it may be that the photopolymerisable liquid diluents are themselves hazardous materials and there is also the disadvantage that it is virtually impossible to obtain a dry (tack-free) coating. A dry or tack-free coating is most desirable since it makes possible the use of an appropriate patterned mask (commonly a photographic negative) in contact with the layer of photopolymeπizable material, thereby making it possible to obtain high resolution and definition, a matter of increasing importance with increasing miniaturisation and complexity of printed circuits.

It has now been found, in accordance with the present invention, that a dry (tack-free) layer of resist precursor may suitably be formed on the surface of the board by a powder coating process.

According to one embodiment of the invention, therefore, there is provided a process for the production of a patterned resist layer upon a substrate by the steps of:

(i) forming a layer of radiation-sensitive material upon the substrate; (ii) imagewise exposing the layer of radiation- sensitive material to radiation to thereby cure portions of the layer exposed to the radiation; and

(iii) removing uncured (unexposed) portions of the layer to form a patterned layer of cured material upon the substrate;

in which method the layer of radiation-sensitive material is formed upon the substrate by applying a layer of a powdered solid radiation-sensitive material to the substrate, heating the powdered material to cause it to melt or soften to form a continuous layer of radiation-sensitive material upon the substrate, and allowing the continuous layer to solidify.

In the process of the invention, the layer of radiation-sensitive material (resist precursor) is formed upon the substrate by a powder coating process. Accordingly the coating powder may be applied to the substrate by any of the usual powder coating techniques, for example as a charged spray (i.e. by an electrostatic coating method) or as a fluidized bed. When applied by an electrostatic method it is especially preferred that the substrate (board) to which the powder is applied be itself provided with a static electric charge and this can be effected, in accordance with a preferred embodiment of the invention, by the use of a highly charged backplate placed behind the board during spraying so as to attract powder and induce static charge over the board. The powder may be applied to the board using voltages of 30-100 kV. preferably 40-90 kV. Alternatively a corona discharge unit may be used to induce charge on the board which may then be sprayed with powder using a compressed air gun. The thickness of the layer of powder will depend upon the intended purpose of the layer. Generally, the layer of powder will suitably have a thickness of from 15 to 100 microns. The layer is preferably 25 to 70 μm thick for later use as a plating resist; 15 to 60 μm thick for an etch resist; and 15 to 60μu thick for a solder resist. In order to form a continuous layer of resist precursor upon the board it is necessary to heat the board bearing the layer of powder and then allow it to cool. Heating is most conveniently carried out in an oven maintained at an appropriate temperature, which temperature will, of course, depend upon the nature of the coating material, but will typically be from 70 to 140°C. especially 100 to 120°C.

In order to promote an even spread of change over the head, and hence improve the quality of the final coating, the board may be pre-treated with an ionic or nonionic surfactant such as those sold under the Trade Names "Synperonic", "Micro" and "Decron" . The surfactant may be applied, for example, by dip-coating, spray-coating or wipe-coating.

Once the layer of resist percursor has been formed upon the board, the subsequent processing steps are conventional in the printed circuit board manufacturing art and do not require particular description. Generally the radiation to be used to cure the precursor layer will be ultraviolet light but. of course, other radiation, such as electron beam radiation may be employed. The solvent to be employed in the development step will, of course, depend upon the nature of the resist material to be removed.

The starting material used in the process of the invention is a solid photocurable material in powdered form, suitably having an average particle size of from upto 100 microns, preferably from 30 to 90 microns. The material will almost invariably be a curable composition comprising two or more components, one essential component being a solid polymerizable substance. The polymerizable substance will generally be one which is polymerizable by virtue of the presence therein of ethylenically unsaturated groups, especially terminal ethylenically unsaturated groups of the formula -CH=CH . Such unsaturated materials are conveniently obtained by reacting an ethylenically unsaturated compound containing a chemically reactive group (such as a carboxy, hydroxy, amino or epoxy group) with a suitable base or substrate material having complementary reactive groups; the base or substrate material having a molecular weight such that the final reaction product, with the ethylenically unsaturated compound, is a solid. Typical examples of such base or substrate materials include so-called expoxy resins (polyepoxy compounds) such as bisphenol A/epichlorohydrin condensation products or epoxy novolak resins.

A particularly preferred class of polymerizable substance is one which is developable using an aqueous alkaline solution (thereby making it possible to wholly avoid the use of organic solvents in the process of the invention) and which comprises the reaction product of an expoxy resin and an ethylenically unsaturated carboxylic acid (typically acrylic or methacrylic acid), which reaction produced has been further reacted with an anhydride of a docarboxylic acid to introduce free carboxyl groups into the material, thereby to render it soluble in aqueous alkaline solution. Alkali developable materials of this sort are described, for example, in US-A-398044-83. ^Other ^base or substrate materials are viny_l a_ddition polymers derived from "inert" monomers having no chemically reactive groups (other than ethylenically unsaturated groups) and. generally, minor amounts of other unsaturated monomers having chemically reactive groups. Examples of the first, "inert", monomers include alkyl esters of unsaturated carboxylic acids such as acrylic and methacrylic acid, vinyl hydrocarbons, vinyl aromatic hydrocarbons and halogenated vinyl hydrocarbons. Typical monomers containing reactive groups include hydroxyalkyl and glycidyl esters of unsaturated carboxylic acids such as acrylic and methacrylic acid, ethylenically unsaturated acids such as acrylic acid and methacrylic acid. etc.

In order to be polymerisable on exposure to radiation, the powder composition will, if it is to be cured by reaction of ultra-violet light, contain a photopolymerisation catalyst or photoinitiator such as ethyl anthraquinone. The photoinitiator or photopolymerisation catalyst is suitably present in an amount from 0.1 to 12 % by weight, preferably 0.8 to 8% % by weight of the total composition.

Other components which may be present in the powder include colouring agents, such as pigments or dyestuffs, and inert fillers such as barium sulphate or talc. Colouring agents are suitably present in amounts of up to 2% by weight and the inert fillers, if present, are suitably present in amounts of from 2 to 18%, preferably 8 to 15% by weight. The powder may also contain a flow aid, such as poly(butyl/acrylate) to promote even film production, suitably in an amount of from 0.5 to 3% by weight.

The powders may be prepared by melting the unsaturated polymerizable material, adding the ingredients to the melt and dispersing them therein, allowing the melt to cool and then converting it into a powder, e.g. by means of granualtion using a flaking roller followed by fine grinding and classification to give the desired particle size range. The melt may also be produced by a hot melt extrusion technique.

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

EXAMPLE

A mixture of 83 parts of a carboxylated epoxy acrylated resin [obtained by firstly reacting a technical bis-phenol A epoxy resin with an epoxy equivalent weight of 450-500 and a softening point of 50°C-70°C. (Epikote 1001, Shell) with acrylic acid (reactive ratio of acid groups to epoxy groups 1;1) and then reacting the acrylated epoxy resin with tetrahydrophthalic anhydride to give a resin with a final acid value of 90 mg KOH/g and a softening point of 85.3°CJ; 3.8 parts of a mixed photoinitiator, comprising xanthone, ethanone and morpholino type photoinitiators; 7.4 parts talc. 3.6 parts of a flow out additive (modaflow) adsorbed on silica; 1.6 parts of a degassing agent, (benzoin); and 1.6 parts pigment (phthalocyanine greed), was melted in a glass flask at a temperature of 95-100°C, and thoroughly mixed. After cooling the solid mass was milled and sieved to give a powder with a particle size of less than 90 microns. The powder was sprayed to an IPC test panel (backed by a steel plate) using an electrostatic spray gun (Ransberg Gema 705 CVP gun) at 100 kV. After reflow. at 100°C for 15 minutes, a non-porous film was obtained, about 60 mincrons thick. The film was then UV exposed through the desired

2 art-work at 2000 mj/cm ; developed with aqueous 0.6% sodium carbonate solution; given a post-dry at 140^*C for

2 one hour, and finally given hard UV cure (1500 mj/cm under 120 /σm metal halide lamps).

The printed circuit produced by this method was coated with Fry' s R8 flux and immersed in solder for 10 seconds. No visible changes occurred in the resist film.

EXAMPLE 2

A powder was produced as described in Example 1 except that the carboxylated epoxy acrylate resin was replaced by a carboxylated epoxy novolak resin produced by reacting an epoxy novolak resin with an epoxy equivalent weight of 180 and softening point of 69^*C (Quatrex 2410, produced by DOW Chemicals) with acrylic acid in equivalent amounts, and then reaction of the resin produced with trimellitic anhydride to yield a resin with an acid value of 88.6 mg KOH/g and a melting point of 87'C.

The powder produced, with a softening range of 92^*C-99^*C, was sprayed onto a board consisting of glass-epoxy FR4 laminate, clad on both its faces with 12 copper foil, using an electrostatic powder spray gun at 80 kV. After reflowing the powder layer at 100°C for 15 minutes a pore-free of 55 microns thickness was obtained.

The film was exposed through a photomask, developed in 0.6% w/W aqueous potassium carbonate solution and given a drying bake of 15 minutes at 120°C. The board was then cleaned and degreased using McDermids Metex acid cleaner 9771, water rinsed, micro-etched with Metex G2, water rinsed again and dipped into a 5% w/w sulphuric acid solution pre-dip before electrolytically plating the copper areas bared by development. The plating solution used was McDermid's Macuspec 9241. The resist was then stripped using a strong aqueous solution of sodium hydroxide, leaving a circuit pattern built up by plating which was faithful to the artwork and free from surface blemishes.

EXAMPLE 3

A mixture of a carboxylated epoxy cresol novolak acrylate resin [produced by reacting an epoxy cresol novolak resin with an epoxy equivalent weight of 215 and a softening point of 83°C (Quatrex 3710, Dow Chemicals) with an equivalent amount of acrylic acid, and then reacting the resin with- itaconic anhydride to yield a resin with a final acid value of 97 mg KOH/g and a melting point of 95°C]; 2.9 parts of an anthroquinone photoinitiator; 8.0 parts of inorganic filler, made up of talc and air flowed silica; 3.6 parts of a flow aid (Modaflow adsorbed on silica); 1.7 parts of benzoin, degassing agent; and 0.9 parts of Victoria blue dye was mixed by passing it through a Werner-Pfleiderer ZSK extruder. The mixture was then ground and sieved to yield a powder of less than 90 microns particle size.

The powder produced had a melting range of 104°C - 111°C and was sprayed with an electrostatic powder spray gun (Ransberg-Gema Type 705) at a voltage of 80 kV on to an FR4 glass-epoxy plate with copper foil laminated to both its faces. The board had through holes of varying diameters (0.5 to 0.9 mm) drilled in it and these had electrolessly deposited copper through them to enable them to be built up by electrolytic plating. The plast was earthed with respect to the spray gun. After reflow at 120°C for hald and hour, a non-porous film of around 56 microns is formed, all the through holes were free of resist.

The plate was then exposed to 1800 mj/cm 2 through a photomask from a 5 kW metal halide lamp, developed in

0.6% w/w potassium carbonate and dried at 100°C for 15 minutes. The board was then plated with electrolytic copper as in Example 2 and water rinsed before it was dipped in a 5% w/w fluoroboric acid bath before plating a layer of tin/lead allow onto the circuit pattern using Schloetter LA solution for plating. The resist was then stripped away using sodium hydroxide solution. The copper uncovered by this operation was then etched away using acidified cupric chloride to yield a tin/lead covered circuit free of blemishes and faithful to the photo-tool.

EXAMPLE 4

A powder lacquer was produced as in Example 3. except that the acrylated epoxy cresol novolak resin was reacted with maleic anhydride to yield a resin with a final acid value of 96 mg KOH/g and a melting point of 856°C. The powder produced had a melting point of 94-98°C.

The powder was sprayed onto a board of FR4 glass epoxy plate with copper foil laminated to one side. The board was attached to wire which earthed it with respect to an electrostatic spray gun (Ransberg-Gema type 705, operating at 90 kV) . After powder reflow at 110°C for

20 minutes a pore-free film of approximately 60 microns

2 was formed. This film was then exposed to 2000 mj/cm through a photo-mask (with a pattern suitable for an etch resist) solution; and dried at 100°C for 15 minutes. The copper bared by the development was then etched away using acidified cupric chloride leaving the circuit pattern beneath the resist unaffected. The resist was then stripped using a strong solution fo sodium hydroxide, leaving a copper circuit which was accurate and faithful to the original art-work.

EXAMPLE 5

A powder was prepared as in Example 1. The IPC test panel to be coated was covered with a film of a liquid charge dispersant (Synperonic, Ellis & Everard) by wiping with a soaked cloth. The board was then sprayed with powder, using a Ransberg-Gema 705 spray gun at 100 kV and an earthed steel plate placed behind the panel to induce a static charge all over it. The film given on reflow at 100°C for 15 minutes was pore-free and and more even than that produced in Example I. The panel was then processed as in Example I and could be soldered for ten seconds over a molten solder wave without deleterious effect to the resist film.

Claims

CLAIMS : -

1. A process for the production of a patterned resist layer upon a substrate by the steps of:

(i) forming a layer of radiation-sensitive material upon the substrate;

(ii) imagewise exposing the layer of radiation- sensitive material to radiation to thereby cure portions of the layer exposed to the radiation; and

2. A process as claimed in claim 1 in which a charged back plate is placed behind the substrate whilst spraying the powder onto the substrate.

3. A process as claimed in claim 1 or claim 2 in which the radiation-sensitive solid material comprises an alkali-soluble radiation sensitive material.