IE50821B1 - Process for the selective chemical deposition and/or electrodeposition of metal coatings,especially for the production of printed circuits - Google Patents

Abstract

In a process for the selective chemical deposition and/or electrodeposition of a metal coating onto an activated non-conducting surface, the part or parts of the surface that are to remain free of metal are anodically passivated. Passivation may be brought about by applying an anodic reverse potential to the surface(s) during chemical deposition. This process is suitable for the production of printed circuits having very fine conductors on a very small area and having optimum electrical characteristics.

Description

The invention relates to a process ^or the production of printed circuits. Processes are already known for the selective deposition of metals from solutions of electrolytes ξ onto conductive surfaces or surfaces rendered conductive The known processes are based on two fundamentally different techniques: according to one, which avoids the use of plating tanks, the articles to be electroplated are brought into contact with the electrolyte only at to the desired places, this being achieved, for example, by using rollers (DE-PS 186654), wheels (DE-PS 2324834) and open hollow bodies (DE-PS 1807481); according to the other, conventional tanks are used, but the supply of metal ions and the distribution of the electric )5 field to the surfaces to he treated are controlled by inserting, for example, screens (DE-PS 2263642), masking arrangements (DE-PS 2362489), electrically insulating belts running on rollers (DE-PS 2009118), cages (DE-PS 2230891) or layers of lacquer (DE-PS 2253196).

However, these known processes are unsatisfactory insofar as they either make possible what is, in most cases, only an insufficient supply of metal ions, which results in an inadequate metal coating, or involve great expenditure in terms of costs, materials and time, since 2.Γ each of the masks used must first be applied and then -2removed and from time to time renewed because of signs of wear. These processes are therefore not suitable for the production of printed circuits.

The processes usually used for the production of printed circuits are, on the other hand, affected by certain disadvantages.

One disadvantage of the so-called subtractive technique is that large amounts of the cladding of the base material must be removed after the conductor pattern has been built up. This involves the simultaneous undercutting of the conductive features with all the known damage that this entails, this damage being the more serious and increasing the more rapidly in percentage terms, the narrower the distance separating the tracks. These occurrences thus preclude further miniaturisation when using the subtractive technique.

A disadvantage of the so-called additive technique, on the other hand, is that a base material coated with a bonding agent must be used. After chemical fusion and activation, the bonding agent forms the base for the selectively applied chemically deposited copper and, in comparison with the epoxy resin or other base material used, has clearly less good electrical characteristics after the wet treatment, which likewise limits the layout of miniaturised circuits.

There is therefore a need for a process that, while avoiding the disadvantages of the known processes, 5Q82J -3makes possible the chemical deposition and/or electrodeposition of metal coatings onto insulating surfaces activated in the usual manner and that is suitable especially for the production of printed circuits having very fine conductors on a very small area and having optimum electrical characteristics.

We have found that good results can be achieved by a process in which those places on the surfaces that are to remain free of metal are anodically passivated.

Accordingly, the present invention provides a process for the production of printed circuits wherein a copper-clad non-conducting substrate is drilled, cleaned, activated and optionally reduced, the copper-clad surface of the treated substrate is anodically passivated by applying an anodic potential while the substrate is * treated with a chemical nickel, cobalt or nickel-cobalt bath, to deposit metal exclusively in the drill-holes, the circuit pattern is then applied positively or negatively by a screen or photographic printing technique using a resist, the unmasked copper is then etched away and the remaining resist is removed, the circuit pattern is then masked by a soldering resist lacquer with the soldering eyes and drill-holes being left free, and the uncovered eyes and drill-holes are provided with a copper layer using a chemical copper bath. 5082/ -4Before the substrate is treated with the chemical nickel, cobalt or nickel-cobalt bath it may be treated with a chemical and/or electrolytic copper bath.

The circuit pattern may be applied preferably 5 positively by means of a resist, for example, the resist is exposed to light through a film pattern and the exposed resist selectively removed by its developer, leaving the desired resist pattern on the clad laminate. The exposed copper may then be etched away to leave the circuit pattern, and the remaining resist removed in a manner known per se. If desired a tin layer, e.g. a tin-lead layer, is then applied, optionally also chemically, to the copper-plated eyes and drill-holes.

The non-conducting substrate, or base material, is especially a glass fibre-reinforced epoxy resin.

The chemical nickel, cobalt and nickel-cobalt baths may contain, for example, as the main constituents; in the case of the nickel bath: - a nickel salt, a citrate and an alkali metal hypophosphite, or - a nickel salt, an alkali metal diphosphate, an alkali metal hydrogen phosphate, and hydrazine or a derivative thereof, in the case of the cobalt bath: - a cobalt salt, a citrate and an alkali metal hypophosphite, or - a cobalt salt, an alkali metal diphosphate, an alkali metal hydrogen phosphate, and hydrazine or a derivative thereof, 508 2i -5in the case of the nickel-cobalt bath: - a nickel salt, a cobalt salt, a citrate and an alkali metal hypophosphite.

Advantageously, t.he nickel, cobalt or nickel-cobalt layer is applied in a thickness of from 0.1 to 1.5 μπ», more especially from 0.3 to 0.8 pm.

Passivation may be brought about by applying an anodic reverse potential to the surface(s), preferably with a current density of at least 0.8 mA/cm 10 at a voltage of approximately 200 mV, and using as a cathode preferably copper wire.

The chemical copper bath is preferably stabilised and preferably contains, as the main constituents, a copper salt, a complex former, formaldehyde, and, as stabilisers, an alkali metal cyanide and, if desired, a selenium compound. -6In this process, it is especially surprising that the metal layer is deposited exclusively in the drillholes and not, however, on the copper-clad surface.

Consequently, there is no need for a special step to remove metal (for example nickel, cobalt or nickelcobalt) deposited on the cladding. Chemically, this would be removed during the etching attack on the copper but the copper under the electrolessly deposited thin metal layer would be etched only slowly and nonuniformly: with mechanical removal, damage to the transition from drill-hole to cladding could not be avoided. These disadvantages are avoided and the metal that would otherwise be deposited electrolessly (unnecessarily) on the cladding is saved since,as a result of anodically passivating the cladding, surprisingly the metal layer is deposited exclusively in the drill-holes.

By the process according to the invention it is possible, in a manner previously not achieved, to produce high quality miniaturised circuits. Furthermore, the process has the great advantage that, starting from a copper-clad base material, it is possible to produce very fine conductive tracks having a width of less than 100 urn and having optimum insulation and surface resistances.

A further major advantage is the saving in copper which is a valuable raw material.

Suitable insulating substrates, or base materials. 5082! -7are, for example, phenolic resin-bonded paper, epoxy resin paper and, especially, glass fibre-reinforced epoxy resin.

The substrate may first be drilled and cleaned in 5 the usual manner, activated by one of the conventional activator systems and optionally reduced, and may be after-treated in the usual manner. It may then ba washed and dried in a manner known per se.

The circuit pattern may be applied negatively or positively using a screen or photographic printing technique.

It is also possible to apply the soldering pattern (soldering eyes and drill-holes) negatively by means of a screen or photographic printing technique using a resist. Metallisation may be carried out chemically with a nickel, cobalt or nickel-cobalt bath to a layer thickness of from 0.1 to 1.5 μΐη. With the copper surfaces that have remained free being anodically passivated, only the inside surfaces of the drill-holes are coated. Before this operation chemical and/or electrolytical pre-copper-plating may be carried out using a conventional bath. After stripping the previously applied resist, which may be removed in a manner known per se by the action of an organic solvent such as, for example, methylene chloride, the copper that has been laid bare can be etched away in the usual manner. The resist used is advantageously a conventional 5082! -8photosensitive lacquer or film. In many cases the developed circuit pattern is then masked negatively by printing with soldering resist lacquer and the soldering eyes and drill-holes remaining free are chemically copper-plated.

The copper bath used is preferably a stabilised chemical copper bath that contains, as the main constituents, a copper salt, a complex former, formaldehyde, and, as stabilisers, an alkali metal cyanide and, if desired, a selenium compound.

The alkali metal cyanide stabiliser in the copper baths is especially sodium cyanide in a concentration of from 15 to 30 mg/litre.

Suitable selenium compounds are the organic, inorganic and organic-inorganic mono- and di-selenides, and, of these, especially the alkali metal selenocyanates, such as, for example, potassium selenocyanate, which are usually used in low concentrations, especially from 0.1 to 0.3 mg/litre.

The following Examples illustrate the invention.

EXAMPLE 1 A conventional base-board consisting of glass fibre-reinforced epoxy resin clad with copper on both sides is drilled in the usual manner, and is etched and cleaned using a stabilised sulphuric acid solution of hydrogen peroxide. Then the board is activated by 0821 -9treatment with an aqueous alkaline solution of a palladium complex, such as, for example, palladium sulphate in 2-aminopyridine, which is then reduced by the action of a reducing agent, such as, for example, diethylamino— borane.

While anodically passivating the entire copper surface, the walls of the drill-holes are chemically nickel-plated by the action of a chemical nickel bath having the following composition: g/litre nickel sulphate WiSO^.7 HjO g/litre sodium hypophosphite Ν&ΗΡΟ2.Η_,Ο g/litre succinic acid HOOC(CfH2) jCOOH g/litre sodium borate Na2B^0^.10 HjO For the purposes of anodic passivation, an anodic reverse potential with a current density of at least 0.8 mA/cm is applied to the cladding, resulting in a voltage of 200 mV with respect to the reference electrode. A copper wire spaced at about 5 mm is advantageously used as cathode. The effective angle onto the board is at most 80°, so that with relatively large board dimensions several copper wires may have to be provided.

The so-called batch process may also be used. In this case, the cage receiving the boards is constructed in such a manner that each individual board makes lateral contact and the whole cage may be designed as -loan anode. An insulating hoop which is mounted on the frame of the cage and carries fine copper wires in such a manner that the copper wires lie between the boards at a distance of about from 4 to 8 mm in each case, serves as cathode. The wires must be-replaced from time to time according to the extent of their metallisation.

The treatment is carried out for 5 minutes at a pH of 8.5 and a temperature of 35°C. The layer thick10 ness obtained on the walls of the drill-holes is 0.2 um. The copper cladding is not nickel-plated again.

The surface is then positively printed with the circuit pattern, the cppper cladding is etched away, the resist removed, the surface is negatively printed with soldering resist lacquer with the soldering eyes and the drill-holes being left free, the soldering eyes and drill-holes are then chemically copper-plated ard. if desired, a tin-lead layer is applied. By this means, a printed circuit is obtained having optimum electrical characteristics of at least 1 . 1012jti/cm.

EXAMPLE 2 A conventional base-board consisting of glass fibre-reinforced epoxy resin clad with copper on both sides is drilled in the usual manner, and is etched and cleaned using a stabilised sulphuric acid solution of hydrogen peroxide. Then the board is activated by I -11treatment with an aqueous alkaline solution of a palladium complex, such as. for example, palladium sulphate in 2-aminopyridine which is then reduced by the action of a reducing agent, such as, for example, di ethylaminobo ran e.

Cobalt is chemically deposited on the surface of the board and on the walls of the drill-holes by the action of a chemical cobalt bath having the following composition: g/litre cobalt sulphate CoSO^.6 H^O g/litre sodium hypophosphite NaHPO^.H^O g/litre succinic acid HOOC(CH^)jCOOH g/litre sodium borate NajB^O^.IO H^O The treatment is carried out for 5 minutes at a pH of 8.5 and a temperature of 35°C. The layer thickness obtained is 0.2 urn.

The cladding is inhibited, as described in Example 1, by applying an anodic potential.

Then the surface is positively printed with the circuit pattern, the copper cladding is etched away, the resist removed, the surface is negatively printed with soldering resist lacquer with the soldering eyes and the drill-holes being left free, is chemically copper-plated and, if desired, a layer of tin is chemically applied. By this means also, a printed -12cireuit is obtained having optimum electrical character1 2 istics of a least 1 .10 Jl/cm.

If desired, there may also be treatment with a copper bath before the treatment with the chemical nickel or cobalt bath. 5082(

Claims (16)

1. A process for the production of printed circuits wherein a copper-clad non-conducting substrate is drilled, cleaned, activated and optionally reduced, the 5 copper-clad surface of the treated substrate is anodically passivated by applying an anodic potential while the substrate is treated with a chemical nickel, cobalt or nickel-cobalt bath, to deposit metal exclusively in the drill-holes, the circuit pattern is •jO then applied positively or negatively by a screen or photographic printing technique using a resist, the unmasked copper is then etched away and the remaining resist is removed, the circuit pattern is then masked by a soldering resist lacquer with the soldering eyes 1.5 and drill-holes being left free, and the uncovered eyes and drill-holes are provided with a copper layer using a chemical copper bath.

2. A process as claimed in claim 1, wherein the substrate is treated with a chemical and/or 20 electrolytic copper bath before the chemical nickel, cobalt or nickel-cobalt bath.

3. A process as claimed in claim 1 or claim 2 wherein, after the eyes and drill-holes have been provided with a copper layer using a chemical copper -14bath, a tin layer or a tin-lead layer is applied to the copper-plated eyes and drill-holes.

4. A process as claimed in anyone of claims 1 to 3 wherein the passivation is brought about by applying an anodic reverse potential with a current density of at 2 least 0.8 mA/cm and a voltage of substantially 200 mV.

5. A process as claimed in any one of claims 1 to 4, wherein the non-conducting substrate is a glass fibrereinforced epoxy resin.

6. A process as claimed in any one of claims 1 to 5 5 wherein a chemical nickel bath is used containing a nickel salt, a citrate and an alkali metal hypophosphite.

7. A process as claimed in any one of claims 1 tc 5 ? wherein a chemical nickel bath is used containing a nickel salt, an alkali metal diphosphate, an alkali metal hydrogen phosphate and hydrazine or a derivative thereof.

8. A process as claimed in any one of claims 1 to 5, wherein a chemical cobalt bath is used containing a cobalt salt, a citrate and an alkali metal hypophosphite.

9. A process as claimed in any one of claims 1 to 5, wherein a chemical cobalt bath is used containing a 5082 1 -15cobalt salt, an alkali metal diphosphate, an alkali metal hydrogen phosphate and hydrazine or a derivative thereof .

10. A process as claimed in any one of claims 1 to 5, 5 wherein a chemical nickel-cobalt bath is used containing a nickel salt, a cobalt salt, a citrate and an alkali metal hypophosphite.

11. A process as claimed in any one of claims 1 to 10, wherein the nickel, cobalt or nickel-cobalt layer is iq applied in a thickness of from 0.1 to 1.5 μπ.

12. A process as claimed in claim 11, wherein the layer is applied in a thickness of from 0.3 to 0.8 pm.

13. A process as claimed in any one of claims 1 to 12, wherein the chemical copper bath is stabilised. •j 5

14. A process as claimed in claim 13, wherein the chemical copper bath contains a copper salt, a complex former, formaldehyde, and, as stabilisers, an alkali metal cyanide and if desired a selenium compound.

15. A process as claimed in claim 1, carried out 2o substantially as described in Example 1 or Example 2 herein.

16. A printed circuit board produced by a process which comprises a process as claimed in any one of claims 1 to 15.