CA1311210C - Production of copper-clad dielectric boards - Google Patents

Abstract

ABSTRACT

Copper substantially free of micro-pores is electrodeposited on a polished surface of a stainless steel, titanium, or chromium-plated steel press plate.
The copper layer is then provided with a matte surface of copper of dendritic structure which is subsequently bonded to a dielectric material under the application of heat and pressure in a laminating press. The resulting copper-clad dielectric board separates from the press plate, which can then be re-used.

Description

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PRODUCTION OF DIELECTRIC_BOARDS

TECH~ICAL FIELD

This invention relates to a process for producing copper-clad dielectric boards and to material for use in such a process.

BACKGROUND ART

The backgrouna to the invention is as follows.
Copper clad laminates are at present manufactured by taking copper foil produced generally in accordance with the teaching of U.S~ Patent 3 674 656, laying i~ on top of one or more sheets of partially cured resin impregnated base material, and placing the two ~aterials between press pl-ates on a laminating press. Under heat and pressur2 the partially cured resin adheres to the copper foil so that, when removed from the press, the ; ~ two materlals are firmly bonded together.

Such copper foil as is used in this process is available in unsupported form in a thickness range of 9 ~m upwards to in excess of 105 ~m. Since such foil ; ~ ~ 20 Is frequently in excess of~l metre wide, handling sheets of it can be difficult and, particularly in thicknesses between 9 and 20 ~m, a grea~ deal of scrap is genera~ed in the laying up process. ~n order to preserve ~he surface quality of the~laminate great care 25 has to be taken to exclude all dust ~articles from between the surface of ~he copper foil and the press plates, which are used to separate the laminates in the press during manufacture.

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In order to facilitate the handling of thin copper foils it has been proposed to manufacture such materials by continuously depositing such copper onto a carrier foil of aluminium or chromium-pla~ed copper and processes for so doing are disclosed in U.S. Patent 4 113 576 and U.K. Patent Specifications 1 460 849, 1 458 260, and 1 458 259. In practice foils produced by these techniques are costly and unreliable and have found little favour in the industry.

U.S. Patent 3 984 598 describes a process in which a stainless steel press pla~e known as a caul plate is coated with a silane as a release agent and is then electroplated with copper. The exposed surface of the copper is then oxidised and treated wi~h a silane as a bonding agent. The copper-clad caul plate is then laminated to resin-impregnated base material in a laminating press. hfter the laminate is removed from the press, the caul plate is removed from the copper-clad dielectric board which has been produced.
The copper coating is found to have a variable thickness of about 5 to 12 ~m.

It is notoriously difficult to uniformly electroplate a surface provided with an organic parting layer such as silane. As a result, the deposit will suffer from porosity. During laminating, the resin will therefore seep through the pores, both causing adherence to the caul pla~e and contaminating the surface of the laminate.

What is desired is a method by which thin copper layers of 3 microns and upwards can be successfully and economically laminated to dielectric base materials with great reliability.

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The present invention provides a process for producing copper-clad dielectric boards, comprising the sequential steps of (a) depositing a first layer of copper substantially free of micro-pores directly on a polished surface of a flat metallic press plate, the polished surface having a uniform finish of a surface roughness not exceeding 0.2Jum centre line average;
tb) depositing on the first copper layer a second copper layer with a matte surface of copper of dendritic structure;
(c) bonding the matte surface to a dielectric material while applying heat and pressure to the press plate and the dielectric material in a laminating press and subsequently allowing the press to cool, the forces generated at the interface of the press plate and the first copper layer, owing to the penetration of the dielectric material into the dendritic structure under heat and pressure and the subsequent cooling of the dielectric material, being sufficient to overcome the adhesion of the first copper layer to the polished surface of the press plate and thereby to cause the copper layer to be detached from the press plate; and (d) removing the resulting copper-clad dielectric board from the press and separating it from the press plate.
The press plate may then be returned to step (a), repeating steps (a) to (d).
One or both of the surfaces of the press plate may be used.

In the preferred process the press plate is 1.5 to 3 mm thick and is made of stainless steel, titanium, or chromium-plated steel. The press plate is polished by , .

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an abrasive brush or spray to provide a uniform finish of a surface roughness no~ exceeding 0.2 ~m (preferably O.l ~m) centre line average (C.L.A.).

After polishing, all traces of abrasive and products of abrasion are removed by washing.

The polished press plate is immersed in a copper plating bath vertically disposed and parallel to a suitable anode where it is rendered cathodic. A current is applied so as to plate on the press plate a fine 1U grain copper deposit substantially free of micro-pores.
The so plated press plate is removed from the bath, washed, and placed in a strong copper sulphate bath, again vertically and parallel to an anode. By controlling the conditions in this bath a further copper layer is deposited in such a way as to cause a somewhat coarser crystalline layer to be deposited on the fine grain deposlt already present.

Subsequent plating in further copper baths under controlled conditions can be carried out to create a microcrystalline dendritic structure which has a high surface area suitable for bonding to typical dielectric base materials. When the plating sequence is complete the copper plated press plate is washed, passivated in weak chromic acid, washed again, and dried.

The pla~e is then taken to a laminating press and laid on top of suitable base material such as epoxy resin impregnated glass cloth. When the laminating press is closed and heat is applied, the resin in the base material is forced into the microcrystalline dendritic structure of the copper. During the subsequent cooling of the resin there is created a suf~icient force to disturb the adhesion between the 2 ~ ~

copper and the carrier plate so that when the press is opened it will be found that the copper layer is completely detached from the cacrier plate and is firmly adherent to the base. If the plating conditions in the first bath are properly related to the surface texture of the carrier plate, the dstachment of the copper happens so cleanly that the plate can immediately be passed through the plating cycle again.

Such a method of making laminates avoids completely the hazard of reeling and unreeling rolls of copper foil, eliminates the common problems of surface defects on finished laminates, and because of the fine crystal deposit of the first layer eliminates the problems of porosity commonly to be found in electroformed copper foil. Use can be made of polished caul plates which have previously been used in a conventional laminating process.

The total thickness of copper on the press plate is preferably 3 to 12 ~m, more preferably about 5 ~m.

The first copper layer deposited on the press plate can be very thin, e.g. l to 2 ~m. It may be deposited by electrolysis, e.g. from a copper cyanide bath or a copper pyrophosphate bath, preferably containing 25 to 35 g/l of copper, 150 to 310 g/l of P207, l to Z g/l ammonia, and having a pH of 8 to 9.

If the first strike of copper is carried out from a nea~ neutral plating bath with high throwing power, and the metal carrier plate has the correct surface finish, the dense crystal structure of the first layer virtually guarantees that the foil as eventually plated will be free from pinholes or micro-porosity. In conventional foil making technology porosity ~ thin foils is a major ~L3~L2~

problem because the copper foil is deposited and plated all *rom the same bath and, in the interests of leconomical production and so that a matte structure can be achieved, the bath used is an aqueous copper sulphate solution. Such baths, operated at the high current densities required to a~hieve econ~mical levels of production, always pose difficulties in maintaining control of the nucleation sites of the copper at the start of the plating process. Nicro-contamination of the drum surface or the solution can resultin intercrystalline porosity which permits resin to bleed through it if such material is laminated. Rigorous testing is carried out by the foil producers and laminator~ so that the high standards required result in high scrap levels in }5 the industry. The production of foil in a multi-stage sheet-by-sheet process as now proposed allows the copper core to be plated at high speed from a similar bath to that used in typical drum foil processes, but the nucleation site problem is avoided by plating this layer after a first strike. The near-neutral pyrophosphate bath also assists in obtaining an oxide free surface on the finished laminate.

Even if a few micro-pores are present in the copper plated on the press plate, resin bleed-through is effectively prevented because there is no space between the copper and the press plate into which the air entrapped in the micro-pores can escape, so that the resin cannot even enter the micro-pores.
-EXAMPLE

A press plate consisting of a ~heet of titanium 2 mm thick was polished to pro~ide a uni~orm surface of between 0.1 and 0.2 ~m C.L.A. The polished sheet was placed in a -:
plating tank containing copper cyanide ~` ~3~2~

solution and plate~as described in Table 1 below.

Table 1 Copper Cyanide 3.0 - 16 g/l Sodium Cyanide 4.5 - 18 g/l Sodium Carbonate 2.0 - 4 g/l Rochelle Salt 0 - 6 g/l pH 12 - 13 Temperature 32 - 82C
Current Dansity 1 - 7 A/dm2 Time 3 - 30 s Anode Material Copper or steel The plated sheet was removed from the bath and thoroughly washed in a warm water spray. The sheet was then placed in a copper sulphate plating solution, rendered cathodic and plated in conditions as described below in Table 2.

Table 2 Copper (as metal) 25 - 110 g/l Sulphuric acid 60 - 110 g/l Temperature 45 - 65C
Current Density 2 - 110 A/dm Anode Material Lead Sheet The plating time depends on the ~hickness of copper required.

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After the foregoing plating step the sheet was transferred to a further copper sulphate plating bath and subjected to conditions as follows in Table 3. In determining the precise conditions to be used, it is important that copper crystals plated are not of a powdery oxidised character but are pure metallic copper dendrites firmly adherent to the surface.

Table 3 Copper (as metal) 15 - 45 g/l Sulphuric acid 60 ~ 90 g/l ~rsenic (as metal) 200 - 500 mg/l Temperature 18 - 50C
~node material Lead Sheet Current Density 5 - 220 A/dm2 The sheet was placed in this bath disposed parallel and in close proximity to the lead anode and subjected to a continuous but variable curren~ in a range of time and current densities so as to produce a strongly adherent microcrystalline dendritic deposit of high surface area.

The so plated sheet was removed from the plating bath, thoroughly washed, passivated in a weak chromic acid solution, washed again, and dried. The total thickness of the plated layer was lZ ~m.

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This sheet was taken to a laminating press and laid upon 5 sheets of epoxy impregnated glass cloth of a type commonly used in the production of copper clad laminates. ~fter the press had been closed and heat and pressure applied in accordance with the requirements of the resin impregnated base material, the press was allowed to cool and the laminate removed. It was immediately apparent that the titanium sheet had separated from the copper layer and could be lifted off 10 the laminate that had been made: it was ready for return to ~he initial plating bath.

The resultant laminate demonstrated a particularly clean, stain free copper surface and the copper layer was firmly adherent to the base. The laminate was 15 subjected to test procedures typical for the industry and was found to be satisfactory in every respect.

Instead of the copper cyanide solution specified in Table l above, a cop~er pyrophosphate solution may be used under the following conditions:

20 Plating solution:
Copper (as metal) 30 g/l Pyrophosphate as P207 l~0 g/l ~mmonia l g/l pH 8.6-8.8 Temperature50-55C
Current density2.2 - 4.3 A/dm2 Anode-cathode gap 7-12 cm ~node material copper.

Potassium hydroxide is used to regulate the pH. The 30 plating time depends on the current density and required thickness (generally 1-2 ~m). During the plating process continuous aeration of the anode/cathode ~ 3~L~2~

interspace is carried out to prevent the copper deposit from 'burning'.

The pH of the bath is regulated continuously to maintain it in the range 8.6-8.8. Variations on ei~her side of these levels may result in copper which adheres too strongly to the press plate or is porous or both.

The process of the invention described above has clear advantages over the prior ar~ process represented by U.S. Patent 3 984 598. In the prior art process a 10 silane release agen~ has to be used ~o facilitate removal of the laminate from the caul plate after lamination; even so, it is clear that separation of the laminate from the caul plate does not occur automatically in the laminating press. The known use of temporary (disposable or re-usable) substrates has always required stripping of the foil from the substrate mechanically (i.8. by peeling) or chemically (i.e. by dissolving the substrate). The present invention is a radical departure, in that adhesion between the laminate 20 and the substrate is destroyed during the laminating process, so that the caul plates and lamina~es can be separated in the same way as conventional caul plates and laminates.

In the present process an initial copper layer substantially free of micro-pores is surmounted by a copper dendritic structure, thereby achieving a copper ~oil into which the dielectric material can penetrate so as to produce strong bonding (high peel strength) but ~hich foil is impermeable to the dielectric material.
In contrast, the prior~art process uses a high current density to achieve a copper layer with a rough surface, ~with the inevitable result that the thin copper layer is micro-porous; furthermore, it is still found necessary .

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to roughen the surface further by oxidization. Such a surface provides a much weaker bond than a dendritic structure.

In the prior art process, after removal of the substrate from the laminate, it is expected that the substrate (caul plate) will still contain the release agent as a thin film. However, there is a risk that the film will become so thin, perhaps locally, that i~ will no longer facilitate removal of the substrate.
Therefore, checking of recycled caul plates would be necessary to ensure that the release layer is continuous and undamaged. The release layer will, in general, be more easily damaged than a polished metallic surface.
It will be difficult to detect imperfections in a release layer, whereas imperfections in a polished surface (such as roughening or scratching) are very easily detected.

In order to operate economically, a laminator would use not only copper-clad caul pla~es (in the production of laminates wit~ very thin copper, i.e. less than 20 ~m) but also ordinary polished caul plates with self-supporting copper foil (for producing laminates with thicker copper layers). If a release agent is used (as in the ~rior art process) there is a risk that caul plates with and without a film of release agent will be mixed up, whereas the present process can make use of pollshed caul plates which have been used in a conventional laminating process.

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Claims (9)

1. A process for producing copper-clad dielectric boards, comprising the sequential steps of (a) depositing a first layer of copper substantially free of micro-pores directly on a polished surface of a flat metallic press plate, the polished surface having a uniform finish of a surface roughness not exceeding 0.2 µm centre line average;
(b) depositing on the first copper layer a second copper layer with a matte surface of copper of dendritic structure;
(c) bonding the matte surface to a dielectric material while applying heat and pressure to the press plate and the dielectric material in a laminating press and subsequently allowing the press to cool, the forces generated at the interface of the press plate and the first copper layer, owing to the penetration of the dielectric material into the dendritic structure under heat and pressure and the subsequent cooling of the dielectric material, being sufficient to overcome the adhesion of the first copper layer to the polished surface of the press plate and thereby to cause the copper layer to be detached from the press plate; and (d) removing the resulting copper-clad dielectric board from the press and separating it from the press plate.

2. A process as claimed in claim 1, in which the press plate is made of stainless steel, titanium, or chromium-plated steel.

3. A process as claimed in claim 1, in which the first copper layer is 1 to 2 µm thick.

4. A process as claimed in claim 1, in which the first copper layer is deposited electrolytically from a non-acidic solution.

5. A process as claimed in claim 4, in which the first copper layer is deposited electrolytically from a copper cyanide solution.

6. A process as claimed in claim 4, in which the first copper layer is deposited electrolytically from a copper pyrophosphate solution.

7. A process as claimed in claim 1, in which the total thickness of copper on the press plate surface is 3 to 12µm.

8. A process as claimed in claim 1, further comprising returning the press plate to step (a) and repeating steps (a) to (d).

9. A process as claimed in claim 1, in which, in step (a), the polished surface has a uniform finish of a surface roughness not exceed 0.1 µm centre line average.