GB2185757A - Dendritic surface treatment of metal layers - Google Patents

Abstract

A metal layer, e.g. a foil or the surface layer of a metallic object, which is to be bonded to a plastics material, is provided with a matte surface by depositing in sequence a copper layer of dendritic form, a layer of a zinc-based alloy containing iron and conforming to the copper layer. The metal layer may be carried by a temporary, removable substrate. The invention is particularly applicable to the treatment of copper foil which is to be bonded to a plastics material in the manufacture of printed circuit boards.

Description

SPECIFICATION Dendritic surface treatment of metal layers This invention relates to a method of providing a metal layer with a matte surface suitable for bonding to a plastics material.

The invention also relates to a process whereby a relatively smooth metal sheet can be provided with a controlled electrolytically deposited microcrystalline layer of copper which significantly increases the surface area of the smooth sheet, in such a way that, with appropriate adhesives, it will adhere strongly to dielectric base materials used in the production of printed circuit boards and particularly that the copper layer will have a further outer layer which is inert to any chemical reaction on the surface of the dielectric base.The invention is aimed particularly at enhancing the bond strength of electrolytically produced copper foil commonly used in the manufacture of laminates for printed circuit applications, but the invention may also be used to improve the adhesion of other metal layers to plastics or plastics-covered base materials whether such metals are produced by electroplating or by rolling, for example. Other metal sheets which could be provided with such dendritic structures are aluminium, brass, gold, silver, nickel and iron.

The background to the invention is as follows. In the manufacture of printed circuit boards copper clad laminates are made by causing electroformed copper foil, produced generally in accordance with the teachings of U.S. Patent 3 674 656, to be bonded by heat and pressure to various dielectric base materials such as epoxy impregnated glass cloth or phenolic impregnated paper.

The degree of adherence between the copper foil and the base material is of critical importance because, in the course of conversion from copper clad laminate to printed circuit board, the laminate is exposed to drilling, punching, etching, and hot solder baths so that, after manufacture, much of the original copper has been removed and what is left is commonly in the form of narrow (250 micron) tracks on the surface of the base material. It is important that such tracks, which play a vital part in the function of the circuit proper, should be well adherent to the surface and that, in the areas from which the copper has been etched away, there should be no residual traces of copper that might cause electrical short-circuits between closely spaced tracks.

In order to promote adhesion between the copper foil and the base material is it usual to treat the foil by means of electrolytic processes such as those described in U.S. Patents 3 918 926,3 857 681,4 131 517, and 3 585 010. These processes require that copper foil be passed through a series of tanks containing different strength solutions in such a manner that fine copper particles are deposited on the copper and thereby increase the surface area so as to provide a surface into which the adhesive used in laminating can penetrate. Although these processes are capable of increasing the bond strength between the two materials, they have problems which can be outlined as follows.

In the case of treatments which consist only of the deposition of copper particles two types of difficulty can occur in the production of laminates. One of these is that such copper particles may not be totally adherent to the base copper and so become detached during laminating; these particles can remain embedded in the surface of the base material or become encapsulated therein so that when the unwanted copper is etched away they threaten to cause shorting between adjacent tracks.

The other problem is that there can be a chemical reaction between copper and some of the commonly used polymers so that when copper is etched away the exposed base material is discoloured in such a way as to make it difficult to determine whether its surface is free of particles or not.

To overcome these difficulties it has been proposed that another metallic layer be interposed between the copper particles and the base material.

Such layers would be electro-deposited onto the copper dendrites in such a way as to isolate the copper from the base. U.S. Patents 3857 681 and 3 585 010, for example, propose methods of effecting such so-called barrier layers. The state of the art is that the preferred layers for accomplishing this separation are either brass or zinc.

Both of these processes have disadvantages. The brass barrier layer is produced from a cyanide plating solution and usually encapsulates copper dendrites produced from an acid plating solution.

The combination in proximity of these different types of plating baths is chemically hazardous and, although - with great care - the process can be carried out, this adds to the cost of manufacture in time, equipment, and treatment of effluent. A zinc barrier layer, although plated from an acid plating solution, poses other problems for the manufacturer in a different way because the zinc and copper have different electrode potentials and, during the etching of the printed circuit board, this difference can accelerate the etch rate of the zinc layer in such a way as to increase the undercutting by the etchant of the circuit track. This phenomenon occurs because etching can take place for two reasons.

One is a chemical replacement reaction whereby the etchant causes the dissolving of a metal effectively by absorption of the metal into the etchant and the other process occurs owing to electrolytic reaction between two materials of differing electrode potential in intimate proximity to each other. In the case of, for example, a zinc micro layer of 2-10 microns in thickness plated over a dendritic copper deposit where such material has been laminated to a plastics base material and then etched into a circuit pattern, it can easily be established that, when the etchant has removed unwanted copper and the base material is exposed, a reaction will have been started between the zinc and copper, as will now be explained in more detail.

The copper foil provided with the dendritic copper layer and the zinc barrier layer is bonded to the dielectric base material and has an acid resistant coating applied wherever the foil is not to be etched away. As the etchant reaches the base material the force of the spray by which the etchant is propelled at the material is of sufficient magnitude to maintain the etchant active against the lower edges of the copper track. Here the edges of the zinc layer are exposed to the etchant and commence to dissolve.

As the etchant dissolves a small quantity of zinc beneath the etched copper a cell like reaction is set up which adds an electrolytic reaction to the chemical one, dissolving the zinc at a faster rate than the adjacent copper and producing undercuts.

When such a track is peeled back from the laminate and the underside is examined it can easily be seen that the zinc barrier layer has been etched inwards from the edge of the track, leaving a line of copper visible. Such a phenomenon results in a low peel strength for the track width and in extreme circumstances the track can detach from the base completely.

In order to overcome this problem, our U.K.

patent application GB~A~2 151 660 describes a method in which an iron barrier layer is deposited on the dendritic copper layer before the zinc barrier layer.

We have now found that the two barrier layers can be replaced by a single barrier layer comprising iron and zinc and also that it is possible to co-deposit the iron and zinc from an electrolytic zinc plating bath using an irnncontaining anode. In this way fewer process steps are required.

Accordingly, the present invention provides a method of providing a metal layer with a matte surface, comprising depositing on the metal layer in sequence a copper layer of dendritic form, and a layer of a zinc-based alloy containing iron and conforming to the copper layer, the iron being deposited simultaneously with the zinc.

Preferably, the total thickness of the copper layer and the zinc-iron alloy layer is 2-10 microns, more preferably 2-5 microns.

The zinc-iron layer encapsulates and anchors the copper dendrites and serves as a barrier layer. The predominant zinc content resists atmospheric oxidation. Both iron and zinc are compatible with the usual etchants for copper and do not cause undue contamination.

An advantage, in relation to the production of printed circuits, is that the zinc-iron layer has an electrode potential that lies between the electrode potentials of copper and zinc. It therefore has less tendency than a zinc layer to undercut under the conditions of etching.

Table 1 below gives the electrode potential, in volts, of the three relevant metals.

TABLE 1 Copper CU2+iCu + 0.337 Iron Fe2+JFe - 0.440 Zinc Zn2+1Zn - 0.763 The preferred iron content of the zinc alloy layer is 5 to 20 wt.% (more preferably about 10 wt.%). More than 20 wt.% might induce a risk of atmospheric oxidation.

Less than 5 wt.% might be insufficient to reduce undercutting during etching.

Preferably, the zinc-iron layer is electrodeposited from an acidic zinc bath, using a soluble anode of iron or an iron-based alloy. The anode preferably has an iron content of at least 75 wt.%, more preferably 80 wt.%; a stainless steel anode is preferred.

Although the invention is primarily directed to the treatment of copper foil, the metal layer may be constituted by any metallic surface layer on which a copper layer of dendritic form can be deposited, the surface being of any shape. The various layers will normally be deposited electrolytically, but other deposition techniques are not precluded.

The metal layer, e.g. a copper layer, may be carried by a temporary substrate which can subsequently be removed from the layerd product, e.g. by mechanical separation or chemical dissolution. In this case the metal layer, applied to the temporary substrate, need only be a very thin layer, e.g. 1 to 2 microns. The temporary substrate may be flexible or rigid.

The invention will be described further with reference to the following examples of copper foil treatment processes.

Example 1 35 micron copper foil produced generally in accordance with the prior art teachings of U.S.

Patent 3 674656 was placed vertically in a plating bath of aqueous copper sulphate solution made up as in Table 2 below.

TABLE 2 Copper (as metal) 5 gil Sulphuric Acid 60--90 gil Temperature 1S50 C Current Density available 9220 Alum2.

The copper foil was connected to the negative side of a DC rectifier and disposed parallel and in close proximity to a lead anode. The plating solution was caused to circulate in the anodelcathode interspace and the foil was subjected to a series of plating steps as follows: Current density Time 1.27Aidm2 4 6s 2.BAidm2 1 & 20s 3.22Aidm2 4 6s 4.8Aidm2 1 S20 s.

Thus-plated with copper of dendritic form, the foil was washed thoroughly and placed in another plating bath, containing zinc sulphate plating solution generally as described in Table 3 below.

TABLE 3 Zinc (as metal) 560 gil Sulphuric Acid pH 1.54.5 Temperature 1 & 8"C Current Density available 0.55 Alum2.

Plating time S10 s.

With the foil rendered cathodic and parallel and in close proximity to a lead anode, zinc was plated over the copper dendrites so as to cover them completely. The foil was then washed thoroughly and passivated in a solution of 2 g/l chromic acid, washed again, dried, and set aside.

EXAMPLE 2 Similar copper foil was taken and passed through the same copper plating bath and plating conditions as described above in Table 2 and washed. In accordance with GB~A~2 151 660 the foil was then plated in an iron plating bath in conditions as described in Table 4 below.

TABLE 4 FeSO4 ~ 7H2O 180~220 gil FeCI2 ~ 4H2O 3040 gil NH4CI 15--189/1 pH 4.5~6 Temperature 90 C Current Density 511 A/dm2.

Plating Time 3--10 s.

The washed foil was plated with iron in conditions which provided a homogeneous micro-layer to cover all the copper dendrites present after the first stages. After the sample had been washed it was treated in a Zn bath as in Table 3 above in an equivalent time to the previous example.

EXAMPLE 3 Similar copper foil was taken and passed through the same copper plating bath and plating conditions as described above in Table 2 and washed. The foil was then plated in a Zn bath as in Table 3 above in an equivalent time to the previous Examples but using a stainless steel anode (iron content about 80 wt.%). The resulting zinc-iron micro-layer had an iron content of approximately 10 wt.%.

After washing, stainproofing, and drying, the samples resulting from Examples 1 to 3 were laminated onto an epoxyiglass base material under typical laminating conditions. The samples were selectively masked by acid resists in 250 micron tracks and spaces and exposed to etching by ammonium persulphate in a typical spray etching machine. When the exposed copper areas had been cleared of copper the etching was stopped and the samples were examined.

On removing tracks from the laminate by physically peeling them off it was readily determined that undercut was visible on the first sample (Example 1) and no undercut was visible on the other samples (Examples 2 and 3). The adhesion of the samples to the base material was measured by recording the force required to strip them from the base; the adhesion of all samples was within the specifications set for such products but the second and third samples required 12% more force to peel than the first.

Claims (14)

1. A method of providing a metal layer with a matte surface, comprising depositing on the metal layer in sequence a copper layer of dendritic form, and a layer of a zinc-based alloy containing iron and conforming to the copper layer, the iron being deposited simultaneously with the zinc.

2. A method as claimed in claim 1, in which the metal layer is a copper layer.

3. A method as claimed in claim 1 or 2, in which the metal layer is in the form of a foil.

4. A method as claimed in claim 1 or 2, in which the metal layer is carried by a temporary substrate which can subsequently be removed from the metal layer.

5. A method as claimed in claim 4, in which the metal layer is 1 to 2 microns thick.

6. A method as claimed in any of claims 1 to 5, in which the iron content of the zinc alloy layer is 5 to 20wit.%.

7. A method as claimed in any of claims 1 to 6, in which the zinc-iron layer is electrodeposited from an acidic zinc bath, using a soluble anode of iron or an iron-based alloy.

8. A method as claimed in claim 7, in which the anode is of stainless steel.

9. A method as claimed in claim 1, substantially as described in Example 3.

10. An article comprising a metal layer provided with a matte surface by a method according to any preceding claim.

11. An article having a matte surface, comprising a metal layer, a superposed copper layer of dendritic form, and a superposed layer of a zinc-based alloy containing iron and conforming to the copper layer.

12. An article as claimed in claim 11, in which the metal layer is a copper layer.

13. An article as claimed in claim 11 or 12, in which the metal layer is in the form of a foil.

14. An article as claimed in claim 13, in which the substrate is a temporary substrate removable from the first copper layer.

14. An article as claimed in claim 11 or 12, further comprising a substrate carrying the metal layer.

15. An article as claimed in claim 14, in which the substrate is a temporary substrate removable from the metal layer.

16. An article as claimed in claim 15, in which the metal layer is 1 to 2 microns thick.

17. An article as claimed in any of claims 11 to 16, in which the iron content of the zinc alloy layer is 5 to 20 wt.%.

18. An article as claimed in claim 17, in which the said iron content is 10 wt.%.

Amendments to the claims have been filed, and have the following effect:~ (a) Claims 1,2,4,5 and 11 to 15 above have been deleted or textually amended.

(b) New or textually amended claims have been filed as follows:- (c) Claims 3r 3, 6, 7, 8, 17 and 18 above have been re- numbered as 2,5,6,7, 16 ans 17 and their appendancies corrected.

1. A method of providing a copper layer with a matte surface suitable for bonding to a plastics material, comprising depositing on the copper layer in sequence a copper layer of dendritic form, and a barrier layer of a zinc-based alloy containing iron and conforming to the dendritic copper layer, the iron being deposited simultaneously with the zinc directly on the dendritic copper layer.

3. A method as claimed in claim 1, in which the first-mentioned copper layer is carried by a temporary substrate which can subsequently be removed.

4. A method as claimed in claim 3, in which the first-mentioned copper layer is 1 to 2 microns thick.

8. A method as claimed in any of claims 1 to 7, in which the dendritic copper layer is electrodeposited from an acidic copper bath, using a higher current density followed by a lower current density.

11. An article having a matte surface suitable for bonding to a plastics material, comprising a first copper layer, a superposed second copper layer of dendritic form, and a directly superposed barrier layer of a zinc-based alloy containing iron and conforming to the second copper layer.

12. An article as claimed in claim 11, in which the first copper layer is in the form of a foil.

13. An article as claimed in claim 11, further comprising a substrate carrying the first copper layer.