GB2151660A - 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 iron conforming to the copper layer, and a layer of zinc conforming to the iron 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 it is usual to treat the foil by means of electrolytic processes such as those described in U.S. Patents 3 918926,3857681, 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 electrodeposited onto the copper dendrites in such a way as to isolate the copper from the base. U.S. Patents 3 857 681 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 phenomenen 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 absorbtion 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 which manifests itself as shown in the accompanying drawing.

In the drawing the sole Figure is a fragmentary section through the copper foil and the dielectric base material during etching.

The copper foil 1 provided with the dendritic copper layer 2 and the zinc barrier layer 3 is bonded to the dielectric base material 4 and has an acid resistant coating 6 applied wherever the foil is not to be etched away. As the etchant (arrows 7) reaches the base material 4 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 8 of the copper track. Here the edges of the zinc layer 3 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 9.

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 3 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.

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, a layer of iron conforming to the copper layer, and a layer of zinc conforming to the iron layer.

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

The iron layer encapsulates and anchors the copper dendrites and serves as a barrier layer. The zinc layer also acts as a barrier layer and in addition protects the iron layer against atmospheric oxidation. Both iron and zinc are compatible with the usual etchants for copper and do not cause undue contamination.

Another advantage, in relation to the production of printed circuits, is that the iron layer introduced between the zinc and the copper dendrites has an electrode potential that lies between the electrode potentials of copper and zinc. This has the effect of stabilizing the product under the conditions of etching which would otherwise result in severe undercut.

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

TABLE 1 Copper Cu2#Cu+0.337 Iron Fe2+/Fe-0.440 Zinc Zn2+/Zn-0.763 It can be seen from the above that iron falls roughly midway between the electrode voltages of copper and zinc so that in the presence of a common etchant such as ammonium persulphate solution the iron acts as buffer between zinc and copper to minimize the cell-like reaction that would otherwise occur when the ammonium persulphate solution acts as an electrolyte during the etching process.

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 layered 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.

Such a technique, when applied to a sheet of stainless steel, titanium or chromium-plated steel, can be used to produce a copper layer of such relative thinness that it can, quite uniquely, be transferred under heat and pressure from the carrier sheet onto the dielectric base in such a way that the matte surface provided on the micro layer of copper becomes firmly adherent to the base and the carrier sheet can be removed and used again for the same purpose. Such a system allows for the transfer of extremely thin (3 micron) layers of copper to dielectric bases with reliability and low cost in a way that has hitherto not been possible. The same technique can be used for plating thicker copper layers too, the only criterion being the economic break even point between plating carrier sheets discontinuously and plating copper onto a rotating mandrel continuously as described in U.S.Patent 3 674 656. Micro thin copper clad laminates have previously been manufactured by techniques as disclosed in U.S. Patent 113576, and U.K. Patent Specifications 1 460 849, 1 458 260, and 1 458 259, but such processes have proved costly to operate and unreliable in their utility.

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) 545 girl Sulphuric Acid 60~909/l Temperature 1 5500C Current Density available 5220 A/dm2.

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 anode/cathode interspace and the foil was subjected to a series of plating steps as follows: Current density Time 1. 27 A/dm2 4 - 6 s 2. 8A/dm2 1 & 0s 3. 22 A/dm2 4 - 6 s 4. 8A/dm2 1 & 0s.

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 girl Sulphuric Acid pH 1.5~4.5 Temperature 18~28 C Current Density available 0.55 Alum2 Plating Time 4--10 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 git 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. The foil was then plated in an iron plating bath in conditions as described in Table 4 below.

TABLE 4 FeSO4 7H2O 180 - g/l FOCI, ~ 4H20 30-40 gIl NH4CI 1518 girl pH 4.5~6 Temperature 900C 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.

After washing, stainproofing, and drying, the two samples resulting from Examples 1 and 2 were both laminated simultaneously onto an epoxy/glass base material under typical laminating conditions. Both 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 two 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 second sample (Example 2). The adhesion of both samples to the base material was measured by recording the force required to strip them from the base; the adhesion of both samples was within the specifications set for such products but the second sample required 12% more force to peel than the first.

EXAMPLE 3 A third sample was produced by taking a clean sheet of 2 mm thick polished stainless steel plate and subjecting it to the same plating sequence as in Example 2, with the exception that prior to the plating carried out from the copper bath described in Table 2 a first strike of copper on the stainless steel was made from an aqueous copper sulphate solution made up as in Table 5 below.

TABLE 5 Copper (as metal) 25110 gIl Sulphuric Acid 60~110 gIl Temperature 45#-65C Current Density 2-110 A/dm2 When the plating sequences had been concluded, the washed, stainproofed, and dried plated sheet was taken and placed in a laminating press on top of an epoxy/glass base material, where heat and pressure were applied. After the press cycle had finished and the plate had cooled, it was found that the copper layer had detached from the stainless steel and was firmly adherent to the base. The resultant laminate was tested in accordance with typical procedures laid down by the industry and found to be satisfactory in every respect.

Claims (15)

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, a layer of iron conforming to the copper layer, and a layer of zinc conforming to the iron layer.

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 claim 1, substantially as described in Example 2 or 3.

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

8. An article having a matte surface, comprising a metal layer, a superposed copper layer of dendritic form, a superposed iron layer conforming to the copper layer, and a superposed zinc layer conforming to the iron layer.

9. An article as claimed in claim 8, in which the metal layer is a copper layer.

10. An article as claimed in claim 8 or 9, in which the metal layer is in the form of a foil.

11. An article as claimed in claim 8 or 9, further comprising a substrate carrying the metal layer.

12. An article as claimed in claim 11, in which the substrate is a temporary substrate removable from the metal layer.

13. An article as claimed in claim 12, in which the metal layer is 1 to 2 microns thick.

14. A process for applying a metal layer to a plastics base, in which the matte surface of an article according to claim 12 or 13 is caused to adhere to the plastics base under the application of heat and pressure, and the temporary substrate is subsequently removed from the metal layer.

15. A process as claimed in claim 14, substantially as described in Example 3.