GB1593206A - Plasticsmetal composite and method of making same - Google Patents

Description

(54) PLASTICS-METAL COMPOSITE AND METHOD OF MAKING SAME (71) We, CALIFOIL INC., a Corporation organised and operating under the laws of the State of California, United States of America, of 8983 Complex Drive, San Diego, California, United States of America, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- The present invention generally relates to composites and more particularly to composites of thin copper foil and plastics, which composites are useful in the manufacture of electrical and electronic components and the like.

Copper foil that has been laminated to plastics is commonly used in the manufacture of printed circuit boards. These boards are employed in a wide variety of commercial electronic applications such as in television sets, radios, computers, and automobile dashboards. In such applications it has been unnecessary to design printed circuits with conducting elements smaller than about 1/16 inch in width or with gaps between adjacent conducting elements smaller than about 1/32 inch. Such coarse dimensions have not required great precision in the preparation of the printed circuits nor much uniformity in the width of their conductors. Special extra-thin copper foil weighing 1/8 oz, to 1/2 oz.

per sq. ft. is now made using processes that employ temporary carriers such as metal metal foils and plastics.

Typically, however, conventional copper foil-clad laminates are made using copper foil that weighs between about 1/2 oz. per sq. ft. and about 2 oz. per sq. ft.

and that has been formed by electrodeposition of copper onto the surface of rotating drum. The drum surface has been pretreated so that the thus formed copper foil layer can be easily peeled away. The side of the thus formed copper foil, which is exposed, that is, is away from the drum, is much rougher than the side facing the drum and it is this rough side which is then bonded to plastics to form the typical composite or laminate referred to above.

Before the lamination step, however, the rough (matt) surface of the copper foil is usually treated to form thereon a plurality of small projections or nodules which enhance the bondability of the foil to the plastics. Thus, these nodules or projections comprise a myriad of microscopic particles of copper and/or copper oxide, some of which are only loosely adherent to the copper foil surface.

One such plastics adhesion-increasing treatment which causes formation of the nodules is described in U.S. Patent No. 3,220,897, issued November 30, 1965 to C. C. Conley. An improvement of that method is disclosed in B. Luce's U.S.

Patent No. 3,293,109, issued December 20, 1966. In the latter patent, layer of copper is placed over and around the nodules or projections of copper-copper oxide particles (formed as by the method of U.S. Patent No. 3,220,897) so as to lock those particles more securely to the matt surface of the copper foil.

Although both of the described patented methods are still used to manufacture copper foil for printed circuit laminates, problems are encountered as a result of their use. Thus, when the treated copper foil surface is brought into contact with the uncured plastics under high heat and high pressure during the lamination step, the copper or copper oxide particles tend to be broken away from the copper foil surface and to become embedded below the plastics surface. When the copper is then etched away, as during the preparation of the printed circuit, there remains behind a discoloration of the plastics surface, which is called "staining". Such discoloration comprises a plurality of the copper and/or copper oxide particles as embedded in the plastics, and adversely affects the dielectric properties of the plastics. Thus, the over-all performance of the printed circuit is impaired as well as its physical appearance.

More recent applications of copper foil clad laminates for printed circuits have been in areas requiring miniaturization of the printed circuits. New electronic components such as light emitting diodes and miniature assemblies of semi-conductors, such as integrated circuits, have created more severe performance requirements for the manufacturers of printed circuits. Printed circuits with conducting elements having a width of only a few thousandths of an inch and spacings between elements of similar dimensions, have become common and have shown up the deficiencies of the ordinary copper foil adhesionincreasing treatments.

In order to insure that a copper conducting element of only a few mils width remains firmly attached to the plastics substrate during the fabrication of the printed circuit, the described adhesion-increasing treatment must be extensive and it must also be uniform. Despite such precautions, more numerous occurrences of "staining" or "brown spotting" usually are experienced as copper conductor widths are reduced. Moreover, an additional difficulty arises in the form of a phenomenon called "line lifting", where the conducting elements tend to separate over small areas from the plastics substrate. Examination of the separated areas under the lifted conducting elements often shows that the particular area of copper surface so affected has not been sufficiently nodularized.

An improvement in the treatment of copper foil surfaces to enhance bonding and reduce "staining" has been described by Luce in U.S. Patent No. 3,585,010 issued June 15, 1971. In that patent, the copper layer called for in U.S. Patent No.

3,293,109 is replaced by a layer of one of a selected few other metals, namely, indium, nickel, tin, cobalt, bronze, zinc or brass. The layer is electroplated over the copper-copper oxide particles on the treated foil surface to form a barrier layer.

Some reduction in "staining" is often noted when this method is employed.

However, because of the substantial differences in the chemical characteristics of the metals indium, zinc and brass, from those of the copper over which they are deposited, certain other substantial problems are introduced when this method is used.

The zinc, whether present in the form of the pure metal or as the alloy brass, is highly soluble in conventional ferric chloride and copper chloride etching solutions used in the manufacture of the circuit boards, and, thus, is attacked much more rapidly during etching than is the copper. Therefore, when the excess copper being etched away to form the conducting elements, the zinc-rich layer over the copper-copper oxide particles is exposed to the chloride etching solution and is stripped away quite rapidly. Since this encapsulating layer is at the bonding interface between the copper foil and the plastics, its more rapid attack results in undercutting of the conducting element.

If the conducting element is narrow, such as in the more recent circuit designs, then rapid etching of the small zinc-rich bonding interface severely reduces bond between the copper conducting element and the plastics. This often results in low peel strength and/or line lifting. This susceptibility of the zinc to the chloride type etchants is well known and it is for this reason that zinc or brass treated copper foil is not often used for fine line circuits that are to be etched in chlorides. Moreover, copper foil with such zinc-rich layer on its bonding surface requires special handling to prevent its dezincification, which can result even from casual contact with fingerprints.

The other metals listed in the Luce patent, have even greater drawbacks as stain resistant barrier layers. Indium, cobalt, and nickel, for example, have etching characteristics that are markedly different than copper and if used commercially they would require specialized etching procedures of two or more steps and different solutions to remove the composite metals.

Accordingly, there is a need for an improved copper foil for use in printed circuit laminate manufacture, which foil readily adheres to but reduces both staining of the plastics substrate and fine line lifting of copper conductor (made from the foil) from the substrate during such manufacture. There is also a need for an improved laminate of copper foil and plastics substrate having such properties and which can be processed into printed circuits without having to resort to special etching solutions.

According to one aspect of the invention there is provided a plastics-metal unitary circuit composite having reduced staining and line lifting characteristics, comprising: a) copper foil, a side thereof having a rough irregular surface with improved bondability to plastics but substantial staining characteristics; b) a layer in the range 5 to 80 microinches thick of at least one of cadmium, cadmium alloy of tin, cadmium alloy of zinc, or cadmium alloy of copper, covering said rough irregular surface; and c) a plastics substrate bonded to the covering layer.

According to a second aspect of the invention there is provided a method of making a plastics-metal circuit composite having reduced staining and line lifting characteristics, comprising: a) forming a copper foil having a side thereof with a rough irregular surface which exhibits improved bondability to plastics but substantial staining characteristics; b) covering said rough irregular surface with a layer, in the range 5 to 80 microinches thick, of at least one of cadmium, cadmium alloy of tin, cadmium alloy of zinc, or cadmium alloy of copper; and c) bonding the covering layer to a plastics substrate, whereby said composite has a reduced tendency to stain.

According to a third aspect of the invention there is provided a plastics-metal circuit composite whenever made by a method as hereinbefore defined.

The composite and method are substantially as defined above. Such composite has substantially reduced staining and fine line lifting characteristics and yet can be processed into printed circuitry utilizing standard cleaning and etching solutions. Moreover, the present method results in increased adhesion between the copper foil and substrate through the action of the covering layer and thus assures unitary integrity throughout processing. The method embodying the invention is relatively simple, direct, inexpensive and highly efficient. It has been found that the increased bondability of the foil to the plastics substrate is preserved while the tendency of the described particles to separate from the foil and become embedded in the plastics is greatly suppressed.

Unlike other covering techniques, the composite produced by the present method can be cleaned and etched, during fabrication of printed circuits therefrom, using standard single step solutions and procedures.

A composite and a method of producing the same are hereinafter described, by way of example, with reference to the following specific description and Examples.

The present improved method of making a plastics-metal composite having reduced staining and line characteristics and useful for printed circuit manufacture comprises as a first step, forming a copper foil having a rough irregular surface which exhibits improved bondability to plastics but also a substantial staining characteristic. The copper foil can be formed in any suitable manner, as by rolling or the like. However. as previously described, electrodeposition or electroplating of copper on the surface of a specially treated drum is the conventional well-known way of forming such foil, at about 1/2-2 oz.

per sq. ft. such as utilized in the usual preparation of laminates for the manufacture of standard printed circuitry and the like. Ultra-thin foil that is electrodeposited on temporary carriers such as heavier metal foils or plastics films can also be used with the method taught herein, and such foil will be from 1/8 oz. per sq. ft to 1/2 oz. per sq. ft.

Modification on the foil to increase the bondability of the side thereof which is initially exposed, that is, away from the drum (or carrier) is also contemplated in accordance with the present method and can be carried out in any suitable manner. for example, one such conventional foil modifying technique often referred to as "treatment" is described in U.S. Patent No. 3,220,897 to Conley, referred to above, wherein projections or nodules are produced on the already rough or exposed side of the copper foil by exposing it as a cathode to a current density of from 60 amps per sq. ft. (a.s.f.) to 125 a.s.f. for about 1-60 seconds in an aqueous acidic copper sulfate bath containing 1540 p.p.m. of halogen ion and 0.2-1 grams per liter of water-dispersible proteinaceous material. The net result is substantially increased bondability of that side of the foil due to the formation thereon of the described nodules or projections comprising coppercopper oxide particles, but at the expense of a substantial staining characteristic.

Other comparable adhesion-increasing techniques can be employed.

In any event, the first step of the present method contemplates formation of the copper foil, by one or more conventional techniques, such as those described above, so that the foil has the desired bondability to plastics, but unfortunately also the undesired staining characteristic.

The second step of the present method involves greatly reducing the described staining characteristic while avoiding the introduction of undesired side effects, including those produced by the application of prior art techniques, namely, fine line lifting and inability of the plastics foil composite to be satisfactorily etched by standard etching solutions in standard single step etching modes.

In this regard, the second step of the present method comprises covering the nodule-bearing or otherwise rough, irregular, plastics bondable surface of the copper foil with a thin layer of metal comprising cadmium, or cadmium alloy of either tin or of zinc or of copper, or a mixture thereof, without substantially depreciating the bondability of that surface to a plastics substrate. The layer of metal may preferably be between about 5 and about 80 microinches thick and, most preferably, for most purposes, about 15 to about 35 microinches. Of course, the most desired thickness will vary, depending on the particular metal(s) or alloy(s) employed in the layer, and the extent of the nodularization of the surface.

Thus, to achieve the full benefits described for this invention utilizing cadmium as the metal, there should be at least about 5 microinches of cadmium covering the nodularized surface of the copper foil. Because the nodularized surface is irregular, direct measurement of thickness is difficult. Thickness is best measured by a weight difference determination. With less than the recommended about 5 micro inches of cadmium, some improvement in stain resistance and peel strength will be achieved but the improvements will not be maximized, in fact, will generally be relatively small.

Not all nodularization (oxide) treatments are the same, and some may produce oxides that are more loosely adhering than others. Therefore, the amount of cadmium or cadmium alloy to cover the particular oxide treatment should be empirically determined for each treatment process. It has generally been found, however, that covering with more than about 80 microinches of cadmium offers little or no additional stain resistance or improved bonding properties. For most properly applied oxide treatments, cadmium or cadmium layer of about 25 microinches in thickness is preferred. The layer so applied suitably covers and locks the nodules securely to the copper foil surface and insulates the plastics resin from the copper.

The covering procedure of the second step is preferably carried out utilizing a standard electroplating technique. However, other conventional techniques such as vacuum evaporation, ion plating, thermal decomposition of metal compounds and the like well known in the metal deposition art are also suitable for use in applying the encapsulating barrier layer of metal in the second step of the method.

It has been determined that the desired improved bonding characteristics can readily be achieved by depositing thereon the thin layer, preferably of cadmium or cadmium-copper alloy. The copper foil preferably has the described nodular copper-copper oxide projections when covered with the cadmium or cadmiumcopper alloy.

The etching speed and behaviour of cadmium and cadmium-copper alloys is remarkably similar to pure copper in all of the commonly used printed circuit etchants. Cadmium, therefore, provides excellent resistance to staining or brown spotting and its presence on the copper foil does not require special handling.

There is also a discernible improvement in the peel strength between the cadmium covered copper and at least one type of epoxy plastics substrate, over similarly oxide-treated copper foils not having the cadmium encapsulation.

Although the mechanism for this is not understood, it is believed that the curing of some epoxy resins is adversely affected by contact with copper and/or copper oxide. When the cadmium layer embodied in the invention is interposed between the copper foil and the epoxy resin, curing of the resin proceeds in a way which yields improved bonding results, as evidenced by peel strength increases of up to 20 percent. Because of this phenomenon, it has also been found useful to employ the cadmium layer directly over the rough surface of the copper foil, without the use of an oxide or nodularizing treatment. The rough surface of one ounce copper foil laminated to a glass-epoxy without oxide treatment will often exhibit peel strength of 2 to 4 pounds per inch. By interposing the cadmium between copper and plastics, the untreated peel strengths can be raised to a range of 5 to 6 poundstinch, which is high enough for many applications.

Thin cadmium and cadmium-copper alloy encapsulating layers on the copper foil etch at essentially the same rate as does pure copper foil. Therefore, undercutting of narrow conducting elements, in conventional printed circuit etchants is minimal and line lifting from etch undercutting of narrow conductors is eliminated.

Although pure cadmium can be used to cover the oxide, i.e., the nodules, or the otherwise roughened foil surface, the heat applied during the subsequent laminating procedure (bonding of foil to plastics substrate) may cause some codiffusion to occur at the cadmium-copper interface. This results in the formation of an alloy between the copper and cadmium which provides stronger chemical similarity in the etching characteristics.

Alternatively, cadmium already alloyed with copper and/or zinc or tin can be used as the covering metal. Thus, cadmium codeposited electrochemically along with copper, zinc, and/or tin can be used in place of pure cadmium, if desired. In any event, the alloy should contain a major proportion, by weight, of cadmium.

Because of thermal diffusion, the alloy content of such an encapsulating barrier layer changes less than does pure cadmium upon exposure to heat during the laminating cycle. The cadmium alloy barrier also provides improved resistance to other more severe thermal exposure such as is encountered during soldering or bonding in the multilayer assembly of circuit boards, etc.

As the third step in the present method, the covering layer, now in place over the rough and irregular surface on the copper foil as a result of the second step, is bonded to the plastics substrate. Such substrate may be any suitable plastics, but usually is one of the conventional types of thermosetting epoxy resins or phenolic or phenol-nitrate resins utilized for such purposes. As a specific example, epichlorohydrin, -2,2'-di(p-hydroxyphenyl) propylidine condensation product can be used. So also can poly (phenolformaldehyde, vinyl butyral) resin.

Application of suitable heat and/or pressure usually sets the resin and effects the desired bonding to form the unitary foil-plastics composite. Thus, epoxy resins typically are bonded at about 200400 F. and at 1001000 p.s.i. over the course of a few minutes to an hour or more. In the case of epoxy resin, a hardening agent such as selected polyamine is also used.

The substrate has a high dielectric strength and may include suitable glass fibres, glass fabric, paper fibres or the like to provide the desired physical strength. The plastics substrate may itself be, if desired, an adhesive which joins the foil to a sub-layer. Alternatively, a separate adhesive layer may perform the joining of the plastics and foil. Such adhesive could be, for example, phenolformaldehyde condensate and butadiene-acrylonitrile-rubber with inert particles mixed therein. For most purposes, however, epoxy resins, reinforced with glass fabric, without intervening layers of adhesives, are utilized as the plastics substrate in the present method.

Once the bonding step is effected, the present method is complete and the desired unitary plastics-foil composite, having improved resistance to staining and line lifting, is obtained and is ready for use in conventional printed circuitry manufacture and the like. The following specific examples further illustrate certain features of the invention.

EXAMPLE I Three separate sheets (A, B, and C) of copper foil are produced by conventional electrodeposition on a rotating drum. Each sheet weighs about 2 oz.

per sq. ft. In a separate apparatus, the rough surface of each of these thusproduced foils is then treated to enhance bonding to plastics by exposing it to one of the following electrolytic treatments, with the rough surface contacting an electrolyte, the copper foil being the cathode and a lead plate being the anode: Treatment I: (Sheet A) a. Electrolyte is mixture of: Copper Sulfate-6 oz/gal and Sulfuric Acid-13 oz/gal b. Current Density=125 to 175 amps per sq. ft.

c. Treatment Conditions: Room Temperature No Agitation Time-30 Seconds Treatment II: (Sheet B) a. Electrolyte is mixture of: Copper Cyanide--13 oz/gal and Sodium Cyanide--15 oz/gal b. Current Density--125 to 175 amps per sq. ft.

c. Treatment Conditions: Room Temperature Mild Agitation Time-3 minutes Treatment III: (Sheet C) a. Electrolyte is mixture of: Copper Sulfamate--6 oz/gal, Sulfamic Acid-20 oz/gal and Sodium Dihexyl Sulfosuccinate-.03 oz/gal b. Current Density=125 to 175 amps per sq. ft.

c. Treatment Conditions: Room Temperature Mild Agitation Time-l minute The small projections formed on the rough foil surface of sheets A and C, respectively, by treatments I and III are composed of mixed copper and copper oxide, while that formed by treatment II on the rough surface of sheet B is relatively pure copper. The thus treated sheets A and B of foil are each divided into 2 sub-sheets A' and A", B' and B" and each of the 4 sub-sheets plus sheet C are then exposed to a separate one of the following covering procedures to provide improved stain resistance and fine line lifting resistance and to enhance bonding, each procedure comprising electrolysis: Covering I: (Cadmium stain resistant barrier-S RB) (Sheet A') Electrolyte Bath.

Cadmium Fluoborate-32 oz/gal Ammonium Fluoborate-8 oz/gal Boric Acid-3.5 oz/gal Licorice 0.15 oz/gal The pH of the bath is maintained in the range from 3-3.5 and current density is 30 a.s.f. The temperature is held ina range from 70" F. to 900 F. and the foil sheet is plated for 30 seconds to a thickness of 30 microinches. Cast aluminum anodes are used and positioned opposite the treated foil surface in a deep rectangular tank. Cadmium fluoborate is added periodically to maintain the cadmium metal content of the bath at 12.6 oz. per gal.

Covering 2: (Cadmium SRB) (Sheet A") Electrolyte Bath: Cadmium Oxides.0 oz/gal Sodium Cyanide--13.0 oz/gal Current density is held at 25 amps per sq. ft., and the bath temperature is held in the range of 75-900F. The anodes are high purity cadmium for approximately two-thirds of the anode area and insoluble steel for the remaining one-third of the anode area. The foil sheet A" is plated for 30 seconds to 60 seconds to a thickness of 30 microinches.

Covering 3: (Cadmium-Copper Alloy SRB) (Sheet B') Electrolyte Bath: Cadmium Oxide 5 oz/gal Copper Cyanide 1.0 oz/gal Sodium Cyanide4.5 oz/gal Sodium Carbonate-2 oz/gal Copper anodes are used to maintain the copper metal content of the bath while additions of cadmium oxide concentrate dissolved in sodium cyanide are made to keep the cadmium metal content uniform. Current density and bath make-up are varied to keep the alloy which is deposited whitish in color. Chemical analysis of the deposited alloy should show an alloy of about 50% cadmium50% copper, which produces best results. The best thickness is approximately 30 microinches.

Covering 4: (Cadmium-Tin Alloy SRB) (Sheet B") Electrolyte Bath: Potassium Stannate (K2Sn(OH)6)-l 4 oz/gal Cadmium Oxide1 oz/gal Potassium Cyanide (total94 oz/gal Potassium Hydroxide-2 oz/gal This bath operates at 650C. with a current density of 35 amps per sq. ft.

Approximately 35 microinches of alloy of 50:50 is obtained in 1 minute and is maintained through control of the anode composition. Thus, the anode utilizes an alloy of the same composition (50:50). Temperature and current density are also used to control the deposition ratio. This alloy has a low melting point and is best used in applications having relatively low plastics laminating and curing temperatures.

Covering 5: (Cadmium-Zinc Alloy SRB) (Sheet C) The treated copper foil sheet C is first passed through the bath of Covering I, for 20 seconds to deposit approximately 20 microinches of cadmium and is then rinsed, after which it is passed through the following bath: Electrolyte Bath Zinc Chloride--15 oz/gal Ammonium Chloride-20 oz/gal at room temperature with a current density of 20 amps per sq. ft. for 15 seconds to deposit about 15 microinches of zinc thereon.

After laminating treated sheet C to a glass reinforced epoxy at about 300"F.

and 200 p.s.i. for 60 minutes, examination of the SRB layer thereon shows that it comprises a 60:40 alloy of cadmium with zinc together with a small concentration (a few percent) of copper. The laminate exhibits no visible staining and, when used in the fabrication of thin line conductors in printed circuitry exhibits no tendency to delaminate (line lifting). Essentially the same results are obtained upon laminating sheets A', A", B', and B" to epoxy resin at 3004000 F. and 100--500 p.s.i. and thereafter utilizing the laminate in the fabrication of printed circuit boards. Moreover, standard cleaning and etching solutions are employed on the laminates without undercutting of the conductor lines thereof during manufacture of the printed circuitry, even in instances where the conductor lines are substantially narrower than 1/100 inch. Accordingly, improved results are obtained by the present method.

EXAMPLE II Treatments I and III, Example I, are applied to copper foil sheets X and Y, after which improved adhesion of the thus formed copper-copper oxide nodules produced in these treatments, is attained by subjecting sheets X and Y to a 2 minute cycle at a current density of 40 amps per sq. ft. in the same solution as that used to form the oxide (Treatments I and III). This additional step deposits pure copper over the copper-copper oxide nodules to better anchor them to the foil surface. The remainder of the procedure of Example I, Covering I, is then applied to sheets X and Y with further improvements in plastics adhesion and line lifting suppression, as contrasted with Example I.

After the application of the SRB layer, the thus-treated copper foil is treated with a suitable corrosion inhibitor, namely, it is dipped in a solution of 2 grams per liter of benzotriazole in water at 150"F.

Accordingly, an improved method of covering roughened, preferably nodularized copper foil surfaces to reduce staining thereof while not reducing the plastics bondability thereof is provided. The method is simple, inexpensive and direct. Line lifting is suppressed and in most instances no change whatsoever in standard cleaning and etching solutions and techniques need be made in order to successfully apply the composite product of the method in the fabricating of high quality standard and miniaturized circuitry. Various other advantages of the method and product

Claims (13)

**WARNING** start of CLMS field may overlap end of DESC **. with a suitable corrosion inhibitor, namely, it is dipped in a solution of 2 grams per liter of benzotriazole in water at 150"F. Accordingly, an improved method of covering roughened, preferably nodularized copper foil surfaces to reduce staining thereof while not reducing the plastics bondability thereof is provided. The method is simple, inexpensive and direct. Line lifting is suppressed and in most instances no change whatsoever in standard cleaning and etching solutions and techniques need be made in order to successfully apply the composite product of the method in the fabricating of high quality standard and miniaturized circuitry. Various other advantages of the method and product are set forth in the foregoing. WHAT WE CLAIM IS:

1. A plastics-metal unitary circuit composite having reduced staining and line lifting characteristics, comprising: a) copper foil, a side thereof having a rough irregular surface with improved bondability to plastics but substantial staining characteristics; b) a layer in the range 5 to 80 microinches thick of at least one of cadmium, cadmium alloy of tin, cadmium alloy of zinc, or cadmium alloy of copper, covering said rough irregular surface; and c) a plastics substrate bonded to the covering layer.

2. A composite according to Claim 1, wherein said rough irregular surface includes adherent nodules comprising particles of copper and/or copper oxide, wherein said reduced staining comprises a reduced tendency of said particles to become detached from said rough irregular surface and embedded in said plastics during manufacture of said circuit and wherein said reduced line lifting comprises increased resistance of fine copper lines made from said foil to separate from said substrate during fabrication of said circuit.

3. A composite according to Claim 1 or Claim 2, wherein said layer of metal is in the range 15 to 35 microinches thick.

4. A composite according to any preceding Claim, wherein said copper foil weighs between about 1/8 and about 2 oz. per sq. foot.

5. A method of making a plastics-metal circuit composite having reduced staining and line lifting characteristics, comprising: a) forming a copper foil having a side thereof with a rough irregular surface which exhibits improved bondability to plastics but substantial staining characteristics; b) covering said rough irregular surface with a layer in the range 5 to 80 microinches thick, of at least one of cadmium, cadmium alloy of tin, cadmium alloy of zinc, or cadmium alloy of cooper; and c) bonding the covering layer to a plastics substrate, whereby said composite has a reduced tendency to stain.

6. A method according to Claim 5, wherein said rough irregular surface includes nodules comprising adherent particles of copper and/or copper oxide, wherein said reduced staining comprises a reduced tendency of said particles to become detached from said rough irregular surface and embedded in said plastics during manufacture of said circuit, and wherein said reduced line lifting comprises increased resistance of fine copper lines made from said foil to separate from said substrate during fabrication of said circuit.

7. A method according to Claim 5 or Claim 6, wherein said layer of metal is formed to be about 15 to 35 microinches thick.

8. A method according to any of Claims 5 to 7, wherein said bonding of said encapsulating layer to said plastics substrate is effected at elevated temperature and/or pressure.

9. A plastics-metal unitary circuit composite having reduced staining and line lifting characteristics, substantially as hereinbefore described.

10. A plastics-metal unitary circuit composite having reduced staining and line lifting characteristics, substantially as hereinbefore described with reference to either of the Examples.

11. A method of making a plastics-metal circuit composite having reduced staining and line lifting characteristics according to Claim 5, substantially as hereinbefore described.

12. A method of making a plastics-metal circuit composite having reduced

staining and line lifting characteristics, substantially as hereinbefore described with reference to any Example.

13. A plastics-metal circuit composite whenever made by a method according to any one of Claims 5 to 8, 11 or 12.