WO2001032963A1

WO2001032963A1 - Stainproof capable of protecting copper foil

Info

Publication number: WO2001032963A1
Application number: PCT/US2000/041190
Authority: WO
Inventors: Charles B. Yates; George Gaskill; Ajesh Shah; Chinsai T. Cheng; Adam M. Wolski
Original assignee: Yates Foil Usa, Inc.
Priority date: 1999-11-01
Filing date: 2000-10-18
Publication date: 2001-05-10
Also published as: AU1965901A

Abstract

Copper foil having electrodeposited on at least one thereof a stainproofing layer containing chromium, zinc and nickel, and providing improved protection from oxidation at elevated temperatures, e.g., 400 °F and higher, while being easily removable by an alkaline solution.

Description

STAINPROOF CAPABLE OF PROTECTING COPPER FOIL

FIELD OF THE INVENTION

This invention relates to electrodeposited copper foil provided with a chromium-zinc-nickel stainproofing layer which protects the foil from oxidation at elevated temperatures, and to copper-clad laminates made with such foil.

BACKGROUND

As with many other materials used in high technology applications, electrodeposited copper foil is a composite, i.e., it has a near-surface region with properties differing from those of the bulk material. In this sense of the word, the bulk of the copper foil (core) serves in printed circuit boards (PCBs) as a conductor of electricity. In copper clad laminates, one of the outer surfaces of the foil serves as a substrate for image patterning and soldering to ensure the necessary electrical connection between components, while the opposite side of the foil, the bonding side, is responsible for permanently bonding conductor lines to a polymeric substrates and may be provided with an electrodeposited copper bonding treatment. In addition, in the case of copper foil destined for fabrication of multi-layer printed circuit boards (MLBs), the same side of the foil that is used for image patterning, serves also as a substrate for application of brown oxide treatment that is used for B-stage lamination.

A choice exists as to whether the shiny side (the side facing the drum) or the matte side (the side facing the electrolyte) of the foil should be provided with the bonding treatment. Each choice has its advantages and disadvantages. Moreover, it will depend on which segment of the PCBs industry the foil is destined: for printed circuit boards that are manufactured with rigid, single sided or double sided copper clad laminates, or multilayer boards. Both require high quality copper foil, but while PCB manufacturers who use rigid copper clad laminates use copper foil with bonding treatment applied to the matte side of the foil, the MLBs segment of the PCB industry may prefer copper clad laminates with bonding treatment applied to the shiny side of the foil, since in this case matte side of the foil forms the outer surface of the laminate, and the "natural" micro-roughness of the matte side contributes toward quality and reliability of finished MLB.

The side of the foil which is provided with the bonding treatment should assure the highest possible bond strength of the copper foil-polymeric substrate interface. Conversely, the opposite side of the foil, the processing side, which forms the top surface of copper clad laminate should assure good adhesion between this surface and photo-resist used for image patterning. These two requirements should be balanced against each other, with the view of achieving the optimum functional quality and performance of the PCB.

It will be appreciated that both sides of the foil may be stainproofed to assure a foil's shelf life, but the present invention is especially useful for stainproofing that side of the foil which forms the top surface of a copper clad laminate.

Requirements of the stainproof layer protecting the "processing" or "imaging" side of the foil are:

*- extended shelf-like, i.e., resistance to atmospheric corrosion;

* resistance to oxidation by hot air during lamination and post- bake; and

* easy removability by means of alkaline cleaning, before application of photo-resist, and later brown-oxide treatment.

Major foil manufacturers practice a stainproofing technique capable of forming conversion layers that contain chromates, or chromates and zinc, in a process which provides the face side of the copper foil with a protective layer resistant to both elevated temperature corrosion and to atmospheric "wet corrosion". (See United States Patent Nos. 3,853,716 and 5,447,619) The latter process is particularly advantageous to the foil user because the zinc component of the conversion layers increases foil resistance to wet corrosion as well as in addition to providing heat resistance. When the shiny side of the foil is coated evenly with a very thin layer of zinc, this layer is more active (anodic) than the copper. In the presence of corrosive conditions, e.g. moisture, it is the zinc that corrodes anodically and the copper is cathodic and protected. The continuity (and thus thickness) of the film of zinc is not essential to this type of protection. Even if small pores or discontinuities develop in the coating, the copper does not corrode for some time, until practically all the zinc has disappeared from the immediate location of the pore. Because the zinc is "sacrificed", corroding as it protects the copper, such coatings are often termed sacrificial coatings. In effect, the Cr+Zn stainproof for the shiny side of the foil takes advantage of the phenomenon of galvanic corrosion. When two dissimilar metals (zinc and copper are both present on the shiny side of the foil) in electrical contact with each other are exposed to an electrolyte formed by condensation of water, under ordinary atmospheric condition, a current flows from one metal to another and is called a galvanic current. Obviously, zinc sites form the local anodes, while copper behaves as the cathode. Galvanic corrosion is that part of the corrosion of the anodic member of such a couple directly related to the galvanic current by Faraday's law. Simultaneous additional corrosion taking place on the anode will be called local corrosion; corrosion taking place when there is no contact with a dissimilar metal will be called normal corrosion. When galvanic corrosion takes place, the local corrosion of the anode may be equal to the normal corrosion, or it may be altered. Such a change is often called the difference effect, which may be positive, if the local corrosion decreases when the galvanic current flows, or otherwise, negative. A galvanic current generally causes a reduction in the total rate of corrosion of the cathodic member of the couple. This is called galvanic or cathodic protection. In addition to the electrolytic protection, zinc may retard corrosion of exposed copper by the fact that the solution of zinc (or the products of its corrosion) in water increase the pH so as to retard the corrosion of copper. Zinc film alone would not, of course, offer adequate protection to the shiny side of the foil, especially at high humidities, since the corrosion of zinc itself would be too rapid. Chromates, on the other hand, retard the corrosion of zinc, and thus the combination of chromates and zinc represent an excellent type of protective layer. Stainproofing layers which protect copper foil against oxidation have more function than just extending the shelf-life of the foil. Once the copper clad laminates are ready for further processing, protective layers have to be easily removed from the shiny side of the foil by standard means used in PCB fabrication, i.e. brushing, pumice scrubbing, acid cleaning or microetching. The complete removal of stainproofing compounds assures good adhesion of photoresists, good solderability, unhindered response to the etchants and good acceptability of brown-oxide treatments. Cr+Zn stainproof offers an advanced degree of protecting the foil during the shipping and storing of the foil, but is also easily removed from the surface of the foil by the simple PCB's processing procedures.

Oxide films formed at higher temperatures may pass through well- known interference colors, producing layers of oxide and scale, responsible for "window-frame" oxidation. Metallic copper exposed to hot air produces a film of both cuprous (Cu₂O) and cupric (CuO) oxides which are porous and do not protect the foil from further oxidation. The production of oxide film should not be regarded simply as the union of metallic atoms with oxygen atoms, but rather as an exchange of electrons. Cuprous oxide crystals consist not of copper atoms and oxygen atoms arranged in a lattice, but of cuprous ions (copper atoms with electron missing) and half the number of oxygen ions (oxygen atoms with electron missing) and half the number of oxygen ions (oxygen atoms with two extra electrons each) - thus preserving electrical neutrality.

At a glance it appears that direct oxidation of copper

Cu + ¹/₂ O₂ = CuO does not involve electrochemical mechanisms, but on closer study, it turns out that even this form of corrosion basically depends on an electronic exchange mechanism involving a flow of current. Molecular oxygen is initially absorbed on the metal surface. Here it decomposes into atoms, which ionize according to the reaction V≥- O₂ + 2e — > O. The other half of the circuit is provided by the ionization of the metal: Cu = Cu⁺⁺⁺ 2e. The oxygen and metal ions combine, forming an initial layer of the oxide film. Metal ions continue to be formed at the surface and electrons diffuse through the oxide layer and ionize the oxygen at the surface. The oxide ions diffuse into the oxide layer and react with the metal ions; thus the oxide layer gradually increases in thickness. In some cases it may be the metal that ionizes and the metal ions then diffuse to the surface with the same result.

Stainproofing is perhaps a part of the overall foil manufacturing process which has to be reformulated most often to keep up with the evolution of the techniques used in the manufacture of printed wiring boards. To complicate matters further, the function of the stainproof layers on the shiny side of the foil and the foil's bonding surface, are not the same. It is the fast growth of MLBs with its evolving manufacturing technology that forces foil manufacturers to constantly improve the stainproofing process. In the fabrication of MLBs copper foil is laminated (bonded) to polymeric substrates twice. First thin, double-sided copper clad laminates are produced. These laminates are then subjected to image patterning and etching away of unwanted copper to produce the desired patterns of circuitry. Several layers of double-sided boards prepared in such a manner are then stacked together, with sheets of prepreg (glass reinforced polymeric resin composites) inserted in between to separate dielectrically each inner board from the other. Such a stack of circuit boards and prepreg is then laminated together to form a monolithic multi-layer board. Later, holes are punched or drilled through the board in pre-arranged places and so-called thru-hole plating of copper is used to ensure the electrical interconnection between all layers of coppertrack conductor lines. Increasing sophistication of multi-layer boards (narrower widths of track-lines, narrower spacing between the lines, increasing number of inner layers) create the ever-increasing demand on the quality of copper foil used in the fabrication of MLBs (i.e. HTE foil), and particularly on these aspects of foil's quality that depend on the properties of the stainproof layer that protects the shiny side of the foil from oxidation and corrosion.

To achieve a perfect "registration" of individual inner layers (the lining up of the drilled holes for "thru hole" plating) the original double-sided thin laminates are subjected to a so-called post-bake. Laminates are "baked" in ovens for 15 hours, at a temperature of 325° F to achieve dimensional stability. It has to be appreciated that heat expansion of epoxy prepreg and copper foil are not the same. Post-bake assures that the copper-clad laminate acquires its final and permanent dimensions, before the "art work" and subsequent second stage lamination are accomplished. This dimensional stability, in turn, assures good "registration" of punching or drilling and thru- hole plating. It is obvious that unprotected copper foil, when subjected to contact with hot air for any length of time oxidizes and discolors quite easily. Yet, manufacturers of MLBs require that copper clad laminates, when exposed to post-bake, remain pristine looking and free of any discoloration. Obviously, it is the role of the stainproof layer applied to the foil to protect it from heat oxidation.

Naturally, a question arises of ensuring good bonding between the top surfaces of track lines (the surface which was used for image patterning) and the sheets of prepreg, in the course of second (so-called B stage) lamination.

It is a practice in fabrication of MLBs to subject the inner layer boards, with their patterns of circuitry, to so-called brown oxide treatment, which changes the micro-topography of the top surfaces of the track lines to improve their bondability to the polymeric prepreg. This brown oxide treatment is produced by immersing the boards in an alkaline solution of sodium chlorite which, by its oxidizing action, causes the conversion of metallic copper on top surfaces of exposed copper tracks, into cupric oxide CuO, with a possible admixture of cuprous oxide Cu₂O, depending on the type of the bath and operating conditions. This oxide coating grows in the form of dendritic crystals, perpendicular to the surface of the copper tracks, thus the surface area available for bonding to polymeric substrates is increased and improved "bondability" is achieved. The role of the stainproof layer on the shiny side of the foil is paradox. This layer is absolutely necessary while foil is stored and later on laminated to polymeric substrates. However, this protective layer is a hindrance in fabrication of printed circuit boards. It forms an impenetrable shield between the surface of the foil and the processing chemicals. As an example, photoresists do not adhere to the stainproof layer. Therefore photoimaging cannot be accomplished without first removing the stainproof layer and revealing a surface of pure metallic copper underneath. Clearly, the question arises as to what constitutes a good, easily removable stainproof layer and how to produce it, while not diminishing its protective ability. The answer lies in the proper chemical composition and the thickness of the stainproof layer.

Stainproofing methods and the resultant stainproof layers disclosed in prior art e.g., U.S. Patent 5,447,619 protect the top surface of copper clad laminates against tarnishing, oxidation and discoloration in the course of laminating and post-baking steps of PCB's manufacturing.

However, this protective ability is limited to a temperature not higher than 400° F, and exposure time not longer than 10 hours. These limitations of the prior art allow fabrication of MLBs based on epoxy laminates, which represent a bulk of MLBs, with epoxy glass transition temperature T_g being 325° F.

The most common material for making MLBs is known as FR-4. This is a highly cross-linked brominated epoxy resin reinforced with woven glass cloth.

There is, however, a proliferation of new materials that are now commonly used in the manufacture of MLBs. The materials have glass transition temperatures (T_g) substantially higher than epoxy, and that is because many modern MLBs often operate at elevated temperatures due to the heat output from devices in which they are used. When certain systems are turned off, the board sees a large thermal cycle. As boards become thicker and holes smaller, these thermal cycles result in an increasing threat to the reliability of plated holes. For example, small holes have been shown to fail in 100 or fewer cycles when subjected to multiple thermal cycles up to temperatures near the T_g of the MLB. These cycles can occur when a high- power device is turned on and off. The best solution for systems subjected to such stresses is to use materials with a higher T_g. This explains the rapidly growing use of polyimide resins in the fabrication of MLBs. This group of resins has a T_g in excess of a high degradation temperature. Polyimide resin are used to form a composite material by coating glass fabric to produce MLB substrate that processes like epoxy FR-4.

Fabrication of copper clad polyimide laminates require a 450°F laminating temperature, as compared with 325°F for epoxy, and a laminating time of 8 hours, as compared with 3 hours for epoxy.

Polymers such as polyetherimides, polyamide-imide, polyphenylene sulfide have glass transition temperatures in excess of 480°C, while Union Carbide's Udel (polysulfone) resin requires a laminating temperature of about 700°F.

In addition, post baking operations are now practiced commonly in order to improve dimensional stability of printed circuit boards. By this practice copper clad laminates, e.g., epoxy based, are typically kept in ovens at temperatures of 380° for 16 hours.

The idea is that any dimensional changes of copper clad laminates, shrinkage, warp, etc., will occur in the course of the post-bake. Thus the subsequent processing in the fabrication of MLB's will produce boards that will be faultless in terms of registration and precision.

Thus, there is a need for a stainproofing method capable of endowing the processing side of copper foil, which forms the top surface of copper clad laminate, with an ability to protect the copper surface against oxidation by hot air resulting from the use of materials with high T_g in the fabrication of MLBs.

When copper foil whose processing side is stainproofed according to prior art, e.g. the method disclosed in U.S. Patent 5,447,619, is subsequently laminated to FR-4 prepreg, the protection afforded by the method is excellent, and no visible oxidation, tarnishing and discoloration takes place. Even if the laminate is post-based in the oven for 16 hours to simulate PCB industry procedure, the top surface of the laminate remains pristine.

If, however, the above foil is laminated to polyimide prepreg, with an attendant temperature of 450°F and a duration of 8 hours, the oxidation of copper surface is very obvious. The formation of copper oxides causes discoloration of the top surface of the laminate, particularly along the periphery of the laminate, since it is there that air can most easily come in contact with the copper surface during lamination. This is referred to in the trade as "window frame oxidation," and the PCB industry does not easily tolerate it. Copper oxides formed at elevated temperatures may pass through several orders of well-known interference colors. These oxide films are not merely a cosmetic nuisance, but also a hindrance in further PCB processing steps.

To solve these problems, the surface of heat tarnished, copper-clad laminates can be cleaned with mineral acids prior to use of the laminates in the manufacture of printed circuits; however, these cleaning procedures are both costly and troublesome.

Therefore, there is a need for an improved stainproofing system to better protect copper foil surfaces from corrosion which occurs during the lamination process.

The stainproof films formed according to prior art protect the surface of copper adequately at the temperatures encountered in the course of fabricating and post-baking epoxy laminates, but fail when these temperatures are exceeded.

As is well known, formation of such protective layers is achieved by

* forming an electrolyte comprising anions of hexavalent chromium and cations of zinc;

* immersing copper foil in said electrolyte;

* rendering copper foil cathodic while in electrolyte, and passing electric current (D.C.) through the electrolyte to form on the surface of the foil, by means of electrolytic reduction, a protective layer composed of the hydrated compounds of trivalent chromium and metallic zinc. Thus, the result of the stainproofing method is a protective layer, adherent to the copper surface, water insoluble amorphous film, which, due to the structure of hydrated trivalent chrome compounds, is tight and hermetic, thus ensuring an advanced degree of insulation of the copper surface from the hot air environment. Due to the presence of metallic zinc the film is capable of providing sacrificial protection to the copper surface.

Chromium hydroxides that form the bulk and structure of protective layer are usually referred to as Cr(OH)₃, CrH₃O₃, Cr(OH)₃ • 3H₂O, and Cr₂O₃ • 9H₂O.

A possible mechanism of formation of protective layer might be:

(1) Adsorption of dichromate ion together with two protons as ions of opposite charge.

(2) Transition of two electrons from the adsorbent.

(3) Attraction of two further protons.

(4) Elimination of water

After triple repetition, this cycle should lead to the reduction of Cr₂O₇ ^2" to Cr₂O₃ without causing hydrolysis of the — Cr— O— Cr— bond.

The zinc component of such protective layers offers sacrificial protection, as well as prevention of oxidation and tarnishing by selective oxidation (oxidation being a form of corrosion). Zinc, when in contact with more a noble (less reactive) metal in the electromotive series, such as copper, will be oxidized by hot air, thus protecting copper from oxidation. Thus, when used together, hydrated compounds of trivalent chromium and zinc can form thin, virtually invisible films capable of protecting copper surface against various forms of corrosion, including oxidation by hot air.

But, naturally, there is a limit to this protective ability.

Hydrated compounds of Cr⁺³ will be subject to de-hydration at elevated temperatures, and they begin to dehydrate at temperatures as low as 150°F. The loss of water from the protective layer, causes, in turn, its cracking and the loss of structural integrity.

This cracking in itself is not fatal, since even though access of hot oxygen to the copper is now facilitated, it is the zinc component of the layer, as a baser, more reactive metal, that gets oxidized rather than copper.

The limitation of the prior art comes from the fact that in the process of sacrificially protecting copper from oxidation, zinc atoms preferentially oxidize, and thus are consumed by Zn+¹/4O₂→ZnO process, and zinc oxide does not, naturally, have an ability of sacrificial protection, and finally the oxidation of the copper surface takes place when the supply of atoms of metallic zinc is exhausted.

One might suggest that perhaps the situation could be improved by increasing the zinc content in the protective layer, but this does not really work, since the "gel" of hydrated compounds of trivalent chromium can "hold" and "contain" only a finite amount of zinc atoms in a state of homogeneous dispersion. If both the amount of Cr⁺³ compounds and Zn were increased this would lead to the formation of a thicker protective layer which would be less easily removed by diluted alkaline solutions used in PCB processing.

We believe that the stainproofing methods disclosed in the prior art provide for the protection of copper surfaces against hot air oxidation only up to about 400 °F as a result of the fact that zinc is too reactive. Because of this, zinc is too easily consumed (or selectively oxidized), while protecting copper, leading to the limitations of this protection. It has been stated that the formation of zinc oxide at 400 °C (750 °F) follows a straight line when the square of weight increase is plotted against time. The slope of this line is 0.88 X 10-¹⁰ (grams/sq cm)² hr¹.

Thus, the use of zinc for sacrificial protection has been confined, by and large, to the protection of nobler metals, particularly steel, against wet corrosion, rather than oxidation by hot air.

Cadmium, the only other metal capable of sacrificial protection, and less reactive than zinc, is practically banned from industrial use for ecological reasons.

This is why we have searched for an alloy, rather than a single metal, which, when incorporated into a conversion layer of trivalent chromium, provides a long-lasting protection of copper surfaces against oxidation by hot air at temperatures of 400°F and higher.

SUMMARY OF THE INVENTION

A primary object of the present invention is a stainproofing layer for copper foil which provide improved protection from oxidation at elevated temperatures, e.g., above 400°F, and to overcome drawbacks of prior art stainproofings. Other objects of the invention may become apparent from the following description and practice of the invention.

The above and other objects of the invention may be achieved by electrodepositing on one or both sides of a copper foil a corrosion-resistant protective layer containing chromium, zinc and nickel, preferably from a phosphoric acid solution having a pH in the range of from 3 to 4.5 and containing chromium ions, zinc ions and nickel ions.

DESCRIPTION OF PREFERRED EMBODIMENTS

As a result of our work, we have found a combination chromium, zinc and nickel to be surprisingly effective in achieving the object of the invention. We believe that nickel is well-suited for this purpose because it is a metal:

* whose position is electromotive series is half-way between copper and zinc Standard Electrode

Potential E°(V),25°C

Cu/Cu²⁺ 0,337

Ni/Ni²⁺ -0,250

Zn/Zn²⁺ -0,763

* capable of slowing down the rate of consumption of zinc in the process of sacrificial protection of copper surface of copper surfaces.

We believe that nickel-zinc alloy particles suspended in the film of hydrated compounds of Cr⁺³ corrodes (oxidizes) more slowly than zinc alone, which is attacked by hot oxygen too rapidly, and that is why the stainproofing layer and stainproof method of our invention afford protection of copper surfaces against oxidation at temperatures in excess of 400°F. Providing the "processing" or "imaging" side of the foil (it can be either shiny or matte side of the foil, as discussed above) with the stainproofing of our invention involves the simultaneous reduction of hexavalent chromium anions to hydrated compounds of trivalent chromium and the codeposition of zinc and nickel. As a result of this electrolytic process a protective stainproof layer is formed over the copper surface. This layer offers the protection of the copper surface against atmospheric corrosion (thus assuring a very long lasting shelf life), as well as outstanding protecting against oxidation due to elevated temperatures encountered in laminating and post-baking operations.

The factor which makes the codeposition of chromates and metallic zinc possible is the pH of the electrolyte. At low pH values, e.g., pH2 (which is the value of 3 g/l CRO₃), hexavalent chromium compounds are very strong oxidants, thus counteracting cathodic reduction of zinc and nickel. At such pH values the standard electrode potential Eo has a value of +1.33v for the reaction

Cr₂O₇ ^2" + 14H⁺ 6e = 2 Cr³" + 7 H₂O Under such conditions codeposition of zinc is impossible. In basic solutions, chromates rather than dichromates, are the prevailing species and are by and large, much less oxidizing. The reaction:

CrO₄ ^2" + 4 H₂O + 3e = Cr(OH)₃ + 5 OH^" E₀ = -0.13v is much closer to the standard electrode potential of zinc (E₀ = -0.76) and of nickel

(E₀ = -0.25v), and thus enables codeposition of hydrated compounds of trivalent chromium with metallic zinc and nickel to become possible. The pH of the stainproof electrolyte is about 4. The pH4 is, of course, far from basic, but it refers to the bulk of the electrolyte. The PH at the foil-solution interface exceeds a pH value of 7. Whenever there is a flow of current there is necessarily a reduction of some chemical species at the cathode (foil). In the present process the cathodic reactions are: reduction of Cr⁶" (written above) reduction of zinc Zn²⁺ + 2e = Zn reduction of nickel Ni²⁺ + 2e = Ni reduction of water 2H₂O + 2e = 2 OH^" + H₂. It is the last reaction, e.g. evolution of hydrogen at the foil surface which is responsible for the local increase of pH and thus allows for simultaneous precipitation of the chromate layer and the deposition of zinc. The foregoing outlines the basic principles of the zinc-chromates stainproof technique, which has been found to successfully avoid window-frame oxidation in the fabrication of copper clad laminates.

However, since its inception, the printed circuit industry has kept evolving and two trends are very obvious:

Growth of the multilayer segment of the PCB market; and

Decline in the use of mechanical methods of cleaning the laminates

(brushing) in favor of chemical cleaning (micro etching). This calls for the formulation of a stainproof process that produces conversion layers which are endowed with good protective abilities, and which can be easily removed.

We have considered what constitutes a good, "processible" stainproof layer to meet today's needs and how to produce it, while not endangering the shelf life and corrosion resistance of the foil. By studying the chemical composition of experimental stainproof layers, we have found, using instrumental methods of surface analysis, that stainproof layers containing chromium, zinc and nickel are capable of good protective action while also being easily removable by immersion in alkalies. The weight ratio of compounds of tri-valent chromium to metallic zinc should be at least 1 :1 , preferably at least about 2:1 , and the weight ratio of metallic nickel to metallic zinc should be in the range of from about 1 :10 to about 1 :1. Such layers which contain, by weight, about 10-20% of chromium, about 20-40% of zinc and about 4-8% of nickel (the balance is water) have been found to be preferable. The ratio of chromium to the combined content of zinc and nickel is also very important. Zinc and nickel typically are dispersed as particles of Zn/Ni alloy in the film of trivalent chromium hydrated gel. A high proportion to zinc in the layer assures that the layer is easily attacked by alkalies; due to the amphoteric character of this metal it dissolves in alkaline solutions, such as an aqueous sodium hydroxide solution, forming, e.g., sodium zincate with the copious evolution of hydrogen. Since the atoms of zinc and nickel are uniformly dispersed within the lattice of the chromium hydroxide component of the protective layer, alkaline cleaners attack and dissolve atoms of zinc, hydrogen is formed and this combined affect of attack and "fizzing" lifts chromium compounds off the surface of the foil leaving it, after being rinsed, pure and clean, and ready for further PCB processing.

The stainproofing layers of the present invention may be produced by passing a direct electric current from an anode to copper foil (cathode) through an aqueous phosphoric acid solution containing chromium ions, zinc ions and nickel ions to electrodeposit the stainproofing layer on the foil. Electrolyte composition and plating parameters we have found to be especially effective are shown in the following tables, wherein the electrolyte is an aqueous solution containing the indicated amounts in grams per liter (g/l), of the indicated ingredients, and the current density of direct electric current is indicated in amperes per square foot (A/ft²).

Electrolyte

Plating Parameters

The following Examples illustrate advantages achieved by the present invention compared to prior art stainproofing.

Examples

A web of "base" (or "raw") copper foil, 35 microns thick (so-called one ounce foil in terms of weight per square foot of surface area) was produced by means of electrodeposition of copper on a rotating drum-cathode, using the electrolyte, grain refining agents and plating parameters described in U.S. Patent No. 5,863,410 to Yates et al., which is incorporated herein by reference, except that only primary anodes were used, and the second anode was not used.

This "base" foil had one top surface which was smooth or shiny, and another opposite top surface which was "matte" because of its complex micro-topography. The second surface was composed of micro-peaks and micro-valleys, which together formed the matte side's micro-roughness.

A sample of the base foil described above was, in turn, passed through a "treater" machine in order to electrodeposit on the shiny side of the foil a plural-layer (copper dendritic layer, copper gilding layer, and a barrier layer) bonding treatment, and to provide the matte side of the treated foil with an easily removable stainproofing layer.

The multi-layer bonding treatment applied to the shiny side !of the foil employed the techniques, plating parameters, and the electrolytes described in U.S. Patent No. 4,572,768 to Wolski et al., which is incorporated herein by reference, to produce a treated side.

The matte side of the foil was provided with an electrodeposited heat resistant, easily removable (by means of dissolution in 5% solution of sodium or potassium hydroxide) stainproofing film according to this invention employing the above most preferred electrolyte and plating parameters.

The foil processed as described above is designated SAMPLE 1.

Subsequently, another sample of the base foil described above was in turn passed through a "treater" machine in order to provide the matte side of the foil with a plural-layer (copper dendritic layer, copper gilding layer and a barrier layer) bonding treatment, and to provide the shiny side of the foil with an easily removable stainproofing layer.

This multi-layer bonding treatment applied to the matte side of the foil employed the techniques, plating parameters, and the electrolyte described in U.S. Patent No. 4,572,768 to Wolski et al., which is incorporated herein by reference, to produce a treated matte side.

The shiny side of the foil had electrodeposited thereon a heat resistant, easily removable (by means of dissolution in 5% solution of sodium or potassium hydroxide) stainproofing film according to this invention using the above most preferred electrolyte and plating parameters. The foil processed as described above is designated as SAMPLE 2.

The stainproofing layers of SAMPLES 1 and 2 were examined and found to comprise metallic zinc and nickel and chromium compounds and to have a zinc to nickel and to chromium ratio of 2.1 : 0.4 : 1.0.

Subsequently a third web of the base foil described above was passed through the treater machine in a manner identical to SAMPLE 1 , except that the stainproof layer electrodeposited on the matte side was formed according to U.S. Patent 5,447,619 to Wolski et al.

This foil is designated as SAMPLE 3.

Subsequently a fourth web of the base foil described above was passed through a treater machine in a manner identical to SAMPLE 2, except that the stainproof layer was formed according to U.S. Patent 5,447,619 to Wolski et al. This foil is designed as SAMPLE 4.

The stainproofing layer of SAMPLES 3 and 4 were examined and found to comprise metallic zinc and chromium compounds and to have a zinc to chromium ratio of 2.0 : 1.0.

SAMPLES 1 , 2, 3, 4 were laminated, through the side having the bonding treatment applied, to FR-4 epoxy prepreg which involved exposure to hot air at a temperature of 340°F for 2 hours. The opposite side of the foil formed the top surface of the copper clad laminate.

After lamination was completed, all the copper-clad laminates were examined for appearance of top copper surfaces. All laminates were pristine, with no signs of oxidation.

Then the laminates prepared with SAMPLES 1 , 2, 3, and 4 were post- baked, in an oven containing air, for 16 hours. After the post-bake was completed, the laminates were examined for appearance of top copper surface.

All four samples were found to be in excellent condition.

SAMPLES 1 , 2, 3, 4 were then laminated through the side having the bonding treatment applied to a polyimide prepreg, a process that involved a temperature of 450°F and duration of 8 hours.

After lamination was completed the resulting clad laminates were examined for appearance of their top surfaces.

Laminates prepared with SAMPLES 1 and 2 were entirely free of oxidation, while laminates prepared with Samples 3 and 4 displayed "window frame oxidation."

As an arbitrary test, SAMPLES 1 , 2, 3, and 4 were placed in an oven heated to 600 °F, stainproofed side-up, and they were observed through a window of the oven.

SAMPLES 1 and 2 resisted oxidation for about 3 hours, while Samples 3 and 4 were heavily oxidized within 5 minutes.

Thus, we believe that the present invention enables a stainproofing which protects copper foil from oxidation at higher temperatures than possible with prior art stainproofing, while at the same time being easily removable from the foil.

Having described preferred embodiments of our invention, it is to be understood that variations and modifications thereof falling within the spirit of the invention may become apparent to those skilled in the art, and the scope of our invention is to be determined by the appended claims and their equivalents.

Claims

WHAT IS CLAIMED IS:

1. Copper foil useful in the manufacture of copper-clad laminates, which comprises:

(a) electrodeposited copper foil having a first side with a matte surface and an opposite second side with a shiny surface; and

(b) a corrosion-resistant protective layer electrodeposited on one or both sides of the copper foil, said protective layer containing one or more chromium compounds, zinc and nickel.

2. The copper foil of claim 1 , wherein the protective layer is formed of a trivalent chromium hydrated gel having dispersed therein particles of a zinc- nickel alloy.

3. The copper foil of claim 1 , wherein the protective layer contains, by weight, about 10-20% chromium, about 20-40% zinc and about 4-8% nickel.

4. The copper foil of claim 1 , wherein the protective layer has a chomium:zinc:nickel ratio of from about 1 :2:0.4 to about 1:4:0.8.

5. The copper foil of claim 3, wherein the zinc and the nickel are present in the form of a zinc-nickel alloy.

6. The copper foil of claim 1 , further including a copper bonding treatment electrodeposited on one or both of the first side of the foil, and wherein the protective layer is electrodeposited on one or both of the bonding treatment and the opposite second side of the foil.

7. The copper foil of claim 6, wherein the bonding treatment includes a zinc or zinc alloy barrier layer electrodeposited on the copper bonding treatment, and wherein the protective layer is deposited on the barrier layer.

8. The copper foil of claim 6, wherein the protective layer is also electrodeposited only on the side of the foil opposite the side having the bonding treatment.

9. A copper-clad laminate comprising the copper foil of claim 1 laminated to a polymeric substrate.

10. A copper-clad laminate comprising the copper foil of claim 8 laminated to a polymeric substrate through the bonding treatment.

11. A process for electrodepositing a protective layer on metal foil, which comprises:

(a) immersing the foil in an electrolyte solution containing chromium ions, zinc ions and nickel ions; and

(b) passing an electric current from an anode through the electrolyte to the foil, under electrodeposition conditions effective to electrodeposit on the foil a protective layer containing chromium, zinc and nickel.

12. The process of claim 11 , wherein the electrolyte is an aqueous phosphoric acid solution having a pH of from about 3.5 to about 4.0.

13. The process of claim 12, wherein the electrolyte contains from about 1.0-1.5 g/l Cr0₃, from about 0.5 to about 1 g/l Zn, and from about 0.2 to about 0.5 g/l Ni.

14. The process of claim 13, wherein the electrodeposition conditions include a current density of from about 0.5 to about 20 amperes per square foot, and a plating time of from about 1 to about 10 seconds.