Stock material or miscellaneous articles – All metal or with adjacent metals – Foil or filament smaller than 6 mils
Reexamination Certificate
1999-09-13
2001-05-01
Zimmerman, John J. (Department: 1775)
Stock material or miscellaneous articles
All metal or with adjacent metals
Foil or filament smaller than 6 mils
C428S612000, C428S675000, C428S935000, C205S111000, C205S170000, C205S182000, C205S269000
Reexamination Certificate
active
06224991
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to electrodeposited copper foil for use in the manufacture of copper-clad laminates for printed circuit boards (PCB's), the foil having a copper bond-enhancing treatment electrodeposited on a bonding side of a base foil and a thin cobalt barrier layer electrodeposited on the treatment wherein the cobalt is deposited as a continuous layer on micro-peaks and micro-valleys forming the surface of the treatment. This invention also relates to a process for producing such foil and an electrolyte useful in such process.
BACKGROUND OF THE INVENTION
Copper foil used in the manufacture of PCB's is typically produced on a rotating drum cathode machine partially immersed in a sulfuric acid/copper sulfate electrolyte, wherein an electric current is passed through the electrolyte from one or more anodes to the drum cathode to electrodeposit the copper foil on the surface of the cathode. The resulting foil had one side with a relatively smooth (shiny) surface, often referred to as the drum side, and the opposite side, which has a velvety (matte) surface, is often referred to as the electrolyte side. The microprofile of the electrolyte side of the raw foil is formed of micro-peaks and micro-valleys when observed under a microscope.
In the field of electrodeposited copper foil destined for printed circuits, “barrier layer” is a term to describe a metallic coating plated over a copper bond-enhancing treatment deposited on a bonding side of a copper raw, or base, foil.
Thus, the barrier layer forms the outer perimeter of the bonding treatment, and as such interfaces with polymeric substrates in the manufacture of copper clad laminates. The basic raw material for the manufacture of printed circuits is a laminate clad with copper foil which comprises of a thin copper foil firmly bonded to a polymeric, dielectric (insulating) substrate material. This “bonding” operation is carried in laminating plants and involves heating and cooling cycles. Sheets of copper foil are laid upon sheets of “prepreg” (e.g. glass fabric impregnated with epoxy resin). Both materials are placed in a hydraulic press, having pressing plates which are heated, while the two materials are pressed together (high psi). At selected temperatures, the resin liquefies and is forced, by the pressure, to flow into the micro-irregularities of the foil surface. This is followed with a second cycle, when both materials are cooled, while the pressure is maintained. The resin solidifies in the irregularities on the foil surface and both materials are firmly bonded together and are very difficult to pull apart. The “peel strength” between both materials is high, because the bonding side of the copper foil is provided with the bond-enhancing treatment. High peel strength is a characteristic of the highest importance since the mechanical support of the circuit elements, as well as the current carrying capability of PCB's, is provided by the copper foil - polymer joint. It is essential that the foil is bonded very tightly and securely to the laminate and also that such an adhesive joint can withstand all the manufacturing steps in PCB's fabrication without a decrease of the initial adhesion, which, moreover should remain constant through the service life of the PCB.
The bonding treatment, usually formed in two steps, typically includes a first dendritic copper deposit, followed by an encapsulating, or gilding, layer of copper, as disclosed in U.S. Pat. Nos. 3,857,681, 4,572,768 and 5,207,889, and is composed of copper, while the barrier layer deposited on the treatment is composed of zinc or brass, a zinc-nickel alloy, or another metal essentially chemically inert to the polymeric substance, which, in the process of lamination is in a semi-liquid, flowing state to effect the bonding to the treated side (surface) of the copper foil. The most common polymeric substrate used in the fabrication of printed circuits is a glass fabric impregnated with epoxy resin. Epoxy curing agents (catalysts, hardeners) belong usually to the class of organic derivatives of ammonia, are highly reactive chemically, and typically they are: amine complexes, e.g., tertiary amines, polyamines, aromatic polyamines, etc. Probably the most known epoxy curing agent is dicyandiamide NH
2
C(HH)(NHCN).
Reactivity of copper with ammonium compounds (amines, amides) is well known, and explains the need for the barrier layer. The purpose of the barrier layer is to prevent direct copper resin contact.
If the bonding treatment composed of copper only (no barrier layer) is subjected to lamination with an epoxy substrate, the metallic copper reacts with the amino catalysts present in the resin. These reactions are particularly harmful to the quality of the printed circuits. They create moisture at the interface between the copper and the resin, causing harmful effects of measling and possibly de-lamination. In search of excellent dimensional stability, dielectric properties and long service life of PCB's, there has been a growing importance of new polymeric substrates that are superior in these respects to epoxy based materials.
Most of the new materials that are now commonly used in the manufacture of multilayer printed circuit board (MLB's) have glass transition temperatures (T
g
) substantially higher than epoxy. Fabrication of copper clad polyimide laminates require a 450° F. laminating temperature compared with 325° F. for epoxy, and a laminating time of 8 hours, 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, again 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° F. for 16 hours.
The idea is that any dimensional changes of copper clad laminate, shrinkage, warp, etc., will occur in the course of the post-bake. Thus the subsequent processing ir the fabrication of MLB's will produce boards that will be faultless in terms of registration and precision.
The practices described above impose very harsh conditions on the copper foil-polymer interface, conditions that threaten the forces of adhesion that join the two materials. Since the outer surface of the bonding treatment is a barrier layer, the harsh conditions of the interface particularly threaten, chemically, integrity and performance of
Barrier layers on polyimide-grade treatments have to withstand much higher laminating and post-bake temperatures, compared to the treatments destined for epoxy applications. High temperature at the metal-polymer interface can subject the metal surface to oxidation, with the attendant partial loss of adhesion. A well designed barrier layer will be self-protected, along with the underlying all-copper treatment, from heat oxidation and the loss of bond.
A good barrier layer should offer reasonable permanence and survival ability of the adherence under various conditions encountered during a PCB's manufacturing steps, as well as during PCB's service life. Successful stain proofing is synonymous with forming a stable film on the surface which promotes good adhesion and improves resistance to disbonding by various chemical environments, thus assuring durability to the foil-resin interface. During the manufacturing process of printed circuits, barrier layers are attached in a variety of ways. Narrow tracks of the foil are exposed to etching solutions, acids and hot water rinses, thermal shocks, etc. The chemicals and/or water tend to penetrate from the sides underneath the lines into the foil-polymer interface. If that happens, and it always happens to a degree, the real “functional” width of the track bonded to the polymer is diminished and thus the peel strength of the conductor line is diminished. It is t
Cheng Chinsai T.
Dufresne Paul
Gaskill George
Shah Ajesh
Wolski Adam M.
Finnegan Henderson Farabow Garrett & Dunner L.L.P.
Yates Foil USA, Inc.
Zimmerman John J.
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