Electrolysis: processes – compositions used therein – and methods – Electrolytic coating – Coating moving substrate
Reexamination Certificate
1999-10-22
2001-08-07
Wong, Edna (Department: 1741)
Electrolysis: processes, compositions used therein, and methods
Electrolytic coating
Coating moving substrate
C205S182000, C205S152000, C205S111000, C205S112000
Reexamination Certificate
active
06270648
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to electrolytic copper foil, to a process and apparatus for producing such copper foil having a matte side provided with a bond-enhancing treatment, and to a laminate using such foil.
BACKGROUND OF THE INVENTION
A conventional method for the production of copper foil for electronic application, i.e., copper-clad laminates for printed circuit boards, typically involves two steps, first, electrodeposition or plating, of a “base” or “raw” foil on a rotating drum-cathode and, second, passing the “base” foil through a “treater” machine, in order to provide the matte side of the foil with a bondable surface suitable for bonding to a polymeric substrate. The latter step is sometimes called the bonding treatment. Traditionally, these two steps are separated by the foil manufacturers, since they seem to be mutually exclusive in that formation of base foil calls for a concentrated, hot copper sulfate/sulfuric acid electrolyte, in order to yield strong, ductile and compact depositions which form the body of the foil, while the bonding treatment usually requires a more dilute and colder electrolyte which yields fragile, powdery deposits whose role is to enhance the true surface area of the matte side of the foil and this enhance the bonding ability of the foil.
In the typical process, the first step, fabrication of the base foil, or “core,” a primary objective is to impart to the bulk of the foil the combination of physical, metallurgical and electrical properties desired in the printed circuit industry. Obviously, those properties are determined by the microstructure, which in turn is determined by purity and conditions of the plating process. Typical properties of the core of the foil sought by printed circuit board manufacturers are suitable tensile strength, yield strength, elongation, ductility and resistance to fatigue. Many of the desired properties relate to the maximum load the material may withstand before failure, and are usually derived from stress-strain curves. Similarly, electrical conductivity is considered an important property of copper foil. All these properties of copper foil depend on the foil's microstructure, but particularly on the microstructure of the core of the foil. This microstructure, responsible for foil's properties, is in turn determined by the electrodeposition conditions.
Similar to other materials used in high technology applications, copper foil is a composite; i.e., it has a near-surface region with properties differing from those of the bulk material. Thus, the bulk of the copper foil (core) serves in printed circuit boards as the conductor of electricity. The bonding side of the foil is responsible for promoting a permanent bond to the polymeric dielectric (insulating) substrate or prepreg, e.g., glass fabric impregnated with epoxy resin.
The above-mentioned first manufacturing step utilizes a large cylindrical drum-cathode which rotates, partially immersed in a copper sulfate-sulfuric acid electrolyte. The drum cathode is adjacent to and facing a pair of curved anodes, which may be formed of lead, lead, lead-antimony, platinized titanium, iridium or ruthenium oxides. Both the drum and the anodes are connected electrically to heavy buss-bars to a DC power source, and currents of up to 50,000 amps or more are commonly used. As the drum rotates in the electrolyte, an electrodeposit of copper forms on the drum surface, and as the latter leaves the electrolyte, the electrodeposited copper is continuously stripped from the rotating drum in the form of thin foil, which is lit to size and wrapped around a take-up roll. The outer surface of the drum is usually formed of titanium or other durable metal.
Foil produced in such a process, prior to being treated, is usually referred to as raw foil. The raw foil is pale pink in color and has two distinctly different looking sides—a “shiny side”, the side which was plated onto the drum surface and then stripped is quite smooth, while the other side, the side which was facing toward the electrolyte and the anodes, is referred to as the “matte” side, since it has a velvety finish, due to differences in the growth rate of differing crystal faces during electrodeposition of the “base” foil. The matte side surface, at this stage, has a very fine scale micro-roughness and a very specific micro-topography. Viewed under high magnification of a scanning electron microscope, it is composed of peaks and valleys. The peaks are closely packed cones or pyramids, the height, slant, packing and shape of which depend, as is well known, upon closely controlled independent variables of foil thickness, current density, electrolyte solution composition and temperature, as well as upon the type and concentration of the addition agents in the electrolyte and the like.
The surface quality (profile) of the matte side of the base foil determines its suitability for application as a cladding for copper-clad laminates destined for fine line circuitry and the multi-layer printed circuit boards. The criteria of suitability depends upon the quantitative evaluation of the matte side's surface roughness. A variation which gives useful information about the surface is called “Rz,” which is the average deviation from the mean line of the five highest peaks and the five lowest valley within the roughness sampling length.
The matte side of the base foil provides the basic shape of the foil surface for embedding in the resin of a substrate to promote adhesion in the copper clad laminates used in the manufacture of printed circuit boards (PCB's).
While the matte side of the foil has a certain micro-roughness it is not nearly good enough to satisfy industry need for foil's bendability. This is why copper foil manufacturers use the second manufacturing step in which a surface bonding treatment is applied to the matte side of the base foil. The term “bonding treatment” is universally used to describe the changing of the morphology of the matte side of the base foil to make it suitable for bonding to laminate resins.
The bonding treatment operation is conducted in machines called “treaters” where in rolls of raw foil are unrolled in a continuous manner and fed into the treater by means of driven rollers (similar to the way in which a web of paper is handled in a printing machine), rendered cathodic by means of contact rollers and passed in a serpentine fashion through a plurality of plating tanks, facing, in each tank, a rectangular anode. Each tank has its own supply of appropriate electrolyte and its DC power source. Between the tanks the foil is thoroughly rinsed on both sides. The purpose of this operation is to electrodeposit on at least one side of the foil, usually the matte side, copper microprojections of complex shape which ensure that the foil will be firmly anchored to the base polymeric materials used in fabricating the copper clad laminates.
High peel strength (the force necessary to pull apart the copper foil and the supporting insulating substrate material) 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 substrate 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 throughout the service life of the PCB.
This bonding operation is carried out 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 heated pressing plates, and the two materials are pressed together under high pressure. At elevated temperatures the resin liquefies and is forced, by the pressure, to flow into the micro-irregularities of the foil surface, and both materials are firmly
Bodendorf Keith
Cheng Chinsai T.
Gaskill George
Shah Ajesh
Wolski Adam M.
Finnegan Henderson Farabow Garrett & Dunner L.L.P.
Wong Edna
Yates Foil USA, Inc.
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