Electrolysis: processes – compositions used therein – and methods – Electrolytic coating – Coating is dendritic or nodular
Reexamination Certificate
1999-09-13
2002-04-16
Zimmerman, John J. (Department: 1775)
Electrolysis: processes, compositions used therein, and methods
Electrolytic coating
Coating is dendritic or nodular
C205S296000, C205S297000, C428S607000, C428S612000, C428S675000, C428S687000, C428S935000
Reexamination Certificate
active
06372113
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to electrolytic copper foil useful in the fabrication of printed circuit boards (PCBs), especially multi-layer printed circuit boards (MLBs), to a process for producing such foil, and to copper-clad laminates made with such foil. More particularly, this invention relates to such foil having an electrodeposited copper corrective/bond-enhancing layer applied to a matte surface of the foil.
BACKGROUND
Like many other materials used in high technology applications, electrodeposited copperfoil 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 copper foil (core) serves in PCBs as a conductor of electricity. One of the outer surfaces of the foil serves as a substrate for image patterning and soldering to ensure necessary electrical connection between components, while the opposite side of the foil is responsible for permanently bonding conductor lines to the polymeric substrates. In addition, in the case of copper foil destined for fabrication of 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 necessary for B-stage lamination.
A conventional process for making electrolytic copper foil consists essentially of two steps: first, electrodeposition or plating of a “base” foil on a rotating drum-cathode and second, passing the foil through a “treater” machine in order to provide one of the outer surfaces of the “base” or “raw” foil with a bonding treatment suitable for bonding to a polymeric substrate. 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 the difference 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 cones' height, slant, packing and shape depend, as is well known, upon closely controlled independent variables of foil thickness, current density, solution composition and temperature and the type and concentration of the addition agents and the like.
A choice exists as to whether the shiny side or the matte side 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 PCB 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 might 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, as will be explained, toward quality and reliability of finished MLB.
While both rigid boards and MLB circuits these days conform to the needs of miniaturization and are manufactured with copper foil conductor or track lines that are as narrow as 5 mils, or less, it is MLBs that are the fastest growing segment of the industry, since they permit achieving the highest functional density in electronic packaging. The considerations that govern the choice whether bonding treatment should be applied to the matte side of the foil (in which shiny side of the foil forms the outer surface of the laminate), or the shiny side of the foil (in which case matte side of the foil forms the outer surface of the laminate) depend on the fundamental roles the two outer surfaces of copper foil play in the fabrication of PCBs.
The side of the foil which is provided with the bonding treatment should assure the highest possible bond strength of copper foil-polymeric substrate interface. Conversely, the opposite side of the foil which forms the top surface of copper clad laminate should assure good adhesion between this surface and photo-resist. These two requirements should be balanced against each other, with the view of achieving the optimum functional quality and performance of PCB.
The basic raw material for manufacturing printed circuits is a laminate clad with copper foil, i.e., thin copper foil firmly bonded to a substrate, e.g., a polymeric, dielectric (insulating) base material. This “bonding” operation is accomplished in laminating plants and involves heating and cooling cycles. Sheets of copper foil are laid on sheets of “prepreg” (e.g., glass fabric impregnated with epoxy resin). Both materials are placed in a hydraulic press with heated pressing plates and pressed together under high pressure. At elevated temperatures, the resin liquefies and is forced, by pressure, into micro-irregularities of the foil surface. This is followed by a second cycle where both materials are cooled while pressure is maintained. The resin solidifies in the irregularities of the foil surface to firmly bond materials together, making them very difficult to pull apart.
The “peel strength” between both materials, i.e., a mechanical force required to separate two bonded materials, is increased if the bonding side of the copper foil is provided with a bonding treatment. Such bonding treatment technology and processes developed by major copper foil manufacturers are well known.
High peel strength is an extremely important characteristic since the mechanical support of circuit elements, as well as the current carrying capability of PCBs, is provided by the copper foil/polymer joint. It is essential that the foil is bonded very tightly and securely to the substrate so that the adhesive joint can withstand all PCB manufacturing steps and remain constant throughout its service life—without a decrease in initial adhesion strength.
Traditionally, in rigid, single sided or double sided copper clad laminates the “shiny” side (drum side) of the foil represents the metallic side of copper clad laminate, while the matte side (electrolyte side), responsible for permanently bonding conductor lines to the polymeric substrates. Since the highest possible bond strength (peel strength) was the most important desideratum in rigid boards technology, it was logical to combine the original micro-roughness of the matte side of the foil with the further micro-roughening effect of the electrodeposited bonding treatment.
In the case of multilayer printed circuit boards, the considerations of bondability are more complex. 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 stacked together, with sheets of prepreg (glass reinforced polymeric resin composites) inserted between in order to dielectrically separate adjacent boards form one another. Such a stack of circuit boards and prepreg is then laminated together, in the so-called “B-stage lamination”, to form a monolithic multi-layer board. Later, holes are punched or drilled through the board in prearranged placed and so-called thru-hole plating of copper is used to ensure the electrical interconnection between all layers of copper-track conductor lines. Obviously, both outer surfaces of copper foil are subjected to bonding in the fabrication of MLBs and the bond strength of b
Cheng Chinsai T.
DuFresne Paul
Gaskill George
Shah Ajesh
Wolski Adam M.
Yates Foil USA, Inc.
Zimmerman John J.
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