Stock material or miscellaneous articles – All metal or with adjacent metals – Foil or filament smaller than 6 mils
Reexamination Certificate
1999-09-29
2002-01-29
Zimmerman, John J. (Department: 1775)
Stock material or miscellaneous articles
All metal or with adjacent metals
Foil or filament smaller than 6 mils
C428S612000, C428S626000, C428S658000, C428S675000, C205S111000, C205S176000, C205S177000, C205S182000, C205S239000
Reexamination Certificate
active
06342308
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to copper foil for use in manufacturing printed circuit boards, and, especially, to electrodeposited copper foil having a bonding side to be bonded to a polymeric substrate wherein the bonding side is provided with a copper bond-enhancing treatment layer, a copper-arsenic layer deposited on the bond-enhancing layer, and a zinc-containing layer deposited on the copper-arsenic layer. This invention also relates to copper clad laminates made with such foil, and to a process for producing such foil.
BACKGROUND OF THE INVENTION
Typically, a bonding treatment is effected by subjecting the bonding side, usually the matte side, of a “raw” electrodeposited copper foil to four consecutive electrodeposition steps. The first consists of the deposition of a microdendritic copper layer which enhances, to a very large degree, the real surface area of the matte side, and thus enhances the foil's bonding ability. It is followed by electrodeposition of an encapsulating, or gilding, copper layer, whose function is to reinforce mechanically the dendritic layer and thus render it immune to the lateral shear forces of liquid resins occurring in the laminating stage of printed circuit board (PCB) fabrication. The encapsulating step of the treatment is very important, since it eliminates the foil's tendency toward “treatment transfer” and the resulting “laminate staining” which can cause a decrease of the dielectric properties of copper-clad laminates. The shape, height, mechanical strength and the number of dendritic microprojections per unit of surface area which constitute the dendritic deposit are the factors instrumental in achieving adequate bond strength of the foil, after all stages of the treatment are completed, when the foil is bonded to a polymeric substrate. The role of the second treatment stage, is to reinforce mechanically, the fragile dendritic layer, by overplating it with a thin layer of sound and strong metallic copper, which locks the dendrites to the base foil structure. Such a dendrites-encapsulation composite structure ought to be characterized by high bond strength and the absence of treatment transfer. The treating parameters which assure just that are relatively narrow. If the amount of the encapsulating, or gilding deposit is too low, the foil will be given to treatment transfer, and, if on the other hand, the gilding layer is too thick, a partial loss of peel strength may be expected. In these first two steps of the treatment the layers are composed of pure copper, in the form of microscopic, spherical micro-projections.
The electrodeposition of the copper bonding treatment is typically followed by deposition of a very thin layer of zinc or zinc alloy, a so-called barrier layer. Its purpose is to prevent direct copper-epoxy resin contact and that is why the zinc-alloy layer (which during lamination is converted to alpha brass), is called the barrier layer. If the bonding treatment composed of copper only is subjected to lamination with epoxy resin systems, it tends to react with amino groups of the resin, at the high laminating temperatures. It, in turn, creates moisture at the foil-resin interface, causing the harmful effect of “measling” and possibly delamination. A barrier layer which is plated over all-copper treatment prevents these harmful effects entirely. All three stages of the treatment mentioned above, effected by means of electrodeposition, change the geometry and morphology of the matte side of the foil, assuring the desired mechanical strength of the surface region, as well.
The electrodeposition of the treatment is usually followed by an electrochemical stainproofing which changes the surface chemistry. As a result of this step, the bonding surface is rendered chemically stable. This operation removes weak surface films, which can greatly decrease the adhesion of the solids, and replaces the films with a stable film of controlled thickness, which is responsible for imparting “durability” of its properties to the treated surface. The film serves as an undercoat for subsequent bonding. The same stainproofing step protects the opposite shiny side of the foil against atmospheric oxidation.
Contemporary bonding treatments were invented in the early 1970's and major foil manufacturers are using the same techniques today. The changes that have occurred in the intervening years pertain, by and large, to the composition of the barrier layers, to accommodate technical needs imposed by the emergence of new polymeric dielectric substrates used in the manufacture of PCBs. For example, polyimide substrates introduced to the printed circuit industry fairly recently require a much higher laminating temperature than the epoxy pre-pregs. Consequently, foil manufacturers had to modify a portion of the overall treating processes in order to achieve the composition and performance of barrier layers for the foils that are destined for polyimide applications. Simply speaking, 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 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.
Other changes in the technology of the bonding treatment are continuing to occur. For example, some major foil manufacturers build their new treaters with a larger number of individual plating tanks, in order to apply twice the sequence of dendritic deposit followed by encapsulating deposit. Thus, quite often, the first four tanks of the treater are designated and devoted to the application of micro-roughening treatment that consists of a dendritic layer followed by an encapsulation layer and this composite plural layer is repeated twice. This practice is aimed at being able to run the treater at greater speeds, since the initial capital outlay for the construction of the treaters is very high today. Conversely, the larger the number of tanks, with the treater run at more traditional speeds, permits deposition of a greater mass or weight of the treatment to assure acceptable peel-strength on so-called “difficult to bond to” polymeric substrates that aim at higher glass transition temperatures. These substrates, which often are blends that involve multifunctional epoxies, BT resin, polyimide, etc., usually require an increased amount of bonding treatment to assure adequate peel-strength. It should be remembered that aside from its bond-enhancing microstructure, the amount of treatment per surface area of copper foil is also an important factor.
It is estimated that foil manufacturers usually electrodeposit about 5 grams of dendritic deposit per square meter of copper foil and about the same amount of encapsulating deposit, while the mass of the barrier layer is usually about 1 gram per square meter. The amount of treatment deposited on the matte side of the foil, depends on the current density and the plating time as determined by Faraday's Law. Current density cannot be increased excessively to accommodate higher treater speeds, since the copper foil has to carry the current between the contact rollers and the plating electrolyte.
Excessive currents will cause over-heating of the foil with resulting wrinkling, cosmetic defects, etc. It follows then that the plating time cannot be rendered too short, to accommodate high treater speeds, because the amount of the bonding treatment that has to be deposited is an important consideration. As a result, the treater speeds, although they vary among major foil manufacturers, do not exceed about 100 ft/min. Since a drum-cathode machine operated with a current of about 50,000 amps, produces about 60″ wide, 1 ounce foil at a rate of about 10 feet/min. and twice that speed for ½ ounce foil, it follows that one modern treater can process the output of 5 to 8 drum machines, depending on the mix of foil gauges, down times, etc. Aside from
Cheng Chinsai T.
Gaskill George
Shah Ajesh
Yates Charles B.
Finnegan Henderson Farabow Garrett & Dunner L.L.P.
Yates Foil USA, Inc.
Zimmerman John J.
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