Treated copper foil and process for making treated copper foil

Details Treated copper foil and process for making treated copper foil Treated copper foil and process for making treated copper foil

2000-02-10

2002-07-30

Dawson, Robert (Department: 1712)

Stock material or miscellaneous articles

Composite

Of epoxy ether

C428S220000, C428S343000, C428S344000, C428S447000, C428S548000, C428S607000, C428S450000, C205S197000

Reexamination Certificate

active

06426146

ABSTRACT:

TECHNICAL FIELD
This invention relates to a treated copper foil, and to a process for making the treated copper foil. The treated copper foil has a thin layer of zinc oxide adhered to the base surface of at least are side of the foil and a layer of a trivalent chromium oxide adhered to the layer of zinc oxide. The treated copper foil is useful in making laminates and printed circuit boards.
BACKGROUND OF THE INVENTION
Copper foil is used in the production of printed circuit boards. Although an excellent electronic conductor, there are problems inherent with the use of such foil. Copper is easily oxidized and corroded. In the production of printed circuit boards, it is generally necessary to bond the copper foil to dielectric substrates to provide the foil with dimensional and structural stability. As plated or rolled, the adhesion of copper foil to such substrates is generally insufficient. Copper is also known to accelerate or catalyze the decomposition of dielectric substrates. For these reasons, copper foil is typically sold with one or more protective layers applied to its surface.
The current practice for applying protective layers to copper foil typically involves the following sequence of steps. First: a nodularized or dendritic copper layer is deposited on the foil surface. This dendritic layer can be applied to either the matte side or the shiny side of the foil, or to both sides of the foil. The dendritic layer is applied to increase mechanical interlocking between the dielectric substrate and foil surface to thereby increase the adhesion strength of the foil. Second: a barrier layer typically comprised of brass is then deposited on the dendritic layer of copper. This barrier layer is added to prevent thermal degradation of the metal-resin interface, thereby maintaining adhesion of the foil to the resin. Third: a stabilization layer typically comprised of zinc and chrome is then applied to both sides of the foil. The stabilization layer aids in oxidation resistance, shelf-life and humidity durability.
The foregoing practice has a number of disadvantages. The nodularized copper layer increases foil profile as well as the etching time required to etch circuitry using the foil. The nodularized layer also decreases foil quality by increasing the pit and dent count, and it reduces treat line speed. The application of the barrier layer requires the use of a caustic, cyanide-containing bath which is difficult and costly to waste treat. Application of the barrier layer also requires the use of soluble anodes which contribute to poor foil quality and anodes that are susceptible to polarization. During the application of the stabilization layer undesirable precipitates form in the bath. The present invention overcomes many of these problems by providing a copper foil that does not require the nodularized or dendritic copper layers or the barrier layers required by prior art copper foils, but still possess initial peel strengths and thermal degradation resistance properties that are comparable to prior art foils.
The dielectric substrates used in the market place, which are sometimes referred to as prepregs, are often made using epoxy resins. Many epoxy resin based prepregs that are available are made using amine curing agents, such as dicyandiamine. There are however, a number of problems are associated with the use of such amine curing agents including environmental, safety and handling concerns. Recently, new prepregs based on epoxy resin systems that are made without such amine curing agents have been introduced into the market place. These new epoxy prepregs are sometimes referred to as “non-dicy” prepregs. While these non-dicy prepregs are beneficial, a problem with the use of such prepregs relates to the fact that the initial peel strength between the copper foil and the non-dicy prepreg that is typically achieved is generally less, in some instances about 10% less, than when conventional epoxy prepregs are used. The present invention also overcomes this problem by providing a treated copper foil that can be used with non-dicy prepregs and still provide desired initial peel strength levels.
SUMMARY OF THE INVENTION
This invention relates to a treated copper foil, comprising: a copper foil with a layer of zinc oxide adhered to the base surface of at least on side of said copper foil, said layer of zinc oxide having a thickness of about 3 Å to about 80 Å; and a layer of a trivalent chromium oxide adhered to said layer of zinc oxide. In one embodiment, the foil has a layer of a silane coupling agent adhered to the layer of trivalent chromium oxide. The invention also relates to a process for applying the layer of zinc oxide and the layer of trivalent chromium oxide to said copper foil. The invention also relates to laminates comprised of a dielectric substrate and the foregoing copper foil adhered to the substrate. In one embodiment, the dielectric substrate is comprised of an epoxy resin made with a curing agent that is other than an amine curing agent; that is, the dielectric substrate is a non-dicy prepreg.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The copper foils used with this invention are made using one of two techniques. Wrought or rolled copper foil is produced by mechanically reducing the thickness of a copper or copper alloy strip or ingot by a process such as rolling. Electrodeposited foil is produced by electrolytically depositing copper ions on a rotating cathode drum and then peeling the deposited strip from the cathode. Electrodeposited copper foils are especially useful with this invention.
The copper foils typically have nominal thicknesses ranging from about 0.0002 inch to about 0.02 inch. Copper foil thickness is sometimes expressed in terms of weight and typically the foils of the present invention have weights or thicknesses ranging from about ⅛ to about 14 oz/ft
2
. Especially useful copper foils are those having weights of ½, 1 or 2 oz/ft
2
.
Electrodeposited copper foils have a smooth or shiny (drum) side and a rough or matte (copper deposit growth front) side. The zinc oxide and trivalent chromium oxide layers can be applied to either side of the foil, and in some instances they are applied to both sides.
The side or sides of the foil to which the zinc oxide and trivalent chromium oxide layers are applied can have a “standard-profile surface,” “low-profile surface” or “very-low-profile surface.” The term “standard-profile surface” is used herein to refer to a foil surface having an R
tm
of about 7 to about 10 microns. The term “low-profile surface” refers to a foil surface having an R
tm
of about 4 to about 7 microns or less. The term “very-low-profile surface” refers to a foil surface having an R
tm
of about 4 microns or less. R
tm
is the mean of the maximum peak-to-valley measurement from each of five consecutive sampling measurements, and can be measured using a Surftronic 3 profilometer marketed by Rank Taylor Hobson, Ltd., Leicester, England.
The layer of zinc oxide has a thickness of about 3 Å to about 80 Å, and in one embodiment about 5 Å to about 60 Å, and in one embodiment about 10 A to about 50 Å, and in one embodiment about 15 Å to about 40 Å, and in one embodiment about 20 Å to about 35 Å, and in one embodiment about 25 Å to about 32 Å. The thickness of this zinc oxide layer is critical to achieving the desired peel strength properties of the invention. The zinc oxide layer is applied to one or both sides of the copper foil as a layer of zinc metal. The zinc metal layer is then oxidized as discussed below. Prior to oxidation, the zinc metal layer has a thickness of about 2 Å to about 60 Å, and in one embodiment about 2 Å to about 50 Å, and in one embodiment about 5 Å to about 40 Å, and in one embodiment about 10 Å to about 35 Å, and in one embodiment about 1 5 Å to about 30 Å, and in one embodiment about 20 Å to about 25 Å.
The zinc metal layer is applied to the base sur

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