Electrolysis: processes – compositions used therein – and methods – Product produced by electrolysis involving electrolytic...
Utility Patent
1999-03-01
2001-01-02
Gorgos, Kathryn (Department: 1741)
Electrolysis: processes, compositions used therein, and methods
Product produced by electrolysis involving electrolytic...
C205S318000, C205S319000, C205S243000, C205S244000, C205S155000, C205S156000, C428S674000
Utility Patent
active
06168703
ABSTRACT:
TECHNICAL FIELD
This invention relates to a process for treating copper foil and, more particularly, to a process for applying a protective, stabilizing layer to at least one side of copper foil comprising contacting said side of copper foil with an electrolyte solution comprising zinc ions, chromium ions and at least one hydrogen inhibitor. The foils treated by this process are 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 a dielectric substrate 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 adhesion enhancement and protective layers to copper foil typically involves the following sequence of steps. (1) 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 roughen and thereby increase mechanical interlocking between the dielectric substrate and foil surface in order to increase the adhesion strength of the foil. (2) A barrier layer is then deposited on the dendritic layer of copper from step (1). This barrier layer is added to prevent thermal degradation of the metal-resin interface, thereby maintaining adhesion of the foil to the resin. (3) A stabilization layer of zinc and chromium is then applied to both sides of the foil. The stabilization layer aids in oxidation resistance, shelf-life and humidity durability. (4) A silane layer is applied over the stabilization layer to enhance adhesion and to improve humidity durability.
In copper foils having a metallic stabilization layer deposited from an electrolyte solution containing no hydrogen inhibitor, two weaknesses have been noted. The first such weakness is the problem of high temperature oxidation (“HTO”), which generally exists at some temperature for any copper foil. The stabilization layer is intended to reduce the HTO problem, but the benefit obtained is limited in known systems.
The second such weakness results from evolution of hydrogen at the metal surface during electrodeposition of the stabilization layer onto the metal surface. Hydrogen evolved at the surface causes local increases in pH, which in turn cause precipitation of dissolved species onto the metal surface, resulting in spots on the finished copper foil.
The prior art has failed to adequately address these weaknesses.
SUMMARY OF THE INVENTION
This invention relates to a process for applying a stabilization layer to at least one side of copper foil comprising contacting said side of said copper foil with an electrolyte solution comprising zinc ions, chromium ions and at least one hydrogen inhibitor. This invention also relates to copper foils treated by the foregoing process, and to laminates comprising a dielectric substrate and the inventive copper foil adhered to said dielectric substrate. An advantage of this invention is that by virtue of the use of the hydrogen inhibitor, the evolution of hydrogen during the inventive process is reduced and the properties of the resulting plated foil are enhanced. Not only does the invention solve the precipitation and spot problems, it also provides an unexpectedly large increase in HTO performance of the treated copper foils.
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 foil 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 ounces per square foot (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 stabilization layer applied by the inventive process can be applied to either side of the foil, and in some instances it is applied to both sides. In one embodiment, the layer applied by the inventive process is applied to the matte side of the foil. In one embodiment, the layer applied by the inventive process is applied to the shiny side of the foil.
The side or sides of the foil to which the layer applied by the inventive process overlies can be a “standard-profile surface,” “low-profile surface” or “very-low-profile surface.” Useful embodiments involve the use of foils with low-profile surfaces and very low-profile surfaces. The term “standard-profile surface” is used herein to refer to a foil surface having an R
tm
of about 10 microns or less. The term “low-profile surface” refers to a foil surface having an R
tm
of 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 vertical measurement from each of five consecutive sampling measurements, and can be measured using a SURTRONIC 3 profilometer marketed by Rank Taylor Hobson, Ltd., Leicester, England.
The source of zinc ions for the electrolyte solution can be any zinc salt, examples include ZnSO
4
, ZnCO
3
, ZnCrO
4
, etc.
The source of chromium ions for the electrolyte solution can be any hexavalent chromium salt or compound, examples include ZnCrO
4
, CrO
3
, etc.
The hydrogen inhibitor can be any additive for the electrolyte solution that inhibits hydrogen evolution during the inventive plating process. These include the following ions: P
+3
, W
+6
, V
+5
, As
+3
, Pb
+2
, Pb
+4
, Hg
+1
, Hg
+2
, Cd
+2
or quatemary ammonium ions. P
+3
, W
+6
and V
+5
are particularly useful and P
+3
is especially useful. Sources for these ions include H
3
PO
3
, Na
2
WO
4
, Na
3
VO
4
HAsO
3
, Pb(NO
3
)
2
, Pb(NO
3
)
4
, Hg
2
SO
4
, HgSO
4
, CdSO
4
, and the like.
The quaternary ammonium ions can be any compound represented by the formula
wherein R
1
, R
2
, R
3
and R
4
are, independently hydrocarbon groups of 1 to about 16 carbon atoms, and in one embodiment 1 to about 8 carbon atoms, and in one embodiment about 4 carbon atoms, and X
−
is an anion such as Cl
−
, OH
−
or other such anion, which acts as a counterion to the quaternary ammonium. Sources of these quaternary ions include tetrabutyl ammonium hydroxide.
The concentration of zinc ions in the electrolyte solution is generally in the range of about 0.1 to about 2 g/l, and in one embodiment about 0.3 to about 0.6 g/l, and in one embodiment about 0.4 to about 0.5 g/l, and in one embodiment about 0.45 g/l. The concentration of chromium ions in the electrolyte solution is generally in the range of about 0.3 to about 5 g/l, and in one embodiment about 0.5 to about 3 g/l, and in one embodiment about 0.5 to about 1.0 g/l, in one embodiment about 0.65 to about 0.85 g/l in one embodiment about 0.6 g/l, in one embodiment about 0.75 g/l, in one embodiment about 1.0 g/l, and in one embodiment about 1.9 g/l. The concentration of hydro
Czapor Edward
Lee Chin-ho
Centanni Michael A.
GA-TEK Inc.
Gorgos Kathryn
Nicolas Wesley A.
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