US 3677906 A
Description (OCR text may contain errors)
13, 1972 CHlH-CHUNG WANG 3,577,0
METHOD AND APPARATUS FOR PRODUCING THIN COPPER FOIL Filed Dec. 17. 1970 LIFTING MECHANlSM 2| INVENTQR CH|H-CHUNG WANG BY L,
ATTORNEYS United States Patent 7 Claims ABSTRACT OF DISCLOSURE A process and apparatus for producing thin copper foils by electroplating the copper onto a rotating drum which is connected as a cathode. The surface of the rotating drum is rhenium.
FIELD OF THE INVENTION This invention relates in general to a method and apparatus for manufacturing thin copper foils and more particularly to an electrolytic process using a rotating cathode drum to produce ultra-thin copper foils.
BACKGROUND OF THE INVENTION Thin copper foils having a thickness of 0.003 inch or less, free of pinholes, and having a purity of greater than 99 percent are required for printed circuit applications. Foils for this purpose are generally produced in an electrolytic process in which copper from a copper anode or a copper containing electrolyte is plated onto a rotating drum, which serves as a cathode, and the foil is peeled off the drum as it rotates. The initial processes developed for this purpose employed a lead rotating drum as the cathode. However, the softness of the lead reulted in contamination of the produced copper foil which lead particles. Subsequently stainless steel cathode drums were used to overcome this problem. However, the stainless steel drum must be vary carefully prepared and, even when so prepared, non-conducting stainless steel oxides which are not uniform result in preferential sites for nucleation of the deposited copper resulting in pinhole forma tions in the ultra-thin foils, for example, less than 1 mil.
Various techniques have been employed to overcome the deficiency of the pinhole formation resulting from the drums. Platinum has been employed as a surface for cathode drums. Nickel coated steel mandrels have also been used to produce copper foils. Some success in producing pinhole free copper foils at thicknesses down to about 0.0007 inch has been achieved using especially prepared chromium-coated drums. All the above materials have a common shortcoming of having an electrically nonconductive oxide fihn which causes preferential nucleation of deposited copper resulting in pinholes.
SUMMARY OF THE INVENTION DESCRIPTION OF THE DRAWINGS The single figure is a schematic representation of an electrolytic plating apparatus suitable for use in the practice of this invention.
3,677,906 P atented July 18, 1972 DESCRIPTION OF PREFERRED EMBODIMENTS With reference now to the drawing, an electrolytic tank 11 contains an electrolytic solution 12 in which is inserted an anode 18 and a cathode 16, the latter including a rotating drum 14 from which electrolytically plated copper foil 20 is peeled by means of a lifting mechanism 21. The drum 14 is kept rotating at a relatively slow speed in a counterclockwise direction by means of a motor (not shown). It will be understoodthat the apparatus illustra'tedis schematic in form and that the actual physical apparatus may take any of several conventional forms well known in the art. In one example, the anode 18 was formed of soluble copper and the electrolytic solution was 1 mole of CuSO .5H O, 1 mole of H and 1000 cc. of H 0. The lifting mechanism can simply be a driven roller mechanism on which the produced copper foil is wound.
In one embodiment, the cathod drum 14 is a rhenium plated copper rod. The copper rod is first wetted with oil and polished using a series of abrasive papers with grit numbers 180, 320, 420, 500, 600 2/0 in succession. The polished copper rod is then degreased with a solution of trichlorethylene followed by an alcohol rinse and a cold water rinse. The rod is then electro-cleaned with trisodium phosphate and rinsed with cold water. Following this the rod is dipped in a hot 2 percent citric acid solution and again rinsed with cold water. The rod is next plated with a gold strike, such as that manufactured under the trade name Sol-Rex Aurobond by Sel-Rex Corp., Nutley, NJ. and then rinsed with a deionized cold water. The rod is then further plated with a gold plate to an approximate thickness of of a mil and once more rinsed with cold water. The plated gold is polished with water and alumina to a 1 micron finish, washed with green soap and rinsed. It is then buffed with microcloth, degreased with trichlorethylene followed by an alcohol rinse and a deionized cold water rinse.
The initial step in the rhenium plating is to dip the gold plated copper rod in a 2 percent citric acid solution and then use a distilled water rinse. Rhenium is then plated on at about 65 C. with a current of about ma./ cm. for 18 hours using a platinum anode and a solution of ACR Rhe'nplate (trademark of American Chemical & Refining Co., Waterbury, Conn.) until the rhenium plate has a thickness of 1.5 mil.
In one example, the plating current for producing the copper sheet was 62 amps/ sq. ft. and the resultant copper sheet deposited uniformly on the rhenium plated roll and peeled off readily. Consistently good copper foils of 0.2 mils were thus produced. While in the above example the cathode drum was a rhenium plated copper rod, high density rhenium tubing or a solid rhenium rod made from rhenium powder may also be employed. Using a high density rhenium tubing, copper foils of thicknesses down to 0.113 mil have been successfully produced.
What is claimed is:
1. In a process for preparing thin copper foils, the improvement comprising,
electro-depositing the copper on a rotating electrode having a rhenium surface.
2. A process in accordance with claim 1 wherein the rotating rhenium electrode is formed of a copper roll having rhenium plated thereon.
3. A process in accordance with claim 1 wherein the rotating electrode is formed of a high density rhenium tubing.
4. A process in accordance with claim 1 wherein the copper is plated from a soluble anode in a plating solution of 1 mole CuSO .5H O, 1 mole H 80, and 1000 cc. of H 0.
5. Apparatus for the electrolytic production of copper foil comprising,
an electrolyte tank, a copper anode having at least a portion of its surface within said electrolyte tank, a rotatable drum having a portion of its surface disposed within said tank, said rotatable drum having a rhenium surface and means to rotate said drum. 6. Apparatus in accordance with claim 5 wherein said rotatable drum is formed of an underlying surface of copper on which there has been deposited gold plate with a surface finish of 1 micron, on which a rhenium surface has been plated.
7. Apparatus in accordance with claim 5 wherein said rotatable drum is formed of a high density rhenium tubing or high density rhenium. I
References Cited UNITED STATES PATENTS Wilkins 204-28l Yates 204l3 Macon 204l3 Sternfels 204-43 Brown 204-'6 Harrison 204l3 Edison 204-13 US. Cl. X.R.