Apparatus and Method for Regeneration of Chromosulfuric Acid Etchants

An aqueous solution of sulfuric acid and a hexavalent chromium ion containing compound is often used as an etchant for removal of metal. This etchant is especially useful in dissolving copper from copper clad laminates in forming electronic printed circuit boards. During the etching process, the hexavalent chromium is converted to trivalent chromium and the dissolved copper forms as a copper sulfate in the etchant. With continued use, the rate of copper removal is slowed due to the increased concentration of the copper in the solution and the reduction of the hexavalent chromium until eventually the etchant must be replaced. The spent etchant presents a disposal problem due to the toxicity of the copper and the chromium. In U.S. Pat. No. 3,615,957 to Konstantouros, a chromosulfuric etching solution for metallic copper is regenerated in a different container from the etching container by use of an electrolytic cell separated by a diaphragm. Copper sulfate is precipitated out of the used etchant, dissolved in water and placed in a cathode space in a cell. The spent etchant is then placed in the anode space, an electric current passed between the electrodes to regenerate the etchant in the anode space and to deposit the copper on the cathode. It is also known to regenerate an aluminum etching chromic-sulfuric etchant in an electrolytic cell where an anode is immersed in the etchant container and a cathode is immersed in sulfuric acid. The dissolved aluminum is precipitated out of the etchant in the form of an aluminum sulfate and the etchant is regenerated. It was discovered that a chromic-sulfuric etchant for copper may be continuously regenerated while in the etchant container, and the dissolved copper may be continuously deposited on a cathode in an electrolytic cell having a diaphragm separating the anode immersed in the etchant and the cathode immersed in sulfuric acid.

A container for etching a copper or copper containing article by immersing the article in a chromo-sulfuric etchant acts as an anode chamber for electrolytic regeneration of the etchant. A cathode chamber filled with an aqueous solution of an alkali metal sulfate or with sulfuric acid is in communication with the anode chamber through a permeable membrane. When electric current is passed between electrodes located in the two chambers, the etchant is regenerated and the copper dissolved by the etching process is deposited on the cathode electrode.

It is an object of this invention to provide an apparatus for copper etching and for regenerating the etchant simultaneously in the same container.

It is not yet another object to provide a method to continuously etch copper, to continuously regenerate the etchant and to recover dissolved copper.

It is still another object to continuously maintain copper etch rate during the etching.

FIG. 1 shows a perspective view of the apparatus of this invention.

FIG. 2 is a section taken along lines 2--2 of FIG. 1.

FIG. 3 shows a plan view of the permeable membrane of this invention.

The apparatus 10 of this invention has a tank 12 which is divided into compartments 14 and 16. Compartment 14 contains an aqueous etching solution 18 of a hexavalent chromic oxide-sulfuric acid. The hexavalent chromic acid is obtained from an alkali metal dichromate or from chromic acid, but preferably from sodium dichromate. Compartment 16 contains an aqueous solution 20 of sulfuric acid or of an alkali metal sulfate. The tank is divided into compartments by a barrier 22 which holds a permeable membrane or diaphragm 24 which communicates between the two compartments. This ion-selective member preferably is of polyester or perfluorosulfonic acid; however, it is not desired to limit it to those materials as any material resistant to the chemicals involved may be used. The diaphragm preferably is of a non-woven spun-bonded fabric having interstices to impart permeability to chromium ions of from about 0.02 to about 17.4 grams per square foot per hour when a voltage is impressed across the diaphragm. An anodic electrode 26 is placed in compartment or chamber 14 which contains the etchant, and a cathodic electrode 28 is located in compartment 16 which contains the aqueous solution of sulfuric acid. A direct current power supply 30 impresses a voltage to impart a D.C. current between the electrodes through conductors 32 and 34. A copper or copper containing article such as a circuit board 36 is inserted by use of cable 38, which is controlled by a hoist 40 which is supported overhead by a means not shown.

In operation, the circuit board is lowered into the etchant 18 for a period sufficient to remove the desired amount of copper. As the copper is dissolved and goes into the solution, the following chemical reaction is taking place:

the etchant may be regenerated after the circuit board has been removed or regeneration may be performed at the same time etching is taking place. It is preferred, however, to etch and regenerate at one and the same time as this makes it possible to maintain a nearly constant etching rate with a resultant better control of the etching. In either condition, the etching and the regeneration preferably take place in the same container. To regenerate, a direct current is passed through the electrodes and the following reactions take place:

during regeneration, the tri-valent chromic ion is regenerated back to the hexavalent state, the sulfuric acid is reformed and copper dissolved in the anodic chamber 14 passes through the membrane and is deposited on the cathodic electrode 28. it is preferred to use a current density across the diaphragm of from about 110 to 150 amperes per square foot of diaphragm area.

Etching of copper in the etchant, recovery of the dissolved copper by deposition on the cathode and regeneration of the etchant was simultaneously accomplished in a 7 day test, the results of which are shown in TABLE I. The etchant or anode chamber contained about 19% sulfuric acid, 10% chromium oxide and 71% water by weight. The cathode chamber contained about 27% sulfuric acid and 73% water by weight for the first 6 days. On the seventh day, the cathode chamber was emptied and then filled with a saturated aqueous solution of potassium sulfate. A diaphragm of perfluorosulfonic acid (Du Pont Nafion 36-3080) was used between the two chambers. Each day a copper panel was immersed in the anode chamber and the current was turned on during the hours shown. On the third day, however, the copper panel was not immersed, but the current was turned on.

The etching of the copper panel, the regeneration of the tri-valent chromium ion back to the hexavalent state and the deposition of the dissolved copper onto the cathode electrode all took place simultaneously in the same container.