"ALKALINE PERMANGANATE ETCHANT REGENERATION PROCESS AND APPARATUS"
BACKGROUND OF THE INVENTION 1. Field of the Invention - This invention relates to permanganate etching solutions for printed circuit boards in which electrochemical oxidation of the spent permanganate is employed.
2. Description of the Prior Art - There are several documented uses for permanganate etching solutions.
U.S. Patent No. 3,457,107 discloses compositions for chemically polishing stainless steel. These compositions are aqueous solutions containing alkali metal hydroxides, and alkali metal permanganate. Typical solutions contain 3.9% NaOH, 3.2% KMn0 4 and 9.6% Na 2 C0 3 .
U.S. Patent No. 3,489,625 discloses acid pickling of carbon steel, followed by treatment with an alkaline permanganate solution. U.S. Patent No. 3,506,397 discloses the treatment of ABS plastic with a composition containing phosphoric acid and sodium permanganate.
U.S. Patent No. 3,833,414 discloses a method for removing aluminized coatings from metal by treating the metal with a mixed acid solution followed by treatment with an alkaline permanganate solution and then a second treatment in mixed acid. The alkaline permanganate solution contains 8-11% NaOH, 8-11% Na 2 C0 3 , and 4-6% KMn0 4 . U.S. Patent No. 4,294,651 discloses the etching of a semiconductor substrate with a composition containing a fluorine compound, an oxidizing agent such as KMn0 4 , and an alkali metal hydroxide. The solution contains 7-38 of a fluorine compound, 2.5-7% KMn0 4 , and 1-10% KOH.
U.S. Patent No. 4,430,154 discloses a method for removing an adhesive medium from printed circuit boards without corroding the base material by treating the board with an alkaline permanganate solution. The alkaline permanganate solution contains 5% KMn0 4 and 5% NaOH.
U.S. Patent No. 3,625,758 discloses permanganate as an oxidizing agent used in the manufacturing of printed circuit boards. No specific examples regarding the composition of the permanganate solution are provided. U.S. Patent Nos. 4,042,729; 4,054,693? and 4,073,740 relate to treating resinous surfaces with a solution of manganate and permanganate where the molar ratio of manganate to permanganate is up to 1-1.2, and the pH of the solution is 11-13. U.S. Patent No. 4,425,380 discloses a process for removing resin smear (desmear) from an interior wall of a hole in a resinous substrate. The process involves contacting the substrate with an alkaline permanganate solution having a pH of 11-13, at an elevated temperature. The permanganate used is any metal salt which is stable and soluble in water to lOg/1. The upper practical limit of solubility for the permanganate salt is set at 60g/l.
U.S. Patent No. 4,592,929 discloses a process for the metalization of plastic using an oxidizing solution as a pretreatment. One example of solution contains 4% KMn0 4 , 3% NaOH, and 0.05% surfactant.
In all of the aforementioned patent references, the limiting factor as to the long term effectiveness of the permanganate etching solutions has been the propensity of Mn0 4 _1 to be reduced to Mn0 ~ 2 or Mn0 2 . U.S. Patent Nos. 4,592,852; and 4,629,636 address this shortcoming by the use of a secondary oxidant such as sodium hypochlorite to oxidize Mn0 4 ~ 2 to MnO ^ -. ""1 . A typical solution using this principle contains 6% KMn0 4 , 5% NaOH, and 10% NaOCl.
U.S. Patent No. 4,698,124 discloses a method for regenerating spent permanganate ions in alkaline permanganate solutions by adding a combination of inorganic oxidizing agents to oxidize Mn0 4 ~ 2 to Mn0 4 -1 . The preferred embodiment contains a mixture of inorganic peroxydisulfate and inorganic hypochlorite.
As was previously mentioned, one of the problems encountered in using permanganate etching solutions is the conversion of the permanganate to manganese species having a lower oxidation state. The use of secondary oxidizing agents has proven to be a significant advantage over the prior art, but with several disadvantages. The use of a secondary chemical oxidant introduces reaction products to the solution, which over time will increase in concentration to the point of interfering with bath performance.
The present invention overcomes the deficiencies of the prior art in that electrochemical oxidation can be utilized to oxidize non-permanganate manganese species in permanganate etching solutions, thereby eliminating the necessity of replenishing or replacing the bath. This regeneration technique makes permanganate etching solutions much more practical and economical to operate, and does not contaminate the solution with potentially harmful by-products from the regeneration process or with extrinsic oxidizing agents.
Sϋ MARY OF THE INVENTION It is a general purpose of the present invention to provide a method for regenerating non-permanganate manganese species in permanganate etching solutions by contacting the solution with an anode cathode pair with an anode cathode potential such that non-permanganate manganese species are oxidized to permanganate at the anode while in solution.
BRIEF DESCRIPTION OF THE DRAWINGS
Other objects of the present invention and many of the attendant advantages of the present invention will be readily appreciated as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings:
FIG. 1 illustrates a perspective view of apparatus for performing the method of the invention.
DETAI ED DESCRIPTION OF THE INVENTION The permanganate solution used according to the invention contains alkali metal hydroxide in an amount of about 0.1%-9.0% by weight. It also contains 1.0% by weight, or more, of a soluble, stable permanganate salt such as sodium, potassium permanganate, or a like alkali metal salt. The preferred solution has a pH of 13 or above and contains Mn0 4 ions. The Mn0 4 ""1 anions to the sum of the Mn0 4 -1 and Mn0 4 ~ 2 anions is about 1/0.65. It will be apparent in view of this disclosure that the configuration and composition of the anode cathode pair can be of any like structure for the success of the electrochemical regeneration of permanganate.
The chemical reactions involved are summarized as follows:
2K 2 Mn0 4 + 2e~ + 2H 2 0 2KMn0 4 + H 2 + 2K0H or 2Na 2 Mn0 4 + 2e~ + 2H 2 0 2NaMn0 4 + H 2 + 2Na0H The cell voltage is a function of the relative cathode surface area, the extent of electrode shielding, the distance between the electrodes, the conductivity of the electrode materials, or the presence of a membrane around an electrode.
DESCRIPTION OF THE PREFERRED EMBODIMENTS Example: An aqueous solution was prepared which contained 222g/l NaMn0 4 and 38g/l NaOH. Samples of unclad laminate similar to that used in the manufacturing of printed circuit boards were immersed in the above solution for 24 hours at 99°C. Significant degradation of the laminate was observed. The solution was then reanalyzed and found to contain 184g/l NaMn0 4 and 49 g/1 Na 2 Mn0 4 . This is typical of what occurs in printed circuit board processing.
The above solution was then recirculated through an anode cathode pair for electrochemical oxidation with an anode-cathode potential of 4.0 volts DC. The anode was stainless steel wire mesh in the shape of a cylinder, and the cathode was a solid copper rod. The anode surface area was approximately 30 times greater than the cathode surface area. After the solution had been circulating for several minutes, the current (DC) flow was stopped and the solution was again analyzed. The solution contained 228g/l NaMn0 4 and 6g/l Na 2 Mn0 4 .
FIG. 1 illustrates a perspective view of a manganate regenerator 10. It contains an anode 11, anode connector bars 12 and 14, and a cathode 16 in a tank 18, which has an inlet 20, an outlet 22 and a cover 24 with vent holes 26a-26n. Holes 28, 30 and 32 in the cover 24 accommodate anode connector bars 14 and 12 and the cathode 16, respectively, by sliding over the upper ends of the anode connector bars 14 and 12 and the cathode. The wires from the DC source 28, of course, would be connected to the anode connector bars 14 and 12 and the cathode 16 after the cover 24 is fitted over the anode connector bars 14 and 12 and the cathode 16. The anode 11 can be a 316 stainless steel screen about 26" high and 9" in diameter, and the cathode 16 can be copper, 26" high and 1 1/4" in diameter. A DC current source 28 can supply 0-8 volts at 1-100 amps by way of example and for purposes of illustration only and is not to be construed as limiting of the present invention.
Various modifications can be made to the present invention without departing from the apparent scope thereof.
