US3794571A - Regeneration of ferric chloride copper etching solutions - Google Patents

Abstract

ELECTROLYTIC REGENERATION OF FERRIC CHLORIDE COPPER ETCH SOLUTIONS UNDER, ELECTROLYTIC AND CONCENTRATION CONDITIONS EFFECTING FORMATION OF CHLORINE ATOMS AT THE ANODE FOR OXIDATION OF FERROUS IONS IN THE SOLUTION TO FERRIC IONS.

Description

United States Patent 01 fice 3,794,571 Patented Feb. 26, 1974 3,794,571 REGENERATION OF FERRIC CHLORIDE COPPER ETCHING SOLUTIONS Stanley J. Beyer and Robert M. Lukes, Louisville, Ky., assignors to General Electric Company No Drawing. Filed May 10, 1971, Ser. No. 142,065 Int. Cl. C22d 1/16; C01g 47/10 US. Cl. 204-94 3 Claims ABSTRACT OF THE DISCLOSURE Electrolytic regeneration of ferric chloride copper etch solutions under electrolytic and concentration conditions effecting formation of chlorine atoms at the anode for oxidation of ferrous ions in the solution to ferric ions.

The present invention relates to the regeneration of ferric chloride etchant used for the etching of copper in the manufacture of printed circuit boards, printing rolls and the like.

Ferric chloride is probably the most widely used etchant for such purposes. The etching reaction involves the oxidation of metallic copper by the ferric ion, the products of the reaction being ferrous ions and cupric ions. When the ferric ion content of the solution becomes too low for adequate etching action, it has been a common practice to dispose of the solution and substitute a fresh etchant bath. Because of the pollution problems resulting from the disposal of the solutions, the disposal thereof has become a substantial problem.

It has been proposed to chemically regenerate the etching solutions by oxidation of the iron with hydrogen peroxide or with chlorine gas and the removal of dissolved copper by precipitation. None of these approaches have been in wide spread use, probably because the cost of the chemicals or the batch regeneration are greater than the value of the regenerated bath.

The electrolytic regeneration of the spent or partially spent solutions has also been proposed. For example, an elaborate and rather expensive electrolytic regeneration cell for this purpose is disclosed in US. Pat. 2,748,071- Eisler. In this cell, the cathode is enclosed in a porouswalled chamber which separates the anolyte and catholyte sections with the results that the liquid etchant must eventually pass through the porous walls which in a short time become clogged with suspended solids such as the insoluble or slightly soluble copper or iron compounds.

To avoid these problems and to dispense with porous walls, Pat. No. 2,964,453Garn et al. specifically describes the use of a cupric chloride bath which according to the patentees can be easily regenerated electrolytically employing an anode-to-cathode ratio such that the current density is high at the cathode and is low at the anode. This cathode-to-anode ratio is primarily employed so the metallic copper will be deposited on the cathode by a cathode potential great enough to reduce cuprous ions to metallic copper. The Garn et al. cupric chloride bath must also include an excess of chloride ions which function to stabilize the cuprous valence state in solution and favor a reaction in which cupric ions oxidize copper to the cuprous state and are themselves reduced to the cuprous state. Due to the large anode-to-cathode ratio required in carrying out the Garn et al. process, proportionately large cells or anode configurations are required. -In addition the requirement for an excess of chloride ions increases the chemical cost of the solutions. The Garn et al. patent also suggests, without elaboration, that a ferric chloride bath could be used in place of the cupric chloride bath consistent with the spirit of the Garn et al. in-

ride as suggested by Garn et al. in a regeneration cell free of a porous separator wall or diaphragm does not provide an eflicient low cost and effective method for regenerating the commonly used ferric chloride copper 5 etching baths. More specifically, an electrolytically regenerating cell designed consistent with the spirit of the Garn et al. invention requires a relatively large anode area, because regeneration is based on an anode-to-cathode area ratio of at least three and preferably larger than five. The cell itself must therefore be large to provide a large anode area. Regeneration under these conditions is limited to the direct oxidation of the ferrous ion at the anode, which in turn requires a substantial concentration of ferrous ion in the spent solution.

The present invention does not require a high anodeto-cathode area ratio and, in fact, can be operated with equal anode and cathode areas. Also it does not require a high concentration of ferrous ion, or its direct oxidation at the anode for regeneration. Instead, our invention accomplishes regeneration first by forming an active species of chlorine in situ through oxidation of chloride ion at the anode, and then permitting this active species to oxidize ferrous ion to ferric ion in the bulk of the solution. Thus oxidation of ferrous ion to ferric can be effected even at low concentration of ferrous ion, an achievement not possible under the teachings of Garn et al.

With the present invention, there is no visible formation or liberation of gaseous chlorine, and oxidation of ferrous ion in this manner is not the same as oxidizing by adding gaseous chlorine. By utilizing chloride ions already in solution, there is no gaseous chlorine formed, the chloride ions are used over and over, and the chlorine as formed at the anode is much more active than chlorine gas.

The etching solution to be used with a cell that does not require a high concentration of ferrous ion is not bound to use ion concentration which develop from an etching solution which begins as ferric chloride and hydrochloric acid and acts upon metallic copper. Instead, all ion concentrations can be controlled independently.

Contrary to the teachings of Garn et al., the subject iron chloride etching solution does not require an excess chloride concentration to stabilize or solubilize other ionic species, as for example cuprous ion. However, excess chloride ion is in no way detrimental to the operations of either regeneration of ferric ion or dissolution of copper.

As stated before, the various ion concentrations that can be utilized while practicing the invention are relatively independent of each other. Preferably, the following concentration limits should be observed:

If for some reason not connected with the process of regeneration of ferric ion, one wishes to use a concentration of chloride ion in excess of that required to balance the concentration of Fe+++, Fe++, and Cu++, then the addition of other inert metallic chlorides is acceptable; provided, however, that such addition is consistent with solubility requirements. Such chlorides include but are not limited to MgCl CaCl NaCl, KCl, BaCl, LiCl, ZnCl and HCl.

As discussed above, the teachings of Garn et al. require an anode-to-cathode area ratio of at least three. The present invention does not have this limitation,

operating at lesser anode-to-cathode area ratios. For best operation, the following limits are preferred:

High limit Low limit Anode/cathode area ratio 2:1 Anode current density (amperes/sq. it.) l, 000

Thus electrons are transferred from the copper to the ferric ion in the process of dissolving the copper. In time, the reduction of the concentration in the etching solution slows the etch rate and finally dissolution of copper stops. When the etchant is about 85% spent, its ability to dissolve copper slows drastically and the etch solution is discarded.

In conventional practice, wherein one starts with 2.25 M or 3.75 m FeCl the concentrations of Cu++, Fe+++ and Pe at any one time are highly interdependent, and change as the etching process proceeds. Furthermore, the etch rate is highly dependent upon the relative concentrations of these ions.

With the present electrolytic regeneration process, the opportunity exists to formulate an etch solution containing the desired concentrations and ratios of Cu++, Fe+++ and Fe++ to get a desired etch rate, and to maintain these ratios and concentrations by continuous regeneration and copper removal to balance the copper dissolution.

Rapid and efficient copper removal from solution by plating requires a relatively high Cu++/Fe+++ ratio. This can be achieved without lowering the concentration of Fe+++ below the level required for an adequate etch rate, since the Cu++ and Fe+++ concentrations can be set independently, as described above.

Continuous removal of copper metal requires a special cathode design. Such designs include, but are not limited to:

(1) A moving metal belt cathode that passes through the solution, and from which copper is removed by a doctor blade or other mechanical means.

(2) A rotating drum cathode, the axis horizontally oriented, from which the copper is removed continuously by a doctor blade or other means.

(3) A moving chain of cathode plates, only a fraction of which are immersed in the solution at any one time, and from which copper is removed mechanically or chemically external to the solution.

(4) Use of the tank itself as the cathode, and remove copper powder as a sludge from the bottom continuous 1y.

(5) Removable cathodes that are exchanged periodically and are stripped of copper in a separate operation.

Since cathode designs of these types are well known, many of them being shown and described in the aforementioned Garn et al. patent, no further discussion thereof is believed necessary.

Under certain circumstances, as in making printing rolls for the gravure industry, other requirements for the solution may supervene. In preparing such gravure rolls, the etching of copper is done thrOugh a resist of gelatin or other semipermeable material. Thus the desired etching characteristics of the ferric chloride solution are a function not only of the Fe+++, Fe++ and Cu concentrations, but also of the solution viscosity and osmotic pressure, as well as the diffusion and permeation rates of the virgin and spent solutions into and out of the semipermeable resist.

For such purposes, monovalent or divalent metal chlorides can be added to the etch solution to adjust viscosity, osmotic pressure and total ionic strength. For example, calcium chloride, magnesium chloride or barium chloride could be used without interfering with the electrode processes.

In this way one can set the Fe+++, Fe++ and Cu++ ionic concentrations for optimum etch rate and regeneration, and then add the inert metal chlorides to adjust the viscosity, osmotic pressure, diffusion rate and permeation rate of the solution.

Because oxidation is accomplished by direct electrolytic generation of chlorine at an anode, it is essential to have a minimum quantity of chloride ion present in the solution. Chloride ions may be supplied in part by metal chlorides other than iron chlorides (e.g. calcium, magnesium or barium chloride) to obtain a viscosity and osmotic pressure suitable for etching copper through a semipermeable resist or membrane of the types used in etching gravure cylinders or by iron chlorides alone as may be desirable for etching where resists are not permeable (e.g. printed circuit boards).

Where semipermeable resists are used, the basic iron chloride solution can also be modified with 0.1% to 10% of an organic nonionic Water soluble polymer such as polyethylene oxides, carboxymethylcellulose, or hydroxyethylcellulose.

Within the above limits there are, of course, certain preferred compositions. From the standpoint of chlorine generation, high concentration of chloride ion is favorable. The presence of ferrous ion appears to be inconsequential. If ferrous ion is present in high concentrations, it becomes oxidized; and ferric ion becomes regenerated directly. If ferrous ion is present in low concentrations, it becomes oxidized indirectly by the generation of chlorine and subsequent chemical reaction with chlorine to produce ferric chloride:

From the standpoint of the electrolytic removal of copper from the iron chloride solution, high concentrations of copper are desirable.

The amount of ferric ion present in the etching solution is important. High concentrations are unfavorable in that cathode current efficiency for electrolytic deposition of copper is reduced with increasing ferric ion concentration. More and more electron transfer at the cathode is accomplished by the reduction of ferric ion rather than copper ion as the concentration of ferric ion increases. On the other hand, ferric ion is required to obtain a satisfactory etch rate.

The balancing of etch rate and regeneration eificiency will be a matter of preference which may vary depending on the requirements of the user. However, a preferred point of control of electrolyte composition is as follows:

FeCl 1.0 M FeCl 2.5 M CuCl z 1.0 M

with -0 to 1.0 M of other chloride and 0 to 10% organic polymer added only to affect the etch rate through a particular semipermeable resist.

Electrolytic regeneration can be accomplished in the same vessel used for etching, but it is preferable to circulate the etching solution through a separate electrolytic cell, because solution flow past the electrodes carries away the heat generated and the products of electrolytic action, while supplying fresh chloride ion to the anode and copper ion to the cathode. Also the solution can be more readily monitored using a reference electrode to initiate and stop electrolytic regeneration when the solution is recirculated.

Electrolytic removal of copper from the etching solution can be obtained using cathode current densities of As previously indicated, the deposition, removal, and

recovery of copper can be accomplished in several ways. One method is to use a slowly rotating drum cathode immersed about one-third into the etch solution. Rotation is such as to provide about two minutes of electrodeposition at any point on the circumference of the drum. The drum surface should be constructed of a metal which is not chemically attached by the etch solution and does not receive an adherent copper deposit (e.g. tantalum). A doctor blade is used to scrape the loosely adherent powdered copper from the rotating drum. The powder will then drop to the bottom of a running water rinse tank onto a moving belt conveyor, through a drying oven, and into a collecting container.

Another method of recovering copper is to use a moving metal belt cathode which conveys the copper powder on its surface through rinsing and drying. The dried copper powder can than be scraped off into a collecting container. A modification of this procedure is to pass the moving belt through an acid copper or other copper plating solution as an anode and collect the copper as pure copper cathode sheet.

A third method for recovering copper is to provide a conveyor of separate moving cathodes. As one cathode is conveyed out of the etch bath, another cathode enters to maintain a given cathode area. The cathodes are then conveyed through water rinses and then pass through a copper plating solution as anodes to recover the copper as pure cathode sheet.

The method of removing and recovering copper is incidental. What is important is that the etch solution be controlled within the above prescribed chemical concentration limits so that good cathode efliciency for depositing copper is obtained and the bath retains a standard etching capability only through the passage of electric current through an electrolytic cell rather than using fresh ferric chloride solution until it is nearly spent, discharging the solution, and making it fresh again as is current industry practice.

While there has been shown and described certain embodiments of the present invention, it is obvious that it is not limited thereto and it is intended in the appended claims to cover all modifications falling within the spirit and scope of the invention.

What we claim is:

1. The method of continuously regenerating a ferric chloride copper-etching solution containing ferrous ions during use of the solution for copper etching which comprises electrolytically maintaining the ion concentrations of said solution within the limits of 0.7 to 2 M ferric ion; 0 to 3 M ferrous ion, 0.2 to 1.5 M cupric ion and 2.5 to 11.5 M chloride ion employing an anode-to-cathode ratio of 2:1 to 0.5:1 and an anode current density of from 25 to 1000 amperes per square foot whereby copper is plated from said solution at the cathode and chloride ions are oxidized at the anode for oxidation of ferrous ions in said solution to ferric ions.

2. The method of claim 1 in which the copper deposited on said cathode is at least periodically removed therefrom.

3. The method of claim 1 in which said anode is composed of graphite.

References Cited UNITED STATES PATENTS 272,391 2/1883 Thiollier 204106 3,394,060 7/1968 Douglas 20494 1,969,678 8/1934 White et al 156-19 2,886,420 5/1959 Jones et al. 156-19 3,083,129 3/1963 Jones et al. 156-19 OTHER REFERENCES Regenerable Etchant for Copper by Sharpe et al., Ind. & Eng. Chem., vol. 51, No. 3, pp. 293-298, March 1959.

RCA Tech. Notes by Custman, 2 pp., pub. March 1970.

F. C. EDMUNDSON, Primary Examiner U .5. Cl. X.-R. 15619