US3784455A - Methods of electrolytic regenerative etching and metal recovery - Google Patents

Abstract

Methods and apparatus are disclosed for electrolytically recovering metal from an electrolyte and/or an etchant on specially constructed and configured cathodes. The metal deposits advantageously may be subsequently removed from the cathodes with no mechanical or chemical cathodic stripping operation being required. In a more specific application, the invention relates to a continuous cupric chloride-hydrochloric acid etching system for use in recovering etched copper from articles, such as printed circuit boards, wherein the spent etchant is continuously, or intermittently, electrolytically regenerated and etched copper simultaneously recovered as loose, spongy, fine grain deposits on elongated cathodes preferably having arcuate or curvilinear plating surface profiles. The recovered copper deposits advantageously may be readily removed from the cathodes by passing them through a water spray.

Description

United States Patent [191 Parikh et al.

[ 1 Jan. 8, 1974 METHODS OF ELECTROLYTIC REGENERATIVE ETCHING AND METAL RECOVERY Inventors: Girish D. Parikh, Columbus;

William C. Willard, Reynoldsburg, both of Ohio Western Electric Company, Incorporated, New York, N.Y.

Filed: Dec. 28, 1971 Appl. No.: 212,911

[73] Assignee:

U.S. Cl 204/130, 156/19, 204/202 Int. Cl. B01k l/00, BOlk 3/00 Field of Search 204/10, 130, 202,

[56] References Cited UNITED STATES PATENTS OTHER PUBLlCATlONS Western Electric Technical Digest No. 15 July 1969 Apparatus for Recovering Copper from a Cupric- Chloride Etching Solution by R. H. Haralson pgs. 11 and 12. 1

Primary Examiner-T. Tufariello Attorney-W. M. Kain et al.

ABSTRACT Methods and apparatus are disclosed for electrolytically recovering metal from an electrolyte and/or an etchant on specially constructed and configured cathodes. The metal deposits advantageously may be subsequently removed from the cathodes with no mechanical or chemical cathodic stripping operation being required. In a more specific application, the invention relates to a continuous cupric chloridehydrochloric acid etching system for use in recovering etched copper from articles, such as printed circuit boards, wherein the spent etchan't is continuously, or

intermittently, electrolytically regenerated and etched copper simultaneously recovered as loose, spongy, fine grain deposits on elongated cathodes preferably having arcuate or curvilinear plating surface profiles. The recovered copper deposits advantageously may be readily removed from the cathodes by passing them through a water spray.

13 Claims, 8 Drawing Figures WATER CLEAN CLEAN AIR AIR 22 HYDROCHLORIC ACID QQ\ E I: 7 I: (7,

PLATING 73 BRusHme REGENER I 76 (0PT.) A WASE a T FUME 55 WATER 77 SCRUBBER SCRUBBER SPRAY REMOVAL 57 0F PLATED COPPER COPPER P l SETTLING 23 I 3 NuOH TANK L 39 STORAGE CHILLER 79 TANK P l 24 W 1 i r W H CHLOFHNE FUMES 1 a: SPENT ETCHANT E ETCHER [TETCHER REGENERATED ETCHANT 66 m somum HYDROXIDE CHLORINE 66 M 1 SODlUM HYDROXIDE m 2 OVER FLOW OF SPRAY WATER 66 66 H mm WATER 2 RINSE m SPRAY WATER WITH COPPER ETCHER \UETCHER O TANK as; HYDROCHLORIC ACID FUMES/ I z cuPRlc CHLORIDE MIST p 24 26 PATENTEDJAK 81214 I 3.784.455

METRO! 4 ca 5 I I 6.5 :2 0: 6.0 W A 3 [I '1 I i i 1 g 9 1517392325 3 59 H|5|7|92325293|| 2 6 8 I CALENDAR DAYS 5 'I 700 I O A 8J6OO Lu 2 l I l l I r l l 1 I z I 1 r r 512 I6 18 22 2 8 l2 22 5 7 I5 I? I9 23 25 3 5 9 II IS IT IS 25 25 29 3! 2 6 8 g CALENDAR DAYS j /G. L9

in 26 i E I I i f "s; .D 25 it? LIN} l A 22 23 24 25 26 29 30 3| I 2 5 6 7 8 l2-l3 l4 CALENDAR DAYS METHODS OF ELECTROLYTIC REGENERATIVE ETCHING AND METAL RECOVERY BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to the electrolytic deposition of metals and, more particularly, to a method of and apparatus for the continuous regeneration of spent etchant, with simultaneous electrolytic metal recovery.

2. Description of the Prior Art In the manufacture of printed circuit or wiring boards, the print-etch process is most generally employed. Such a process starts with a non-conductive substrate, such as of phenolic resin, phenol fiber, ceramic or some other electrically insulating material, having a thin layer of copper either electroplated or applied as an adhesive-backed foil on one or both sides of the substrate, depending on whether single or duoblesided circuitry is desired. Copper-clad, phenolic resin substrates have become the most extensively used printed wiring substrates to date, primarily because of physical, electrical and chemical inertness characteristics, as well as cost. The thin layer(s) of copper is normally selectively coated with a protective etch resist to form a positive pattern which conforms to the desired printed circuit configuration. The copper not covered by the resist (negative pattern) is then chemically removed by immersing the circuit board in a suitable etchant which chemically reduces the exposed copper. Removal of the resist is accomplished by either a vapor degreasing or a solvent cleaning operation. This yields the final desired conductive printed circuit pattern. As a final circuit processing step, a thin layer of gold is normally plated on the terminal ends or fingers of the circuit.

While the print-etch process readily lends itself to the mass production of printed circuits, primarily by taking advantage of reliable and precision photo printing and engraving techniques, there is a serious collateral disadvantage that arises when utilizing this process, namely, the problem involved in disposing of the spent etchant.

The etchant most commonly employed heretofore has been a ferric chloride-hydrochloric acid solution. Such an etchant has a number of disadvantages, the most important of which is that it is not capable of being readily regenerated and, as such, normally necessitates continuous disposal of the spent solution. While certain solvent extraction and copper salt reducing techniques have been proposed for use in the regeneration of spent ferric chloride solutions and the recovery of dissolved copper therein heretofore, they involve systems which are quite complex, expensive, and entail careful handling of certain of the spent constituents and critical control of a number of sensitive operating parameters. Even then, a substantial portion of the original etchant, separated during the extraction process, must be disposed of, with considerable expense being involved in both rejuvenating the spent etchant with additives and in the disposal of that portion not reusable.

As such, etching with ferric chloride heretofore has generally involved a simple batch process. In such a process, as the ferric chloride etchant is used, it continuously necessitates a longer period of time to obtain the same degree of etching as is realized when the etchant is fresh, and once the etchant becomes spent, it must be disposed of. The utilization of such a batch process unfortunately results in considerable machine down time while the spent etchant is being replaced by a fresh supply thereof. Compounding the disposal problem is the fact that spent ferric chloride etchant contains considerable amounts of cupric ions dissolved therein, which makes the solution very corrosive.

Another disadvantage in the use of ferric chloride is that the etched copper, reduced. to the form of salt complexes in the spent solution, cannot be readily recovered in its pure state, as previously mentioned, without considerable expense being involved in carrying out several additional chemical process steps.

.To bring the seriousness of this problem into better perspective, by way of example only, in one typical batch process etching application involving the etching of 100,000 printed circuit boards of a given type per week, about 4,000 gallons of ferric chloride etchant were required, with the spent etchant having to be disposed of through hired scavengers. Also, the spent etchant included over 2,000 pounds of unrecoverable etched copper in the form of complex copper salts.

Accordingly, it can be readily seen that considerable financial expense is incurred not only in the mere physical removal and replacement of spent ferric chloride etchant, but also in the loss of the unrecovered copper contained therein. Equally important, however, is the fact that with the ever increasing emphasis being placed on ecology today, and with a constantly expanding body of law being enacted to cope with this problem, it becomes readily apparent that the disposal of corrosive solutions, such as spent ferric chloride etchant, presents a continuously increasing dilemma for not only the users, but the scavengers hired to handle such solutions.

Another approach to etching copper on a continuous basis employed heretofore utilizes ammonium persulfate etchant in a process designated CAPER (for Continuous Ammonium Persulfate Etchant Recovery), developed by the FMC Corp. in this process, the copper is not recovered in its pure state, but in the form of salt crystals. As such, additional chemical reduction operations are required in order to recover the copper in a pure, reusable state. Moreover, considerable quantities of ammonium persulfate, in the form of salts, must be added to the spent etchant in order to renew or replenish the etchant to a fresh state, as the spent etchant is not actually regenerated, such as in a balanced reversible chemical reaction process. The CAPER process, however, does have the advantage of not having to dispose of corrosive spent etchant.

In order to obviate the aforementioned problems and disadvantages, a method of and apparatus for actually regenerating all of the spent etchant, and simultaneously recovering the copper directly in a substantially pure, granular state was developed and is disclosed in US. Pat. No. 2,964,453, of P. D. Garn-L. H. Sharpe, assigned to Bell Telephone Laboratories, Inc. As therein described, the etchant. contains a metallic chloride capable of both being an etchant for copper and chemically changing from the oxidation state of its exhausted condition back to the oxidized state of its fresh condition, and having an excess of chloride ions. One preferred form of the metallic chloride is cupric chloride, with a preferred source of excess chloride i925 ses.

Considered more specifically, regeneration of the etchant is due to an electrochemical reversal of the etching reaction. During etching, cupric chloride (CuCl reacts with copper to form cuprous chloride (CuCl), which is soluble in hydrochloric acid. Conversely, as an electrolyte, the cuprous chloride is oxi dized to cupric chloride at the anode(s) and is reduced to substantially pure metallic copper at the cathode(s).

Ideally, when such an etchant is electrolytically regenerated, there is a conversion of equal amounts of cuprous ions to copper and cupric. While such a balanced chemical conversion is not directly possible, as cupric ions in a concentrated chloride solution are reduced to cuprous ions more readily than cuprous ions to copper, the desired effect can be obtained by using a polarizable cathode and a comparatively nonpolarizable anode in a cupric chloride-hydrochloric acid solution having an excess of chloride ions. Such electrolytically operated electrodes are referred to in the above-referenced patent as producing a forced disproportion (the reverse of the etching process) and, hence, regeneration of the spent etchant.

A more detailed description of the electrochemical reversal phenomenon that takes place in an etchant of the type in question will be described in greater detail hereinafter, with an even more detailed discussion of the specifics thereof being found in the cited patent.

In order to practice the method of continuous electrolytic regenerative etching, the reference patent also discloses several preferred structural embodiments of apparatus for carrying out the process. The prior art apparatus in one illustrative form utilizes a thin, continuous tape of platinum, for example, which successively passes through an etching bath, wash water bath, and then a stripping bath, with the tape being electrically biased selectively so as to have a polarity which results in the tape functioning as a cathode relative to an anode while in the etching bath, and as an anode relative to a cathode while in the stripping bath. With such an apparatus, it is readily appreciated that the etched copper removed from a printed circuit board in the etching bath, for example, will be electrolytically deposited on the continuously moving tape while passing through the etching bath, with the deposited copper subsequently being stripped from the tape while the latter passes through the stripping bath.

In another structural arrangement disclosed in the cited patent, mechanical scrapers are employed in place of the electrolytic stripping bath for removing the copper deposited on the tape while the latter is biased continuously as a cathode in passing through the etching bath.

It has also been proposed heretofore to utilize a retractable copper bar as a cathode, with the bar being slowly withdrawn from the etchant as the latter is being regenerated and the copper electrolytically recovered.

The above-described cupric chloride-hydrochloric acid etchant process and different types of apparatus for use therewith in effecting continuous electrolytic etchant regeneration with simultaneous copper recovery quite obviously afford many advantages over prior art non-regenerative etching systems. Notwithstanding such advantages, however, there has been a need for a more simplified and effective electrolytic copper recovery system for use on a mass production basis. This stems in part from the difficulty encountered heretofore in electrolytically recovering the copper as fine,

loose grain, spongy deposits on a planar surface configured cathode, such as in the form of a continuous metallic tape, plate, or elongated rectangular bar. More specifically, it has been found that a cathode having a planar plating surface area, with associated sharp corners and/or edges, results in not only an uneven copper build-up thereon, but a copper build-up exhibiting variable degrees of looseness (or hardness) over the plated surface area. It is believed that this condition results from excessive variable current density gradients that are established by such electrodes.

In addition to the problems encountered with cathodes having planar surface areas, a cathode comprising a continuous moving tape adapted for both electrolytic plating and stripping operations must necessarily be made out of an inert metal, such as a precious metal. Such a tape, of course, is quite expensive, particularly when of the size required for high volume etching and copper recovery applications.

In addition, the utilization of an etchant compatible metal cathodic tape, such as of copper, which would rule out chemical stripping, presents problems in periodically removing therefrom any accumulated hard, coarse metallic deposits. Such hard deposits, if allowed to build-up appreciably, would seriously impair the effectiveness of mechanical scrapers or brushes.

Another problem involved with either a traveling cathodic tape or a retractable and slowly withdrawn cathodic bar is that chlorine gas is readily generated at the solution interface, particularly when relatively high operating currents are employed. Accordingly, it is very desirous to maintain the active cathode surface area substantially below the solution interface during electroplating. This, of course, is impossible to do with either a traveling cathodic tape or a slowly retracting cathodic bar. With respect to the latter, it is also impossible to maintain a constant anode-to-cathode surface area ratio within the solution, which ratio is vary important if a high degree of etchant regeneration is to be realized.

As for the operating parameters that are involved in a cupric chloride-hydrochloric acid etchant regeneration process, the aforementioned patent discusses a number of parameters that are very important and should be maintained within certain ranges. It has been found, however, that there are several additional operating parameters that are likewise very important and that should at least be periodically monitored and adjusted (directly or indirectly) within predetermined limits, if satisfactory etching and copper recovery are to be achieved in a continuous, regenerative, closed loop system designed for high volume, mass production applications.

SUMMARY OF THE INVENTION It, therefore, is an object of the present invention to provide a method of and apparatus for performing continuous, or intermittent, high volume recovery of electrolytically recoverable metal from an etchant and/or electrolyte, in a manner that is reliable, efficient and relatively inexpensive.

It is another object of the present invention to provide a system allowing for the continuous etching of copper and for independently controlled and timed regeneration of the spent etchant while simultaneously electrolytically recovering the etched copper as loose, fine grain granular deposits on specially constructed and configured cathodes, with the copper deposits being readily removable from the cathodes for reuse.

It is a further object of the present invention to provide electrolytic cathodes for use in a metal deposition or plating system which minimize the liberation of gases, such as chlorine, out of solution, are readily manufactured to size initially, readily machined to size periodically thereafter as required, or on a routine maintenance schedule, and which cathodes may advantageously be constructed of two or more different and coaxially disposed metal portions.

In accordance with the principles of the present invention, an electrolytic metal deposition or plating system is provided wherein one or more uniquely configured cathodes are employed for cyclically recovering electrolytically recoverable metal(s) out of an etchant and/or an electrolyte as loose, easily removed fine grain deposits. The metal deposits advantageously may be subsequently removed from the cathodes with no mechanical or chemical cathodic stripping operations being required, and collected in substantially pure, granular free form for reuse.

In accordance with one preferred method and embodiment of the invention for use in recovering etched copper from printed circuit boards, an electrolytically regenerable cupric chloride-hydrochloric acid etchant is recirculated in a closed loop system comprising one or more spray etchers coupled through conduits to a plating tank having associated therewith a plurality of anodes and a plurality of peculiarly configured cathodes. More specifically, at least the major portion of the total plating surface area of each cathode is arcuately formed in at least two dimensions so as to be nonplanar. Regeneration of the etchant is based on the electrochemical reversal of the etching reaction that takes place in accordance with the teaching in the aforementioned Garn-Sharpe patent. As such, the etched copper is initially recovered as electroplated deposits on the cathodes, while regeneration of the spent etchant, i.e., restoration of the etching capacity, is effected at the anodes. It should be understood that etching and electrolytic etchant regeneration may take place in the same tank or vessel, if space permits.

In accordance with an aspect of the invention, and in order to insure that the etched copper is initially electrolytically recovered as loose, fine grain, spongy cathodic deposits, a plurality of elongated, cylindrical rod-shaped copper cathodes are employed in one preferred embodiment. Such a configuration, by having no major active planar surfaces or sharp edges and/or corners extending along the axial length thereof, has been found to substantially minimize, if not completely eliminate, the problem of hard, crusty deposits accumulating over a period of time on the outer surface areas of the cathodes. This appears to be due, at least in part, to minimizing current density gradient variations at the cathodes. The cathodes by being cylindrical in crosssection also readily facilitate their manufacture to close tolerances initially, and subsequently allow any isolated electroplated build up thereon to be removed periodically, such as by a conventional machining operation.

As the cathodic deposits of recovered copper are advantageously loose and spongy, a water spray may be effectively utilized (rather than a mechanical or chemical stripping operation) to subsequently remove the copper deposits cleanly from the cathodes. The use of water also provides an effective and economical way of rinsing and separating out the recovered copper in loose granular form for reuse (or resale) in a settling tank.

The cathode rods are also preferably formed with a narrow band of non-conductive inert material, such as of plastic, positioned along an intermediate region thereof so as to extend partially beneath and partially above the surface of the etchant while the cathodes are in the plating tank. This has been found to substantially minimize the liberation of chlorine gas that would otherwise occur at the surface of the solution in the areas of the anodes.

In one preferred composite embodiment of the etching system, a plurality of rod-shaped copper cathodes are transported by a closed loop conveyor system between the plating and copper removal stations. At the plating station, the cathodes are automatically immersed in the plating solution in groups, being positioned between mutually disposed graphite anodes. Each group of cathodes is in the plating solution for approximately five minutes.

After being plated, each successive group of copper laden cathodes is successively indexed to the copper removal station, where the plated! copper deposits are advantageously removed from the cathodes by water sprays in accordance with the principles of the invention. The recovered copper is then collected at the bottom of a settling tank.

Hydrochloric acid fumes and any chlorine liberated from the solution in the plating tank are removed in a fume scrubber, as are the hydrochloric acid fumes and cupric chloride mist generated in the spray etchers.

In accordance with still another aspect of the invention, the elongated rod-shaped cathodes may be comprised of an inner metallic core, such as of copper because of its excellent electrical conductivity, and of a thin outer layer ofa different inert metal, such as a pre cious metal, or titanium. An advantage of such a cathode construction is that the outer metal layer may be of a type that is inert to the etchant. As such, there is no danger that the surface of the cathode may react with the etchant should be current normally supplied to the cathode-anode circuit he accidentally cut off. It has been found that any etching of the cathode surface has the deleterious effect of changing the surface grain structure thereof, which often leads to hard plated deposits building up thereon.

To insure continuous highly efficient regenerative etching, the following operating parameters must also be maintained within certain limits: plating current density, operating etchant temperature, HCl content, anode-to-cathode surface area ratio, copper content, oxidation reduction potential (ORP) and specific gravity (degrees Baume). A more detailed discussion of these parameters and of their significance will be described in greater detail hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a flow chart illustrating the manner in which continuous etchant regeneration of a cupric chloridehydrochloric acid solution with simultaneous copper recovery is accomplished in accordance with the principles of the present invention;

FIG. 2 is a plan view illustrating one preferred embodiment of the invention for regenerating the spent etchant received from one or more etching tanks and for electrolytically recovering etched copper as loose, fine grain granules;

FIG. 3 is a partial, detail front elevational view of the overhead conveyor depicted in FIG. 2 for transporting a plurality of elongated, cylindrical cathodes in groups to the various operating stations employed in the etchant regeneration system;

FIG. 4 is a symbolic representation of the electrolytic regeneration reaction that takes place in a cupric chloride-hydrochloric acid plating tank;

FIG. 5 is a fragmentary, detail perspective view, partially in section, of an alternative cathode construction advantageous for use in practicing the present invention, and

FIGS. 6-8 are graphs depicting typical operating ranges of certain of the parameters involved in the regenerative etchant process.

Before describing the illustrative etchant regeneration system in detail, a description of the regenerative etching process itself will be given at this point, as a clear understanding of the process will aid in understanding and appreciating the significance of the salient features embodied in the present etchant regeneration system.

Theoretically, any liquid agent capable of oxidizing copper and reducing it into a dissolved copper salt can be used as an etchant for copper. The particular choice of etchant for any given application is, of course, dependent on a number of significant and important factors. To mention only a few relative to circuit board manufacture: (lThe etchant should be regenerable for continuous use. (2) The etchant should etch copper in the shortest time possible in order to avoid excessive undercutting of or the reduction in the effective widths of the printed circuit paths and terminals. (3) The etchant should not chemically attack the screened resist. (4) The etchant should not permeate the printed circuit substrate, which would lower the insulation resistance thereof. (5) The etchant should be safe to handle.

At present there are basically five etchants which are well known and utilized in the electronics industry, these being: ferric chloride, perhaps the most extensively used etchant heretofore (inherited from the photo engraving art), cupric chloride, chromic acid, ammonium persulfate and M-U alkaline etchant. The latter three etchants are actually special purpose solutions normally employed where the chloride ion is undesirable and, thus, have had limited use.

As for cupric chloride and ferric chloride, they both basically satisfy the last four of the five factors mentioned above and, thus, have much more extensive and universal application in copper etching operations. With respect to the first factor listed above, however, cupric chloride, as previously mentioned, has been found to provide a very significant and beneficial advantage over ferric chloride in that it may be inexpensively and reliably regenerated, with the etched copper being directly recoverable electrolytically. This, of course, provides a means for not only realizing considerable cost savings in the recovery of the etched copper, but in realizing even greater savings by eliminating the need of having to continuously replace spent ferric chloride, and by obviating the problems involved in the disposal thereof because of its corrosive nature. Continuous regeneration also eliminates the need for etcher down time, insures a more uniform rate of etching and, finally, lends itself to providing a cleaner environment.

With attention now being directed specifically to cupric chloride-hydrochloric acid etchant, in particular, regeneration of the etchant is due to an electrochemical reversal of the etching reaction depicted symbolically in FIG. 4. During etching, cupric chloride (C u C l2) reagts with copper to form cuprous chloride lows:

CuCl Cu 2CuCl In the plating tank, cuprous chloride is oxidized to cupric chloride at the anodes, whereas at the cathodes it is reduced to metallic copper in accordance with the following equation:

2CuCl--Cu CuCl At the cathodes:

From equations (3) and (4) above it is readily seen that one-half of the cuprous ions in the etchant are oxidized to cupric at the anodes, whereas an equal number of cupric ions are reduced to cuprous at the cathodes! Therefore, no net chemical reaction occurs.

Next, assume that the total anode surface area and the plating current are held constant while the total cathode surface area is gradually reduced. The cathode current density will then increase until a point is reached where the following chemical reactions take place. AT the anodes:

( 3 At the cathodes:

If reaction (3) is multiplied by a factor of two, and then added to reaction (5), the following results:

Or, we have returned to equation (2):

2 CuCl--ICuCl Cu At some high current density, typically above 3,000 amperes, all of the cupric and cuprous ions which diffuse to the cathode surface will be reduced to copper. As the current density at the anode has not changed, the chemical reaction that takes place at this electrode will still be oxidation of cuprous to cupric ions. This, as previously mentioned, is referred to in the above-cited Garn-Sharpe patent as constituting a forced disproportion (the reverse of the etching process) using a polar izable cathode and a comparatively non-polarizable anode and, hence, regeneration of the spent etchant. For a more detailed discussion of the chemical reaction phenomena that makescontinuous spent etchant regeneration possible in a cupric chloride-hydrochloric there are a number of associated operating parameters that are of considerable importance if continuous and reliable operating results are to be achieved. In particular, it has been found that the current employed for best results in a typical plating operation should produce at least 250 amperes per square foot of cathode surface area, and preferably 1,000-1100 amperes per square foot, for a typical operating voltage of 6.3 volts DC. The minimum current of 250 amperes per square foot prevents the cathodes, when of copper or some other metal compatible with the etchant, from dissolving into the solution, and also prevents hard, coarse grain copper deposits from forming on the cathode surfaces, which deposits would be very difficult to remove therefrom. At very high operating currents, such as above 1,200 amperes per square foot of cathode surface area, the solution becomes more progressively heated, with an attendant increase in the generation of acid fumes. At still higher currents, even free hydrogen may be generated which is very detrimental not only to the etching process, but in terms of pollution control. Operating currents considerably above the preferred range may also generate excessive heat which can prove detrimental to either the anodes or the cathodes, or both. 1

Relatively low operating etchant temperatures in the range of 100 F i 2 have been found to be the most effective. This compares with typical operating temperatures of about 130 F for most other commercially available etchants. It has been found that a temperature of 100 F provides a balance between the low temperatures desired and required for efficient plating and the higher temperatures desired and required for optimum etching. The relatively low operating temperature of 100 F also aids in minimizing the generation of undesirable hydrochloric acid fumes, cupric chloride mist and chlorine gas which, as will be seen hereinbelow, are all effectively removed in the operating system embodied herein.

The copper content preferably should be within a range of approximately 8-15 oz. per gallon of solution which, of course, is equivalent to a solution 1 to 2 molar in cupric chloride. However, it has been found that variation within this range is not critical in obtaining satisfactory etching results.

Another important operating parameter relates to the acid concentration in the etchant solution. Specifically, it has been found that the hydrochloric acid content should be maintained between 5 and 6 N (i.e., 24.34-29.21 oz. per gallon) of hydrogen chloride per gallon of solution. Typical controlled variations of acid concentration relative to calendar day time which existed in a commercial printed circuit board etching operation are depicted in the graph of FIG. 6.

1n the referenced etching operation, which will be described in greater detail hereinafter, about 3 gallons of hydrochloric acid l8.5 Baume) and about 2 gallons of water are added every hour to the solution, initially comprising about 920 gallons, so as to compensate for drag out and evaporation losses, as well as to compensate for losses from chlorine gas generated at the anodes during etchant regeneration. With such additions of l-lCl and water, the concentration thereof was readily maintained between 5 and 6 normal (N) as depicted in FIG. 6.

1t is also important that a relatively high anode-tocathode surface area ratio, preferably five or six to one be employed. The significance of this parameter is that as the anode-to-cathode surface area ratio is increased, a point is reached of diminishing r'etums in terms of regeneration efficiency warranting any further increase in the surface area ratio in question. While this point is theoretically reached for a ratio of approximately 45 to one, from a practical and economic standpoint, the anode-to-cathode surface area ratio should normally be in the range of five or six to one.

The above-mentioned parameters are either specifically or generally disclosed in the above-cited patent. in addition, the optimum desired relationships between etching time and both the molarity of cupric chloride and the concentration of hydrochloric acid in solution, relative to different solution temperatures, are set out in graphs in the referenced patent.

lt, of course, should be readily appreciated that in the electroplating operation it is primarily the combination of a relatively high plating current density, a relatively low solution temperature and a relatively high chloride concentration, in conjunction with peculiarly configured cathodes (described in greater detail hereinafter), that produces the desired fine grain, loose, spongy plated copper deposits on the cathodes.

In accordance with the principles of the present invention, there are two additional parameters which have also been found to be of particular significance, primarily in terms of being able to indirectly monitor the condition of the etchant at any given time and, thereby, to ascertain what etchant constituents, if any, should be altered, or what operating current should be employed, or what degree of plating relative to etching (or vice versa) should be carried out temporarily in order to optimize regenerative etching efficiency.

One of these additional operating parameters relates to what is herein referred to as the oxidation reduction potential (ORP). This parameter indicates the ratio of free cupric to cuprous ions in the solution or, in other words, provides an indication of the state of regeneration of the solution at any given time. Spent etchant tends to lower the ORP value whereas the regeneration of the etchant increases the ORP value. During a typical and continuous etching run, the ORP value should remain between 400-500 millivolts, as evidenced by,

the graph depicted in FIG. 7.

Laboratory analysis has shown that, on the average, 92 percent cupric ions and 8 percent cuprous ions are present in the solution. Stated another way, regeneration is approximately 92 percent complete when the value of ORP falls within a range between 400-500 millivolts. A commercially available instrument known as the Beckman Zeromatic ll may be employed to readily monitor the value of ORP at any given time.

The second additional operating parameter that has been found to be very useful in maintaining the regenerated cupric chloride-hydrochloric acid etchant continuously effective relates to the specific gravity exhibited thereby. This parameter, as used in the plating industry, is measured in degrees Baume. The value of Baume (specific gravity) is representative of and controlled by the total copper and hydrochloric acid concentrations that exist at any given time in the etchant solution. This parameter advantageously is readily measured with a conventional hydrometer. After considerable investigation, it has been found that the etchant should be maintained within a range of 2326 Baume, and preferably above 23 Baume. Below 23 Baume, the etching rate markedly slows down. FIG. 8 depicts a control graph illustrating typical variations of Baume relative to calendar dya for a representative continuous etching operation.

As the HCl concentration is normally kept nearly constant, the change in specific gravity therefor is actually indicative of the change in total copper concentration. If, for some reason, etching of copper is continued without any electrolytic plating (necessary for regeneration of the spent etchant) for a short period of time, a rise in degrees Baume will result. This requires a higher initial operating current when plating is commenced than would normally be required in order to prevent the copper cathodes from dissolving in the etching solution. The initial current required, however, also varies directly with the amount of cupric chloride in the etchant at any given time. Accordingly, as the concentration of cupric chloride increases, less current is effectively available for plating and, hence, for regeneration of the spent etchant. Thus, plating efficiency, or

copper plated per ampere hour, is reduced as the concentration of cupric chloride increases. Periodic monitoring of the value of Baume (specific gravity), preferably every 2 hours, thus provides a simple and reliable means by which an operator may readily adjust the copper plating rate, for example, even though the exact concentrations of cupric chloride and/or hydrochloric acid at any given time are not accurately known or readily measurable at the plating site (as distinguished from in a laboratory using samples).

It is thus seen that the oxidation reduction potential and the specific gravity exhibited by the etchant are parameters that may be readily measured, as well as recorded (e.g., on vu-graphs) with conventional and commerically available instruments.

As for the specific concentrations of the solution constituents, they may be readily ascertained periodically, as required, from laboratory analysis of solution samples. For example, the copper concentration may be measured with a Bausch and Lomb Spectronic 20; and the cupric chloride concentration may be measured with a DuPont 400 Photometric Analyzer. The hydrochloric acid concentration may similarly be measured with a ph meter, such as one designated Model 800, of Orlon Research, Inc. It should be understood that these commercially available instruments are cited by way of example only, and that there are many other similar measuring instruments available which may be employed with equal effectiveness.

It, of course, is also readily apparent that not only the operating parameters in question may be readily monitored, but may also be readily controlled or altered by an operator through simple selective constituent additions and/or by adjusting the operating plating current as the case may be. What operating additions or adjustments need to be made at any given time are readily ascertained from both the known cause and effect relationships that exist between the various constituents of the etching solution, and the preferred ranges for the associated operating parameters, as described herein and as illustrated by the graphs of FIGS. 6-8.

Considering the composite regenerative etching system now in greater detail, and with particular reference to an illustrative embodiment thereof designed for use on a mass production basis in etching printed circuit boards, the system designated generally 20 in the flow chart of FIG. 1 comprises an etching station 21, a plating station 22 and a copper removal station 23. The etching station includes two tandem pairs of spray etchers 24, and two respectively associated rinse tanks 26. The plating station 22 includes an electrolytic plating tank 27, a plurality of pairs of mutually disposed planar graphite anodes 28 and a plurality of peculiarly configured cathodes 30 (seen in FIGS. 2 and 3). The cathodes are supported on and transported by a closed loop conveyor mechanism 33 between the plating and copper removal stations 22 and 23, respectively. The copper removal station 23 includes a copper recovery tank 31 and a plurality of water sprayers 34.

The spent etchant from the spray etchers 24 is pumped to the plating tank 27 in the illustrative system through a suitable conduit 37 at the rate of approximately 30 gallons per minute, based on a total quantity of approximately 920 gallons of etchant, and is returned to the tandem etchers by gravity flow through a suitable conduit 39.

The plating tank 27 preferably comprises a heavy, rubber lined steel tank and, in the illustrative embodiment, accommodates twenty-two of the stationary, planar graphite anodes 28, positioned along the sides of the tank so as to form respective, mutually disposed and linearly aligned pairs.

It should be appreciated, of course, that while the illustrative embodiment is described as a closed loop, recirculating regenerative etching system (etchant flowing between the tandem etchers and the physically separated plating tank), etching and plating may just as readily be accomplished at the same time and actually in a single etcher tank, if space permitted.

In accordance with the principles of the present invention, the cathodes 30 are formed as elongated cylindrical rods, such as of copper in one preferred embodiment. These elongated cathodes are preferably formed with a narrow, non-conductive band 41 of plastic material, preferably plastisol, positioned along an intermediate region thereof, as depicted in FIG. 3, so as to extend partially beneath and partially above the surface of the etchant-electrolyte while immersed therein. This nonconductive band 41, advantageously made possible through the utilization of elongated cathodes, has been found to substantially minimize the liberation of chlorine gas that would otherwise occur at the surface of the solution in the areas of the graphite anodes.

The cathode rods 30 are suspended in groups of three from the conveyor mechanism 33 in the illustrative embodiment by means of simple hook and eye fixtures 42 (best seen in FIGS. 2 and 3). By way of example only, in the illustrative embodiment there are fifty-seven of the elongated rod-shaped cathodes 30 employed, with the plating tank being dimensioned so as to accommodate 27 of the cathodes at any one time. The cathodes may successively or in groups be immersed in the plating tank solution in any one of a number of ways, such as by simply forming an overhead track 43 of the conveyor mechanism 33 with a descending portion 43a, a horizontal portion 43b and a rising portion 43c along its length in the area overlying the plating tank 27, as depicted in FIG. 3. It, of course, should be readily understood that retractable cathode support means may be associated with the conveyor mechanism and selectively actuated to accomplish the same results with a continuous, horizontal overhead track.

Electrical continuity to the cathodes is preferably completed through the conveyor track 43 and the support fixtures 42, as evidenced by the negative potential, shown symbolically, applied to the track in FIG. 2. If desired, only a section of the track 43 overlying the plating tank 31 need be biased negatively. The anodes 28, of course, are biased at a positive potential, as also indicated symbolically in FIG. 2. Any power supply (not shown) capable of generating the necessary amperage and voltage would be acceptable for use in the plating operation.

Associated with the plating tank 27 is a water inlet 44 and an HCI inlet 45, as well as the etchant inlet and outlet conduits 37 and 39, respectively. As previously. mentioned, the I-ICl inlet is necessary to maintain the acid concentration preferably within a range between 5 and 6 N (24.24-29.21 oz. of hydrogen chloride per gallon of solution).

After each group of three copper laden cathodes 30 is withdrawn from the plating tank 27, that group is transported to the copper recovery tank 31 whereat these cathodes are respectively aligned with a different mutually disposed pair of the water sprayers 34. While thus aligned during each dwell period of the conveyor mechanism 33, relative movement is imparted between the water sprayers 34 and the aligned cathodes along the axis of the latter by any suitable means (not shown). This movement results in the sprayed water very effectively removing the loose, fine grain copper previously electrolytically deposited on the aligned cathodes while in the plating tank. It, of course, should be readily appreciated that a plurality of pairs of sprayers 34 axially disposed with respect to each aligned cathode, or one or more pairs of sprayers with vertically directed oscillatory nozzles, for example, could also be utilized to remove the copper deposits on the cathodes with equal effectiveness.

The resulting copper-water mixture collected in the recovery tank 31, as depicted in FIG. 1, is pumped through a suitable conduit 53 to a conventional copper settling tank 57 wherein the copper gravitates to the bottom, and the water which overflows therefrom is returned through a conduit 61 back to the interconnected copper recovery tank 31. The fine grain copper which gravitates to the bottom of the settling tank 57, may then be collected in suitable shipping drums (not shown) through a drain at the bottom of the settling tank for reuse or resale as raw material. This reclaimed copper is approximately percent pure, containing about 4 percent oxygen and 6 percent chlorides that have been dragged out of the plating solution.

It should be appreciated, of course, that the electrolytically recovered copper temporarily carried by the cathodes could be collected not only through the utilization of a water spray and settling tank technique, as in the illustrative system, but also through the use of suitable mechanical scrapers or brushes, with or without the aid of water if desired in a given application.

Notwithstanding the fact that the water spray technique has proven to be very effective in substantially completely removing the recovered electroplated copper from the cathodes, over long periods of use isolated areas on the cathodes may nevertheless have a tendency to build up with minute but relatively hard deposits of plated copper. Such minute copper deposits, of course, are normally not detrimental unless allowed to accumulate over a relatively large surface area of each cathode over a long period of time. To minimize, if not eliminate, this problem in certain etchant regeneration applications, it may prove desirable as an optional operating step to pass the cathode rods through a mechanical brush cleaning station 62, outlined in phantom in FIG. 2 to denote that it is an optional operating station. This station includes a plurality of mutually disposed pairs of rotating wire brushes 63 which can very effectively mechanically clean the surfaces of the cathodes once during each operating cycle. The copper deposits removed from the cathodes in this manner would preferably be collected in dry granular form in a brush-cleaning tank 65 within which the mechanical brushes are confined. Any suitable opening in the bottom of the tank 65 could be employed to remove the small quantities of recovered copper therefrom.

In the etching and simultaneous plating operations involved in the etchant regeneration process, there'are a number of fumes, mists and gases generated which must be removed from the atmosphere. More specifically, due to the elevated temperature and the effect of spray etching, hydrochloric acid fumes and cupric chloride mist are generated from the etching machines. These fumes and the mist are captured at the source and exhausted through a conduit 68 (FIG. 1) to a commercial, extended surface type fume scrubber 71, whereat these fumes are removed by water and only clean air is exhausted.

Similarly, cupric chloride mist, hydrochloric acid fumes and a small amount of chlorine gas are also evolved during the electrolytic recovery of the copper. These effluences are removed, by being drawn through an exhaust system 73 to a commercial, vertically packed, sodium hydroxide tower type fume scrubber 78. Separate interconnecting conduits 76 and 77 are only shown to symbolically represent the effluences actually picked up at the plating and copper recovery stations 22 and 23, respectively, This scrubber, in the illustrative embodiment, is used with a 1,200 gallon storage tank 81 connected thereto in a recirculating manner.

Sodium hydroxide is used as a scrubbing medium because it advantageously reacts with chlorine and forms sodium chloride and sodium hypochlorite. This scrubbing solution is preferably changed about once a month. During this period the ph of the scrubbing solution is maintained above 9.5. The spent solution may advantageously be used to treat, for example, cyanide waste from other plating processes.

By way of further illustration, the preferred embodiment of the composite system was designed to etch up to approximately 100,000 of a given type of printed circuit board per week, based on a day, two shift basis. The copper to be etched from each board in question has a thickness of 0.0028 inch and weighs 2 oz. per square foot. Etching time for such a circuit board is approximately 3 minutes, based on a total quantity of approximately 920 gallons of cupric chloridehydrochloric acid solution, initially formulated with the following constituent amounts:

62 gallons of cupric chloride dihydrate (sp. gr.

2.54g/ml) 552 gallons of hydrochloric acid (20 Baume) 306 gallons of water 920 gallons of solution For every 51 of the printed circuit boards in question etched, approximately 1 pound of copper is placed in the solution. Thus, for a typical etching run of approximately 70,000 of such circuit boards per week, approximately 1,372 pounds of copper is etched and recovered for resale. Not only is this amount of recovered copper per week substantial and signficant from a cost standpoint, but the present system advantageously obviates the need for periodic replacement of the etchant. By comparison, approximately 2,600 gallons of ferric chloride would be required per week if used in a batch process to etch the same number of printed circuit boards, with about 1,400 pounds of unrecovered copper being retained in the spent etchant per week.

As previously mentioned, both the etching and plating of copper is performed at 100 i 2 F, compared to commerically available etchants which normally operate at 130 F. The lower temperature provides a balance between the lower temperature required for efficient plating and the higher temperature required for optimum etching conditions. Concomitantly, the relatively low temperature reduces evaporation, fuming and stress on the equipment.

1n the illustrative embodiment, each group of 27 rodshaped cathodes 30 immersed in the etchant at any one time have a total surface area of 12 square feet, whereas the planar graphite anodes have a total surface area of 72 square feet to produce an anode-to-cathode surface area ratio of 6 to l. The plating current is normally maintained at approximately 12,600 amperes, at 6.3 volts DC. The transfer mechanism 33 indexes the cathodes at a rate of 90 transfers per hour, resulting in a total plating time of about 5 minutes for each cathode while immersed in the plating tank 27.

As plating at 12,600 amperes generates a considerable amount of heat in the solution, the temperature thereof is controlled through the use of five bundles of Teflon cooling coils positioned near the bottom of the plating tank (not shown). Chilled water is circulated through these coils from a 480,000 BTU (40 ton) commercial chiller, designated 79 in FIG. 1. A temperature controller (not shown) automatically provides steam heating and chilled water cooling to'maintain the entire 920 gallons of etching-plating solution at the desired temperature of approximately 100 i F.

Considering now in greater detail the significance and important of the particular construction of the cathode and the manner in which they are selectively immersed in the etchant-electrolyte, extensive investigation into the various ramifications involved in etchant regeneration of a cupric chloride-hydrochloric acid solution led to the following factors that have been found to significantly affect the character of the copper deposits on the cathodes: l size and shape of the electrodes, (2) current density, (3) quantity and concentration of copper, and (4) volume, temperatures and rate of circulating of the solution. The last three listed factors have been discussed in detail hereinabove, and will thus not be considered further at this point.

With respect to the size of the electrodes, as previously pointed out hereinabove, and as discussed in greater detail in the referenced Gam-Sharpe patent, an anode-to-cathode surface area ratio of five or six to one has been found to produce the best results in terms of etchant regeneration and plating efficiency, assuming all of the other operating parameters are likewise maintained within optimum ranges. What has not been fully appreciated heretofore, however, is the importance of utilizing a (preferably elongated) cathode having a major non-planar exterior plating surface profile, such as is realized with a cylindrical rod-shaped cathode. Only with such plating surface profiles has it been found possible to achieve consistently loose, fine grain spongy deposits on the cathodes. While this appears to be attributable, at least in part, to minimizing variations in the plating current density gradients that exist between the anode-cathode structure, other phenomena not appreciated at this time may also contribute to the very beneficial plating results realized in accordance with the principles of the present invention. In any event, it has been found that cathodes having planar surfaces with sharp corners and/or narrow edges adversely affect the degree of looseness or spongy nature of the copper plates on the cathodes.

Concomitantly, it has also not been fully appreciated that continuous plating on a cathode over a relatively long period of time within the plating solution may produce permanent, isolated non-uniform build up of what are referred to as tree formations. Such irregular plated profiles have been observed to readily fall off a cathode during plating. This condition, of course, would effectively mask any variation in the plating rate which is due to variations in the copper concentration. It is thus seen that cathodes formed as thin traveling metallic tapes or elongated bars that are slowly retracted pose problems in the recovery of copper in an etchant regeneration system of the type in question, not only in terms of permitting easy removal of the plated copper from such cathodes, but in terms of plating out the maximum amount of copper in a given period of time.

Through the utilization of a plurality of cylindrical, rod-shaped cathodes, they may also advantageously be successively transported to, immersed in and withdrawn from the solution during short indexing periods, without in any way adversely affecting the many inter related operating parameters that must be maintained within close predetermined limits or ranges in order to optimize both etching and plating operations, as well as copper recovery.

The utilization of cylindrical rod-shaped cathodes has also been found to be advantageous in terms of being readily formed initially to the desired dimensions and periodically machined, such as on a lathe, to the desired nominal diameter during routine maintenance. The rod-shaped cathode configuration also readily lends itself to simple mounting in any one of a number of ways on and removal from an indexable conveyor, such as by the inexpensive hook and eye arrangement depicted in FIGS. 3 and 4.

While the cathodes disclosed and described herein are cylindrical in cross-section, it should be appreciated that they may have other cross-sections, such as elliptical, which would also provide arcuate plating surfaces. Similarly, the cathodes may take the form of spheres or ellipsoids, as they likewise would not have planar plating surfaces, sharp corners or edges. Such alternative cathode configurations, however, would not be as advantageous as cylindrical rod-shaped cathodes with respect to mechanical brush cleaning, and/or to subsequent periodic refinishing of the surfaces thereof through an inexpensive machining operation.

A modified version of the elongated cathodes 30 depicted in FIGS. 2 and 3 is illustrated in FIG. 5. More specifically, the cylindrical rod-shaped cathode designated generally by the reference numeral 85 in FIG. 5 has an inner metallic conductive core 87 of one material and a thin, outer (preferably plated) metal layer or coating 89 of a different material that is chosen to be inert to the etching solution, but which is conducive to the electrolytic deposition of copper on the surface thereof. In accordance with one preferred embodiment, it has been found advantageous to form the basic inner core 87 of copper, because of its excellent electrical characteristics and relatively low cost, and to form the outer layer 89 out of a precious metal or titanium. Cathodes with a titanium outer layer having a thickness, for example, of approximately .05 inch have proven to be very effective for the application described herein.

It, of course, is readily apparent that cathodes of the type and size described herein would be prohibitive from a cost standpoint if made completely out of a pre- 4 cious metal. As for solid cathodes made out of titanium, they would normally prove to be impractical for high current density electroplating applications because of the high resistance (or low conductivity) of titanium relative, for example, to copper.

While the cathode 85 in FIG. 5, as cathode 30 in FIGS. l3, is shown with an eye support at the upper end thereof, it should be appreciated that the cathodes may be readily secured more rigidly to the overhead conveyor mechanism, if desired, in any one of a number of ways. For example, to further augment cathode alignment and/or electrical continuity, each cathode support fixture, whether it be in the form of an eye or comprise some other suitable upwardly extending bracket, may be bolted to an associated bracket forming a part of the conveyor mechanism 33.

In summary, the present invention in its broader applications relates to methods of an apparatus for plating out electrolytically recoverable metal(s) from an etchant and/or an electrolyte as loose, easily removed deposits on specially constructed and configured cathodes. In a more specific application, the invention relates to a continuous cupric chloride-hydrochloric acid regenerative etching system for etching copper from articles, such as printed circuit boards, wherein the spent etchant is continuously, or intermittently, electrolytically regenerated and the etched copper simultaneously recovered. In such a system, having particular utility in the mass production etching of printed circuit boards, it has been shown that such factors as the configuration of the anode-cathode structure, the surface area ratio thereof, the etchant temperature, current density, chloride concentration, copper content, specific gravity, and oxidation reduction potential employed are all very important. Particularly is this true if high regenerative etching efficiencies are to be attained and if loose, spongy, fine grain copper deposits are to be readily recovered from the cathodes in substantially pure, reusable form. A composite etchant regeneration system of the type embodied and described herein affords considerable cost savings in the recovery of the etched copper, and affords: even greater savings by eliminating the need of having to continuously replace or renew spent ferric chloride (or any of the other etchants employed heretofore), and in eliminating the need of having to dispose of the spent etchant. Such a system also contributes to the constant ecology battle by greatly minimizing, if not eliminating, troublesome pollutants from the environment.

What is claimed is:

1. In a process of electrolytically recovering metal from an electrolyte, the process comprising the steps of:

electrolytically plating out the recoverable metal from the electrolyte on a plurality of elongated, cylndrical cathodes of a cathode-anode assembly, the arcuate profile of the plating surface of each of the cathodes exposed to the electrolyte minimizing wide variations in plating current density gradients between the anodes and the cathodes, the anodes being of graphite, having planar surfaces, and being mutually disposed in pairs within the electrolyte, and the active anode-to-catho de surface area ratio being at least five to one;

selectively moving the cathodes into and out of the electrolyte and transporting the cathodes in a closed-loop manner to and from a location for recovering the metal deposited on the surface of the cathodes, and

spraying water against the cathodes, at the location for recovering the metal therefrom, to remove the metal deposited thereon.

2. In an electrolytic plating process in accordance with claim 1, mechanically cleaning the outer surfaces of said cathodes at least periodically following a given water spray step of removing recovered metal depos ited thereon. 3. In a process of etching copper from piece parts with a cupric chloride-hydrochloric acid solution maintained at an operating temperature of F. i 4, with the copper content between I and 2 molar in cupric chloride and an acid concentration between 5 and 6 N, and electrolytically regenerating the spent etchant while simultaneously recovering the etched copper, the process comprising the steps of:

etching the copper from the piece parts and thereby causing the etchant to be reduced to at least a partially spent state;

electrolytically regenerating the etchant by plating out the copper in the etchant on a plurality of elongated, cylindrical cathodes of an electrolytically biased anode-cathode assembly, the cathodes being selectively immersed in the etchant and the arcuate surfaces of the cathodes minimizing wide variations in plating current density gradients between the anodes and the cathodes, the anodes being a plurality of graphite anodes having planar surfaces and being mutually disposed in pairs within the etchant, the active anode-to-cathode surface area ratio being at least five to one, and the plating current density being at least 250 amperes per square foot of cathode so as to result in the etched metal being plated out on the cathode as loose, spongy deposits; selectively moving the cathodes into and out of the etchant and transporting the cathodes in a closedloop manner to and from a location for removing therefrom the copper plated thereon, and

spraying water, at the location for removing the copper from the cathodes, against the cathodes to remove the copper plated thereon.

4. ln a process of etching copper in a regenerable etching solution maintained at an operating temperaure between 96 and 104 F., wherein the solution contains a metallic chloride capable of both being an etchant for copper, and of going from the oxidation state of its exhausted, spent condition back to the oxidized state of its fresh, regenerated condition, and having an excess of chloride ions, the process comprising the steps of:

etching copper from an article immersed in the etching solution, thereby causing the solution to be transformed to its spent condition;

electrolytically regenerating the spent etching solution with an anode-cathode assembly including at least one anode and one cathode, the cathode being formed with an outer surface having a continuous curvature about an elongated axis, the arcuate profile thereof minimizing wide variations in the plating current density gradients that are established between the anode and cathode when immersed in the etching solution and electrolytically biased, the active anode-to-cathode surface area ratio being at least four to one and the cathode plating current density being at least @amperes per square foot to cause cuprous and cupric ions to be reduced to copper at the cathode and cuprous ions to be oxidized to cupric ions at the anode which, together with the curpic ions in the spent etching solution, are physically free to migrate to the cathode so that all of the etched and free copper in solution are plated out of solution and deposited on the cathode surface as loose, spongy, fine grain deposits;

forming a relatively narrow band of non-conductive,

inert material circumferentially about and in adherence with the outer surface of the cathode at a position along the cathode to extend a short distance below and above the surface of the etching solution when the cathode is immersed therein during a plating operation, to minimize the generation of chlorine gas from the etching solution during electroplating;

spraying water against the copper electrolytically deposited on the cathode to remove the copper therefrom, and

collecting the removed copper in substantially pure form as fine grain granules.

5. In the process of etching copper in accordance with claim 4, said step of electrolytically regenerating said spent etching solution is accomplished with the said etching solution and transporting said cathodes to and from the location where the copper deposited on said cathode is removed therefrom.

6. In a process of etching copper in accordance with claim 5, the step of removing the copper from the cathodes by a water spray also includes separating the copper from the resulting copper-water mixture in an associated settling tank, and said process further comprises the steps of:

removing any hydrochloric acid fumes, cupric chloride mist and chlorine gas generated during etching and electroplating in at least one fume scrubber, and

periodically selectively adding water and hydrochloric acid to the etching solution in amounts sufficient to compensate for both any evaporation and cathode drag-out thereof.

7. A process of etching copper in an electrolytically regenerable cupric chloride-hydrochloric acid solution with simultaneous recovery of the etched copper, the process comprising the steps of:

initially mixing in a vessel cupric choride and hydrochloric acid in sufficient amounts to establish an initial copper concentration of between I and 2 molar in cupric chloride, hydrochloric acid concentration between 4 and 7 N, and a specific gravity between 21 and 2 8 BAUME; H

etching copper from an article immersed in the ini tially established etching solution to cause the solution to be transformed into a spent condition including a soluble chloride complex and an excess of cuprous ions;

electrolytically regenerating spent etching solution by converting the cuprous ions to cupric ions and free copper at a plurality of cathodes, formed with cylindrical plating profiles, immersed at least in part into the etching solution in spaced relationship and negatively biased with respect to a plurality of graphite anodes having planar surfaces and being mutually disposed in pairs to form an aligned array, the anodes also being immersed at least in part into the etching solution, the cylindrical plating profile of the cathodes minimizing wide variations in the current density gradients established between the anodes and the cathodes to result in the etched free copper being recovered thereon as loose, spongy, fine grain deposits when an electrolytic cathode plating current density of at least amperes per square foot is established, and with the active anode-to-cathode surface area having a ratio of at least five to one;

selectively immersing the cathodes into the plating solution for plating the etched free copper thereon;

transporting the cathodes having the etched free copper plated thereon to a location where the copper is removed therefrom, and

directing a water spray against the copper plated on the cathodes, at the copper removing location, to remove the electroplated deposits of etched copper from the cathodes.

8. A process of etching copper in accordance with claim 7 wherein the steps of etching and electrolytic regeneration take place in separate, interconnected etching and plating tanks which together form a common vessel, first and second conduit means interconnecting the etching and electrolytic regenerating tanks so as to allow continuous feeding of spent etchant from the etching tank to the plating tank and the feeding of regenerated etchant from the plating tank to the etching tank.

9. A process of etching copper in accordance with claim 8, further including the steps of:

removing any hydrochloric acid fumes and cupric chloride mist generated during etching in a first fume scrubber, and

removing any hydrochloric acid fumes, cupric chloride mist and chlorine gas generated during electroplating in a second fume scrubber.

10. A process of etching copper in accordance with claim 7, further including the step of:

minimizing the generation of chlorine gas from the etching solution during electroplating by forming a relatively narrow band of non-conductive, inert material circumferentially about and in adherence with the outer surface of each of said cathodes, said band being positioned along each cathode so as to extend a short distance below and above the surface of the etching solution when each cathode is immersed therein during a plating operation.

11. A process of etching copper in accordance with claim 7 wherein each of said cathodes is comprised of an inner core of a first conductive material and of an outer layer of a second conductive material, said outer layer being inert to the etching solution and encapsulating said inner core at least over the surface area thereof which would otherwise be exposed to the etching solution, and further including the step of:

periodically selectively adding water and hydrochloric acid to the etching solution in amounts sufficient to compensate for both any evaporation and cathode drag-out thereof.

12. A process of etching copper in accordance with claim 9 wherein each of said cathodes is formed with an inner core of copper and a thin outer layer of titanium, the outer layer encapsulating said copper core at least over the surface area thereof which would otherwise be exposed to the etching solution, and each of said cathodes further including a narrow band of nonconductive, inert material circumferentially disposed about and in adherence with the outer plating surface of the layer of each cathode, said band being positioned along the axial length of each cathode so as to extend a short distance below and above the surface of the etching solution when each cathode is immersed therein during an electroplating operation, and further including the step of:

periodically selectively adding water and hydrochloric acid to the etching solution in amounts suffcient to compensate for both any evaporation and cathode drag-out thereof.

13. ln a process of etching copper in a regenerable etching solution, wherein the solution contains a metallic chloride capable of both being an etchant for copper and of going from the oxidation state of its exhausted, spent condition back to the oxidized state of its fresh, regenerated condition, and having an excess of chloride ions, the process comprising the steps of:

etching copper from an article immersed in the etching solution to cause the solution to be transformed to its spent condition;

electrolytically regenerating the spent etching solution with an anode-cathode assembly including at least one anode and one cathode, the cathode being formed with at least the major plating surface area thereof having a smooth, arcuate profile to minimize wide variations in the plating current density gradients that are established between the anode and cathode when immersed in the etching solution and electrolytically biased, the active anode-to-cathode surface area ratio being at least four to one and the cathode plating current density being at least amperes per square foot to cause cuprous and cupric ions to be reduced to copper at the cathode and cuprous ions to be oxidized to cupric ions at the anode which, together with the cupric ions in the spent etching solution, are physically free to migrate to the cathode so that all of the etched and free copper in solution are plated out of solution and deposited on the cathode surface as loose, spongy, fine grain deposits;

forming a narrow band of non-conductive, inert material circumferentially about and in adherence with the outer surface of the cathode, the band being positioned along the cathode so as to extend a short distance below and above the surface of the etching solution when the cathode is immersed therein during a plating operation,

removing the copper electrolytically deposited on the cathode and collecting the removed copper in substantially pure form as fine grain granules. to minimize the generation of chlorine gas from the etching solution during electroplating, and

removing the copper electrolytically deposited on the cathode and collecting the removed copper in substantially pure form as fine grain granules.

UNITED STATESPATENT @FEIQE er TIFICATE or REQHN ParenrN 3,784,455 Dated January 8. 1974 Girish Do Parikh-William Co Willard lnventor( s) It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

i C olumn 22 delete lines 44-48 in their entirety, which read as -i follows: 'removing the copper electrolytically deposited on the cathode and collecting the removed copper in substantially pure form as fine grain granules. to minimize the generation of chlorine gas from the etching solution during electroplating,

and",

gigned and ficaled this 6 twenty-sixth Day Of August 1975 [SEAL] Attest:

c RUTH C. MASON C. MARSHALL DANN ff Commissioner uj'Patents and Trademarks

Claims (12)

1. 2. In an electrolytic plating process in accordance with claim 1, mechanically cleaning the outer surfaces of said cathodes at least periodically following a given water spray step of removing recovered metal deposited thereon.
2. 3. In a process of etching copper from piece parts with a cupric chloride-hydrochloric acid solution maintained at an operating temperature of 100* F. + or - 4*, with the copper content between 1 and 2 molar in cupric chloride and an acid concentration between 5 and 6 N, and electrolytically regenerating the spent etchant while simultaneously recovering the etched copper, the process comprising the steps of: etching the copper from the piece parts and thereby causing the etchant to be reduced to at least a partially spent state; electrolytically regenerating the etchant by plating out the copper in the etchant on a plurality of elongated, cylindrical cathodes of an electrolytically biased anode-cathode assembly, the cathodes being selectively immersed in the etchant and the arcuate surfaces of the cathodes minimizing wide variations in plating current density gradients between the anodes and the cathodes, the anodes being a plurality of graphite anodes having planar surfaces and being mutually disposed in pairs within the etchant, the active anode-to-cathode surface area ratio being at least five to one, and the plating current density being at least 250 amperes per square foot of cathode so as to result in the etched metal being plated out on the cathode as loose, spongy deposits; selectively moving the cathodes into and out of the etchant and transportiNg the cathodes in a closed-loop manner to and from a location for removing therefrom the copper plated thereon, and spraying water, at the location for removing the copper from the cathodes, against the cathodes to remove the copper plated thereon.
3. 4. In a process of etching copper in a regenerable etching solution maintained at an operating temperaure between 96* and 104* F., wherein the solution contains a metallic chloride capable of both being an etchant for copper, and of going from the oxidation state of its exhausted, spent condition back to the oxidized state of its fresh, regenerated condition, and having an excess of chloride ions, the process comprising the steps of: etching copper from an article immersed in the etching solution, thereby causing the solution to be transformed to its spent condition; electrolytically regenerating the spent etching solution with an anode-cathode assembly including at least one anode and one cathode, the cathode being formed with an outer surface having a continuous curvature about an elongated axis, the arcuate profile thereof minimizing wide variations in the plating current density gradients that are established between the anode and cathode when immersed in the etching solution and electrolytically biased, the active anode-to-cathode surface area ratio being at least four to one and the cathode plating current density being at least 150 amperes per square foot to cause cuprous and cupric ions to be reduced to copper at the cathode and cuprous ions to be oxidized to cupric ions at the anode which, together with the curpic ions in the spent etching solution, are physically free to migrate to the cathode so that all of the etched and free copper in solution are plated out of solution and deposited on the cathode surface as loose, spongy, fine grain deposits; forming a relatively narrow band of non-conductive, inert material circumferentially about and in adherence with the outer surface of the cathode at a position along the cathode to extend a short distance below and above the surface of the etching solution when the cathode is immersed therein during a plating operation, to minimize the generation of chlorine gas from the etching solution during electroplating; spraying water against the copper electrolytically deposited on the cathode to remove the copper therefrom, and collecting the removed copper in substantially pure form as fine grain granules.
4. 5. In the process of etching copper in accordance with claim 4, said step of electrolytically regenerating said spent etching solution is accomplished with the anode-cathode assembly comprising at least two mutually disposed anodes, having planar surfaces, and a plurality of cathodes, and further includes the step of: selectively immersing said cathodes into and out of said etching solution and transporting said cathodes to and from the location where the copper deposited on said cathode is removed therefrom.
5. 6. In a process of etching copper in accordance with claim 5, the step of removing the copper from the cathodes by a water spray also includes separating the copper from the resulting copper-water mixture in an associated settling tank, and said process further comprises the steps of: removing any hydrochloric acid fumes, cupric chloride mist and chlorine gas generated during etching and electroplating in at least one fume scrubber, and periodically selectively adding water and hydrochloric acid to the etching solution in amounts sufficient to compensate for both any evaporation and cathode drag-out thereof.
6. 7. A process of etching copper in an electrolytically regenerable cupric chloride-hydrochloric acid solution with simultaneous recovery of the etched copper, the process comprising the steps of: initially mixing in a vessel cupric choride and hydrochloric acid in sufficient amounts to establish an initial copper concentration of between 1 and 2 molar in cupric chloride, hydrochloric Acid concentration between 4 and 7 N, and a specific gravity between 21* and 28* BAUME; etching copper from an article immersed in the initially established etching solution to cause the solution to be transformed into a spent condition including a soluble chloride complex and an excess of cuprous ions; electrolytically regenerating spent etching solution by converting the cuprous ions to cupric ions and free copper at a plurality of cathodes, formed with cylindrical plating profiles, immersed at least in part into the etching solution in spaced relationship and negatively biased with respect to a plurality of graphite anodes having planar surfaces and being mutually disposed in pairs to form an aligned array, the anodes also being immersed at least in part into the etching solution, the cylindrical plating profile of the cathodes minimizing wide variations in the current density gradients established between the anodes and the cathodes to result in the etched free copper being recovered thereon as loose, spongy, fine grain deposits when an electrolytic cathode plating current density of at least 150 amperes per square foot is established, and with the active anode-to-cathode surface area having a ratio of at least five to one; selectively immersing the cathodes into the plating solution for plating the etched free copper thereon; transporting the cathodes having the etched free copper plated thereon to a location where the copper is removed therefrom, and directing a water spray against the copper plated on the cathodes, at the copper removing location, to remove the electroplated deposits of etched copper from the cathodes.
7. 8. A process of etching copper in accordance with claim 7 wherein the steps of etching and electrolytic regeneration take place in separate, interconnected etching and plating tanks which together form a common vessel, first and second conduit means interconnecting the etching and electrolytic regenerating tanks so as to allow continuous feeding of spent etchant from the etching tank to the plating tank and the feeding of regenerated etchant from the plating tank to the etching tank.
8. 9. A process of etching copper in accordance with claim 8, further including the steps of: removing any hydrochloric acid fumes and cupric chloride mist generated during etching in a first fume scrubber, and removing any hydrochloric acid fumes, cupric chloride mist and chlorine gas generated during electroplating in a second fume scrubber.
9. 10. A process of etching copper in accordance with claim 7, further including the step of: minimizing the generation of chlorine gas from the etching solution during electroplating by forming a relatively narrow band of non-conductive, inert material circumferentially about and in adherence with the outer surface of each of said cathodes, said band being positioned along each cathode so as to extend a short distance below and above the surface of the etching solution when each cathode is immersed therein during a plating operation.
10. 11. A process of etching copper in accordance with claim 7 wherein each of said cathodes is comprised of an inner core of a first conductive material and of an outer layer of a second conductive material, said outer layer being inert to the etching solution and encapsulating said inner core at least over the surface area thereof which would otherwise be exposed to the etching solution, and further including the step of: periodically selectively adding water and hydrochloric acid to the etching solution in amounts sufficient to compensate for both any evaporation and cathode drag-out thereof.
11. 12. A process of etching copper in accordance with claim 9 wherein each of said cathodes is formed with an inner core of copper and a thin outer layer of titanium, the outer layer encapsulating said copper core at least over the surface area thereof which would otherwise be exposed to the etching solution, and each of said catHodes further including a narrow band of non-conductive, inert material circumferentially disposed about and in adherence with the outer plating surface of the layer of each cathode, said band being positioned along the axial length of each cathode so as to extend a short distance below and above the surface of the etching solution when each cathode is immersed therein during an electroplating operation, and further including the step of: periodically selectively adding water and hydrochloric acid to the etching solution in amounts sufficient to compensate for both any evaporation and cathode drag-out thereof.
12. 13. In a process of etching copper in a regenerable etching solution, wherein the solution contains a metallic chloride capable of both being an etchant for copper and of going from the oxidation state of its exhausted, spent condition back to the oxidized state of its fresh, regenerated condition, and having an excess of chloride ions, the process comprising the steps of: etching copper from an article immersed in the etching solution to cause the solution to be transformed to its spent condition; electrolytically regenerating the spent etching solution with an anode-cathode assembly including at least one anode and one cathode, the cathode being formed with at least the major plating surface area thereof having a smooth, arcuate profile to minimize wide variations in the plating current density gradients that are established between the anode and cathode when immersed in the etching solution and electrolytically biased, the active anode-to-cathode surface area ratio being at least four to one and the cathode plating current density being at least 150 amperes per square foot to cause cuprous and cupric ions to be reduced to copper at the cathode and cuprous ions to be oxidized to cupric ions at the anode which, together with the cupric ions in the spent etching solution, are physically free to migrate to the cathode so that all of the etched and free copper in solution are plated out of solution and deposited on the cathode surface as loose, spongy, fine grain deposits; forming a narrow band of non-conductive, inert material circumferentially about and in adherence with the outer surface of the cathode, the band being positioned along the cathode so as to extend a short distance below and above the surface of the etching solution when the cathode is immersed therein during a plating operation, removing the copper electrolytically deposited on the cathode and collecting the removed copper in substantially pure form as fine grain granules. to minimize the generation of chlorine gas from the etching solution during electroplating, and removing the copper electrolytically deposited on the cathode and collecting the removed copper in substantially pure form as fine grain granules.

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