I
LOUIS H. SHARPE and PAUL D. GARN Bell Telephone Laboratories, Inc., Murray Hill, N. J.
Regenerable Etchant for Copper This copper-etching system can be regenerated electrically. Twenty cents worth of electricity regenerates the equivalent of $70 worth of ferric chloride and yields $5 worth of copper. It etches better, too
THE
etched copper foil technique is the most widely used for Froduction of printed \\.iring (J). Intelligent use of a n etchant in this technique must be founded on knowledge of the effects of variables such as concentration, temperature, and dissolved copper on the ability of the etchant to produce a good quality of etch i n a reasonable time. Studies along these lines have been published (2>3, 5). Control of variables and techniques of measurement can be improved. Preliminary studies on ferric salts sho\ved that ferric nitrate and ferric sulfate are unsuitable etchants for copper in the production of printed Liking. Ferric nitrate etches as rapidly as ferric chloride, but its tolerance for added copper is very poor. Ferric sulfate etches copper only about 4 as fast as ferric chloride. .A 2.2i.If ferric chloride solution etches more rapidly than the commercial 3.75.11 solution. T h e increase in etching time ivith added copper is someishat greater than for the 3.75.21 solution? but because the ferric chloride etchant is usually discarded Ivhen added copper concentration reaches 60 grams per liter. the 2.25M solution Lsould provide a n etching time and cost advantage \vitl- only a small sacrifice in etching time control. This advantage has been recognized by manufacturers in this country and in England.
are made to a recording voltmeter (l-arian C-10 recorder \vith appropiAite vcltage divider) from the reference electrode (positive) and the plstinum cylinder (negative). 50 ml. of etchant solution is poured into the cell, the stirring motor is started, and the servo unit of the recorder is switched on. The recorder then indicates the initial potential of the electrodeposired copper ivith respect to the platinum reference electrode in the solution in Lvhich they are both immersed. A s the copper is etched, the patenrial remains steady or decreases slo\vly, but as soon as rhe
underlying platinum is exposed. the potential undergoes a m o r e or lcsj audden decrease (the brestk point). and then decreases more 01'less sharpl!- until all the copper is etched a\\-a!.. a t \\-hich time the potentia.1 has decreasrd to 7xx-o. Thc time a t \vhich the potential has decreased to one half i n initial \ d u e is taken as the time required to strip the original copper deposit. This measurement is \-cry reproducible, on thc order of a fe\v per cent for replicates. The exposed area of the platinum c!.linder is approximately 0.3 square inch. and the copper is deposited from a
Experimental
The apparatus consists of circular stirring elements and a platinum cylinder attached to a 6-inch support rod, a reaction vessel, and a platinum reference electrode. T h e rod \iith stirrers and platinum cylinder is motor-driven through a &inch vertical oscillation. T h e platinum cylinder is made by the rotating cathode in a copper plating bath, electrical connection to the cJ-linder being made through the contact ring on the support rod. After plating. the piece is \sashed and dried. T h e reaction cell is then assembled and placed in a constant temperature bath, and the support rod connected to the stirring motor. Electrical connections
Etching cell Copper i s p l o l e d on the piotinum cylinder and the time required t o dissolve the copper i s measured
VOL. 51, NO. 3
MARCH 1959
293
ic
12
e cn
/*
W
c 3
z
E
2
z w
4
2
4.00M
c
N a C ' L SOLUTION
2
I 1
1
2 3 4 5 MOLARITY OF C U P R I C C H L O R I D E
Figure 1 . Etching time as a function of cupric chloride concentration a t 25' C. The higher chloride ion concentration speeds etching solutions
conventional acid copper plating bath a t a current density of approximately 50 amperes per square foot. The thickness of the deposit used was approximately 0.0009 inch in all cases. Current flo\~in the external circuit during etching is negligible. hlallinckrodt's anhydrous cupric chloride, reagent grade, prepared by drying hydrated cupric chloride (CuC12.2H20) overnight in a 118 C. oven, was used to prepare solutions in hydrochloric acid. All other solutions were prepared from the hydrated material. Mallinckrodt's sodium and ammonium chloride and Baker's hydrochloric acid, reagent grade, were used in preparing solutions.
Results and Discussion The data obtained are presented graphically in Figures 1 through 4. All the curves of cupric chloride concentration us. etching time pass through a minimum (Figures 1 and 2). For convenience the concentration of salt that produces the minimal etching time is termed the "minimal concentration." The striking effect on etching time of addition of hydrochloric acid to the cupric chloride solution is also evident from Figure 1. As the concentration of hydrochloric acid increases, the etching time decreases-for example, the etching time at the minimal concentration of cupric chloride in G A ; hydrochloric acid is less than one third that in water solution and is somewhat less than in minimal concentration ferric chloride a t the same temperature. The tolerance of these hydrochloric acid solutions toward added copper is
294
small. The hydrochloric acid concentration is more important in determining the added copper tolerance of the solutions than the cupric chloride concentration. Any further increase in hydrochloric acid concentration should, therefore, increase added copper tolerance. There are a number of objections to increasing the acid concentration further. The acrid odor of hydrochloric acid in the solution is objectionable at 6.V; it should become proportionately more objectionable a t higher concentrations. Higher concentration should also enhance corrosion. The solubility of cupric chloride decreases rapidly with increasing acid concentration. Hence, if the acid concentration were increased until the rate of increase in etching time with added copper appeared reasonable, the cupric chloride solubility would probably be so low that the added copper tolerance \vould again become impracticably low, and the advantage of increased acid \vould be nullified. The reaction which takes place upon dissolution of copper in cupric chloride is CUCl*
+cu
+
CUSCI,
(1 1
Cuprous chloride is insoluble in \vatu. but soluble in hydrochloric acid solution: CusClr
+ 4C1-
+
2CuC13-2
(2'1
When copper is placed in aqueous cupric chloride solution, Reaction 1 takes place and a cuprous chloride film forms on the copper surface. This film, a n efi'ective deterrent to further reaction. slo\vly dissolves and copper is dissolved a t a rate dependent on the rate of removal of cuprous chloride as the soluble
INDUSTRIAL AND ENGINEERING CHEMISTRY
complex C U C I ~ - ~ As . the hydrochloric acid concentration is increased, all other conditions remaining the same, the rate of removal of cuprous chloride is also increased, leading to a decrease in etching time. Because hydrochloric acid added to cupric chloride solutions favorably decreased etching times, a readily soluble highly ionized chloride should be investigated to determine if it would produce the same effect. Use of the salt lvould, a t least. eliminate the objectionable acrid odor of hydrochloric acid and reduce corrosion. The reaction should proceed as before. .%ccordingly, experiments were carried out a t 35' C. to determine etching times in solutions of cupric chloride of various concentrations, ivith varying amounts of sodium chloride added. These data are presented in Figure 2, Lvith data for aqueous and 6.V hydrochloric acid solutions of cupric chloride. For similar concentrations of cupric chloride the etching times decrease as the sodium chloride concentration is increased, and solutions of cupric chloride in 6 9 hydrochloric acid etch somewhat faster than similar solutions saturated kvith sodium chloride. Figure 3 shoivs the effect of added copper on etching time a t 25" C. for solutions of cupric chloride saturated with sodium or ammonium chloride. Tolerance toivard added copper is increased. for 2.00h.f cupric chloride solution, by keeping the solution saturated with sodium chloride. The situation is similar a t 35" C. (Figure 4 ) . 4 solution 2.00M in cupric chloride and saturated tvith sodium
PRINTED W I R I N G chloride is far more tolerant toward added copper than a solution 2.00M in cupric chloride and 6-1- in hydrochloric acid. Solutions of cupric chloride saturated with ammonium chloride provide even more etching capacity than their counterparts containing sodium chloride (Figures 3 and 4). .4t 35" C. the capacit) of a solution 1.7.bf in cupric chloride and saturated with respect to ammonium chloride is very close to theoretical, as predicted by Equation 1. This solution also exhibits an almost unchanged etching rate over a wide range of added copper, \chich is highly desirable from the standpoint of process control. These solutions appear to be metastable; they will throw down some crystalline cupric ammonium chloride (CuClZ. 2XHdC1) with downward temperature variations. Comparative etching has been carried out on test panels. using ferric and cupric chlorides as etchants in a miniature splash etcher and an air-agitated etching tank. Cupric chloride etchant produces an etch as clean and smooth as ferric chloride. I t etches more rapidly and appears to produce someirhat less undercutting. i
4
+
2C~2C1:! 4 HC1
+
0
2 +
4CuClg
+ 2Hz0
(3)
This requires removal of part of the used solution and addition of hydrochloric acid solution, rather frequently, as the capacity of the system is low. A
1.00 M CUCL2, S A T D . W I T H Nd C L
2 3.00 M 3
system of this type has been described (7). If the system is maintained a t a concentration that gives a reasonably rapid etch, the volume of hydrochloric acid which must be added or used solution removed would be someivhat less than half that of ferric chloride solution used to etch the same amount of copper. This is still a disposal problem. Electrolytic regeneration gives only the problem of disposing of readily marketable copper recovered from the bath. Because 1 mole of cupric ion dissolves 1 mole of copper to form 2 moles of cuprous ion, regeneration should consist ideally of oxidation of 1 mole of cuprous ion to cupric and reduction of 1 mole of cuprous ion to copper. This is not possible, per se, because cupric ion in concentrated chloride solution is reduced to cuprous ion more readily than cuprous ion to copper. The desired effect can be obtained by using a polarizable cathode and a comparatively nonpolarizable anode. When a solution of cupric and cuprous ions with a n excess of chloride ions is electrolyzed between two platinum electrodes of the same area at a moderate current density, cuprous ion is oxidized to cupric a t the anode but cupric ion is reduced to cuprous a t the cathode. There is no net reaction. Cupric is
Although added copper tolerance has been increased for the cupric chloride, this etchant does not compare favorably with ferric chloride in rate of increase of etching time with added copper. Ferric chloride undergocs a markedly slower rate of increase of etching time with added copper. If the cupric chloride etcbant could simply and continuously be regenerated as it was used up, it would merit consideration as an etchant for copper in the production of printed wiring. as no feasible process has been developed for continuous regeneration of any commercially used etchants for copper, including ferric chloride. Continuous regeneration of the cupric chloride etchant by a n electrolytic method is possible. In the cupric chloride-hydrochloric acid system a self-propagating system is also possible, a t least with a spray or splash etching system. This system involves air oxidation of cuprous ion:
C u C L 2 , S A T D . W I T H Nd C L
2.00 M C u CL.,,SATD.
W I T H Na C L 2.00 M CU CL2, SATD. W I T H N H 4 C 1
C O P P E R A D D E D IN O U N C E S PER G A L L O N
21
0
CU
IN OUNCES PER GALLON 4 6 8 10 12
ADDED
2 0
0
20
40
60
80
100
4
16
8
101
12
114
20 40 60 80 100 COPPER A D D E D IN G R A M S PER L I T E R
120
. .gure 4. Etching time as a function of copper added to various cupric chloride solutions at 35" C.
C O P P E R A D D E D IN G R A M S P E R L I T E R
Figure 3. Etching time as a function of copper added to various cupric chloride solutions a t 25' C. For economical control, the etching solution must etch at nearly the same rate over a r a n g e of copper concentrations
Ammonium chloride etches rapidly over a wide range of copper content 1. 1 . 0 0 M CuC12, satd. with NaCl 2. 2 . 0 0 M CuCla, in 6.ON HCI 3. 3.00M CuC12, satd. with NaCl 4. 2.00M CuC12, satd. with NaCl 5. 1 . 7 M CuC12, satd. with NHtCl
VOL. 51,
NO. 3
MARCH 1959
295
the copper ions \\,auld no longer diffuse rapidly enough to maintain the current Ao~v,so the reduction of hydrogen ivould begin. If reduction of h>-drogen \\ere permitted, the solution Lrould become overloaded wirh cupric ions. Eventually: cuprous hydroxide miyht precipitate because of removal of hkdrogcn ions.
9.0
In
5
1.5
I
Cut' +e-+
z
CUO
A
F"z
Operating Expense
1,o
The operaring expense of this process
w t-
can be estimated easily.
a
Consider the electrode prucmses a t various stages of regeneration. -1 hree simplifying assumptions are necessary ; tit.0 are dependent on design and the third will cause no serious error. .'issume first thar the reactions at the cicctrodes are only oxidation of cuprous ion and reduction of cuprous and cupric ionsi.e., 100% current efficiency. This is a matter of design and control of potential. Second. assume that all cupric ions are reduced to copper-the cathode potential is high enough to reduce all copper ions that reach the cathode surface. This, too, is a simple design problem. Third. assume equal diffusion rates for cuprous and cupric. This is probably not true in this solurion. but the error cannot be large. YOM.consider a rather depleted etching bath Xrith a cuprous-cupric ratio of 4 to 1. Let four cuprous ions he reduced, as shown in Equation 4. One cupric ion will also be reduced, using six electrons. As these six electrons must be removed by oxidizing six cuprous ions to cupric (Equation 5), the net reaction (Equation 6) is disproportionation of 10 cuprous ions.
0
1 0.5 -I W
0 0, 0
-I 20
10
30
40
50
f
I
CATHODE CURRENT DENSITY IN AMPERES PER F T ~ Figure 5.
Voltage-current relationship in regeneration cell
The-potential on the cathode determines which reactions occur. constant
reduced to cuprous ion rather than to copper because of the stability of the chlorocuprous complex. Keeping the same anode area, if the cathode area is diminished \vithout change in the current flow, the current density a t the cathode increases until a point is reached a t which cupric ions are reduced to cuprous as rapidly as they diffuse to the electrode surface. There exists, then, a t the cathode surface: a state of concentration polarization. Further increase in current density cannot increase the rate of reduction of cupric ions to cuprous: if the current density is increased by making the cathode area still smaller, some other electrode process must begin-reduction of
The anode potential stays nearly
cuprous ion ro merallic copper. - i t some appropriately high current density all the cupric and cuprous ions which diffuse to the electrode surface \vi11 be reduced to copper. As the currenr density at the anode has not been changed, the reaction at this elactrode \vi11 still be oxidation of cuprous to cupric ion. The net reaction. then: is oxidation of part of the cuprous to cupric ion and reduction of a n equal part of the cuprous to copper. This is simply a forced disproportionation. the reverse of the etching process. and hence. reqeneration of the etching bath. IVhether or not cupric reduction is steprvise is immarerial. If the current \vrre increased further,
+ C u L 2+ 6 r - 5Cu 6Cu" 6Cu-2 + 6elOCu+l-+ 5 C u o + 5Cu-2
4Cu-I
+ .
+ .
-~~. ~-
(4)
(5) (6)
Similarly, as the regeneration nears completion, Equations 7 , 8, and 3 may be set doivn for a cuprous-cupric ratio of 1 to 4. Cu+'
+ 4Cu+2 + 9 e - + 5Cu' ( 7 ) + 9P(8) + ~ C U + ~ (9)
- 9Cu"-, 9cu-2 1 0 C ~ + ~ -5 ,C U 0
+ BATH Z--L Z___-p IB ~~
Figure 6.
- --
ATH -1-T - .- -
.-T-y-
- _-
-
Recovering copper in bulk form by regeneration
This is one of several ways by which the copper can b e recovered
296
INDUSTRIAL AND ENGINEERING CHEMISTRY
The regeneration at this stage requires nine electrons, approaching one electron per cuprous ion as regeneration continues. \Vith a properly designed cell. assuming 50% efficiency of rectification and a n initial cost of 1 cent per kw.-hr., the cost of electricity to regenerate the solution used to etch 1000 square feet of 2ounce copper isill be about 5'1. T o fill a n etching tank (52 gallons) 1i.ith ferric chloride etching solution costs about $70. This \vi11 etch about 200 square feet of 2-ounce copper. Maintaining a good etching rate by addition of hydrochloric acid mill cost about $5. These figures do not include labor, scorage, or disposal charges. Regenerating the cupric chloride etching solution used to etch the same amount of copper xiould
PRINTED WIRING cost about $0.20 for electricit>-. The copper recovered from the solution has a market value of about $5. This solution can he regenerated automatically, so the return from the process may more than pay for maintenance as well as the labor cost of handling recovered copper. A solution with added copper has been regenerated more than 30 times Xvithout apparent change in the solution or its etching charxteristics. .A 2.11 cupric chloride solution saturated with sodium chloride and ivith 30 grams per liter of copper added byas regenerated elecirolytically. The copper was allo\ved to redisso!\-e and the solurion regenerated again. Potenrial measurements during etching sho\ved that the behavior of the etchinq solution \vas the same throughout the experiment. In a production apparatus, the copper must be removed. This can be done in several Lvays. Figure 6 shows one technique by which copper can be recovered as massive copper. This model has heen used successfully in regeneration of used etching solution. The moving electrode serves as the cathode in the etching cell, then moves through a rvash to the stripping cell, lvhere it is the anode. The copper is deposited as a smoorh plate. Another method uses an endless belt electrode. of a material ta which copper adheres poorly; titanium could be used. Again the moving electrode serves as the cathode in the etching bath, but the copper is plated a t relatively high current density as a rough deposit easily removed by scraping. Other mcthods might include withdrawal of a copper bar electrode as it is formed. I t is only necessary that the electrode area remains nearly constant throughout the regeneration. This regeneration process is readily amenable to control. Obviously: the cathode potential must be regulated. direct1)- or indirectly. The lower limit is the minimum deposition potential. The upper limit is the potential needed to reduce hydrogen ion. These are liberal conditions; the controls can be simple. For experimental scudies a potentiostat is the most convenient poiver supply. For process control? this could be replaced by a galvanometer relay arranged to s\vitcli resistances in or out of the l o ~ v voltage circuit or increase or decrease the supply circuit \.oltage. Figure 7 s h o w one control technique using a reference electrode. hlaintaining the cathode at about -0.8 volt against the silver-silver chloride electrode makes reduction of copper certain, but if electrolysis is continued after all the cuprous ion has been consumed, some neiv electrode process would have to start a t the anode to maintain the current flow-oxidation of chloride ion
F:AT
THIS POTENTIAL I S MAINTAINED CONSTANT BY POTENTIOSTAT
REFERENCE
\
\
-
THIS POTENTIAL IS CHANGED BY POTENTIOSTAT IN ORDER TO MAINTAIN CONSTANT CATHODE-REFERENCE POTENTIAL
-
,'
ANODE
'k
THIS POTENTIAL CHANGES RAPIDLY WHEN REGENERATION IS COMPLETE
I Figure 7.
Circuit for control of regeneration (reference electrode method)
This process operates intermittently, regenerating the etchant completely then stopping until the solution i s re-used
to chlorine. This Lvould require a change in anode potential of several tenths of a volt, so this change can be used to shut off the regeneration current. If the control equipment is designed to shut off the process, just as the rapid change begins, the regeneration is virtually complete. The control equipment may be designed to begin regeneration again \\.hen the potential indicates suine particular concentration of cuprous ion. If a silver-silver chloride electrode is used. the silver \$-ill dissolve slowly, but this causes no trouble because the silver \vi11 reduce a t the cathode more readily than copper. TVhen a cell has been designed and its operating characteristics are known, a constant current system is convenient; the currcnt is hcid constant at a value to cause deposition of copper. The cathode potential will not change significantly during electrolysis! because there is a plentiful supply of copper in solution. The anode potential, however: \vi11 change rapidly on depletion of the cuprous ion. Consequently, the total cell voltage \ d l increase. This increase can be used to shut down the regeneration process. The process cannot be conveniently started at a given cuprous
ion concentration unlcss a reference electrode is used. but even this is not necessary. -4 time delay could be used, so that the regeneration cycle Mould start again, say: 30 rninutes after it was shut off. This would be essentially a test process. If the etching bath had not been used in the meantime, the regeneration circuit would be shut off for another half h o u r . LVith a large enough poiver supply and electrodes and using the constant current technique, one apparatus could regenerate a number of etching baths in sequence. It \.iould be designed to complete a regeneration, automatically test the next tank, and, if necessary, carry out regeneration. Some time delay Ivould be required to prevent "hunting" when no rtching was done in any tank. If the etching solution does not become heavily loaded with copper, the anode potential may be controlled as shown in Figure 8. This system could be used to advantage Lvhen continuous regeneration is needed to maintain high cupric and low cuprous concentrations. Cathode potential is controlled indirectly. I n this technique the anode is controllrd a t a potential not quite sufficient to
/
I \ C ATH 0 DE
-I IIIII I I I, I, III I II IIII I I I" II I II III I II IIII I I , I I I
1-11e.1
REFERENCE r.io 2
llitnlut
T H I S POTEhTIAL IS C h A N G E D BY POTENTIOSTAT IN ORDER TO MAINTAIN CONSTANT ANODE-REFERENCE POTENTIAL
1-1-11
*E
I
I1
1
111111
ONE OF THESE POTEhTIALS IS MAINTAINED CONSTANT BY POTEN TI 0 STAT
1-1-11
1-1-11
'
T
A
N
O
D
I
E REFEYENCE N O I
,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,1~illl1ll~~lll
\8.
Figure
1
I
Circuit for control of regeneration (controlled anode method)
This process operates continuously.
The rate of regeneration varies with the amount of cuprous ion
vOL. 51, NO. 3
MARCH 1959
297
evolve chlorine. The current varies with the cuprous-cupric concentration ratio; if a substantial amount of cuprous ion is present, a high current is required to maintain the anode potential. If this current is sufficiently high, cupric and cuprous ions will not diffuse to the cathode rapidly enough to keep the cathode polarization within the required range. This will result in hydrogen evolution. Attempts to use this technique in its simplest form-i.e., with the reference electrode in the position given as Reference I-on depleted solutions might result in precipitation of cuprous hydroxide because of removal of hydrogen ions. -4measure of self-biasing is available by use of the voltage drop through the solution. If the reference electrode, instead of being placed near the anode or shielded from potential gradients, is placed near the cathode (Reference 2): the voltage drop through the solution will appear as part of the anode potential. Now the larger flow of current in a depleted solution \vi11 cause a greater potential drop, causing the actual anode
potential to correspond to some higher cuprous-cupric ratio than the final control point. This, in turn, will cause the controller to appl) a lesser voltage to the cell than otherwise required. Hence, the cathode potential could be limited so that hydrogen ions are not discharged. As the solution is regenerated, the effective control point moves to\irard the final control point. As the solution approaches the final concentration, the curi-enr: decreases, decreasing cathode polarization, This lo\cer polarization permits progressively more reduction of cupric ions only to cuprous ions. At the control point there is no net reaction, because cuprous ion is oxidized to cupric a t the anode and cupric ions arc reduced to cuprous at the cathode. The current is very low.
cutting produces better printed wiring and photoengravings. Elimination of doum time and disposal and handling. of new and used etching solutions, coupled with the negligible cost of electricity and recovery of the etched copper. constitutes an extensive saving in the cost of etching. Continuous regeneration will keep etching rates constant. permitting assembly line etching. literature Cited
(1) Black, 0. D., Cutler: L. H., Isn. EXG. CHEM.50, 1539-40 (1958). (2j Photoengraoers Bull. 1947 (November) 19-24. (3) Photogrufhic J . 1915 (hprili 1 6 6 ~ 7 5 . (4) Swiggett, R. L., .Uodern Plnstrcs 31, NO. 8, 94-.5 (1954). (5) Vaaler, L. E., Photoengiarers Bull. 1948 (May) 41-4. RECEIVED for review hfay 8, 1958 ACCEPTED December 1, 1958
Conclusions
Cupric chloride solutions with excess chloride ion are superior to ferric chloride solutions for etching copper. The clean, smooth etch with less under-
Division of Industrial and Engineering Chemistry, Symposium on Chemical Aspects of Printed Wiring, 133rd hleeting, ACS, San Francisco, Calif., April 1958.
CORRESPONDENCE N e w Method for Etching Copper SIR: TVe disagree with several statements which appear in “New Method for Etching Copper” [Black and Cutler, IND.ESG. CHEM.50, 1539-40 (1958)l. 1. The authors state that hydrochloric acid was used “because it is inexpensive, easily handled, and readily obtainable.” O n the basis of these reasons it is strictly fortuitous that they chose the best acid. As we point out [IND. ENG. CHEM. 51, 293 (1959)], chloride ion is the essential ingredient in this system. We suggest that the authors try to devise a similarly regenerable system using other inexpensive, easily handled, and readily obtainable acids-e.g., sulfuric or nitric acid. 2. The authors conclude from their thermodynamic evaluation of the process that it is feasible and state that “thermodynamically the intermediate steps are unimportant.” it‘hile this may be true, kinetically the intermediate steps are of prime importance and one needs more than just cupric and hydrogen ions. This Ivhole process is based upon the ability of chloride ion to form stable soluble complexes with cuprous ion. The fact that the solution is a good etchant for copper is due to the formation of these chlorocuprous complexes. 3. The errors pointed out in 1 and 2 are a natural consequence of the Edisonian approach of these authors. They state, “therefore, the various ions present in the spent (ferric chloride) bath were
298
determined and simple solutions were prepared and carefully checked. . .” This further strengthens our feeling that the author‘s choice of hydrochloric acid was happily fortuitous and was made with no understanding of the s y s tem. 4. The oxidation of copper(1) to copper(I1) by oxygen is, we believe. well known and should not have been surprising to the authors. 5. The method of control uhich is used cannot be as simple as the authors state The addition of hvdrochloric acid must be based on the amount of copper etched rather than on the number of boards, since the amount of copFer etched mav vary Ttith the tIpe of board Ana1)sis of the solution on a routine basis is necessary.
LOUISH. SH.ARPE PACLD. GARN Eel1 Telephone Laboratories, Inc., Murray Hill, N. .J. SIR: Dr. Sharpe’s point that the chloride ion is necessary for the establishment of a commercially feasible regenerative system for etching copper is M ell taken and amply corroborated by the excellent experimental work done by him and Dr Garn. The formation of the complex ions of copper and chloride icas noted in our puhlication, although i t was not stressed. We were interested in the development of a commercial process to replace
INDUSTRIAL AND ENGINEERING CHEMISTRY
the ferric chloride sytem and \\.hen we found one \ve were surprised, not that the copper(1) is oxidized to copper(II), but that the reaction is so fast. The entire process is so simple that i t seemed odd that it had not been placed in use years before. The kinetics of the reaction had already established themselves as satisfactory, so \ye \yere interested in the thermodynamics only to make sure that kve were not dealing svith an impossible situation which had been caused to happen fortuitously but \vould later stop. The method of control which has been in operation in our commercial process for over 2 years is chat of checking the time required for etching a known thickness of copper. \\.hen this begins to increase appreciably. some spent solution is drained off and acid water added t u make up the required amount. In our setup this \could amount to perhaps 2 gallons‘ change at a time. Chemical checks are made only occasionally, perhaps once in 6 or 2 months, and no particular difficulty has been encountered. \$’e appreciate Dr. Sharpe’s bringing to everyone‘s attention more clearly the necessity of the chloride ion in making a rapid regenerative system.
OTISD. BLACK LEOS.+RDH. CUTLER Radio Corp. of America, Camden 2: N.J.
