US3600244A

US3600244A - Process of etching metal with recovery or regeneration and recycling

Info

Publication number: US3600244A
Application number: US800979A
Authority: US
Inventors: Herbert Wegener
Original assignee: International Business Machines Corp
Current assignee: International Business Machines Corp
Priority date: 1969-02-20
Filing date: 1969-02-20
Publication date: 1971-08-17
Anticipated expiration: 1988-08-17

Abstract

THIS INVENTION REFERS TO A PROCESS WHEREBY THE REACTION PRODUCTS OF ETCHING COPPER, COBALT, IRON, NICKEL, ZINC, OR MAGNESIUM WITH CUPRIC CHLORIDE OR FERRIC CHLORIDE ARE PRECIPITATED AS A DOUBLE SALT OF THE DISSOLVED METAL CHLORIDE AND AMMONIUM CHLORIDE BY COOLING THE SOLUTION. THE DOUBLE SALT IS SEPARATED FROM THE REMAINING SOLUTION, THE

ETCHING SOLUTION IS RECOVERED AND REGENERATED BY THE USE OF AN OXIDANT SUCH AS CHLORINE, MURIATIC ACID AND HYDROGEN PEROXIDE, MURIATIC ACID AND AIR, OR MURIATIC ACID AND OXYGEN.

Description

Aug. 17, 1971 WEGENER 3,600,244

PROCESS OF ETCHING METAL WITH RECOVERY 0R REGENERATION AND RECYCLING Filed Feb. 20, .1969 3 Sheets-Sheet l ETCHING REACTION: 2E8 cw Cu 2% me +cu 0E2 REGENERATION REACTIONI 2Fe CL2 CL2"2FeCL3 COPPER PANELS IN ETCHED CIRCUITRY ETCHER NH4CL REGENERATOR CRYSTALLIZER CHLORINE 6A5 DOUBLE SALT CuCL2-2NH4cL-2H20 BASIC FERRIC CHLORlDE RECOVERY SYSTEM BASIC CUPRIC CHLORIDE RECOVERY SYSTEM ETCHING REACTIONI cu (N2 cu -zcu CL REGENERATING REACTIONI ZCuCL +CL2 -2cucl 2 COPPERS PANELS IN EETCHED CIRCUHRY E m H E R om m NH4CL CHLORINE GAS REGENERATOR CRYSTALLIZER 4 DOUBLE SALT CuCL2'2NH4CL-2H20 MIX TANK INVEN/O/P HERBERT WEGENER AGENT Aug. 17, 1971 Filed Feb. 20, 1969 H. WEGENER g 2 I g i ('TVS/ZOT NOIiVHlNl-TONOO HHddOO I I I I (NV ION) NOILVLHNHONOO LHddOf) 3,600,244 PROCESS OF E'I'CHING METAL WITH RECOVERY 0R REGENERATION AND RECYCLING 3 Sheets-Sheet 2 (MOLAR) TYPTCAL OPERATING CYCLE OF FERRIC CHLORIDE CRYSTALLTZATION PROCESS WITH RESPECT TO MUTUAL SOLUBILITIES OF CuCL2 AND NH4 CL IN 3.0 MOLAR Fe CL? SOLUTIONS PROCESS OF ETCHING METAL WITH RECOVERY OR REGENERATION AND RECYCLING Herbert Wegener, Endicott, N.Y., assignor to International Business Machines Corporation, Armonk, N.Y. Filed Feb. 20, 1969, Ser. No. 800,979

Int. Cl. C23g 1/36; C23f 17/00; Hk 3/06 US. Cl. 15619 9 Claims ABSTRACT OF THE DISCLOSURE This invention refers to a process whereby the reaction products of etching copper, cobalt, iron, nickel, zinc, or magnesium with cupric chloride or ferric chloride are precipitated as a double salt of the dissolved metal chloride and ammonium chloride by cooling the solution. The double salt is separated from the remaining solution, the etching solution is recovered and regenerated by the use of an oxidant such as chlorine, muriatic acid and hydrogen peroxide, muriatic acid and air, or muriatic acid and oxygen.

BACKGROUND OF THE INVENTION The invention is directed to an etching system that permits the recovery or regeneration of a spent etchant and the recycling of the regenerated or reconstituted etching solution, thus enabling the etching process to be carried out under steady state conditions if so desired. More particularly, this invention relates to a system wherein copper-cladded boards may be etched at a constant rate by cupric chloride or ferric chloride etchant and in which the spent etchant may be recovered by the removal of the copper salts therefrom, which salts are the product of the etching process, said recovered spent etchant being either refortified or regenerated for subsequent use.

In the printed circuit art, copper-cladded boards are etched according to a desired pattern to obtain electrically conductive circuitry. The desired conductive circuitry is obtained by coating the surface of the boards with an etch resist in the desired pattern leaving the unwanted metal exposed. The removal of the unwanted metal, normally copper, is accomplished by etching with a solution in which the metal is soluble. The removal of the metal from the board is generally performed in an etching chamber through which a succession of metal-cladded boards are conveyed and sprayed with an etchant to efiect the dissolution of the metal.

The etchant in the case in which copper is the metal to be dissolved is generally selected from ferric chloride, cupric chloride, or ammonium persulfate solutions. The etching proceeds according to the chemical reaction Equations 1 for ammonium persulfate, 2 for cupric chloride, and 3 and 4 for ferric chloride:

CuCl +Cu-9 2CuCl (Equation 2) 2FeCl Cu+ 2FeCl CuCl (Equation 3) CuCl +Cu+2CuC1 (Equation 4) With ferric chloride at lower copper concentrations, e.g. 6 oz. 1 gal., dissolved copper exists in the cupric state because it is oxidized by the ferric ion,

Fe+ +Cu+ +Fe+ -{-Cu" (Equation 5) and therefore, Equation 3 describes the overall etching reaction at low copper concentrations.

The ammonium persulfate, cupric chloride, and ferric chloride etching systems are generally batch systems, i.e., systems in which the etch chamber is charged with a discrete volume of fresh etchant solution. The fresh etchant United States Patent O ice is recycled from the bottom of the etching chamber and is sprayed or splashed onto the surfaces of the coppercladded boards. The exposed copper is etched according to the above chemical equation, and the reaction products are dissolved in the etchant. The etch rate is continuously reduced as the etchant concentration diminishes and reaction products increase. The reaction is continued to a point where the rate becomes prohibitively slow. At this point, the etchant, containing both reaction products and unused etchant, is dumped into waste treatment facilities and the system is recharged with a fresh batch of etchant.

In batch etching systems the disposal of the unused etchant with the spent solution as Waste makes the process an expensive one. Further, steady state etching is, at most, difficult to maintain because as the etchant is contamined with the etching by products its concentration is reduced causing a decrease in the etching rate. To compensate for changes in etching rates, it is necessary to continually vary the speed of the conveyor carrying the copper-cladded boards to insure removal of the exposed copper prior to the boards emergence from the chamber. This is necessary to avoid overetching and undercutting lines due to overexposure and to provide efiicient machine utilization.

In those installations having batch etching systems from which the spent etchant is disposed as waste into streams and rivers, there arises the additional problem of stream pollution. In order to overcome this problem, it is imperative that the copper salts are removed from the spent etchant prior to its discharge into the streams or rivers.

SUMMARY OF THE INVENTION In the past ammonium persulfate etching systems have been used wherein the double salt of the reaction products were removed and the remaining etching solution recovered and fortified with ammonium persulfate. Although ammonium persulfate systems were rather costly and did not produce as good a quality etch as was desired, they were used due to the fact that the copper-cladded boards were provided with a tin-lead plating resist which was not affected by the ammonium persulfate. The tin-lead resist was ultimately replaced by an organic resist and it was then found that superior steady state etching of printed circuit boards could be carried out. In accordance with the present invention, this is accomplished through the combination of an etching system and an etchant recovery system wherein the etchant used may be either ferric chloride or cupric chloride. The ultimate concept in etching is a continuous operation in which a metal is removed from the etchant at a rate in which it is dissolved into the etchant and the etchant consumed during the etching process is replaced at an equivalent rate. These operations have been performed with the present ferric chloride and cupric chloride systems, said systems being characterized by having constant etching characteristics. The present systems provide advantages over prior art systems in that chemical costs are very low because the only chemicals regularly added to the system are ammonium chloride salt and chlorine gas, higher quality etching is obtained with less undercutting whereby better finished boards are produced, and the systems are better adapted for either continuous or batch operation.

Therefore, an object of this invention is the provision of an improved etching system which overcomes the problems of the existent systems.

Another object of this invention is the provision of a continuous etching system in which steady state etching may be accomplished without the need for continuously adjusting the conveyor speed.

Yet another object of this invention is the provision of a steady state etching system in which the reaction prodnets are removed and the spent etchant is refortified or regenerated and reused.

Still another object of this invention is the provision of a steady state etching system in which about 90% or more of the etchant may be advantageously utilized.

Yet another object of this invention is the provision of a steady state etching system in which the etch rate will remain constant.

In addition to the outstanding advantages of obtaining steady state etching, this invention provides additional advantages such as the more efficient utilization of the etchant, e.g., about 90% or more of the etchant is utilized in the system of this invention. Additionally, the etching reaction by-products are removed as solids in the form of saleable salts, thus eliminating the handling of large quantities of solution. Further, the problem of stream pollution is no longer existent, since the contaminating agents are removed as the above state saleable salts.

The foregoing and other objects, features, and advantages of the invention will be apparent from the following more particular description of preferred embodiments of the invention, as illustrated in the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic flow diagram of a preferred basic ferric chloride recovery system.

FIG. 2 is a diagram illustrating a typical operating cycle of the ferric chloride crystallization process shown in FIG. 1.

FIG. 3 is a schematic fiow diagram of a preferred basic cupric chloride recovery system.

FIG. 4 is a diagram illustrating a typical operating cycle of the cupric chloride crystallization process shown in FIG. 3.

DESCRIPTION OF PREFERRED EMBODIMENTS The system concept can be summarized by the schematic presentations shown in FIGS. 1, 2, 3, and 4 for the ferric chloride and cupric chloride recovery systems.

In FIG. 1, ferric chloride having an initial predetermined molar concentration is flowed across a metal part, e.g. copper, in an etcher and an increase in copper concentration and a decrease in ferric chloride concentration thereby results. The solution is then flowed through a crystallizer where a portion of the copper is removed from the solution in the form of a double salt,

CuCl 2NH Cl- 2H O It is desirable to add ammonium chloride, in an amount equivalent to that which will precipitate out in the double salt, in the crystallizer as shown. The solution is next sent to a regenerator where an oxidant such as chlorine gas is used to regenerate the etchant. A typical operating cycle is presented on a stepwise basis in FIG. 2. Mutual solubilities of cupric chloride and ammonium chloride in 3.0 molar ferric chloride are presented for a temperature of 70 F. A conventional 34 Baum ferric chloride etchant contains approximately 3.0 molar ferric chloride and 0.55 molar ammonium chloride and corresponds to point A on FIG. 2.

Copper is dissolved during etching to a concentration of approximately 6 oz./gal., shown as point B. At this copper concentration, the etch rate is nearly the same as that of fresh etchant A. After etching, ammonium chloride is added and the etchant corresponds to point C. During crystallization, the solution is cooled to 70 F etched copper is removed as CuCl -2NH Cl-2H O, and the solution returns to point D. After a regeneration step, copper is again dissolved to point B and the cycle BCD repeated with regeneration on each pass. With the ferric chloride system, heat generated during chlorination raises the etchant to the required etching temperature.

Two important points should be noted concerning the addition of ammonium chloride. First, the quantity of ammonium chloride added must be equivalent to that re- 4 moved in crystallizing the double salt and this amount can be predicted from data in FIG. 2. Second, ammonium chloride can be added either before or during the crystallization step. In most cases, it is desirable to add ammonium chloride during crystallization.

A ferric chloride crystallization system can be operated on a batch, semi-continuous or continuous basis and can tolerate relatively high amounts of etched copper without significantly decreasing the etch rate or forming salt crystals in the etcher. The ferric crystallization system requires control of both copper and iron concentrations. Perhaps the ferric system is best adapted to a batch process be cause of the conveniently high copper concentration that can be tolerated and the requirement that both copper and iron concentrations be controlled.

In FIG. 3, cupric chloride having an initial predetermined molar eoncentration is flowed across a metal part, e. g. copper, in an etcher and an increase in cuprous chloride concentration and a decrease in cupric chloride concentration thereby results. A portion of the solution is then flowed through a crystallizer where a portion of the copper is removed from the solution in the form of a double salt, CuCl -2NH Cl'2H O. It is desirable to add ammonium chloride, in an amount equivalent to that which will precipitate out in the double salt, in the crystallizer. The solution leaving the crystallizer is mixed with that portion of the solution leaving the etcher that did not pass through the crystallizer in the mix tank as shown in FIG. 3. The echant is regenerated with an oxidant such as chlorine gas either before or after crystallization.

The cupric etching system cannot tolerate relatively high amounts of etched copper without significantly decreasing the etch rate. To maintain a high etch rate with the cupric chloride etchant, the amount of etched copper in the system should be kept at a low level. The low copper level requires a high solution flow rate to and from the etcher. This requirement increases the heating and cooling requirements between the 120 F. etcher and the 70 F. crystallizer. The heat transfer requirements for the cupric chloride system can be substantially reduced by using the process flow shown in FIG. 3. Here, the crystallizer is placed in parallel with a larger, uncooled, uncrystallized stream. After crystallization, the streams are mixed, regenerated, and recycled to the etcher.

Sufficient ammonium chloride is added to the crystallizer to reduce the copper concentration by approximately 4 oz./ gal. Because no copper is removed from the bypass stream, the ratio of crystallized to uncrystallized streams can be controlled to provide the desired concentrations in the mix tank. The following condition must be satisfied:

FeACu,,=F,,ACu where F=solution flow rate Subscript e=condition across the etcher Subscript c=condition across the crystallizer This technique permits high solution flows and a low copper concentration increase across the etcher and simultaneously low solution flows and a high copper concentration increase across the crystallizer. Heat transfer requirements are substantially reduced with no detrimental effect on the etching operation.

FIG. 4 shows the step-wise operating cycle for the cupric chloride system using the by-pass technique. Mutual solubilities of cupric chloride and ammonium chloride in 0.1 molar hydrochloric acid are shown at 70 F. Point A represents an initial or reprocessed etchant. Copper is etched to point B. A small portion of solution B is passed through the crystallizer, ammonium chloride is added, and the solution crystallized at 70 F. to point D. Addition of ammonium chloride can be made simultaneously with crystallization. Subsequent mixing of crystallized solution D with the remainder of spent etchant B returns the mixture to point A. After regeneration, etchant A is returned to the etcher.

Although FIG. 4 shows a crystallization temperature of 70 F., it may be desirable to crystallize at a temperature of 50 F. or 60 F. A lower crystallization temperature would allow reprocessed etchant A to remain unsaturated at room temperature and the etchant can then be stored under ambient conditions without crystallization.

For carrying out the present invention as it relates to ferric chloride solutions, a fresh aqueous 3 molar (M) ferric chloride solution is made containing about 487 grams of ferric chloride per liter of solution and 294 grams of ammonium chloride per liter of solution.

The etching solution is then heated to a temperature of about 55 C. and used to etch printed circuit panels.

Etching is continued until the ferric chloride concentration decreases to about 1.6 molar in ferric chloride. At this point, the solution is capable of further etching but the etch rate and quality of etch diminishes.

The spent solution is treated in accordance with the present invention by adding about 1 mole of ammonium chloride and cooling the solution to about 21 C. whereupon a copper chloride and ammonium chloride double salt crystallizes and precipitates from the solution. The exact temperatures and concentrations are not critical, but it is desired that it be such that cupric chloride and ammonium chloride double salt precipitate out without substantial amounts of ferric or ferrous chloride.

The precipitated salts can be collected in a chamber and the solution drained from the crystals.

The resulting solution, about 1.6 molar in ferric chloride, and 1.4 molar in ferrous chloride, 0.24 molar in cupric chloride, and 0.55 molar in ammonium chloride, is sent to the chlorinator where the ferrous chloride is oxidized to ferric chloride. The resulting solution, about 3.0 molar in ferric chloride, 0.24 molar in cupric chloride, and 0.55 molar in ammonium chloride, is returned to the etcher. When the ferric chloride concentration is decreased to 1.6 molar, the solution is sent to the crystallizer where ammonium chloride is added and the solution is cooled as indicated above.

It is possible to add the ammonium chloride either be fore or during the crystallization step. It is desirable, however, to add the ammonium chloride during the crystallization step since the addition of ammonium chloride results in the precipitation of more cupric chloride and ammonium chloride than for the case where the addition of the ammonium chloride follows the crystallization ste F or carrying out the present invention as it relates to cupric chloride solutions, a fresh aqueous 2.54 molar cupric chloride and 1.75 molar ammonium chloride solution is made containing about 342 grams of cupric chloride per liter of solution, and 94 grams of ammonium chloride per liter of solution. The etching solution is then heated to a temperature of about 40 C.-55 C. and use to etch printed circuit panels.

Etching is continued until the copper concentration reaches about 2.62 molar. At this point, the solution is capable of further etching but this etch rate and quality of etch diminishes.

The spent solution is treated in accordance with the present invention by adding about 0.8 mole of ammonium chloride per liter of solution and cooling the solution to about 21 C. whereupon copper chloride and ammonium chloride crystallize and precipitate from the solution. The exact temperatures and concentrations are not critical but it is desired that it be such that cupric chloride and ammonium chloride precipitate out without substantial amounts of cuprous chloride.

The resulting solution, about 1.75 molar in ammonium chloride and 2.54 molar in copper, is sent to the chlorinator where the cuprous chloride is converted to cupric chloride. The resulting solution, about 2.54 molar in cupric chloride and 1.75 molar in ammonium chloride, is

6 sent to the etcher. When the copper concentration is increased to about 2.62 molar, the solution is sent to the crystallizer where ammonium chloride is added and the solution is cooled as indicated above.

Although the above cooling temperature was indicated as 21 C., it may be desirable to crystallize in a temperature range of 10 C. to 15 C. A lower crystallization temperature would allow reprocessed etchant to remain unsaturated at room temperature. The etchant can then be stored under ambient conditions without crystallization.

While the invention has been particularly shown and described with reference to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention.

What is claimed is:

1. A process for precipitating the reaction products of etching copper with ferric chloride in an etching solution containing ferric chloride, ferrous chloride, cupric chloride and ammonium chloride which comprises the steps of cooling the solution to 21 C. to crystallize the double salt of cupric chloride and ammonium chloride from the solution without crystallizing large amounts of ferric chloride or ferrous chloride;

separating said double salt from the remaining solution;

adding ammonium chloride to the solution in an amount equivalent to that which precipitated out in the double salt; and

regenerating the solution by the use of an oxidant such as chlorine, muriatic acid and hydrogen peroxide, muriatic acid and air or muriatic acid and oxygen.

2. A process according to claim 1 wherein before or during cooling of the solution, said ammonium chloride is added in an amount equivalent to that which will precipitate out in the double salt.

3. A process for continuous etching with an aqueous 3 molar ferric chloride solution which comprises the steps of:

etching copper with an etching solution containing ferric chloride, ammonium chloride, ferrous chloride, and cupric chloride;

adding ammonium chloride to said solution;

cooling the solution to 21 C. to crystallize the double salt of cupric chloride and ammonium chloride without crystallizing large amounts of ferric chloride or ferrous chloride;

said ammonium chloride which was added to the solution being added in an amount equivalent to that which precipitates out in said double salt; separating said double salt from the solution; regenerating the solution by the use of an oxidant such as chlorine, muriatic acid and hydrogen peroxide, muriatic acid and air, or muriatic acid and oxygen; and

recycling the regenerated etching solution back to the etcher.

4. A process according to claim 3 wherein after cooling said solution, said ammonium chloride is added in an amount equivalent to that which precipitated out in the double salt.

5. A process for precipitating the reaction products of etching copper with cupric chloride in an etching solution containing cupric chloride, cuprous chloride and ammonium chloride which comprises the steps of;

cooling the solution from 10 C. to 21 C. to crystallize the double salt of cupric chloride and ammoium chloride from the solution Without crystallizing large amounts of cuprous chloride;

separating said double salt from the remaining solution;

regenerating the solution by the use of an oxidant such as chlorine, muriatic acid and hydrogen peroxide, muriatic acid and air, or muriatic acid and oxygen.

6. A process according to claim 5 wherein before or during cooling of the solution, said ammonium chloride is added in an amount equivalent to that which will precipitate out in the double salt.

7. A process for continuous etching with an aqueous 2.54 molar cupric chloride and 1.75 molar ammonium chloride solution which comprises the steps of;

etching copper with an etching solution containing cupric chloride, cuprous chloride and ammonium chloride;

adding ammonium chloride to said solution;

cooling the solution to 21 C. to crystallize the double salt of cupric chloride and ammonium chloride from the solution without crystallizing large amounts of cuprous chloride;

said ammonium chloride which was added to the solution being added in an amount equivalent to that which precipitates out in said double salt;

separating said double salt from the solution;

regenerating the solution by the use of an oxidant such as chlorine, muriatic acid and hydrogen peroxide, muriatic acid and air, or muriatic acid and oxygen; and

recycling the regenerated solution back to the etcher.

8. A process according to claim 7 wherein said solution is cooled from 10 C. to 21 C.

9. A process according to claim 7 wherein after cooling said solution, said ammonium chloride is added in an amount equivalent to that which will precipitate out in the double salt.

References Cited UNITED STATES PATENTS JACOB H. STEINBERG, Primary Examiner US. Cl. X.R. 156-3; 25279.l