Process and apparatus for etching copper and copper alloys

Abstract

Copper and copper alloys, in particular copper-coated laminates, are etched by immersing them in or spraying them with a solution of cupric chloride and regenerating the formed cuprous chloride during continuing operation by adding hydrogen peroxide and hydrochloric acid to the etching solution, the amount of the addition being controlled by measurements of the redox potential by means of a redox electrode and the control being adjusted to add predetermined amounts of the said two additives whenever the copper-I-ion concentration reaches a predetermined value, preferably 0.4 g/l so as to maintain a constant etching speed. An apparatus for use in this method comprises a tank for the etching solution, means for applying the solution to the copper or copper alloy material, a redox electrode immersed in the said tank, supply vessels for holding the regenerating additives, a conduit between said supply vessels and the tank, valve means for passing the regenerating liquid from the supply vessels to the tank and control means for operatively connecting the valve means to the redox electrode.

Description

BACKGROUND OF THE INVENTION

The invention concerns a process for etching printed circuit boards in immersion and spray apparatus.

A variety of methods exists for etching conductor plates, e.g., printed circuits. Known etching agents are aqueous solutions of ferric chloride (FeCl.sub.3), ammonium persulphate (NH.sub.4).sub.2 S.sub.2 O.sub.8), sodium chlorate and hydrochloric acid (NaCIO.sub.3 + HCl), sodium chlorite (NaClO.sub.2), hydrogen peroxide and sulfuric acid (H.sub.2 O.sub.2 + H.sub.2 SO.sub.4), and cupric chloride (CuCl.sub.2) with various regenerating agents such as air and oxygen. Immersion or spray apparatus is used for etching, the latter, in particular, in the form of a continuous process operation particularly suitable for manufacture of conductive sheets on a conveyor belt. An economical conveyor belt-type manufacture is conditional on maximum and constant throughput speed. In most etching processes so far known increasing copper content in the etching agent and decreasing etching agent power meant lengthened etching times. If, for example, the copper content rises from 0 to 50 g Cu/liter the etching time in an ammonium persulphate bath rises from 6 to 33 minutes and in a hydrogen peroxide/sulfuric acid bath from 5 to 21.5 minutes (German published application 1,253,008). In another process using hydrogen peroxide and sulfuric acid plus a catalyst, etching times of 1.5 minutes initially and 3 minutes at approximately 35 g Cu/liter have resulted. At higher copper contents the etching time is further lengthened and becomes uneconomical. With iron chloride as etching agent, initial etching time is about 1 minute, but even when ferric chloride is used as the etching agent the initially high etching rate goes steadily down with rising copper contents and, in spray apparatus, etching times become uneconomical at copper contents of about 50 g resulting in low throughput rates.

It has already been proposed to use copper-II-chloride as etching agent and to regenerate the copper-I-chloride which reduces the etching speed, by means of hydrogen peroxide and hydrochloric acid. However, this type of process was not suited for industrial production since, on the one hand, even small amounts of copper-I-chloride substantially reduce the etching speed and, on the other hand, the addition of hydrogen peroxide is a highly critical matter since, if too much hydrogen peroxide is added, oxygen forms and the etching agent is subject to gassing. This again reduces the etching speed. Besides, the gassing etching agent can no longer be pumped with adequate pressure because the liquid pumps which are in particular used in connection with a spraying operation are not suited for moving liquid-gas mixtures.

If, on the other hand, the addition of hydrogen peroxide is too small, an adequate regeneration of the copper-I-chloride does not take place and the residual copper-I-chloride thus again contributes to a reduction of the etching operation speed.

It is accordingly an object of the present invention to avoid the shortcomings described and to provide for an etching process and etching apparatus which is useful for industrial production, and in particular for the etching of conductive plates in a dipping or spraying operation.

SUMMARY OF THE INVENTION

This object is met by a method of etching conductive plates of which the surface is formed by copper or a copper alloy, the said method comprising the steps of applying an etching solution of copper-II-chloride to the plate by immersion or spraying and regenerating the formed copper-I-chloride in the etching solution by adding hydrogen peroxide and hydrochloric acid to the solution during continuing operation, the amount of the addition being controlled by a continuous measurement of the redox potential of the etching solution by a redox electrode and the said control being adjusted to add predetermined amounts of hydrogen peroxide and hydrochloric acid whenever the copper-I-ion concentration reaches a predetermined value, so as to maintain a constant etching speed.

The invention also embraces an apparatus for carrying out the method just defined, comprising a tank for holding the etching solution, means for applying the solution to the copper or copper alloy surface, a redox electrode immersed in said tank for measuring the redox potential, at least one supply vessel for holding a regenerating liquid, a conduit between said vessel and said tank, valve means for opening and closing said conduit and passing liquid from said supply vessel to said tank, and control means for operatively connecting said valve means with said redox electrode.

The novel features which are considered as characteristic for the invention are set forth in particular in the appended claims. The invention itself, however, both as to its construction and its method of operation, together with additional objects and advantages thereof, will be best understood from the following description of specific embodiments when read in connection with the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1a is a graph plotting the Cu.sup.+ ion concentration of the etching solution against the redox potential identified as u;

FIG. 1b is a partial enlarged view of the encircled portion of FIG. 1a;

FIG. 1c is a graph plotting the etching velocity v against the redox potential; and

FIG. 2 is a diagrammatic view of the apparatus of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The process of the invention permits to shorten the mean etching time obtainable with iron chloride by a substantial amount, that is by about 40%. This is of course of great significance for the economies of the operation.

While the initial etching speed of the process of the invention as compared with the customary iron-III-chloride process is somewhat smaller, this initial speed, by virtue of the process of the invention, can be maintained rather constant during the entire operation irrespective of the number of plates or sheets which pass through the etching apparatus and irrespective of the amount of copper subjected to etching.

In the apparatus of the invention, the dimensions of the conduits passing the regenerating liquid or liquids from the supply vessel or vessels to the tank holding the etching solution should be dimensioned so as to comply with the optimum amount-ratio of the regenerating chemicals.

It is preferred if the valves which control the addition of regenerating liquids are arranged to be actuated in succession so that a small excess of hydrochloric acid may thus be supplied. This will then definitely avoid the decomposition of the hydrogen peroxide into hydrogen and oxygen.

In general, when the etching is effected by using copper-II-chloride, copper-I-chloride will be formed according to the equation Cu + CuCl.sub.2 = 2 CuCl. A comparatively minute amount of copper-I-chloride, such as 4 g Cu.sup.+/l in the etching solution, which latter may for instance contain 120 g Cu.sup.+.sup.+/l in the form of copper-II-chloride, will noticeably slow down the etching speed as appears from the curve in FIG. 1c. However, it is of interest that the absolute value of the copper content does not have to be particularly constant in order to obtain a high etching speed. Rather, it may vary between 80 g Cu/l and 130 g Cu/l. It is rather important to maintain the Cu-I-chloride amount which is being formed during the etching operation at a low value, for instance below 2 g/l, and to reoxidize it as fast as possible to copper-II-chloride.

The regeneration, from the purely stoichiometric standpoint, takes place following the equation:

2 CuCl + H.sub.2 O.sub.2 + 2 HCL.fwdarw.2 CuCl.sub.2 + 2 H.sub.2 O

However, the actual mechanism of the reaction between these components is far more complex than expressed by the equation and does not require discussion here. As the stoichiometric equation shows, it is necessary to add the regenerating chemicals H.sub.2 O.sub.2 and HCl in required ratio, that is, for 34 g of H.sub.2 O.sub.2,2 HCl, i.e. 2 .times. 36.5 g, must be added. For instance, if a 35% concentration H.sub.2 O.sub.2 solution and a 37.5% concentration hydrochloric acid are used, the chemicals must be added at a ratio of 1:2, which can easily be done by means of the proper dimensioning of the conduits from the supply vessels.

The above equation of the reaction occurring during the regeneration also shows that hydrochloric acid is used up during the regeneration, resulting in a change of the pH of the etching solution. One might at first think that the pH could therefore be controlled by a pH electrode which would adjust the amount of addition of the regenerating agent. However, in practical applications, it has been found that in this manner optimum etching conditions cannot be obtained. If, on the other hand, the amount of addition is controlled by a redox electrode, as proposed in the present invention, the desired object can be accomplished with great precision.

During the etching operation there are present in the solution Cu.sup.+.sup.+ ions and Cu.sup.+ ions. The potential of the electrode depends on the concentration ratio, or more properly on the activity ratio, as indicated by the following formula ##EQU1## wherein the symbols have the following meaning: E electromotoric force (EME)

r gas constant

T absolute temperature

F Faraday-equivalent

The potential of the electrode will be positive when the concentration of the higher oxidation stage rises and it will reach the maximum value when the Cu.sup.+ ion concentration is zero. With increasing Cu.INTEGRAL. ion content, the potential of the solution changes. This change will be comparatively stronger in the area of small Cu ion concentration (0 to 4 g/l) than with higher increases in the Cu.sup.+ ion as is illustrated by the curves in FIG. 1a and FIG. 1b.

FIG. 1a illustrates qualitatively the interrelation between the etching velocity v and the redox potential u. The redox potential u, as will be seen, at 530 mV which occurs at a Cu.sup.+ ion concentration of less than 0.4 g/l (see FIG. 1b) will afford an optimally high and almost constant etching speed. It is therefore preferred to practice the invention by maintaining this limit value during the regeneration of the etching agent.

This change of potential is made use of in the apparatus of the invention for controlling the addition of the regeneration chemicals. With reference to FIG. 2 it will be seen that the apparatus 1 may for instance be used to process conductive plates 2 which consist of a copper-coated laminate. The conductive plates are passed by means of a conveyor belt 3 under the spray nozzles 4.

In order to regenerate the etching solution, the hydrogen peroxide and hydrochloric acid are fed from the supply vessels 8 and 9 through magnet valves 10 and 11 and are furthermore pumped by pumps 12 and 13 into the etching solution in tank 5. The magnet valves 10, 11 and the pumps 12, 13 are controlled by the potential as determined through the redox electrode 14 and the control measuring device 15.

The control device comprises, in a preferred form, an amplifier, a measuring device and a recording device which, through amplification, indicate or record the potential differences. The measuring device is provided with two adjustable sensors which cooperate with the hand of the measuring device. The sensors and the hand of the measuring device are designed to generate a signal whenever the hand in response to a change of potential in the etching solution moves from the area of the higher potential with a gradually increasing Cu.sup.+ ion concentration into the area of a lower potential and in so moving passes the sensors and causes a response thereby.

This response thus occurs at a definitely determined value and causes the opening of the magnet valves 10 and 11 in succession, and likewise causes actuation of the pumps 12 and 13 successively. Thus, the regenerating liquids H.sub.2 O.sub.2 and HCl are fed from the supply vessels 8 and 9 into the etching solution in tank 5.

The two sensors of the control device, in order to attain a constant high etching speed, are adjusted to prevent exceeding a potential difference of about 150 mV and thus to maintain the Cu.sup.+ ion content below 2 g/l.

The two sensors are also spaced from each other so that the first valve which adjusts the hydrochloric acid flow from the vessel 9 will open earlier than the second valve which controls the H.sub.2 O.sub.2 flow. Conversely, when the hand moves in the other direction, the hydrochloric acid valve will be closed at a definite time interval prior to the closing of the H.sub.2 O.sub.2 valve. This type of adjustment will assure that there is always a small HCl excess in the solution.

A particular advantage of the process of the invention is that the regenerating liquids can be supplied during the etching operation and that used-up etching solution can be withdrawn likewise during the operation. Thus, an interruption of the process, which heretofore has always been necessary, is avoided.

The spent etching solution also has a high re-use value since it consists of a hydrochloric acid-containing Cu-II-chloride solution without addition of other salts, and can therefore be used in the manufacture of various kinds of copper compounds. The cumbersome and expensive decontamination and neutralization of the drainage water, which in conventional etching apparatus is always necessary, is thus dispensed with.

EXAMPLE

This example illustrates a preferred method of practicing the invention by means of the apparatus above described.

The material treated in this example were laminates with a facing of pure copper. The solution in the tank at the commencement of the operation comprised 100-130 g copper II-chloride per liter and a 10% concentration of hydrochloric acid. The solution was sprayed onto the plates at a temperature of 40.degree.-50.degree.C which developed during the reaction.

After 0.4 g/l Cu.sup.+ -ion-concentration corresponding to a redox potential of 530mV was reached, the Redox-electrode by means of the control device opened the valve of the supply vessel holding HCl and at the same time actuated the pump to feed HCl into the main tank. The delayed action valve and pump of the supply vessel holding H.sub.2 O.sub.2 opened shortly thereafter.

An etching speed of 35.mu. copper was thus reached in about 75 seconds and kept practically constant.

Claims

I claim:

1. A method for continuously etching copper conductive plates which comprises contacting the copper conductive plate to be etched with an etching solution of cupric chloride (copper (II) chloride) and regenerating the etching solution containing cuprous chloride (copper (I) chloride) formed in the etching by reaction between said copper conductive plate and said etching solution by introducing hydrogen peroxide and hydrochloric acid into said etching solution, the amounts of hydrogen peroxide and hydrochloric acid introduced being regulated by continuously measuring the redox potential of said etching solution by means of a redox electrode and automatically introducing said hydrogen peroxide and hydrochloric acid whenever the cuprous ion (Cu(I) ) concentration in said etching solution exceeds 4 g/l in an amount at least sufficient to reduce said cuprous ion (Cu(I) ) concentration below said value whereby the cuprous ion concentration is continuously maintained below said value and a constant etching speed is maintained.

2. Method according to claim 1 wherein hydrogen peroxide and hydrochloric acid are automatically introduced into said etching solution whenever the cuprous ion concentration exceeds 0.4 g/l.

3. Method according to claim 1 wherein hydrogen peroxide and hydrochloric acid are automatically introduced into said etching solution whenever the cuprous ion concentration exceeds 2 g/l.

4. Method according to claim 1 wherein said hydrogen peroxide and hydrochloric acid are introduced in a molar ratio of 1:2 hydrogen peroxide to hydrochloric acid.