DE3935222A1 - ETCHING PLANT AND METHOD FOR ETCHING OBJECTS - Google Patents

Abstract

An etching installation contains an etching machine (1) in which the etching agent is enriched with metal that has been etched away during the etching process. The etching agent is regenerated in an electrolytic cell (11) by electrolytic enrichment in metal. The electrolysis inside the electrolytic cell (11) is controlled by a densimeter (26) which monitors the density of the etching agent leaving the electrolytic cell (11). If this density falls below a given value, the electrolytic cell (11) is deactivated, so that the current supplied by a direct current source (13) falls from an operating value to a low resting value at which the electrolytic deposition of the metal on the electrodes (12) is equal to or slightly greater than the chemical back-etching of the metal from the electrodes (12).

Description

The invention relates to an etching system, in particular for Manufacture of electronic circuit boards, with

• a) an etching machine in which the objects to be etched exposed to an etchant released by the counter metal would etch away and get covered with metal chert;
• b) an electrolysis containing a direct current source cell, which is enriched with metal-etching agent is feasible and in which the etchant by electro lytic deposition of the metal is depleted and then can be fed to the etching machine again if required;
• c) a density control device with a density meter, which is the density of the etchant in the electrolysis cell monitors, this when a certain th density value is activated and if it falls below a certain density value is deactivated.

The invention also relates to a method for etching of objects, in particular for the production of electro African circuit boards, in which the objects an etching be exposed to the medium caused by the etching process enriched with metal and for regeneration in an elec trolysis process is again depleted of metal, whereby the electrolysis process to regulate the density of the waste etching agent alternately activated and deactivated becomes.

Such etching systems or methods for etching are in the Known practice. The deactivation of the elec  trolysis cell to interrupt the electrolysis process a certain problem: the electricity that runs the electrolytic cell flows through, can not simply be turned off, because others if a chemical etching back of the electrodes on the Electrolysis cell of deposited metal by the etchant would be done. The known etching systems or the known Methods therefore work in such a way that the electrolysis cell in an electrolysis chamber and a collection container below the electrolysis chamber is divided. In the electrolysis came the separation electrodes are always located; here the actual electrolysis process instead. Should electrolysis be interrupted because, for example, the desired one Density value of the escaping etchant is undershot, so the etchant from the Electrolysis chamber drained into the collecting container. Here by avoiding the fact that the Electrolytic cells with the metal to the etchant Electrodes comes into contact. To reactivate the electrical The lysis cells then become those in the collecting container Etchant pumped back into the electrolysis chamber, where below the influence of the voltage across the electrodes the electrolysis process starts again. The one with the activation tion or deactivation of the electrolysis cell connected However, the electrolysis chamber is filled or emptied associated with a considerable expenditure of time. This means, that control processes take place with a considerable time delay affect that the density of the electrolysis cell available depleted caustic in proportion wide limits fluctuates. This was in the past acceptable where the precision of the etching process is not so high demands were made. With the increasing However, miniaturization of the structures to be etched will more and more important, the etching process under precisely defined Conditions. Understandably, the Density of the etchant plays a crucial role.

Another disadvantage of alternating draining and how  derfüllens the electrolysis chamber in the known Ätzan was that the shot on the electrodes which metal films partially infiltrated by etching agents are and tend to flake off. In addition, one can some etching back of the metal while draining the Do not avoid caustic from the electrolysis chamber.

The object of the present invention is an etching system of the type mentioned in such a way that the rule fluctuations reflecting the density of the etchant in the etch machine is subject to be kept as low as possible can.

This object is achieved in that

• d) the current emitted by the direct current source optionally a rest value at which the electrolytic Ab separation of the metal on the electrodes and the chemi approximate etching back of the metal from the electrodes balance, or a labor value at which the predominates electrolytic deposition, may have;
• e) the density meter of the electrolytic cell the direct current Source controls so that this at a density value below a certain limit the resting value and above a certain limit the work gives up value.

According to the invention, the electrolysis cell is deactivated no longer drained. Instead, the one in the Electrolytic cell flowing current from a higher, namely the labor value, to a lower value, the rest value lowered. This rest value is measured so that the electrolytic deposition and etching back of the etchant roughly balance that the con concentration of the etchant in the electrolysis cell is not is changed more. To give an indication of the relationship  nisse to offer: With an electrolysis cell that with a Working current of about 250 A is at rest worth about 120 A. The order provided according to the invention switching the current in the electrolysis cell can be very done quickly, is no longer mechanical Inertia of the flowing etchant or the performance the pump, which takes the etchant into the electrolysis chamber feeds, bound. In the etching system according to the invention the control fluctuations are comparatively small because the Density control device has very short response times. This will result in the etching result in the etching machine is clearly improved and reproducible. The danger that the metal deposited on the electrodes is largely excluded by etching agents sen. The metal layer that forms on the electrodes is therefore mechanically more stable and has little tendency to flake off.

As a precaution, the resting value of the of the direct current Source of current delivered so that the elec trolytic deposition of the metal the chemical etching back slightly exceeded by the electrodes. The electrolytic cell is also operated in the phases of deactivation, that the density of the etchant in it at most decrease slightly, but can never increase.

According to a feature of the invention, the density meter is the Electrolytic cell constantly from depleted etchant flowed from the outlet of the electrolytic cell to its Inlet flows. So there is a constant revolution in the Etchant over the density meter instead, so that the density knife always the current value of the density of the elec indicates depleted etchant emerging from the trolysis cell.

A particularly simple and inexpensive way of switching of the operating current from the working to the idle value in that the DC power source contains a servomotor, which is controlled by the density meter of the electrolytic cell  and the tap of a variable transformer with variable Output voltage moves. The regulation is also successful fluctuations in the mains voltage and current fluctuations, the temperature changes of the etchant in the electrical lysis chamber based, particularly good. It goes without saying but it is also possible to control the electronic DC power source with suitable, high-performance Build semiconductor devices.

In the known etching systems mentioned at the outset Regulation of the density of the etchant in the etching machine itself in interrelation with the density control in electronics lysis cell. A density meter monitored the density of the etch means in the electrolysis cell and this led with metal enriched etchant from the etching machine then when the density of the etchant in the electrolytic cell below a certain level dropped. By adding ange Enriched etchant then became the density of the etchant in the electrolysis cell back to the desired level brought. Simultaneously with the supply of caustic The electric became the etching machine in the electrolytic cell depleted etchant in the same amount taken and fed to the etching machine. The removal of Enriched etchant from the etching machine or the Return of depleted etchant to the etching machine was based on the density of the electrolysis cell etchant. So with this operating do not know the density of the etchant in the etching machine could sink too much was another density meter provided the density of the etchant in the etch monitored. When falling below a certain The electrolysis cell was described in density above NEN way by draining the etchant in a catch container shut down. This mutual intermingling of Control processes in the etching machine on the one hand and the electrical system lysis cell, on the other hand, also proves to behave sluggish. In a special embodiment of the  Invention is therefore the etching machine's own density Control device with its own density meter assigned to the of the etchant located in the sump of the etching machine is continuously flowable and if exceeded the removal of angerei at a certain density value etched to the electrolytic cell and the return a corresponding amount of depleted etchant from releases the electrolytic cell to the etching machine. The electro The lysis cell is simply used as a source of depleted Etching agent of certain density understood; the regulation of Electrolysis cell is self-contained and independent of the Regulation of the density of the etchant in the etching machine. Here through are the volumes of the etchants that usually exist gears are moved or influenced by the electrolysis need to be comparatively low, which is small in general fluctuations.

A buffer tank is preferably provided which is connected to the The sump of the etching machine communicates and the density Knife removed from the etching machine flowing through the etchant becomes. In principle, it would be possible without further ado. to ent this etchant directly to the bottom of the etching machine to take. Such a buffer tank can be found, however rides with most common etching machines and can therefore in the manner described for removing the for the etching agent required for density measurement can be used. The corresponding measuring devices and additional units are then particularly easy to access.

In this case it is also recommended that the buffer tank the etchant supplied to the electrolytic cell is removed becomes.

In the same way, etchant can be removed from the buffer tank which are injected via one or more injectors which it with gaseous or vaporous substances is enriched, is returned to the etching machine.  

As a gaseous or vaporous substance comes primarily Ammonia into account, as is the case when etching copper medium must be added frequently.

It is particularly easy when to remove the etchant from the buffer tank for the different purposes (flow Density metering, supply of etchant to the electrical lysis cell, enrichment of the etchant in injectors) only pump is provided.

Electrolysis cells are used, one as an outlet serving overflow and an inlet serving as an inlet have container from which the etchant in the elec trolysis chamber can be pumped, being through the outlet Etchant emerges in the same amount in which etchant enters at the inlet. Such electrolysis cells have ins special the advantage that the etch contained in them is always constant: what at the inlet is discharged at the outlet - depleted - again. With such electrolytic cells it is recommended if

• a) the level of the etchant in the collecting container a level sensor is monitored;
• b) the overflow of the electrolytic cell with the collecting container ter is connected via a line in which a mag net valve lies, in which
• c) the solenoid valve is closed when the level sensor an increase in the level in the collecting container notes.

Such an electrolytic cell is operated so that Etchant constantly between the container and the actual electrolysis chamber via the solenoid valve vice versa  is wallowing. This cycle remains as long as no etchant is supplied from the outside to the collecting container. However, if this is the case, the level increases in the reception area container. The level sensor now closes the solenoid valve in the way between the overflow and the collecting container, so that the level now also in the overflow of the electrolysis cell can increase. A corresponding amount of etchant can emerge from the overflow and, for example, the etch machine can be fed. As soon as the level in the catchment container returned to its original value the solenoid valve is opened again and the etch tel moves back inside the electrolytic cell closed circuit.

The object of the invention is also a method of gangs mentioned so that the density of the Caustic agent kept within narrow control fluctuations that can.

This object is achieved in that the Current flowing in the electrolysis process for deactivation of a work value at which the electrolytic Ab separation of metal significantly exceeds the chemical etchback increases, is reduced to a rest value at which the electrolytic deposition approximately the chemical back etching holds the scales or only slightly exceeds them.

The advantages with this method according to the invention connected correspond to those already above have been set out for the system according to the invention.

An embodiment of the invention is described below hand of the drawing explained in more detail; the only figure shows schematically an etching system for the production of electronic Printed circuit boards with electrolytic regeneration device.

In the drawing, a Ätzma machine is shown with reference numeral 1, which is its basic structure according to well known. Your description can therefore be kept brief. The etching machine 1 comprises an input station 2 , in which the circuit boards 3 to be etched are placed on a roller conveyor system 4 . On this they are guided in the direction of arrow 5 into the actual etching chamber 6 , through this and possibly through downstream processing stations (not shown in the drawing).

In the etching chamber 6 of the etching machine 1 there are 3 nozzles 7 and 8 above and below the path of movement of the printed circuit boards, to which pumps 9 and 10 continuously supply etchant. The nozzle assemblies 7 and 8 spray the printed circuit boards 3 carried past with etching agent, which then collects in the sump 11 of the etching machine 1 . There it is sucked in again by the pumps 9 and 10 and fed again in the circuit to the nozzle assemblies 7 and 8 . Here, the etchant enriches with copper, which is etched from the Lei terplatten 3 .

An electrolysis cell is provided for the regeneration of the etchant, that is to say for the depletion of copper, which is identified overall by reference number 11 . The basic structure of this electrolysis cell is also known. It comprises the actual electrolysis chamber 14 , in which a plurality of plate-shaped electrodes 12 are arranged parallel to one another and on which the copper to be removed from the etchant is deposited. The electrodes 12 are alternately connected to the positive or negative pole of a direct current source 13 , which will be discussed in more detail below.

Below the electrolysis chamber 14 , a collecting container 15 is provided, in which, if required (as will be explained in more detail below), the etchant which is sensitive in the electrolysis chamber 14 can be drained off.

Also in those operating times in which the electrolysis chamber 14 is filled with etchant, there is 15 etchant in the collecting container at a certain level, which is monitored by a level sensor 16 . A pump 17 continuously removes etchant from the sump of the collecting container 15 and circulates it via filters 18 , 19 , which are preceded by valves 20 , 21 , for cleaning. Above all, however, the pump 17 continuously introduces etchant from the bottom of the collecting container 15 into the electrolysis chamber 14 from below; a corresponding amount of etchant emerges in an overflow 22 in the upper region of the electrolysis chamber 14 . This overflow 22 is connected via a line 23 to the sump of the etching machine 1 .

Another line 24 leads from the lowest point of the overflow 22 of the electrolytic cell 11 into the collecting container 15 via a solenoid valve 25 . Parallel to the solenoid valve 25 is a density meter 26 , via which etchant can flow continuously from the overflow 22 into the collecting container 15 .

The solenoid valve 25 is controlled by the level sensor 16 already mentioned above, which monitors the etchant level in the collecting container 15 . Details of the Regelvor process are explained below.

The density meter 26 acts on a servomotor 27 in the direct current source 13 , which in turn is capable of actuating an actuating transformer 28 within the direct current source 13 . The variable voltage output by the variable transformer 28 is rectified by a rectifier 29 and then applied to the electrodes 12 within the electrolysis chamber 14 in the manner already indicated above.

The sump of the etching machine 1 is connected to a buffer tank 31 via a line 30 . A pump 32 continuously removes etchant from the buffer tank 31 and returns it to the sump of the etching machine 1 via a filter 33 and injectors 34 and 35 . In the injectors 34 and 35 , gaseous additives, in particular NH ₃ , are added to the circulating etchant in a known manner.

The etching medium removed from the pump 32 from the buffer tank 31 is also passed through a pH meter 36 , from which it reaches the buffer tank 31 again. The pH meter 36 can regulate, for example, the addition of ammonia by acting on a solenoid valve 37 .

Another portion of the etchant is circulated by the pump 32 via a second density meter 38 , from which it flows back into the buffer tank 31 .

Finally, the etchant removed from the pump 32 from the buffer tank 31 can be introduced into the collecting container 15 of the electrolytic cell 11 via a line 39 in which a magnetic valve 40 is located. The solenoid valve 40 is controlled by the density meter 38 .

The operation of the etching system described is as follows: As already indicated above, the circuit boards 3 to be etched are introduced by the roller conveyor system 4 from the input station 2 into the etching chamber 6 of the etching machine 1 and are sprayed there with etchant through the nozzle assemblies 7 and 8 . The conductor tracks on the printed circuit board 3 are etched out in the desired manner; the copper-enriched etchant collects in the sump of the etching machine 1 and is then circulated again by the pumps 9 and 10 via the nozzle assemblies 7 and 8 . The composition of the etching means in the sump of the etching machine 1 essentially corresponds to the composition of the etching agent in the buffer tank 31 . The composition of this etchant is continuously checked by the pH meter 36 and by the density meter 38 , through which the etchant circulated by the pump 32 flows. The pump 32 also serves, as already mentioned, to add gaseous additives, in particular ammonia, by circulating etching medium between the sump of the etching machine 1 and the buffer tank via the injectors 34 and 35 .

As long as the density meter 38 determines that the density of the etchant in the buffer tank 31 and thus also in the sump of the etching machine 1 is below a predetermined value, it remains in the essentially closed Ätzmit telkreislauf between the sump of the etching machine 1 , the puffing tank 31 , the pump 32 , the filter 33 and the injectors 34 and 35 .

Only when the density meter 38 detects an increase in the copper content in the etchant above the predetermined value, is the solenoid valve 40 opened. The pump 32 now conveys etchant from the buffer tank 31 via the line 39 into the collecting container 15 of the electrolytic cell 11 .

Up to this point, the solenoid valve 25 , which is able to establish a direct connection between the overflow 22 and the collecting container 15 of the electrolytic cell 11 , was open. Therefore, the level in the overflow 22 of the electrolytic cell 11 could never rise so high that etchant returned from the overflow 22 of the electrolytic cell 11 via the line 23 into the etching machine 1 . However, if, as described, the level in the collecting container 15 rises as a result of the supply of (copper-enriched) etchant via the line 39, the level sensor 16 emits a signal which closes the solenoid valve 25 . The previously essentially closed circuit of the etchant in the electrolytic cell 11 between the collecting container 15 , the pump 17 , the electrolytic chamber 14 and the overflow 22 of the electrolytic cell 11 is now interrupted (the relatively low throughput of etchant via the density meter 26 , which also runs from overflow 22 to the collecting container 15 , can be neglected in this context). The level in the overflow 22 of the electrolytic cell 11 rises so far that the same amount of etchant, which was previously pumped via line 39 into the collecting container 15 , can now flow back via line 23 into the etching machine 1 . This via line 23 from the electrolytic cell 11 to the etching machine 1 flowing etchant is depleted by the electrolysis process in the electrolysis chamber 14 to a certain density of copper, so that the supply of low density etchant via line 23 is in the etching machine 1 circulated etchant is brought back into the permissible density range. If, as a result of this supply of depleted etchant into the etching machine 1, the density meter 38 determines that the density of the etchant in the buffer tank 31 has dropped below a predetermined value, it closes the solenoid valve 40 again. The supply of etchant from the buffer tank 31 into the collecting container 15 of the electrolytic cell 11 is stopped; the etchant is now again essentially circulated in a closed circuit between the etching machine 1 and the buffer tank 31 via the pump 32 , the filter 33 and the injectors 34 and 35 , until he then again exceeds the density value by the density by the progressive etching process knife 38 is determined.

The above-described control process for the density of the etching means in the etching machine 1 and the buffer tank 31 presupposes that the density of the etchant available at the overflow 22 of the electrolysis cell 11 is below a certain value, that is to say that the etchant in the electrolysis cell 11 is actually in copper has been sufficiently removed. This is ensured by a second control loop, which is now described:

As already stated above, the etching medium is in circulation in the electrolysis cell 11 , that is, between the collecting container 15 and the electrolysis chamber 14 , as long as the solenoid valve 25 is open. The total amount of the etchant in the electrolytic cell 11 , that is the sum of the contents of the collecting container 15 and the electrolysis chamber 14 , is always constant.

The etchant is passed through the electrolysis chamber 14 from bottom to top, wherein it is progressively freed from copper by the electrolysis process depending on the distance traveled (corresponding to the dwell time between the electrodes 12 ). The density of the etchant within the electrolysis chamber 14 thus decreases from bottom to top.

As long as the density meter 26 , which is continuously flowed through in the overflow 22 emerging, depleted etchant, in this determines a density above a predetermined minimum value, the variable transformer 28 is in a position in which a relatively high current between the electrodes 12 flows: The electrolysis takes place with the deposition of copper from the etchant flowing through the electrolysis chamber 14 .

However, as soon as the enrichment has progressed to such an extent that the etchant flowing through the density meter 26 has reached a density below a certain value, the density meter 26 sets the servomotor 27 in operation. The actuating transformer 28 is now brought into such a position in which a considerably reduced quiescent current flows between the electrodes 12 . This quiescent current is dimensioned such that the electrolytic deposition of copper on the electrodes 12 is essentially in equilibrium with the chemical etching back of the copper from the electrodes 12 by the etchant. For safety reasons, the quiescent current is set so that the electrolytic deposition of copper on the electrodes 12 slightly outweighs the chemical etching back.

The density of the etching circulated in the electrolysis cell 11 now rises again through the supply of copper-enriched etchant from the buffer tank 31 until the density meter 26 finally detects that a density limit value has been exceeded and the motor 27 is acted on. This then leads the variable transformer 28 back to the position in which the working current flows between the electrodes 12 . The electrolytic cell 11 is now functioning again; the etchant crossing the electrolysis chamber 14 from the bottom upwards is again freed of copper electrolytically.

The control of the etching system described thus takes place so that the density control within the etching machine 1 (with a connected buffer tank 31 ) takes place essentially independently of the density control within the electrolysis cell 11 .

The density of the etchant within the electrolytic cell 11 (more precisely at its overflow 22 ) is kept below a certain value by the density meter 26 and the direct current source 13 regulated thereby, so that the electrolytic cell 11 for the control of the etching machine 1 essentially as a source of depleted Etchant below half a certain density value can be understood.

The density of the etchant in the etching machine 1 is kept between certain limit values by the density meter 38 in that enriched etchant is discharged via the line 39 and supplied by the depleted etch by means of the electrolytic cell 11 via the line 23 to a corresponding extent.

The etching system described is characterized by very fast control behavior. This fast response speed of the control devices is based firstly on the fact that the electrolysis chamber 14 does not have to be alternately drained and pumped again when the density in the electrolysis cell 11 is adjusted. The changeover of the current which flows between the electrodes 12 between a working current and a quiescent current can take place very quickly, so that the process of electrolysis can be resumed very quickly if required.

A second reason for the rapid controllability of the density of the etchant in the etching machine 1 lies in the separate regulation of the density values of the etchant in the etching machine 1 and in the electrolysis cell 11 .

If the entire etching system is to be shut down for a longer period of time (for example overnight or on weekends), the etchant located in the electrolysis chamber 14 is drained into the collecting container 15 in the same way as is the case in the prior art was.

Claims (11)

1. Etching system, in particular for the production of electrical circuit boards, with

• a) an etching machine in which the objects to be etched are exposed to an etchant which etches off the metal from the objects and thereby accumulates with metal;
• b) an electrolysis cell containing a direct current source, which can be feasible with metal-enriched etchant and in which the etchant is depleted by electro-lytic deposition of the metal and can then be fed to the etching machine again if required;
• c) a density control device with a density meter, which monitors the density of the etchant in the electrolysis cell, activates it when a specific density value is exceeded and deactivates when it falls below a specific density value, characterized in that
• d) the current emitted by the direct current source ( 13 ) optionally a rest value at which the electrolytic deposition of the metal on the electrodes ( 12 ) and the chemical etching back of the metal from the electrodes ( 12 ) roughly balance one another, or a work value , in which the electrodeposition predominates, may have;
• e) the density meter ( 26 ) of the electrolysis cell ( 11 ) controls the direct current source ( 13 ) in such a way that it emits the rest value at a density value below a certain limit value and the work value above a certain limit value.

2. Etching system according to claim 1, characterized in that the rest value of the current supplied by the direct current source ( 13 ) is dimensioned such that the electrolytic separation of the metal slightly exceeds the chemical etching back from the electrodes ( 12 ). 3. Etching system according to claim 1 or 2, characterized in that the density meter ( 26 ) of the electrolytic cell ( 11 ) is continuously flowed through by depleted etchant which flows from the outlet ( 22 ) of the electrolytic cell ( 11 ) to its A ( 15 ) . 4. Etching system according to one of claims 1 to 3, characterized in that the direct current source ( 13 ) contains a servomotor ( 27 ) which is controlled by the density meter ( 26 ) of the electrolytic cell ( 11 ) and the tap of a variable transformer ( 28 ) with variable output voltage moves. 5. Etching system according to one of claims 1 to 4, characterized in that the etching machine ( 1 ) is assigned its own density control device with its own density meter ( 38 ) which flows continuously from the caustic in the sump of the etching machine ( 1 ) and if a certain density value is exceeded, the removal of enriched etchant to the electrolytic cell ( 11 ) and the return of a corresponding amount of depleted etchant from the electrolytic cell ( 11 ) to the etching machine ( 1 ) are released. 6. Etching system according to claim 5, characterized in that a buffer tank ( 31 ) is provided which communicates with the sump of the etching machine ( 1 ) and from which the density meter ( 38 ) of the etching machine ( 1 ) flowing through the etching medium is removed. 7. Etching system according to claim 6, characterized in that the buffer tank ( 31 ) is removed from the electrolytic cell ( 11 ) supplied etchant. 8. etching system according to claim 6 or 7, characterized. that the buffer tank ( 31 ) etchant is removed, which is fed back into the etching machine ( 1 ) via one or more injectors ( 34 , 35 ) in which it is enriched with gaseous or vaporous substances. 9. Etching system according to claim 7 or 8, characterized in that a single pump ( 32 ) is provided for the removal of etchant from the buffer tank ( 31 ) for the various purposes. 10. Etching system according to one of the preceding claims, in which the electrolysis cell has an overflow serving as an outlet and a collecting container serving as an inlet, from which the etchant can be pumped into the electrolysis chamber, with the same amount of etchant escaping through the outlet in which etchant enters at the inlet, characterized in that

• a) the level of the etchant in the collecting container ( 15 ) is monitored by a level sensor ( 16 );
• b) the overflow ( 22 ) of the electrolytic cell ( 11 ) with the collecting container ( 15 ) via a line ( 24 ) in which a solenoid valve ( 25 ) is located, wherein
• c) the solenoid valve ( 25 ) is closed when the level sensor ( 16 ) detects an increase in the level in the collecting container ( 15 ).

11. Method for etching objects, in particular for Manufacture of electronic circuit boards in which the objects are exposed to an etchant that enriched with metal by the etching process and raining ration in an electrolysis process again from metal is enriched, the electrolysis process for regulation the density of the depleted etchant alternately acti fourth and deactivated, characterized in that the current flowing in the electrolysis process to the desacti vation of a labor value at which the electroly chemical deposition significantly exceeds, is reduced to a rest value, in which the electrodeposition is approximately the same chemical etching or the balance is low compliant.