Abstract:
In order to regulate the metal ion concentration in an electrolyte fluid serving to electrolytically deposit metal and additionally containing substances of an electrochemically reversible redox system, it has been known in the art to conduct at least one portion of the electrolyte fluid through one auxiliary cell provided with one insoluble auxiliary anode and at least one auxiliary cathode, a current being conducted between them by applying a voltage. Accordingly, excess quantities of the oxidized substances of the redox system are reduced at the auxiliary cathode, the formation of ions of the metal to be deposited being reduced as a result thereof. Starting from this prior art, the present invention relates to using pieces of the metal to be deposited as an auxiliary cathode.

Description:
BACKGROUND OF INVENTION 
     1. Field of Invention 
     The invention relates to a method and a device for regulating the metal ion concentration in an electrolyte fluid. The method and the device may particularly be used for regulating the copper ion concentration in a copper deposition solution that serves to electrolytically deposit copper and that additionally contains Fe(II) and Fe(III) compounds. 
     2. Brief Description of the Related Art 
     When the electroplating process is performed using insoluble anodes, it must be made certain that the concentration of the ions of the metal to be deposited is kept as constant as possible within the electrolyte fluid. This may be achieved by compensating for the loss of the metal ions in the electrolyte fluid, which is caused by the electrolytic deposition of metal, by adding the corresponding metal compounds for example. However, the supply and disposal costs for this method are very high. Another well-known method for complementing the metal ions in the electrolyte fluid consists in dissolving metal directly in the fluid with the help of an oxidizing agent such as oxygen for example. For copperplating, metallic copper can be dissolved in an electrolyte fluid that has been enriched with atmospheric oxygen. In this case, ballast salts, resulting among others from the complementation with metal salts, do not enrich in the electrolyte fluid. However, in the process of electroplating, oxygen is produced in both cases at the insoluble anodes of the electrolytic cell. This oxygen attacks the organic additives in the electrolyte fluid, these additives being usually added to the electrolyte fluid for controlling the physical properties of the deposited metal coating. Additionally, the oxygen also causes the anode material to be destroyed by corrosion. 
     In order to avoid the formation of noxious gases such as e.g., oxygen at the insoluble anodes and by using typical sulfuric acid copperplating baths that additionally contain chloride ions, as well as of chlorine, DD 215 589 B5 proposes a method for the electrolytic deposition of metal with insoluble anodes that consists in adding substances of an electrochemically reversible redox system as additives to the electrolyte fluid, Fe(NH 4 ) 2 (SO 4 ) 2  for example, these substances being brought, by means of an intensive forced convection with the electrolyte fluid, to the anodes, where they are electrochemically converted by the electrolytic current, upon which conversion they are led, by means of intensive forced convection, away from the anodes into a metal ion generator in which they are electrochemically converted back to their original state on regeneration metal contained in said generator while, concurrently, the regeneration metal dissolves without the help of external current and, in their original state, they are returned to the deposition tank by means of intensive forced convection. The metal ions resulting from the dissolution of metal pieces in the metal ion generator are conveyed to the electroplating plant together with the electrolyte fluid. 
     In this process, noxious by-products are prevented from forming at the insoluble anodes. Additionally, the metal ions that have been used up in the electrolytic deposition of metal are subsequently produced by the reaction of the appropriate metal pieces with the substance of the electrochemically reversible redox system by causing the metal pieces to oxidize with the oxidized substances and the metal ions to form. 
     DD 261 613 A1 describes a method that uses, for the electrolytic copper deposition, substances of an electrochemically reversible redox system such as Fe(NH 4 ) 2 (SO 4 ) 2  wherein it indicates that organic additives which are customarily utilized in the deposition fluid for the deposition of smooth and high-gloss copper coatings are not oxidized at the insoluble anodes while conducting the method. 
     DE 43 44 387 A1 also describes a method for the electrolytic deposition of copper with predetermined physical properties using insoluble anodes and a copper ion generator arranged outside the electroplating cell as well as substances of an electrochemically reversible redox system in the deposition fluid, the copper ion generator serving as a regeneration space for the metal ions and containing pieces of copper. It indicates that the organic additives contained in the deposition fluid have been observed to decompose while conducting the processes described in DD 215 589 B5 and DD 261 613 A1 so that, as a result thereof, in a deposition bath being in use for a longer period of time, decomposition products of these additives would enrich in said bath. To overcome this problem it suggests to use the substances of the electrochemically reversible redox system in a concentration that precisely leads to maintaining the total content of copper required for electroplating in the electroplating plant and to conduct the electrolyte fluid inside and outside the electrolytic cell in such a manner that the life of the ions of the reversible convertible substance that have been formed by oxidation at the anodes of the electrolytic cell is so limited in time in the overall electroplating plant that these ions are prevented or at least drastically hindered from destroying the additives. 
     The problem with the methods and devices mentioned is that the metal content in the electrolyte fluid cannot be kept constant easily. As a result thereof, the conditions for deposition vary, thus rendering it impossible to achieve reproducible conditions for the electrolytic deposition. One of the causes for the modification of the metal content in the electrolyte fluid is that the metal pieces in the metal ion generator are not only formed under the influence of the substances of the electrochemically reversible redox system, but also, in the case of a copper deposition bath using Fe(II)/Fe(III) compounds as substances of the electrochemically reversible redox system, by the oxygen from the air contained in the electrolyte fluid. 
     Moreover, it has also been found out that the oxidized substances of the electrochemically reversible redox system are not only reduced in the metal ion generator but also at the cathode in the precipitation tank, so that the cathodic current efficiency merely amounts to approximately 90%. 
     On account of the reasons mentioned above, a stationary condition between the formation of metal ions in the metal ion generator and the consumption of the metal ions by way of electrolytic metal deposition does not arise. This effect is still reinforced, specifically when using a higher temperature. Therefore, the content of the metal ions to be deposited in the electrolyte fluid increases continuously. However, the content of the metal ions has to be kept within narrow limits in order to keep up enough good physical properties of the deposited coatings of metal. 
     Among other indications, WO 9910564 A2 asserts in this connection that it is not possible to lower the metal ion concentration in the electrolyte fluid in an additional electrolytic secondary cell utilizing an insoluble anode in a manner which is well-known in conventional electroplating plants utilizing soluble anodes instead of the insoluble anodes employed here. The problem herewith, according to said document, is that the substances of the electrochemically reversible redox system are oxidized at the anode of the secondary cell so that the content of the oxidized species of these substances rises in the fluid. It maintains that, as a result thereof, the metal ion content in the electrolyte fluid continues to rise so that the actual goal aiming at lowering the metal ion concentration is missed. 
     The document mentioned additionally indicates another approach in overcoming the problem that involves diluting permanently the electrolyte fluid. But since this would entail that large quantities of the fluid would continuously have to be discarded and disposed of, this procedure, which is also known under the name of, feed and bleed method, is said to be unsatisfactory. 
     According to this document, the solution of the problem consists in suggesting a method and a device for regulating the metal ion concentration. According to this solution, at least one portion of the electrolyte fluid contained in the electroplating plant is guided through one or several electrolytic auxiliary cells provided with at least one insoluble anode and at least one cathode and a flow of current is set between the anodes and the cathodes of the auxiliary cells, said flow of current being so high that the current density at the surface of the anode amounts to at least 6A/dm′ and the current density at the surface of the cathode to no more than 3 A/dm? The ratio of the surface of the anodes to the surface of the cathodes is set to at least 1:4. 
     By means of this arrangement the metal ion content in the electrolyte fluid can be kept constant over a longer period of time by allowing part of the oxidized species of the electrochemically reversible redox system contained in the electrolyte fluid to be reduced at the cathode of the auxiliary cell. In adjusting the ratio of the current densities at the anode and at the cathode in the auxiliary cell by selecting for example the suitable relationship between the surfaces of the anode and of the cathode, the reduced species of the electrochemically reversible redox system at the anode of the auxiliary cell are oxidized merely to a minor extent or not at all so that the concentration of the oxidized species of the electrochemically reversible redox system can be regulated, which permits to directly influence the rate of formation of the metal ions. 
     The device described in WO 9910564 A2 proved however to be quite complicated since the precipitation tank has to be provided with several secondary cells. It is question of the auxiliary cell mentioned and of the metal ion generator. In production plants, it may be necessary to provide for a plurality of auxiliary cells and metal ion generators. Moreover, metal continuously deposits onto the cathode in the auxiliary cell so that the efficiency of the reduction of the oxidized species of the electrochemically reversible redox system continuously decreases at the cathode, thus requiring an increased electrical power. The rectifiers used for the purpose of supplying the auxiliary cell with current have to be provided with an increased rated capacity, which adds to the prime costs. Moreover, the duration of life of this device is limited on account of corrosive attack of the anode material. 
     Furthermore, the copper deposited on the cathode of the auxiliary cell has to be electrochemically removed from time to time which implies additional consumption of energy and non availability of the device for this period of time. Accordingly, several such auxiliary cells have to be provided to ensure continuous production, some of these cells being utilized for regulating the metal ion concentration while in other parallelled auxiliary cells the copper is being removed from the cathode. The particular disadvantage thereof is that the cathode material that is customarily employed is damaged in the stripping procedure. As a result thereof, the efficiency of reduction is reduced on one hand. On the other, the cathode has to be replaced by a new one after some stripping procedures. 
     Accordingly, the basic problem the present invention is dealing with is to overcome the drawbacks of the known methods and devices and to more specifically discover a device and a method that permit an economical way of operation of the procedure of electrolytic deposition. More specifically, the process of electrolytic deposition is intended to use insoluble anodes and substances of an electrochemically reversible redox system in the electrolyte fluid. The method is intended to be capable of being performed under constant conditions over a very long period of time. The metal ion concentration in the electrolyte fluid in particular has to be kept constant within narrow limits over said period of time. The invention is above all directed to permit to keep the metal ion concentration constant with simple means merely requiring low consumption of energy and low prime costs. 
     SUMMARY OF THE INVENTION 
     The method according to the invention serves to regulate the metal ion concentration in an electrolyte fluid serving to electrolytically precipitate metal and additionally containing substances of an electrochemically reversible redox system in an oxidized and reduced form. It comprises the following steps:
         a. having at least one portion of the electrolyte fluid guided through at least one auxiliary cell, each cell being provided with an insoluble auxiliary anode and with at least one auxiliary cathode,   b. producing a flow of current between the auxiliary cathodes and the auxiliary anodes of the auxiliary cell by applying a voltage; and   c. using pieces of the metal to be deposited for acting as auxiliary cathodes.       

     For this purpose, the electrolyte fluid is continuously conducted through the plant in which metal is electrolytically deposited and through the auxiliary cells in such a way that the fluid flows concurrently or, if need be, subsequently through the plant and the cells at least from time to time. After the fluid has flown through the auxiliary cells it is brought back to the plant over and over again. 
     For electrolytic deposition of the metal, said metal is deposited onto the work from the electrolyte fluid using at least one insoluble main anode which is preferably provided with dimensional stability. For this purpose, an electric current is passed between the work and the main anode. The metal ions are formed by the substances of the redox system in the oxidized form in at least one metal ion generator through which the electrolytic fluid at least partially flows and which serves as an auxiliary cell in causing the metal pieces to dissolve. To this effect, the substances in the oxidized form are converted to the reduced form in producing corresponding substances such as metal ions. The thus produced substances in the reduced form are oxidized again at the main anode in producing the corresponding substances in the oxidized form. 
     The device according to the invention therefore is a metal ion generator serving as an electrolytic auxiliary cell
         a. which can be filled with pieces of the metal to be deposited and   b. which is provided with at least one insoluble auxiliary anode and at least one power supply, preferably a source of direct current, for generating a flow of current between the auxiliary anode and the metal pieces that can be filled in,   c. wherein the metal pieces can be used as auxiliary cathodes.       

     Preferably, the anode spaces surrounding the auxiliary anodes and the cathode spaces surrounding the metal pieces are separated from each other by means that are at least partially permeable to ions. If necessary, the at least partially ion permeable means between the anode spaces and the cathode spaces may also be relinquished, though. In this event, the auxiliary cathodes are accommodated in a section of the metal ion generator in which the fluid has been appeased in order to prevent at least as far as possible the electrolyte fluid contained in the cathode space from mixing with the electrolyte fluid in the anode space. From a constructional point of view, the two spaces may be separated from each other in such a manner for example that mixing hardly occurs. The metal pieces are preferably accommodated in a compartment of the metal ion generator that has a very good through-flow. 
     With the inventive method and device, which more specifically serve to regulate the copper ion concentration in a copper deposition solution serving to electrolytically deposit copper and additionally containing Fe(II) and Fe(III) compounds, the metal ion content in a metal deposition solution can be kept constant within narrow limits so that reproducible conditions can be considerably lower. Furthermore, the deposition solution does not come into contact with an inert auxiliary cathode as this is the case with the plant described in WO 9910564 A2, so that a potential deposit of metal onto the auxiliary cathode cannot cause the problems discussed herein above. Accordingly, the method according to the invention does without substantial maintenance works such as e.g., the intermediary stripping of the metal deposited onto the auxiliary cathode as required by the prior art device, over a very long period of time. The problem created thereby, namely a reduction of the efficiency of the conversion of the oxidized substances of the redox system into the reduced substances on account of a metal coating formed on the auxiliary cathode, does not occur when using the present invention. 
     The lower the content of the substances of the redox system in the oxidized form in the electrolyte has an additional advantage; the work in the electroplating plant is located in an electrolyte fluid that contains a reduced concentration of the substances of the redox system in the oxidized form when performing the method according to the invention. An accordingly reduced quantity of the substances of the redox system is reduced by the electroplating current on the surface of the work. As a result thereof, the cathodic current efficiency in the electroplating plant is improved. The correlated gain of production capacity amounts to up to 10%. 
     A further advantage of the invention is that the anode slime known from electroplating plants with soluble anodes does not occur. In parts, a ,feed a bleed operation of the plant may nevertheless be useful. This is particularly true when organic and/or inorganic additives in the electrolyte fluid are to be exchanged in the long run. As a result of the partial discard of electrolyte fluid, the content of the oxidized metal ions of the redox system is lowered proportionally. The capacity of the metal ion generator may be reduced by this portion. Accordingly, the metal ion content can also be kept constant by having substances of the redox system in the oxidized form reduced in the metal ion generator and concurrently, by having part of the electrolyte fluid removed from the electroplating plant and replaced by a fresh electrolyte fluid. 
     Inert metal electrodes that have been activated with precious metals and/or with mixed oxides, maintained for deposition. The metal deposition solution is continuously conducted from the electroplating plant, e.g., a precipitation tank into the metal ion generator of the invention and from there back again into the electroplating plant. The substances of the redox system that formed in the oxidized form at the main anode in the electroplating plant are reduced again at the metal pieces in the metal ion generator, thereby forming metal ions. Due to the fact that the rate of formation of the substances of the redox system in the reduced form in the metal ion generator can be varied by having the metal pieces provided with a cathodic polarity relative to an auxiliary anode, the rate of formation of the metal ions in the metal ion generator can be regulated. Another oxidation of the reduced substances of the redox system relative to the oxidized substances at the auxiliary anode is largely prevented from taking place in having the anode space surrounding the auxiliary anode separated from the cathode space surrounding the metal pieces. The fluids in the anode space and in the cathode space are largely prevented from mixing so that the reduced substances of the redox system can reach the auxiliary anode to a very little extent only since these substances can reach the auxiliary anode only by diffusion and since the concentration of the substances in the anode space depletes on account of the electrochemical reaction taking place there. 
     In regulating the flow of current in the metal ion generator, the production rate of the substances of the redox system in the reduced form and thus subsequently the rate of formation of the metal ions in the metal ion generator is set to a value which is so large that the quantity of metal ions produced per unit time by oxidation with the redox compounds plus the quantity generated by the dissolution of the metal on account of the oxygen from the air entered in the electrolyte fluid equals the quantity of the metal ions used up at the cathode in the electroplating plant. As a result thereof, the total content of ions of the metal to be deposited in the electrolyte fluid remains constant. In using the method according to the invention the desired stationary condition between the formation of metal ions and their consumption is achieved. 
     As compared to the invention described in WO 9910564 A2, the further advantage of the inventive method and device is that merely one or several secondary cells have to be provided in addition to the electroplating plant and not one or several auxiliary cells and one or several additional metal ion generators. As a result thereof, the expenses for plant engineering are more specifically of precious metals, are preferably used. This material is chemically and electrochemically stable relative to the deposition solution and the substances of the redox system used. The basis material used is titanium or tantalum for example. The basis material is preferably used as perforated electrode material, in the form of rib mesh metal or nets, in order to offer a large surface when little place is available. Since these metals have a considerable overpotential when electrochemical reactions take place, the basis materials are coated with a precious metal, preferably with platinum, iridium, ruthenium or their oxides or mixed oxides. As a result thereof, the basis material is additionally protected against electrolytic stripping. Anodes of titanium coated with iridium oxide that are exposed to radiation by spherical bodies to become compressed so as to become free from pores are permanent enough, thus being provided with a long useful life at the conditions applied. 
     Metal pieces shaped like balls are preferably used. Copper needs not to contain phosphorus as this is the case when using soluble copper anodes. As a result thereof, the formation of anode slime is diminished. Metal balls have the advantage that a reduction in volume of the ball&#39;s bulk in the metal ion generator does not easily cause hollow spaces acting as bridges to form when the metal pieces are dissolving so that it is easier to fill up with new metal pieces. By using balls having an appropriate diameter, the bulk volume in the metal ion generator may be optimized. Again, as a result thereof, the flow resistance or, when the pumping capacity is given, the volume flow of the deposition solution is determined by the formed bulk of the metal balls. However, the metal pieces may also be substantially cylindrical or cuboid in shape. It has to be made sure that the flow through the cathode space is sufficient. 
     In order to further diminish an oxidation of substances of the redox system in the reduced form entering the anode space, the ratio of the surface of the metal pieces to the surface of the at least one auxiliary anode is set to a value of at least 4:1. As a result thereof, the current density at the auxiliary anode is increased so that preferably the water of the deposition solution oxidizes, forming oxygen in the process, and the substances of the redox system in the reduced form only oxidize to a minor extent. A surface ratio of at least 6:1 is preferred, even more preferred being a surface ratio of at least 10:1. Ratios of at least 40:1 are more specifically preferred, above all a ratio of at least 100:1. Such a high surface ratio can be adjusted in selecting for example small metal pieces, more specifically metal balls having a small diameter. Typically, a cathodic current density of 0.1 A/dm 2  to 0.5 A/dm 2  and an anodic current density of 20A/dm 2  ensues. At these conditions, virtually oxygen alone is formed at the anode. Substances of the redox system in the reduced form possibly present in the anode space are virtually not oxidized at these conditions. 
     The metal ion generator can preferably be shaped like a tube. In this case, an advantageous embodiment consists in having the auxiliary anode accommodated above the space that can be occupied by the metal pieces. As a result thereof, the oxygen set free by the anodic decomposition of the water at the auxiliary anode can escape from the deposition solution in the metal ion generator without contacting the metal pieces and without coming into close contact with the solution so that it dissolves in the solution in appreciable quantities, thus reaching the metal pieces. This arrangement allows to prevent the metal pieces from dissolving faster under the action of the oxygen. 
     In an alternative, advantageous embodiment, the metal ion generator may be vertically partitioned into two compartments (anode space and cathode space), the metal pieces being accommodated in the one compartment and the at least one auxiliary anode being arranged in the other compartment. In this case too, oxygen originated at the auxiliary anode emerges from the deposition solution without further contacting the metal pieces. 
     The bulk of the metal pieces preferably rests on an electrode that has the shape of a sieve and consists of an inert material such as titanium for example. The power can be delivered to the metal pieces by way of this electrode. Thanks to the sieve shape of said electrode, the deposition solution can be passed through the sieve to the metal bulk through which it can be delivered. Reproducible flow conditions are thus set in the metal bulk. The deposition solution entering the cathode space can be exited out of the cathode space by being caused to overflow upon flowing through the metal bulk in the upper region of the cathode space. Thanks to the high flow rate set by the bulk, the efficiency of the reduction of the substances of the redox system in the oxidized form at the metal pieces can be increased since the concentration overpotential for these substances at the pieces is reduced. 
     The auxiliary anode is surrounded by an anode space and the metal pieces by a cathode space, the deposition solution being located in said spaces. The two spaces are separated from each other by means that are at least partially permeable to ions. Liquid permeable, nonconducting woven cloths such as polypropylene cloth for example may preferably be used as ion permeable means. This material hampers convection between the electrolyte spaces. 
     In an alternative embodiment, ion exchange membranes may be utilized. These membranes have the additional advantage not only to hamper convection between electrolyte spaces but selectively, migration as well. When utilizing an anion exchange membrane for example, anions coming from the cathode space can arrive into the anode space whereas cations coming from the anode space cannot get into the cathode space. In the event a copper deposition solution with Fe 2+  and Fe 3+  ions is employed, the Fe 3+  ions formed by oxidation in the anode space are not transferred into the cathode space so that the efficiency of the device according to the invention is not impaired. If these ions were transferred into the cathode space, the Fe 3+  ions would be reduced to Fe 2+  ions in a reaction competing with the Cu 2+  reduction. That is why ion exchange membranes used as at least partially ion permeable means are particularly advantageous from a technical point of view. However, these materials are more expensive and mechanically more sensitive than the woven cloths that are permeable to liquid. 
     The metal ion concentration in the deposition solution can be regulated by adjusting the current conduction between the auxiliary anode and the pieces of metal. For this purpose, the current is controlled by way of the electric power supply. A sensor may be additionally provided for the automatic control of the metal ion content, the metal ion concentration in the solution being measured continuously by means of said sensor. For this purpose, the extinction of the deposition solution may be determined by photometry in a separate gauge head in which the solution is circulated and the output signal of the gauge head can be brought to a comparator. The thus obtained regulating variable can then be converted into an actuating variable for adjusting the current to the power supply. This current serves to influence primarily the content of substances of the redox system in the electrolyte fluid. This content again influences the rate of dissolution at the metal pieces. 
     From the electroplating plant, in which the inert main anodes and the work to be plated are located, the electrolyte fluid is delivered in a forced circulation to the metal ion generator from where it is returned to the electroplating plant. Pumps are utilized for this purpose which convey the fluid in the forced circulation through appropriate pipelines. If necessary, a reservoir is employed as well and is arranged between the electroplating plant and the metal ion generator. This reservoir serves to store the electrolyte fluid for several precipitation tanks operated in parallel in an electroplating plant for example. For this purpose, two liquid cycles can be formed, the one being formed between the precipitation tanks and the reservoir and the second between the reservoir and the metal ion generator. Moreover, filtering means can also be inserted in the cycle in order to remove impurities from the electrolyte fluid. On principle, the metal ion generator may also be placed in the very precipitation tank in order to achieve the shortest possible flow paths. 
     The invention is preferably suited for regulating the concentration of the copper ion content in copper baths using inert anodes of dimensional stability in the precipitation tank, said baths containing Fe 2+  and Fe 3+  salts, preferably FeSO 4 /Fe 2 (SO 4 ) 3  or Fe(NH 4 ) 2 (SO 4 ) 2  or other salts for the purpose of maintaining the concentration of the copper ions. On principle, the invention can also be utilized in regulating the metal ion concentration in baths serving to electrolytically deposit other metals such as e.g., zinc, nickel, chromium, tin, lead and the alloys thereof and with other elements such as e.g., phosphorus and/or boron. In this event, other substances of an electrochemically reversible convertible redox system have possibly to be used, the redox system being chosen in dependence on the respective precipitation potential. Compounds of the elements titanium, cerium, vanadium, manganese, chromium for example may also be used. Suitable compounds are titanyl sulfuric acid, cerium(IV) sulfate, alkali metavanadate, manganese(II) sulfate and alkali chromate or alkali dichromate for example. 
     The method and the device according to the invention are particularly suited for use in horizontal through-type electroplating plants in which plate-shaped work, preferably printed circuit boards, which is horizontally or vertically positioned, is conveyed in a linear manner in horizontal direction while being brought into contact with the electrolyte fluid. As a matter of fact, the method can also be used for electroplating work in traditional dip plants in which the work is in most cases submerged in vertical orientation. 
     In the following, the invention is explained in more detail with the help of the Figures. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING FIGURES 
       FIG.  1 : shows a diagrammatic view of an arrangement for electroplating; 
       FIG.  2 : shows a sectional view of the metal ion generator in a first embodiment; 
       FIG.  3 : shows a sectional view of the upper region of the metal ion generator in a first embodiment; 
       FIG.  4 : shows a sectional view of the metal ion generator in a second embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1  shows a diagrammatic view of an electroplating arrangement provided with a precipitation tank  1 , a metal ion generator  2  and a reservoir  3 . The precipitation tank  1  may be of the through-type for treating printed circuit boards, a sump being preferably provided out of which electrolyte fluid is taken to be splashed or sprayed onto or brought into contact in any other way with the printed circuit boards and to which it is returned after contact with the printed circuit boards. In this case, the tank  1  shown in  FIG. 1  is the sump. 
     The discrete receptacles are filled with the electrolyte fluid. A sulphuric acid copper bath can be utilized as electrolyte fluid, said bath containing copper sulfate, sulphuric acid and sodium chloride as well as organic and inorganic additives for controlling the physical properties of the metal deposited. 
     The metal ion generator  2  contains an auxiliary anode  20  and pieces of metal  30 . The metal pieces  30  (a portion thereof only being illustrated) rest as a bulk on a sieve bottom  31  made of titanium. The sieve bottom  31  and the auxiliary anode  20  are connected to a direct current supply  50  by way of electric feed lines  40 ,  41 . The sieve bottom  31  has cathodic polarity and is therefore connected to the negative terminal of the power supply  50 . The auxiliary anode  20  has anodic polarity and is connected to the positive terminal of the power supply  50 . The metal pieces  30  are also given cathodic polarity via the electric contact of the metal pieces  30  with the sieve bottom  31 , a current being conducted between the metal pieces  30  and the auxiliary anode  20  as a result thereof. An ion permeable polypropylene woven cloth  21  is clamped between the anode space  25  surrounding the auxiliary anode  20  and the cathode space  35  containing the metal pieces  30  in order to prevent the convective transport of fluid between the spaces  25  and  35 . 
     The precipitation tank  1  communicates with the reservoir  3  in a first liquid cycle: electrolyte fluid is drawn from the upper region of the precipitation tank  1  through the pipeline  4  and is transferred to the reservoir  3 . The fluid may be drawn from the precipitation tank  1  through an overflow compartment for example. The fluid contained in the reservoir  3  is drawn from the lower region of the receptacle through a pipeline  5  by means of a pump  6  and is channelled through a filter unit  7 , e.g., taped filter candles. The filtered solution is returned to the precipitation tank  1  via the pipeline  8 . 
     The reservoir  3  also communicates with the metal ion generator  2  via a second liquid cycle: fluid is taken from the bottom of the reservoir  3  through the pipeline  9  and is caused to enter the metal ion generator  2  in the lower region underneath the sieve bottom  31 . The fluid is drawn out of the metal ion generator  2  again by way of an overflow in the upper region of the cathode space  35  and is returned to the reservoir  3  through the pipeline  10 . 
       FIG. 2  shows a section of a first embodiment of the metal ion generator  2 . The metal ion generator  2  consists of a tubular housing  15  which is made of polypropylene for example and which is provided with a bottom  16  made e.g., of polypropylene too. On its upper front side, the tubular housing  15  is provided with an opening  17 . A fluid admission  18  for the electrolyte fluid is provided in the lower region of the tubular housing  15 . Correspondingly, a fluid outlet  19  is arranged in the upper region. The cross section of the tubular housing  15  is preferably rectangular, square or circular. 
     In the metal ion generator  2  there are located an anode space  25  and a cathode space  35 . 
     The anode space  25  and the cathode space  35  are separated from each other by a wall  24  and by an ion permeable woven cloth  21 , a polypropylene cloth in this case, that is fastened to the lower border of the wall  24 . This is shown in detail in FIG.  3 . As a result, the convective transport of fluid between the two spaces  25  and  35  is checked to a large extent. The wall  24  forms an upper opening and is fastened to the upper front-sided edge of the tubular housing  15  (not shown). 
     The auxiliary anode  20  is accommodated in the anode space  25 . The cathode space  35  contains the metal pieces  30 , copper balls in this case, that do not contain any phosphorus and that have a diameter of approximately 30 mm for example. The copper balls  30  form a bulk resting on a titanium sieve  31  in the lower region of the tubular housing  15 . The auxiliary anode  20  is connected to the positive terminal and the sieve bottom  31  to the negative terminal of a direct current supply. The place of screwed union  38  for the anodic power lead from the source of direct current to the auxiliary anode  20  and the cathodic place of screwed union  39  for the power lead to the sieve bottom  31  are illustrated schematically in FIG.  3 . In this event, the electric feed lines for the sieve bottom  31  are insulated and guided upward out of the metal ion generator  2 . 
     The pipe  9  leads into the metal ion generator  2  via the fluid intake  18 . The fluid intake  18  is provided underneath the sieve  31 . The sieve prevents pieces of metal or slime from obstructing the pipe  9 . The metal ion generator  2  furthermore communicates with the pipe  10  at the fluid outlet  19 . The fluid outlet  19  is arranged in the upper region of the metal ion generator  2 . In order to make certain that the metal ion generator  2  is always filled up to the liquid level  22 , the fluid outlet  19  is designed as a pipeline  10  that exits the tubular housing  15  and is provided with an exhaust port  11  in the upper region of the cathode space  35 . The electrolyte fluid can exit the cathode space  35  through the exhaust port  11  into the pipeline  10 . Said exhaust port  11  is arranged above the level of the auxiliary anode  20 , thus ensuring that the auxiliary anode  20  is always situated within the fluid. 
     The electrolyte fluid that comes from the reservoir  3  or directly from the deposition tank  1  and that contains, in addition to the copper ions, Fe 3+  ions and possibly additionally Fe 2+  ions formed at the main anode, is pumped into the metal ion generator  2  via the fluid intake  18 . The fluid then traverses the sieve bottom  31  in the direction of the arrow  23  and enters the cathode space  35  containing the copper balls  30 . The Fe 3+  ions react with the copper to form Cu 2+  ions while Fe 2+  ions are produced at the same time. The rate of formation of the copper ions can be regulated by giving the copper balls  30  cathodic polarity via the sieve bottom  31 : increasing the cathodic potential at the copper balls  30  forces back the rate of formation of the Cu 2+  ions. The solution, enriched with Cu 2+  ions, exits the metal ion generator  2  in the upper region of the cathode space  35  through the port  11  via the fluid outlet  19 . The electrochemical reaction is made possible by applying a cathodic potential to the sieve bottom  31  and accordingly to the copper balls  30  and an anodic potential to the auxiliary anode  20  in the anode space  25 . The water of the electrolyte fluid contained in the anode space  25  is anodized liberating oxygen, said oxygen exiting the upper region of the metal ion generator  2  through the opening  17 . If necessary, Fe 2+  ions contained in the anode space  25  are oxidized as well at the auxiliary anode  20 . Since the exchange of fluid between the cathode space  35  and the anode space  25  is strongly impaired by the separation  21 ,  24 , the Fe 2+  ions deplete in the anode space  25  so that their concentration in stationary operation comes near zero. 
       FIG. 4  shows a second embodiment of the metal ion generator  2  according to the invention. In this case, the metal ion generator  2  is a receptacle with side walls  15  which form a rectangular, square or circular ground plan of the metal ion generator  2 . The receptacle is furthermore provided with a bottom  16 . The walls  15  and the bottom  16  are made of polypropylene. The metal ion generator  2  forms an opening  17  at its top. 
     The metal ion generator  2  again is provided with a cathode space  35  and an anode space  25 . Furthermore, the spaces  25  and  35  are separated from each other by an ion permeable wall  21 , an ion exchange membrane in this case, preferably an anion exchange membrane, which is vertically arranged. A perforated wall  26  is also provided, which endows the membrane with the required stability. 
     A sieve bottom  31  is arranged in the lower region in the cathode space  35 , said sieve bottom being constituted by a titanium net. A bulk of metal pieces  30  (shown only in parts) rests on the sieve bottom  31 , the metal pieces here being copper balls having a diameter of approximately 30 mm. An auxiliary anode  20  is accommodated in the anode space. The auxiliary anode  20  is connected to the positive terminal and the sieve bottom  31  to the negative terminal of a direct current supply (not shown). 
     The electrolyte fluid can enter the metal ion generator  2  through the lower fluid intake  18 . The fluid intake  18  is arranged underneath the sieve bottom  31 . Fluid can exit the metal ion generator  2  again through an upper fluid outlet  19 . The outlet  19  is arranged in the upper region of the cathode space  35 . 
     The way of operation of the metal ion generator  2  in this embodiment corresponds to that of the first embodiment shown in the  FIGS. 2 and 3 . In this respect, reference is made to the explanations given herein above. 
     to be deposited as an auxiliary cathode. 
     
       
         
               
             
               
               
             
           
               
                   
               
               
                 List of numerals: 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                  1 
                 precipitation tank 
               
               
                  2 
                 metal ion generator 
               
               
                  3 
                 reservoir 
               
               
                  4, 5, 
                 pipelines 
               
               
                  8, 9, 10 
               
               
                  6 
                 pump 
               
               
                  7 
                 filtering unit 
               
               
                 11 
                 exhaust port 
               
               
                 15 
                 tubular housing of the metal ion generator 2 
               
               
                 16 
                 bottom of the metal ion generator 2 
               
               
                 17 
                 front-sided upper opening of the metal ion generator 2 
               
               
                 18 
                 fluid intake into the metal ion generator 2 
               
               
                 19 
                 fluid outlet out of the metal ion generator 2 
               
               
                 20 
                 auxiliary anode 
               
               
                 21 
                 ion permeable means (woven cloth) 
               
               
                 22 
                 fluid level 
               
               
                 23 
                 direction of flow of the electrolyte fluid 
               
               
                 24 
                 wall separating the anode space 25 from the cathode space 35 
               
               
                 25 
                 anode space 
               
               
                 26 
                 perforated wall 
               
               
                 30 
                 pieces of metal, copper balls 
               
               
                 31 
                 sieve bottom, titanium net 
               
               
                 35 
                 cathode space 
               
               
                 38 
                 electrical contact for leading power to the auxiliary anode 20 
               
               
                 39 
                 electrical contact for leading power to the sieve bottom 31 
               
               
                 40 
                 electric feed line to the auxiliary anode 20 
               
               
                 41 
                 electric feed line to the sieve bottom 31 
               
               
                 50 
                 power supply, direct current source