Electrolysis: processes – compositions used therein – and methods – Electrolytic coating – Treating process fluid by means other than agitation or...
Reexamination Certificate
2000-03-28
2002-02-26
Dawson, Robert (Department: 1712)
Electrolysis: processes, compositions used therein, and methods
Electrolytic coating
Treating process fluid by means other than agitation or...
C205S098000, C205S087000, C204S267000, C204S269000, C204S275100, C204S232000, C204S234000, C204S237000
Reexamination Certificate
active
06350362
ABSTRACT:
The invention relates to a method and an apparatus for regulating the concentration of substances in electrolytes for the deposition of metal. The method is preferably used for the electroplating of printed circuit boards in immersion bath systems and in horizontal or vertical continuous systems, in each case using insoluble anodes.
For the electroplating operation using insoluble anodes, it must be ensured that the metal ion concentration of the metal to be deposited is maintained in the electrolyte and remains as constant as possible. This may be achieved, for example, by supplementing the electrolyte with metal-containing salts. The costs incurred for supplying and disposing of such are very high. An alternative known method for supplementing metal ions in the electrolyte is the direct dissolution of the metal in the electrolyte by means of an oxidising agent such as oxygen. For copper-electroplating, for example, metallic copper is removed from an electrolyte, which is enriched with air oxygen. In such case, there are no residues, such as occur when supplementing with metal salts. However, during the electroplating operation in both cases, oxygen is produced on the insoluble anodes of the electrolytic cell. This oxygen attacks the organic additives of the electrolyte. It additionally causes a corrosive destruction of the anode material.
Methods for the electrolytic deposition of metal, wherein the above-mentioned problems have been solved, and wherein the metal ion concentration in the electrolyte can be kept constant without producing gas on the insoluble anodes are described in DD 215 589 A1 and DE 43 44 387 A1. Compounds of a suitable redox system are then added to the electrolyte and can be electrochemically converted into the oxidised or reduced form respectively. During the electroplating operation the compounds are oxidised on the insoluble anodes in the electrolytic cell so as to avoid gas being produced. During the reduction of the compounds in the oxidised form externally of the electrolytic cell, the metal which is situated in a container and is to be electrolytically deposited is dissolved without auxiliary power. The electrolyte, which is thus enriched with metal ions, is circulated through the electrolytic cell and through said container. In consequence, there is a constant conveyance of oxidised ions of the redox agent from the electrolytic cell into the container and a return conveyance of reduced ions of the redox agent into the electrolytic cell again. Because the metal to be deposited is dissolved in the form of ions in the container, the container is hereinafter called an ion generator.
There is considerable experience for the electrolytic copperplating of printed circuit boards from an electrolytic fluid provided with an iron addition as the redox agent. It is self-evident that the ideal case of complete oxidation of the redox agent on the insoluble anodes cannot be achieved, more especially, with the current densities in the electrolytic cell which are to be used in practice. Equally, the oxidised ions of the redox agent are reduced parasitically not only in the ion generator, but also on the cathode in the electrolytic cell in a small proportion. The consequence of this is that the cathodic current yield drops and is, therefore, only about 90 percent.
Air is constantly introduced into the electrolytic fluid by electrolyte movements so that oxygen, contained in the air, dissolves in the fluid. This oxygen is capable of dissolving copper. In consequence, the metal situated in the ion generator is dissolved in said generator, on the one hand, by reducing the oxidised form of the redox agent and, on the other hand, additionally by means of dissolved oxygen. A balance between the formation of metal ions in the solution by metal dissolution and consumption of the metal ions by electrolytic metal deposition is therefore not set. Rather, the content of the metal ions to be deposited in the electrolytic fluid increases continuously.
However, in order to ensure sufficiently good physical properties of the deposited metal, the metal ion content in the solution must be kept within narrow limits. In the described method with insoluble anodes and using compounds of a redox system, it is not possible to reduce the concentration of the metal ions in the electrolytic fluid by metal deposition in an additional electrolytic secondary cell using an insoluble anode, though such a phenomenon is known from conventional electroplating systems with soluble anodes.
In the case of electroplating systems which operate with insoluble anodes and using a redox system, the electrolytic secondary cell must also be provided with an insoluble anode. During the electroplating operation, in fact, metal works loose from the electrolyte in this secondary cell. At the same time, however, the redox agent is oxidised on the anode of the secondary cell. In consequence, the content of the oxidised ions of the redox agent increases accordingly in the electrolyte. Electrolyte, with the then existent, increased content of metal ions, passes with the electrolyte current into the ion generator. There, correspondingly more metal is electrochemically dissolved by reducing the redox agent.
In consequence, a permanent dilution of the electrolyte solution is only possible with known methods to reduce, or respectively keep constant, the metal content in the solution. For such purpose, large quantities of electrolyte must constantly be rejected and disposed of. When a continuous electroplating system has a length of six metres, approx. 500 litres electrolyte are produced every week during the three-layer operation. In addition, the rejected organic and inorganic additives of the electrolyte must also be supplemented. From economical and ecological viewpoints, this method, known as the “feed and bleed” method, is unsatisfactory.
A prerequisite for the continuous operation of an electroplating system with insoluble anodes and using a redox system in the electrolytic fluid is that a balance is set between dissolving the metal to be deposited and depositing such on the item to be treated.
In known electroplating systems, the electrolyte is conveyed to the item to be treated by means of pumps. Because of the flow movements in the electrolytic fluid, which are produced during conveyance, more especially also during the return of the electrolyte to the electroplating container over vertical paths, air is introduced into the electrolyte. In immersion systems for electroplating purposes, air injection is preferably used for circulating electrolyte. In all of these cases, air oxygen passes into the electrolyte. Considerable technical outlay is required if this introduction of oxygen is to be avoided. One possibility for solving this problem might be, for example, to cover the entire electrolytic fluid with an inert gas. For such purpose, however, the entire electroplating system, including the ion generator, would have to be encased in a gastight manner, thereby incurring considerable technical outlay.
The basic object of the invention, therefore, is to avoid the disadvantages of known methods and apparatuses and, more especially, to provide an economical method and an apparatus which are suitable, during the electrolytic metal deposition, for keeping the content of ions in the metal constant in electrolytic cells, which have insoluble anodes, and in which a depositing electrolyte is used, which contains compounds of a reversible redox system which is to be electrolytically deposited on the item to be treated.
The object is achieved by the method according to claim
1
and by the apparatus according to claim
9
.
In the metal depositing method according to the invention, the metal from the electrolytic fluid is deposited on the item to be treated by using insoluble anodes. Additional compounds of an electrochemically reversible redox system are contained in the electrolytic fluid. With the oxidised form of these redox compounds, metal is dissolved in an ion generator, which is traversed by the electrolytic fluid,
Geisler Jens-Eric
Kopp Lorenz
Maurer Manfred
Wachter Ralf-Peter
Atotech Deutschland GmbH
Dawson Robert
Feely Michael J
Paul & Paul
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