Chemistry: electrical and wave energy – Apparatus – Electrophoretic or electro-osmotic apparatus
Reexamination Certificate
2002-03-07
2004-04-20
Phasge, Arun S. (Department: 1753)
Chemistry: electrical and wave energy
Apparatus
Electrophoretic or electro-osmotic apparatus
C204S634000
Reexamination Certificate
active
06723218
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to a system, apparatus, and method for regenerating an electroless bath electrolyte.
In addition to galvanic plating methods in which an external current is introduced via electrodes that are placed into the plating bath for the purpose of depositing a metal plating on a work piece, so-called electroless plating methods are known. These methods are most often used to plate nonconducting substances, for example, plastic components. Such metal platings, for example, are applied for various reasons to plastic substrates. For one thing, a metal surface may be desired for esthetic reasons; for another, the objective may be to utilize the material properties of the metal with which a substrate is plated. Such properties include, for example, improved resistance to corrosion or the electrical conductivity of the material used. Thus, it is known, for example, that printed conductors can be applied to printed circuit boards made of plastic (for example, epoxy resins) by means of electroless plating techniques.
In particular, nickel metal is frequently deposited by means of electroless plating technology.
To reduce the metal ions contained in the electrolyte to elemental metal when this type of plating technology is used, an appropriate reducing agent which is oxidized itself during the reaction must be added to the electrolyte. In the case of an electroless nickel bath, hypophosphite ions are added. These reduce the nickel ions contained in the precipitation bath to elemental nickel and are themselves oxidized to orthophosphite ions. The equation of the reaction that takes place when an electroless nickel bath is used can be expressed as follows:
NiSO
4
+6NaH
2
PO
2
® Ni+2H
2
+2P+4NaH
2
PO
3
+Na
2
SO
4
In the course of the metal plating process, nickel ions are gradually removed from the electrolyte and are precipitated as elemental nickel on the surface to be plated; at the same time, hypophosphite ions are continuously oxidized to orthophosphite ions. In other words: for one thing, the concentration of the nickel ions dissolved in the electrolyte and the concentration of the hypophosphite ions contained in the electrolyte decrease in the electrolyte, and for another, the concentration of the orthophosphite ions contained in the electrolyte increases. As a result, the electrolyte is being “depleted.” Thus, as the time during which the electrolyte is allowed to stand increases, the quality of a plating deposited with such an electrolyte decreases. This means that the electrolyte can be used only for a certain number of plating runs. Thereafter, the electrolyte must either be replaced or it must be regenerated by means of suitable auxiliary agents. For the nickel precipitation bath, regeneration involves at least the removal of the orthophosphite ions which form as reaction products and, potentially, the addition of spent nickel ions and spent hypophosphite ions.
In addition to the in situ precipitation of undesirable ions in sparingly soluble compounds and the addition of ions which are needed and which are spent in the course of the standing time of the bath, it is also known that electrodialytic methods can be used for the regeneration of electroless precipitation baths. Such methods provide that the depleted bath electrolyte and a regeneration electrolyte which absorbs at least the ions which are to be removed from the depleted bath electrolyte so as to be able to regenerate [the bath electrolyte] are channeled through compartments which are separated from each other by membranes. At the same time, the regeneration electrolyte may contain ions that are to be added to the bath electrolyte. Via electrodes provided in an electrodialysis unit, current is conducted through an electrodialysis unit and an ionic flow is induced. Through the suitable selection of the membranes located between the so-called diluate compartments through which the bath electrolyte flows and the so-called concentrate compartments through which the regeneration electrolyte flows, it is possible to ensure a targeted migration of ions from the bath electrolyte which is passing through the diluate compartments into the regeneration electrolyte which is passing through the concentrate compartments and vice versa.
An example of such an electrodialysis system is described in the German Patent No. DE 198 49 278 C1. In the system described in this printed publication, two separate electrodialysis units are used, each of which comprises diluate compartments and concentrate compartments that are separated from one another by membranes as well as a pair of electrodes, i.e., an anode and a cathode. On the cathode side, the diluate compartments of a first electrodialysis unit are separated from the concentrate compartments of this unit by monoselective cation exchanger membranes and on the anode side by anion exchanger membranes. In the second electrodialysis unit which also comprises diluate compartments and concentrate compartments as well as an anode and a cathode, the diluate compartments are separated from the concentrate compartments by monoselective anion exchanger membranes on the cathode side and by anion exchanger membranes on the anode side. To regenerate the bath electrolyte, this electrolyte is divided into two main streams which are conducted parallel to each other through the diluate compartments of the first electrodialysis unit and the second electrodialysis unit. Similarly, the regeneration electrolyte is divided into substreams which are conducted parallel to each other through the concentrate compartments of the first and the second electrodialysis unit. In the first electrodialysis unit, both orthophosphite ions and hypophosphite ions are removed from the bath electrolyte. Nickel ions still present in the bath electrolyte remain in the electrolyte. In the diluate compartments of the second electrodialysis unit, hypophosphite ions from the regeneration electrolyte are fed into the second substream of the bath electrolyte.
This method is less efficient per run, and it is necessary to recirculate the bath electrolyte to be regenerated several times through the electrodialysis system until the degree of regeneration desired is obtained.
A second system known from prior art is disclosed in EP 0 787 829 A1. This electrodialysis system described in this printed publication also comprises two electrodialysis units which have both diluate compartments and concentrate compartments as well as an anode and a cathode. As to its setup, the first electrodialysis unit of this printed publication is similar to the electrodialysis unit of the German patent specification mentioned above. Again, the diluate compartments of the first electrodialysis unit are separated from the adjacent concentrate compartments by a monovalent cation exchanger membrane on the cathode side and by an anion exchanger membrane on the anode side. The second electrodialysis unit of this electrodialysis system, however, has a setup different from the setup known from the German patent specification above. In this case, the diluate compartments on the cathode side are separated from the adjacent concentrate compartments by a cation exchanger membrane and on the anode side by a monovalent anion exchanger membrane. In the system that is known from this printed publication, both the regeneration electrolyte and the bath electrolyte flow sequentially in one direction through the individual electrodialysis units. In the first electrodialysis unit, the bath electrolyte is depleted of hypo- and orthophosphite ions, and in the second electrodialysis unit, hypophosphite ions are returned in a second step. Thus, the system known from EP 0 787 829 A1 is the starting point for the present invention as disclosed in the precharacterizing clause of main claim
1
.
The system known from the European patent application [Offenlegungsschrift], however, has the disadvantage that its setup is expensive and that the electrodes used in the electrodialysis units are not sufficien
Hadley John Stuart
Verhoeven Peter Anton Adriaan
Enthone Inc.
Phasge Arun S,.
Senniger Powers Leavitt & Roedel
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