1. Field of the Invention
This invention relates to the regeneration of an electroless copper plating bath and, more particularly, to a process and an electrodialytic cell for electrodialytically regenerating an aqueous, spent electroless copper plating bath, where the spent bath contains alkali metal salts as reaction products of the electroless plating process. The improved process utilizes electrodialysis to remove at least a portion of the anionic fraction of the reaction products generated in the chemical plating process.
2. Description of the Prior Art
Electroless chemical copper plating baths commercially used typically contain a water soluble salt of copper, such as copper sulfate, a reducing agent, such as formaldehyde, and an alkali, usually sodium hydroxide, to bring the plating bath into a range where the reducing agent, such as formaldehyde, has strong reducing powers. A complexing or chelating agent is commonly present in the bath to maintain the copper in solution. A commonly used complexing or chelating agent is sodium ethylenediaminetetraacetate, hereinafter referred to in the specification as EDTA. EDTA has so far been found to be highly effective in permitting the electrodialytic removal of at least a portion of the reaction products from the spent electroless copper plating bath without significant loss of the EDTA. Various brightners and stabilizers and the like are generally also included in the copper plating bath. The reducing agent, such as formaldehyde is generally effective at the higher pH values. Four moles of alkali metal hydroxide, such as sodium hydroxide, is consumed in the reduction of a mole of the metal salt, such as copper sulfate: EQU CuSO.sub.4 +2HCHO+4NaOH.fwdarw.Cu+Na.sub.2 SO.sub.4 +2NaCOOH+2H.sub.2 O+H.sub.2
Among the reaction products of the reduction reaction in the electroless copper plating process are alkali metal salts, such as sodium formate and sodium sulfate. The build up of sodium formate and sodium sulfate in the copper plating bath leads to general bath degradation. The generation of the sodium formate and sodium sulfate in the bath also results in a drop in the pH of the cooper plating bath resulting in a loss of effective plating.
Several methods are known and are presently in use to take care of the build up of the waste materials in the copper plating bath. Among these, the simplest and one of the more costly methods is a bail-out or throwing away a portion of the spent copper plating bath; and then adding to the bath the appropriate chemical components to replenish the bath, such as alkali metal hydroxide.
Other methods are known for dealing with the problem of waste material build up in the copper plating bath. For example, U.S. Pat. No. 4,076,618 to Zeblisky discloses a method for separating alkali metal salt by-products of an electroless metal deposition bath from an alkanolamine complexing agent and a complex species of a heavy metal complexed with an alkanolamine complexing agent. In Zeblisky's method, the pH of an alkaline electroless metal deposition bath is lowered so as to render the alkanolamine complexed heavy metan and the alkanolamine complexing agent extractable by an ion exchange medium. The pH adjusted bath is contacted with an ion exchange medium to extract the alkanolamine-complexed heavy metal and the alkanolamine complexing agent from the pH-adjusted bath. A bath liquid is then removed from the exchange medium which includes alkali metal salt by-products. The alkanolamine-complexed heavy metal and the alkanolamine complexing agent is removed 1from the exchange medium. The alkanolamine-complexed heavy metal and the alkanolamine complexing agent is then returned to an alkaline electroless metal deposition bath.
U.S. Pat. No. 4,324,629 to Oka et al. discloses a process for regenerating a chemical copper plating solution. In the method of Oka et al., the pH of a chemical copper plating solution containing copper ions, a reducing agent for copper ions, a chelating agent for copper ions and an alkali metal hydroxide is adjusted to 2-11. The plating solution is led to desalting compartments of an electrodialysis cell, provided alternately with anion exchange membranes and cation exchange membranes. Counter ions to copper ions, ions formed by oxidation reaction of the reducing agent, and CO.sub.3.sup.-2 or HCO.sub.3.sup.-1 are removed. The counter ions to copper ions are at least one of SO.sub.4.sup.-2, HCOO.sup.-1, CO.sub.3.sup.-2 and OH.sup.-1.
U.S. Pat. No. 4,289,597 to Grenda discloses a process for electrodialytically regenerating an electroless plating bath by removing at least a portion of the reacted products. In Grenda, the spent electroless copper plating bath is conducted to a regeneration compartment of an electrodialytic cell. An aqueous electrolyte as the anolyte of the electrodialytic cell is established and maintained in an anode compartment. The anode compartment has in common one wall with the regeneration compartment that is composed of a permselective anionic membrane. Anions of the alkali metal salts in the spent electroless copper plating bath are electrodialytically transferred through the anionic membrane from the regeneration compartment to the anode compartment.
The article "Electrodialysis In Advanced Waste Water Treatment" by Korngold appearing in Desalination, volume 24, pages 129-139 (1978) discusses electrodialysis in the regeneration of chemical copper plating baths. This article mentions that by applying electrodialysis in a continuous mode, the sodium sulfate and formiate can be removed from the spent bath, without affecting the concentrations of formaldehyde and the EDTA-complex.
U.S. Pat. No. 3,562,139 to Leitz discloses a cationic-anionic bipolar ion-exchange membrane. This patent also discloses a method and apparatus for the deionization of electrolyte solutions wherein alternatingly oriented anion-cation bilaminate ion-exchange membranes define chambers of a multi-chamber electrodialysis cell. The anion exhange laminae of the membranes bound the salt diluting chambers and the cation exchange laminae bound the salt concentrating chambers. A direct electrical current, which is periodically reversed, is passed transversely through the chambers.
U.S. Pat. No. 4,024,043 to Dege et al. discloses a single film, high performance bipolar membrane. U.S. Pat. No. 3,388,080 to de Korosy et al. also discloses a process for the production of permselective bipolar membranes.
What is needed, therefore, is a process and an electrodialytic cell which can reduce the number of different types of compartments required for electrodialysis of a spent electroless copper plating bath. What is further needed is a process and an electrodialytic cell which can promote generation of hydroxyl ions and hydrogen ions and substantially eliminate an accompanying oxidation or reduction reaction associated with such generation, the oxidation or reduction reactions occuring at the respective electrodes of the electrodialytic cell. What is additionally needed is a process and an electrodialytic cell which substantially elminates oxygen and hydrogen over voltages in the cell and thereby promotes savings of electrical energy in the electrodialysis of the spent copper plating bath. What is also needed is a process and an electrodialytic cell which substantially eliminates gases generated during electrodialysis in an electrodialytic cell and thereby promotes savings in electrical energy and a more efficient cell construction, with gas generation being primarily limited to the respective electrodes at the extreme ends of the cell.