METHOD OF CONTINUOUSLY REGENERATING AN ELECTROLESS-PLATING BATH AND DEVICE FOR CARRYING OUT THE METHOD

Abstract: An electroless plating bath is continuously regenerated by means of reverse osmosis and selective membranes are used which yield a high retention of metal complexes, complexing agents and surfactants and a low retention of the monovalent anions to be removed, for example, nitrate, formiate anions and bicarbonate anions in the case of a copper-plating bath.

Description: "Method of continuously regenerating an electrolessplating bath and device for carrying out the method". The invention relates to a method of continuously regenerating an electroless-plating bath. The invention relates more specifically to a method of continuously regenerating an electroless copperplating bath. The invention also relates to an electrolessplating device having means for continuously regenerating the electroless-plating bath, which device comprises a treating tank and a dosing device for the bath components consumed in the electroless-plating process. Such a method and device are known from German Patent Specification .DE 3,022,962. In the said Specification it is disclosed how a copper-plating bath is regenerated by means of electrodialysis using cation and anion exchange membranes. However, such membranes only have a limited selectivity, and further, the presence of a relatively small amount of dissolved organic compounds may impair their functioning. It is the object of the invention to provide a method of continuously regenerating an electroless-plating bath having a high degree of selectivity and reproducibility. Selectivity is to be understood to mean herein that metal compounds and bath components which do not have to be supplemented are not removed from the bath. On the other hand, it is not necessary to remove all of the waste products in one run since such a continuous method will lead to a steady state in which the removal of waste products equals the formation thereof during use of the electroless-plating bath. Reproducibility is to be understood to mean herein that in time metal layers of constant quality and composition can be deposited independently of the amount of products to be plated per unit of time, however, within the limits dictated by the permissible bath loading. The object of providing a method of continuously regenerating an electroless-plating bath is achieved in accordance with the invention by means of a method in which a solution which comprises at least metal ions, a reducing agent and a complexing agent for the metal ions is used for eleiroless-plating while the by-products which are formed by and during electroless deposition of metal onto a substrate are removed by means of reverse osmosis. United States Patent Specification US 3,637,467 discloses a method in which reverse osmosis is used in a galvanic plating process, however, said process is an electro deposition process in which no waste products need be removed. In said US Specification reverse osmosis is employed to recover bath components from a rinsing bath by concentrating them. Since the filtrate is reused also, the selectivity of the filter is not very important. The method of the invention can very advantageously be used for the continuous regeneration of an electroless copper-plating bath, the method being characterized in that the metal ions are copper ions and that the solution also comprises an alkali metal hydroxide and one or more surfactants. For continuously regenerating an electroless copper-plating bath, the method in accordance with the invention is preferably carried out so that reverse osmosis yields a retention of at least 90 * of the complexated copper ions and complexing agents. Surfactants which are difficult to analyse must also exhibit the greatest possible retention. With the exception of the reducing agent for copper ions, this group of compounds, which is present in the concentrate of the reverse osmosis device, comprises both the bath components whose amounts remain practically unchanged and the ones which are consumed in and during the electroless copper-plating process. Consequently, the reducing agent, for example, formaldehyde, must be additionally supplied to the copper-plating bath. The reason for this is that there are no suitable filter membranes having a high retention for small organic molecules or ions. The permeate formed in the reverse osmosis process mainly comprises waste products and the reducing agent. The retention value of the waste products should not be too high and is preferably less than 50 %. More specifically, the concentrate after reverse osmosis comprises complexated copper ions, the complexing agent and surfactants, and the permeate comprises monovalent anions. Both the concentrate and the permeate comprise alkali-metal ions. In order to increase the usability of the membranes employed in the reverse osmosis process, it is efficient that the solution which is fed to the reverse osmosis device is neutralized by adding an acid, that the salt thus formed is removed from the concentrate in the reverse osmosis process and that an amount of alkali-metal hydroxide is added to the copper-plating bath exceeding the amount consumed, so as to impart the required acidity to the copper-plating bath. In a preferred embodiment of the method of the invention for continuously regenerating an electroless copper-plating bath, the copper ions to be added to the electroless copper-plating bath to replace the ones consumed in the electroless copper-plating process, are added in the form of copper (II) nitrate and the acid emplpyed for neutralizing the solution prior to reverse osmosis is nitric acid. Another object of the invention is to provide a device for carrying out the method described above. This object is achieved by means of a device as described in the opening paragraph, which additionally comprises a neutralizing tank, a high-pressure pump and a reverse osmosis device, where the treating tank is connected to the neutralizing tank via an outlet and a pump, where the neutralizing tank, which comprises a dosing device for an acid, is connected to the treating tank by means of an outlet, where the neutralizing tank is connected to the inlet of the reverse osmosis device via an outlet and the high-pressure pump, and the outlet for the concentrate of the reverse osmosis device is connected to the neutralizing tank by means of a pressure valve. In order to protect the reverse osmosis device against too high a temperature, it is efficient to use a cooling device. The circulation rate in the circuit of the neutralizing tank and of the reverse osmosis device will generally be much higher than that in the circuit of the treating tank and of the neutralizing tank. By interposing a cooling device between the pressure valve and the neutralizing tank the bath liquid is cooled to a sufficient degree. An alternative embodiment of the device in accordance with the invention is characterized in that a cooling device is interposed between the neutralizing tank and the high-pressure pump. In both cases, the cooling device is arranged at a location where the liquid to be cooled is not at a high pressure. A special embodiment of the electroless copperplating device in accordance with the invention, is characterized in that the plating bath is a copper-plating bath and that the reverse osmosis device comprises a membrane of cellulose acetate. The invention will now be explained in greater detail with reference to examples of embodiments and a drawing, in which Figure 1 is a schematic representation of a device in accordance with the invention. Embodiment of a device in accordance with the invention. Figure 1 shows a treating tank 1 which is filled with a solution for the electroless deposition of metal layers on a substrate, for example, in the form of uniform layers or patterns as in the manufacture of printed circuit boards by means of a fully additive method. For this purpose, the substrates which are activated in a known manner are immersed in the solution. The consumed bath components are supplied to the bath by means of a dosing device 2. Part of the solution can be transferred from treating tank 1 to a neutralizing tank 5 by means of a drain 3 and a pump 4. Neutralizing tank 5 comprises an overflow 7 to treating tank 1 and a dosing device 6 for adding an acid. Neutralizing tank 5 is connected to the inlet of a reverse osmosis device 10 by means of a drain 8 and a high-pressure pump 9. The concentrate is led from reverse osmosis device 10 to neutralizing tank 5 via a drain 11, a pressure valve 12 and a cooler 14. The permeate is discharged from re verse osmosis device 10 via an outlet 13, for example, for further processing or storage. The liquid has to be cooled because it is heated by high-pressure pump 9 and by the supply of warm bath liquid from treating tank 1. In the present example, cooling device 14 is located between the outlet for the concentrate of the reverse osmosis device 10 and neutralizing tank 5, however, it can also be located between neutralizing tank 5 and the inlet of reverse osmosis device 10. Furthermore, treating tank 1 and neutralizing tank 5 comprise measuring devices 15, for example, for measuring the temperature and the pH-value. In addition, several control devices and safety devices may be present, for example, for checking liquid flows, concentrations, pressure and liquid levels. Embodiment 1 of the method in accordance with the invention. A treating tank 1 contains 50 litres of a copperplating solution which comprises 0.03 M of copper (II)-nitrate, 0.05 M of the tetra sodium salt of ethylene diamine tetra-acetic acid (EDTA), 0.05 M of sodium hydroxide and 0.06 M of formaldehyde. EDTA is a complexing agent for copper ions and it is used to prevent the formation of copper (II)-hydroxide in the basic medium and to improve the quality of the copper layers to be formed. Furthermore, complexing prevents the copper ions from being removed from the solution during the reverse osmosis process. Other complexing agents, such as ethylene diamine tetra isopropanol, are also suitable for use in the method of the invention. During use of the copper-plating bath, an amount of salt is formed which consists predominantly of sodium nitrate and sodium formate. If the method in accordance with the invention is used, for example, a steady state is achieved in which the salt content lies between 0.4 and 0.8 M. This is an acceptable level, the values at the lower end of the range yielding the best results as to the quality of the copper layers. The solution also comprises surfactants which serve to influence the quality of the deposited copper layers, for example, as to ductility, selectivity, tensile strength, (re-) solderability and smoothness. These surfactants are hardly consumed in the copper-plating process. A suitable surface-active additive is formed by, for example, the combination of 0.1 s by weight of a wetting agent, such as Triton QS-44 of Rohm and Haas (a phosphate ester of an alkylaryl polyether alcohol having a molecular weight of approximately 800) and 0.02 ffi by weight of a thioether-polyoxyalkylene compound having the following structural formula: : C,H2mt.l - S - (C2H4O)a - (C3H6 )b - (C2H40)c 2 H, in which m has an average value of 12, (a + c) has an average value of 369 and b has an average value of 55. Said compound has the additional advantage that it slightly raises all retention factors, thus bringing about a higher degree of retention of notably the complex copper ions in the reverse osmosis process. Oxygen or air is led through the solution so that it contains between 3 and 5 ppm of oxygen. The solution has a pH-value between 11 and 12 and it is used at a temperature of 700C, the deposition rate being between 2 and 3 /um of copper per hour. Predominantly, the following chemical reactions take place in the treating tank: copper-plating: EMI6.1 Canizzarro side reaction: EMI6.2 absorption of carbon dioxide from the air: EMI7.1 2+ The consumed components Cu , CH2 0 and HO are supplemented to the solution in an amount which depends predominantly on the amount of copper produced. The components are supplemented in the form of aqueous solutions of copper (II) nitrate, formaldehyde and sodium hydroxide. The by-products formed in this process, predominantly formate and carbonate and the increasing amount of sodium ions and nitrate ions, must be removed from the solution in order to prevent the quality of the deposited layers from deteriorating and to preclude a decrease in selectivity of the copper-plating reaction. In the copper-plating solution, the copper ions are present mainly as complexes comprising EDTA and the sodium ions as EDTA salt. The methanol formed in the Canizzarro reaction largely evaporates. By means of drain 3 and pump 4, part of the copper-plating solution (1-31/hr) is fed to a neutralizing tank 5 having a volume of 10 1. Such an amount of 2 M nitric acid is added to the solution in the neutralizing tank that the pH-value is less than 8, preferably, approximately 7. For this purpose, it is necessary to add an amount of acid of approximately 4 * by volume of the volume flow from the treating tank to the neutralizing tank. In the solution, the carbonate ions are then converted into monovalent bicarbonate anions. The solution in the neutralizing tank has a maximum temperature of 30 C, which is achieved by cooling the solution in the reverse osmosis circuit. Part of the solution in the neutralizing tank 5 is fed back to the treating tank 1 by means of an overflow 7. In order to compensate for the fact that the solution fed back from the neutralizing tank is almost neutral, an additional amount of sodium hydroxide is added to the copper-plating solution in the treating tank. The wetting agent used must also be partly supplemented. By means of a plunger pump 9, the neutralized solution (200-300 l/hr) is fed under high pressure (3-4 MPa) to the inlet of a reverse osmosis device 10 which comprises cellulose-acetate membranes having a filter 2 surface area of 0.036 m . Suitable types of membranes are DDS No. 865 of The Danish Sugar Corporation. Membranes of ceramic aluminium oxide or of porous glass are also suitable. The retention factor is the complement of that fraction of the concentration of a component in the inlet stream, which is still present in the permeate. The retention factor of formaldehyde is almost nil, which means that the concentration of formaldehyde in the permeate (and hence in the concentrate) is almost equal to the concentration in the inlet stream. The retention factor of bicarbonate ions, formate ions and nitrate ions ranges from 0.05 to 0.25. On the other hand, the retention factor of EDTA and Cu-EDTA-complexes is high: 0.90 and more. In the example described herein, the cations in the permeate are almost exclusively sodium ions. The volume flow of the permeate is approximately 1 - 2 l/hr. The concentrate is returned to neutralizing tank 5 via a pressure valve 12 (which brings the concentrate to atmospheric pressure) and a cooler 14. In order to reduce the amount of salt formed by neutralization, the circulation rate in the circuit of the neutralizing tank and of the reverse osmosis device is chosen to be higher than that in the circuit of the treating tank and of the neutralizing tank. Consequently, the suppletion of sodium hydroxide can be kept at a low level. Polarization and contamination of the membrane is avoided by the high circulation rate through the reverse osmosis device. Polarization occurs where the reverse osmosis process is rendered ineffective due to reduction of the differences in concentration on both sides of the membrane. The method in accordance with the invention has a number of special advantages. The use of a continuous method brings about a steady state which leads to copper layers of constant quality. The process can easily be controlled by controlling the various liquid flows and the size of the (adjustable) membrane surface area. After a change in the process a new steady state will be established automatically. The pH-value of the copper-plating bath can be adjusted and maintained within wide limits. A high selectivity of the copper-plating process (for copper-plating in a pattern) is achieved at a relatively low pH-value, at a higher pH-value, however, the deposition rate of the copper increases. For salt concentrations of 0.3 M, additively built-up copper layers amply meet the requirements of copper on printed circuit boards. Higher salt concentrations are suitable up to at least 0.8 M. The amounts necessary for supplementation can simply be calculated from the selected copper-production rate, allowing for the side-reactions, and may be verified by analyzing the bath composition. An additional advantage is that components whose amounts are difficult to determine, especially the thio-ether polyoxyalkylene compound, are hardly consumed and are not discharged, consequently, the concentration of these components can be kept constant without any problems. The wetting agent, which has a retention factor of approximately 85 *, can be analysed and supplemented relatively easily. In the method of the invention, copper (II) nitrate is preferably used for supplying copper ions. Copper (II) sulphate, which is more commonly used in the art, cannot be used in this method due to the high retention factor of the bivalent sulphate anions. Solutions comprising cyanide have the disadvantage that the cyanide comes into the permeate, thus rendering further processing of the waste problematic. Preferably, the copper-plating solution is neutralized by means of nitric-acid because this does not result in any new types of anions being added and because the salt formed is removed by reverse osmosis. Other acids having monovalent anions may also be usefu1, but they have their restrictions. The use of hydrochloric acid may lead to, for example; an anacceptably high iron content in the copper-plating solution due to corrosion of the stainless steel component parts and pipes of the reverse osmosis device. Embodiment 2 of the method in accordance with the invention. In an alternative embodiment of the method in accordance with the invention, a solution comprising nickel chloride is used for the electroless nickel-plating process, as described in US Patent Specification 3,946,126. In the way described above, the solution is continuously regenerated by means of reverse osmosis. The borate anion formed in the nickel-plating reaction is then removed from the solution but the nickel ions are retained in the form of a pyrophosphate complex and they are fed back to the nickel-plating bath. The method and the device in accordance with the invention may also be used for continuously regenerating plating baths for the electroless deposition of, for example, tin, silver and gold/copper alloys. The retention factor of the metal ions must then be higher, (as close to 1 as possible) than that of the by-products. A continuous regeneration of a plating bath by means of reverse osmosis is then possible by an appropriate choice of the counterions and the use of complexing agents and surfactants in the bath.

Claims: 1. A method of continuously regenerating an electro less-plating bath, chaI-acterized in that a solution which comprises at least metal ions, a reducing agent and a complexing agent for the metal ions is used for electroless plating while the by-products which are formed by and during electroless deposition of metal onto a substrate are removed by means of reverse osmosis. 2. A method as claimed in Claim 1 for continuously regenerating an electroless copper-plating bath, characterized in that the metal ions are copper ions and that the solution also comprises an alkali metal hydroxide and one or more surfactants. 3. A method as claimed in Claim 2, characterized in that reverse osmosis yields a retention of at least 90 m of the complexated copper ions and complexing agents. 4. A method as claimed in Claim 3, characterized in that after reverse osmosis the concentrate comprises complexated copper ions, the complexing agent and surfactants and that the permeate comprises monovalent anions. 5. A method as claimed in any one of the Claims 2 to 4, characterized in that the solution which is fed to the reverse osmosis device is neutralized by adding an acid, that the salt thus formed is removed from the concentrate in the reverse osmosis process and that an amount of alkali metal hydroxide is added to the copperplating bath, exceeding the amount consumed, so as to impart the required acidity to the copper-plating bath. 6. A method as claimed in Claim 5, characterized in that the copper ions to be added to the electroless copperplating bath to replace the ones consumed in the electroless copper-plating process, are added in the form of copper (II) nitrate and the acid employed for neutralizing the solution prior to reverse osmosis is nitric acid. 7. An electroless-plating device having means for continuously regenerating the electroless plating bath, which device comprises a treating tank and a dosing device for the bath components consumed in the electroless-plating process, characterized in that the device additionally comprises a neutralizing tank, a high-pressure pump and a reverse osmosis device, where the treating tank is connected to the neutralizing tank via an outlet and a pump, where the neutralizing tank, which comprises a dosing device for an acid, is connected to the treating tank by means of an outlet, where the neutralizing tank is connected to the inlet of the reverse osmosis device via an outlet and the high-pressure pump, and the outlet for the concentrate of the reverse osmosis device is connected to the neutralizing tank by means of a pressure valve. 8. A device as claimed in Claim 7, characterized in that a cooling device is provided between the pressure valve and the neutralizing tank. 9. A device as claimed in Claim 7, characterized in that a cooling device is provided between the neutralizing tank and the high-pressure pump. 10. A device as claimed in any one of the Claims 7 to 9 for electroless copper-plating, characterized in that the plating bath is a copper-plating bath and that the reverse osmosis device comprises a membrane of cellulose acetate.

Inventors: SCHOLTENS, ELTJO
Applicants: N.V. PHILIPS' GLOEILAMPENFABRIEKEN
Application year: 1986
Main classification: C23C18
IPC codes: C23C18