FIELD OF THE INVENTION
The present invention relates to a process for the conversion of various forms of metal ion to the metal state, more specifically using an electroless metal solution matrix.
BACKGROUND OF THE INVENTION
The art describes the formulation of ‘electroless’ solutions, such as U.S. Pat. Ser. No. 3,011,920. The purpose of such solutions is to impart a metal coating to a non conductive surface, for example, automotive plastic trim, thus the electron to convert the metal ion must be provided by chemical means instead of electrical. Said matrix is capable of providing an electron(s) to said metal ion for the conversion of said metal ion to the ground or metal state.
Although the discussion below focuses on the application of the invention to copper, it will be clear to a person skilled in the art how variations may be devised for other metals. Also, if more than a single type of metal ion is in solution, further steps may be employed to remove these other ions.
Electroless copper is a solution designed for the chemical deposition of copper metal on a non-metallic surface. Electroless copper metallization is used in a number of industries such as for plating on plastic and printed circuits. A fairly typical example is a double sided circuit board. On both sides of the board there is a copper foil, of varying thickness depending on the manufacturer and the particular function of the board. Some copper foil thickness are about one half ounce per square foot area of circuit board; others may be one ounce and greater. The board material is a combination typically of resin and glass fibres. Over the years resin technology has advanced significantly so the type of resin varies, depending on the dielectric constant desired. The overall structure of the assembly and type of glass fibres would affect drilling and other manufacturing aspects. A hole is drilled straight through this board (called an assembly or laminate) between the two sides. It then becomes desirable to metallize through this hole so the two sides (copper foils) are in electrical communication. There is no technique known whereby electrons may be applied using electrolytic means because the resin itself is not conductive. What is done is to immerse this assembly into a metal-ion containing solution where the metal will adhere to the non-metallic areas by an electrochemical process. This solution is called ‘electroless’ copper.
Typically, the electroless process starts with a cleaning step. The surface to be coated may contain oils, soils, dirt and etc. The purpose of this step is to remove the surface contaminants and provide a nice clean surface. Typically, an acidic cleaner is used, but it might also be alkaline in some cases.
Then an important step, microetch, follows. Electroless copper is applied in the surface of the hole in the assembly as well as on the exposed copper foil surfaces of the board. The surface of a copper foil, depending on the particular foil, may be very smooth. The smoother it is, the less likely that the electroless copper will adhere to it, and it may actually peel away causing potential subsequent manufacturing problems. So it becomes necessary to roughen the surface of the foil before applying the electroless it up, that is to say to cause the surface topology to become pitted on a microscopic level. This involves a chemical process of removing a thin layer of copper from the copper surface of the foil, leaving a roughened surface topography. As a result, when electroless copper is applied, it will adhere tightly.
For this purpose, the assembly is immersed into a microetch solution containing an oxidizer which will take the copper from the ground metallic state and bring it to the Cu+2 state, the copper becoming ionic and moving into solution. This microetch solution contain an oxidizer and Cu+2 in an acidic medium. The question is how much Cu+2 and how much acid, which depend on manufacturing characteristics individual to each manufacturer and job order. There is no way to predict with any degree of precision how much copper is oxidized to the +2 state. As well, the residual acidity that may be present in this solution may vary from moment to moment. As an example, between 20 to 30 grams per liter of copper in the ionic state may be present in an spent microetch solution.
After the assembly is removed from the microetch solution, the residual metal ions present on the workpiece must be removed in order not to contaminate subsequent processes with high levels of copper ion. This is done by attempting to capture much of the ion in a static or standing rinse known as a ‘drag out’. The workpiece is briefly immersed in the drag out, then it proceeds to subsequent water rinses where the remaining metal ion is completely removed. The metal ion which diffuses into the water rinses must somehow be captured and converted into a manageable state; this highly concentrated metal ion solution may not be safely discharged.
This is dealt with historically in any one of several ways. First, being the oldest technique, is simple precipitation: the pH of the rinse is adjusted (raised) to form copper hydroxide (also known as sludge). Subsequently, there may be a possible coagulation step. Sludge, classified as a heavy metal, is typically collected in a filter press and then sent to a hazardous landfill site suitable for heavy metals, at great cost.
The second historical modality is ion exchange. What occurs are: collection, pH adjustment, then the ion exchange. The rinse effluent goes through an ion exchange column, then the upload is disposed (discharged to a sanitary sewer). Periodically the ion exchange resin will become charged with copper ion or metal ion and must be regenerated by typically an acid rinse. After this removal process the acid rinse itself needs to have the metal ion recovered so then a whole other, possibly sub-modality, is required, and that is typically electrowinning, which is electrolytic recovery of the metal ion.
Electrowinning depends upon many factors to operate efficiently. The first is high concentration of metal ion. When the concentration of the metal ion in question drops below a certain level, efficiency drops precipitously to a point of no return where one can literally do electrowinning for literally days or weeks without getting the ion down to the desired concentration. So the concentration parameter is a deficiency in that process.
The second deficiency is the acid itself. The acid has to be back neutralized so every time one conducts this operation to get the metal down to a low enough concentration to meet legal limits, one must do electrowinning for an extraordinarily long period of time, then back neutralize all this large volume of acid.
Third, electrowinning tends to be expensive: the parts rely on platinum electrodes which are very expensive, burn out easily, and are labour intensive. Changing the anodes and cathodes and regular maintenance of the electrowinning cell is labour intensive.
This therefore points to one embodiment of the present invention to remove copper ions from a low pH (acidic) rinse solution.
In some instances, a standing rinse also known as a ‘drag out’ is employed after microetch, the purpose of which is to capture excessive metal ion and diminish the ion concentration in the following rinses. The level of metal ion which is introduced into rinse water is a function of the ratio of the concentration of the copper ion in the microetch to that in the drag out. For example if the concentration ratio is 3:1, two-thirds of the copper ion is captured by the drag out. The drag out is then pumped back into the microetch. A value to having this scheme comes when one possesses the capability to treat volumes of highly concentrated ion solutions, thus introducing the operation of one embodiment of the invention here. Typically, a running microetch solution will contain 20 to 30 grams per liter of copper ion; a drag out may contain levels of copper that are lower than that of the microetch, depending on the frequency and methodology of its management.
Following rinsing, the assembly will be treated by a catalyst. In these processes palladium is typically used as a catalyst, which is a standard industry catalyst. A thin coat at the ppm level is applied which adheres to the resin on the assembly; it also adheres to the surface of the metal (but primarily binds to the resin). There is further rinsing after that, just to rinse off residual palladium which may be present.
After catalysis the assembly goes into the copper electroless solution. An electroless solution comprises copper ions held preferably at high pH with a chelating agent. It is reacted with a source of hydroxide ion (for example, sodium hydroxide) and a reduction agent (for example, formaldehyde), in the following stoichiometrically balanced reaction: (also known as a Cannizzaro reaction)
Cu+2 +2HCHO+4 OH−→Cu0+2HCOO−+2H2 (1)
The electrons are thus provided by chemical means as opposed to electrical means. Electroless copper has had a bad environmental reputation: large volumes of spent solution are generated, which are very difficult to neutralize and yet must be treated down to very low discharge limits.
As copper metal is deposited from a working electroless solution onto the assembly, the concentration of the reactants are decreased and the concentration of byproduct salts are increased. The components are pumped into the solution, which results in solution increase or ‘growth’. The excess solution is withdrawn and treated through various treatment schemes, none of which are very efficient.
Electroless copper is chelated with EDTA, QUADROL or other like chelating or ‘sequestering’ agents. A variety of methods are employed to remove the metal ion including: complexation with various thio (sulphur) derivatives like DTC (sodium dimethyldithiocarbamate, or other carbamates) or other filtration methods employed together with various precipitation methods. Ion Exchange Resins have been used but are not highly reliable because the bond with EDTA can be stronger than that of the resin and thus the copper can bypass the resin treatment process under varying process conditions. Prior art describes the treatment of such ‘electroless’ solutions with ion exchange resins in U.S. Pat. No. 4,076,618, Zeblinsky et al.
Further, when EDTA is to be combined with any waste stream, it necessitates the usage of strong precipitating agents like DTC, because if copper-EDTA is precipitated with traditional means, the newly freed EDTA is able to bond with other metal ions in the waste stream, which can then escape the treatment process. DTC actually forms a copper-sulfur bond, which is sufficiently strong as to resist the very high chelating power of EDTA with copper. However, this use of the DTC results in an insoluble compound (sludge).
All the copper that may contact the solution being neutralized must be precipitated with DTC and not just the copper chelated with EDTA. This results in a large amount of sludge, as well as the precipitation problems associated; also DTC is relatively expensive.
An example of an alternative approach to the neutralization of chelated copper may be found in Japanese patent no. JP54158059, which aims to decompose and remove organic contaminants contained in copper plating waste water at a high removal rate by adding palladium chloride to the waste water, which is kept at high pH, heating the waste water to form a precipitate, removing the precipitate, then subjecting the waste water to oxidizing treatment at low pH, followed by activated carbon. Copper plating waste water containing such contaminants as EDTA, formaldehyde, and copper is treated with an alkali agent to adjust the pH to above 12, followed by palladium chloride, and heated to 30-80 degrees C. After removal of the resulting precipitate, the waste water is treated with a mineral acid to pH 1-3, then treated with an oxidizer, again adding a mineral acid to adjust the pH to within 2-4, then treated with activated carbon, and subjected to solid-liquid separation to provide treated water. Among the numerous problems associated with this approach is that the material must be removed as a fine precipitate and is difficult to remove, dry, handle and dispose of.
SUMMARY OF THE INVENTION
This invention provides for a method for converting metal ions in solution to the metal state comprising the steps of: (a) providing an initial quantity of reaction solution containing a matrix of chemicals, the reaction solution comprising an electroless solution for said ions including a chelating agent for the ions; (b) providing a chamber for containing the reaction solution; (c) delivering the reaction solution to the chamber; (d) feeding the reaction solution in the chamber through a filter constructed of a polymer treated with a catalyst at a pre-determined filter exposure rate for reducing ions to the metal ground state; and (e) repeating steps (c) to (d) one or more times until ion concentration in the reaction solution is less than or equal to a minimal concentration limit.
In one aspect, the step of feeding the reaction solution through a filter above comprises the subsequent step of replenishing hydroxyl ions and reducer.
In a second aspect, the step of delivering the reaction solution to the chamber above comprises the further step of adding an aliquot of aqueous ions to the reaction solution if: the quantity of reaction solution is less than a pre-determined volume; and the ion concentration is greater than or equal to a pre-determined additive point.
In another aspect, the reaction solution is polished if the quantity of reaction solution is equal or greater than the pre-determined volume.
In a further aspect, the ion concentration is determined by an optical element. The optical element may comprise: a light emitter chosen from the group consisting of a light emitting diode and a laser; a wave guide for directing light generated by the light emitter into the reaction solution; a photo diode for receiving light from the wave guide through the reaction solution; and a computer with software to interpret light intensity reading from the photo diode as an indication of ion concentration.
Embodiments also include the variation where the ions are copper ions; and the electroless solution comprises EDTA as the chelating agent, and formaldehyde as a reducer.
In a variation, the initial quantity of electroless solution is derived by steps which include adding a blank solution to aqueous metal ions thereby chelating all metal ions in the electroless solution.
In another variation the method further comprises the step of rinsing the metallized copper to remove the residuals salts and EDTA carried forward.
In a further variation, the catalyst is palladium.
The filter may be constructed of reticulated foam in another variation.
In accordance with one aspect, the pre-determined filter exposure rate is above approximately 300 square feet per liter per minute.
In accordance with another aspect, the method further comprises the step of storing the metal depleted reaction solution for future use as a blank solution.
In accordance with a further aspect, the reaction solution is at a temperature between about 90 degrees and 120 degrees Fahrenheit when fed through the filter.
Embodiments of the this invention also include an apparatus for converting copper ions in a reaction solution, the reaction solution comprising an electroless solution for said ions including a chelating agent for the ions, to the metal state comprising: a chamber for containing the reaction solution; a filter in fluid communication with the chamber providing a catalytic surface for metallic copper deposition from the reaction solution when fed through the filter, the filter surface being a polymer treated with a catalyst; and a heating element for monitoring and controlling the temperature of the reaction solution when fed through the filter; a pump element for circulating the reaction solution after flow through the filter to the chamber.
In accordance to one aspect, the apparatus further comprises elements for replenishing hydroxyl ions reducer, and adding more reaction solution.
In accordance to another aspect of the apparatus, the reaction solution is polished if the quantity of reaction solution is equal or greater than a pre-determined volume.
In accordance to a further aspect, apparatus comprises an optical element for determining the ion concentration in the reaction solution, the optical element comprising: a light emitter chosen from the group consisting of a light emitting diode and a laser; a wave guide for directing light generated by the light emitter into the reaction solution; a photo diode for receiving light from the wave guide through the reaction solution; and a computer with software to interpret light intensity reading from the photo diode as an indication of ion concentration.
In a variation of the apparatus, the ions are copper ions; and the electroless solution comprises EDTA as the chelating agent, and formaldehyde as a reducer.
In a second variation, the initial electroless solution is derived by steps which include adding a blank solution to aqueous metal ions thereby chelating all metal ions in the electroless solution.
In a another variation, the apparatus further comprises an element for rinsing the metallized copper to remove the residuals salts and EDTA carried forward.
In a further another variation to the apparatus, the catalyst is palladium.
In one aspect to the apparatus, the filter is constructed of reticulated foam.
In another one aspect to the apparatus, the pre-determined filter exposure rate is above approximately 300 square feet per liter per minute.
In another one aspect to the apparatus, the heating element maintains the temperature of the reaction solution when fed through the filter at a temperature between about 90 degrees and 120 degrees Fahrenheit.
Embodiments of this invention also includes a method for converting metal ions in solution to the metal state comprising the steps of: (a) providing an initial quantity of reaction solution containing a matrix of chemicals, the reaction solution comprising an electroless solution for said ions including a chelating agent for the ions at a temperature between about 90 degrees and 120 degrees Fahrenheit; (b) providing a chamber for containing the reaction solution; (c) delivering the reaction solution to the chamber; (d) feeding the reaction solution in the chamber through a filter constructed of a polymer treated with a catalyst at a pre-determined filter exposure rate for reducing ions to the metal ground state; and (e) repeating steps (c) to (d) one or more times until ion concentration in the reaction solution is less than or equal to a minimal concentration limit.
The present invention provides a process for conversion of copper ion to the metal state, utilizing an electroless copper solution matrix, said process comprising the steps of:
(a) an aqueous solution containing a matrix of chemicals, containing the components necessary to form an ‘electroless’ solution for said copper ion. Said matrix is capable of providing electrons to said copper ion for the conversion of said copper ion to the metal state, while maintaining the temperature of said matrix between about 90 degrees and 120 degrees Fahrenheit.
(b) transporting and contacting said matrix from step (a) into a metal filter comprising of a large surface area polymer, such as polyether reticulated foam, having previously been treated with palladium chloride catalyst.
(c) repleting the reactants contained in said matrix to optimize and facilitate said ‘electroless’ reaction as said matrix circulates through the metal filter.
(d) maintaining the temperature in said filter between about 90 and 120 degrees Fehrenheit, so as to control the deposition rate of copper ions onto the filter material at a controlled rate but to prevent spontaneous deposition of copper metal onto non filter surface areas.
The present invention also provides a process for the conversion of copper ions from non-chelated sources, such as plating wastes or ion exchange regenerant. As the copper ion concentration is decreased due to deposition as metal within the filter, the copper concentration is maintained by addition of concentrated copper ion, which is present from other processes; and continuing to replace the copper ion which is converted to metal state and to replete the reactants which are consumed in the electroless reaction until either the excess metal ion has been consumed or the volume limit of the filter is reached. When the physical limit of the filter is reached, or the excess metal ion has been consumed, the reactants are repleted, and the solution is transported until such time as the final desired metal ion concentration is achieved (polish).