EP0240589B1 - Process and apparatus for regenerating an electroless copper-plating bath - Google Patents

Description

The invention relates to a method for regenerating an electroless copper plating bath which contains a complexing agent such as ethylenediaminetetraacetic acid or the like.

Chemical, i.e. Copper baths working without connection to an external power source can be found e.g. for the coating of plastic surfaces, for the uniform coating of parts of complicated geometry, especially in the production of printed circuits using the semi-additive or fully additive process. All chemically working baths have in common that the supply of layer-forming metal must be introduced into the bath in dissolved form. In order to achieve a useful deposition, however, the concentration of free metal must be severely limited. Complexing agents are used for this purpose, which mask the metal cation and only make it available in the smallest amounts for the coating reaction within the framework of the complex formation equilibrium. In order to limit the concentration of free metal cations to the necessary extent, the complexing agents are often added to the bath in a multiple excess. The most common complexing agent used is ethylenediaminetetraacetic acid (EDTA).

In order for the copper film deposited by the electroless process to have excellent physical properties, compared to those electrolessly deposited films which only act as a conductive thin film for a lead-through opening and are produced by the subtractive process and on which copper is built up by electrolytic deposition, it is necessary to Control the composition of the electroless copper plating bath as closely as possible so that its concentration is as uniform as possible and to keep the formation of by-products as low as possible. The latter is particularly important when recovering the complexing agent, preferably ethylenediaminetetraacetic acid, which is present in high concentrations in the electroless copper plating bath.

So far, according to European patent application 0 088 852, part or all of the copper plating bath containing the complexing agent has been removed from the copper plating tank, the copper content of the bath has been reduced by precipitation of the copper as metallic copper or as copper oxide or by electrolysis, and the complexing agent has subsequently been precipitated by acidification. The complexing agent thus recovered was returned to the anodic part of a cell with a copper anode, which is separated from the cathodic part with the cathode by an ion exchange membrane. Then direct current was applied to both electrodes and the solution was returned from the anodic part of the cell to the electroless copper plating bath. In this process, the conditions under which the electrolysis for lowering the copper content in the copper plating bath to be regenerated is to be carried out and the influence of these conditions on the purity of the complexing agent to be recovered, preferably the EDTA, are not dealt with in more detail.

FR-A 2 386 491 describes a process for the recovery of bath constituents of aqueous electroless metallization baths, in which the copper content of an exhausted copper plating bath is reduced by precipitation of the copper as metallic copper and the complexing agent is precipitated by acidification. The precipitated EDTA is dissolved in NaOH and recycled. The aqueous residue of the bath, which still contains about 1 g / l EDTA and 10 mg / l Cu, is subjected to electrolysis, in which the remaining EDTA decomposes and the remaining copper is deposited on the cathode.

German Offenlegungsschrift 3 340 305 describes a process for the disposal of chemical metallization baths, in which the heavy metal is removed from the solution by selectively working ion exchangers and the residual solution containing complexing agents can be worked up further. However, this method is only applicable to the separation of metals from solutions containing complexing agents whose complexing constant is lower than that of the exchange resin. This is not the case for EDTA.

The object of the invention is to provide a process for the regeneration of electroless copper plating baths, in which the copper content of the bath is lowered by electrolysis and the conditions for carrying it out are chosen such that a very pure complexing agent, in particular a very complex one, in the further processing of the residual complexing agent solution pure ethylenediaminetetraacetic acid, which is free of by-products, is obtained.

The object of the invention is achieved by a method according to claim 1.

• Figur 1 ein Flußdiagramm für das Verfahren gemäß der vorliegenden Erfindung;
• Figur 2 die Formeln von Zersetzungsprodukten, Aminen und weiteren Produkten;
• Figur 3 die Reaktionen, welche zwischen Anode und Kathode ablaufen;
• Figur 4 die Elektrolysierstromwerte als Funktion der Elektrolysierzeit;
• Figur 5 die Elektrolysezelle mit zwei Überlaufkästen für die interne Zirkulation des Elektrolyten und
• Figuren 6A und 6B der Kupfergehalt des Verkupferungsbades in mg/I in Abhängigkeit von der Elektrolysierzeit in Stunden.

The invention is explained in more detail with reference to the special description and FIGS. 1 to 6. They represent:

• Figure 1 is a flow diagram for the method according to the present invention;
• Figure 2 shows the formulas of decomposition products, amines and other products;
• FIG. 3 shows the reactions that take place between the anode and the cathode;
• FIG. 4 shows the electrolysis current values as a function of the electrolysis time;
• Figure 5 shows the electrolysis cell with two overflow boxes for the internal circulation of the electrolyte and
• FIGS. 6A and 6B show the copper content of the copper plating bath in mg / l as a function of the electrolysis time in hours.

• Kupferionen in zweiwertiger Form;
• Komplexbildner, damit das Kupfer in seiner zweiwertigen Form erhalten bleibt;
• Alkali zur Abpufferung überschüssiger Wasserstoffionen und Erhaltung des pH-Werts
• und Reduktionsmittel, z.B. Formaldehyd.
• Das Bad kann Stabilisatoren, beispielsweise Cyanid und Netzmittel als weitere Zusätze enthalten.

The flowchart for the method according to the present invention, which is illustrated in FIG. 1, is explained below. The electroless copper plating bath 12 in container 11 contains four basic components:

• Copper ions in divalent form;
• Complexing agent so that the copper remains in its divalent form;
• Alkali for buffering excess hydrogen ions and maintaining the pH
• and reducing agents, for example formaldehyde.
• The bath can contain stabilizers, for example cyanide and wetting agents, as further additives.

A container 15 with connecting lines 14 between the two containers is provided for emptying the copper-plating container 11 for cleaning purposes. A pipeline 16 leads from the copper-plating tank 11 to the collecting tank 17. From there, the copper-plating bath to be regenerated reaches the electrolysis unit 19 via feed line 18, in which two electrode blocks 20 and 21 are arranged. The electrolysis unit 19 is provided with overflow boxes 22, a pH measuring probe 24 being installed in one and the addition of sodium hydroxide solution for setting and maintaining the pH value via line 23 into the opposite box. The circulation of the copper bath to be regenerated within the electrolysis unit is described separately with reference to FIG. 5. The number and dimensions of the electrodes in each electrode block are calculated from the current intensity I, the current density i and the container size. The electrodes are mutually arranged in such a way that there is always a cathode between two anodes. The cathodes consist of thin copper foils, the anodes made of stainless steel.

The demetallized bath solution passes through pipeline 25 from the electrolysis unit into a container 26 in which the complexing agent is dropped out into the acidic range by lowering the pH. For this purpose, an acid such as sulfuric acid, hydrochloric acid or the like is metered into the container 26 via the line 27. The pH range suitable for the precipitation is generally below 4.0 and for ED-TA below 2.0, preferably below 1.0. In addition to ethylenediaminetetraacetic acid (EDTA), other complexing agents which are suitable for electroless copper plating can also be worked up, for example potassium sodium tartrate (Rochelle salt), ethylenediaminetetramine, triethanolamine, diethanolamine and the like.

The precipitated EDTA is washed twice with deionized water, the washing water being fed into the container 32 via the pipeline 31. The EDTA can then be dissolved again in sodium hydroxide solution as the tetrasodium salt and purified by reprecipitation with H ₂ S0 ₄ . The purified ethylenediaminetetraacetic acid is dissolved in the same container (26) in sodium hydroxide solution, which is metered in via line 30, to the tetrasodium salt. Via line 28, the solution of EDTA-NA ₄ (tetrasodium edetate) arrives in a storage container 29 and is then fed via line 13 directly into the chemical copper plating bath 12, or a premix with copper sulfate solution is prepared, which then also forms the chemical copper plating bath 12 is supplied in the container 11.

In a preferred embodiment, an electroless copper plating bath is used with the following components, ranges and parameters:

Baths of this composition are described, for example, in U.S. Patent 3,844,799.

The concentrations of the baths are adjusted by adding separately prepared copper sulfate solution, formalin, sodium cyanide solution and sodium hydroxide solution to the bath when their concentration has fallen below a certain value.

The concentrations of the individual bath components are carefully monitored, those of Cu ++, for example, by photometric measurement; that of formaldehyde via a reaction with sodium sulfite, which leads to a change in pH; that of NaCN with an ion-selective electrode and that of NaOH with a glass electrode. The temperature of the bath must also be carefully monitored. The reaction products from the electroless copper plating of activated surfaces of printed circuit boards are essentially Na ₂ S0 ₄ (sodium sulfate) and HCOONa (sodium formate), which reach a constant concentration during the use of the bath.

As was explained with reference to FIG. 1, for the regeneration of the copper plating bath, the copper content of the bath liquid is first reduced to a concentration below about 20 by electrolysis mg / I lowered and then the complexing agent precipitated by acidification.

It has been shown that carrying out the electrolysis for the reduction of the copper content is of crucial importance with regard to the purity of the recovered ethylenediaminetetraacetic acid.

It was found that in the electrolysis processes carried out hitherto, in which constant anodic and cathodic and relatively high current densities were used, the subsequent precipitation of the ethylenediaminetetraacetic acid gave a product which was heavily contaminated with decomposition products and smelled of amines. The decomposition products I represented by the formula in FIG. 2 and the amines II formed by recombination of free radicals could be detected by numerous laboratory tests. Specifically, these are: tetramethylethylenediamine (a), dimethylethylamine (b), N-methyl-N'-dimethyldiaminomethane (c), ethylenediamine (d) and cyclic amines (e). As other products 111, glycine (f), iminodiacetic acid (g) and the like could be detected.

If amines, in particular ethylenediamine (d), are present in the copper plating bath, this has a negative effect on the grain structure of the deposited copper layer. In the presence of amines, a coarse-grained copper layer is deposited from the copper plating bath, in which cracks can form when heated later, for example during soldering. It is also known that amines can react with other components of the bath, for example with formaldehyde, to give s-triazine derivatives. In turn, s-triazine, which has a stabilizing effect on formaldehyde, also has a negative effect on the grain structure of the deposited copper layer.

On the basis of polarographic investigations on samples from the copper plating bath, it was found that about 10% of the EDTA contained in the bath decompose during the electrolysis, which is carried out with a constant current density. This is due to the fact that a strong change of the anodes with oxygen during the electrolysis leads to a change in potential and a potential limit is exceeded from which anodic decomposition of the EDTA begins. The conditions for electrolysis must therefore be selected so that this decomposition of the EDTA is avoided.

During the processing of chemical copper plating baths in the electrolysis cell, in which copper cathodes are arranged in the simplest arrangement between stainless steel anodes, a galvanic deposit of copper is formed on the copper cathodes according to the following reaction equation:

As an undesirable but not suppressible side reaction, water decomposition also takes place on the cathodes according to the following scheme:

Molecular oxygen is formed at the anode in the strongly alkaline solution (pH value from 11 to 12) by electron withdrawal:

This consumption of OH ions during electrolysis results in a drop in the pH. With an initial value of pH = 11.7, for example, a final pH value of 9.4 is obtained after an electrolysis time of about 10 hours. Since the proportion of decomposition products also increases with a falling pH value, the electrolysis unit was provided with a control (24, 23 in FIG. 1) which keeps the pH value constant during the electrolysis.

The reactions at the cathode and anode are shown in a highly simplified manner in FIG. 3. Due to the galvanic deposition of copper on the cathodes, the electrolyte is constantly depleted of copper ions until the electrolysis is stopped at a residual content of about 20 mg / l Cu. In practice, the copper content of the electrolyte is continuously measured during the electrolysis. The system switches off automatically when the desired final value is reached.

• a) Durch die kathodische Abscheidung von Kupfer verarmt der Elektrolyt an Kupferionen. Es besteht deshalb eine Abhängigkeit des Potentials von der Kupferionenkonzentration, welche gegeben ist durch:
  
  wobei e νi die Summe aller Faktoren darstellt, die das Potential mit beeinflussen.
• b) Auch die Stromdichte i ist konzentrationsabhängig, wobei die Grenzstromdichte in Abhängigkeit von der Kupferionenkonzentration und der Temperatur gegeben ist durch:
  

The following electrochemical relationships ad - d apply to the above-mentioned chemical reactions at the cathode and anode:

• a) Due to the cathodic deposition of copper, the electrolyte is depleted of copper ions. The potential therefore depends on the copper ion concentration, which is given by:
  
  where e νi represents the sum of all factors that influence the potential.
• b) The current density i also depends on the concentration, the limit current density depending on the copper ion concentration and the temperature being given by:
  

The dependence of the potential and the current density on the copper ion concentration becomes more important if a lot of copper has already been deposited by electrolysis. Then the values for the limit potential or for the limit current density can be reached at which decomposition of the EDTA takes place.

und

Der Hauptreaktion:

wirkt die EMK, welche eine Funktion von

By covering the electrodes with hydrogen or oxygen during the electrolysis, they become gas electrodes, the electromotive force of which counteracts the copper deposition:

and

The main reaction:

the EMF acts, which is a function of

In electrolysis, which works with constant, relatively high current densities, the electricity and energy yield, based on the copper to be deposited, drops sharply due to the increasing proportion of hydrogen and oxygen during the electrolysis. As a result, a very long electrolysis time is required until the desired residual copper content of about 20 mg / l Cu is reached.

For example, if 15 m3 of additive bath containing 8 g / I CuS0 ₄ x 5 H ₂ 0 (≠ 2.03 g / I Cu) are electrolyzed at a constant current of 6000 amperes and a current density of about 100 A / m ² , is the average cathodic current yield over 24 hours η ( _C u, ₂₄ h) only 18%. The much larger proportion. ie the remaining 82% of the amount of electricity consumed is used for water decomposition and for side reactions. An average cathodic current yield, based on the copper to be deposited, of 69% is calculated for the first 10 hours. After a further 14 hours, in total after 24 hours, an average cathodic current yield of only 18% is calculated. 4, right side, the electrolysis current 1 of 6000 A over the electrolysis time t is shown for this exemplary embodiment. An electrolysis time t of 24 hours is required until the desired copper content of the solution of <20 mg / 1 is reached. As already stated, an average cathodic current yield of only 18% is obtained over 24 hours.

Based on these results, it is necessary to keep the hydrogen and oxygen evolution during the electrolysis as low as possible and to remove the gases formed from the electrodes as quickly as possible in order to improve the cathodic current yield based on the copper to be deposited and the electrolysis time for to shorten the reduction to the desired copper content of 20 mg / I. A reduction in the electrolysis time also means that fewer by-products are formed.

The removal of the gases from the electrodes is best accomplished by high internal bath circulation during the electrolysis, with the electrolyte being circulated with an electrolyte movement of approximately 10 to 50 volumes per hour in the case of a high bath circulation. In the aforementioned embodiment With an electrolytic cell content of 15 m ³ , an electrolyte circulation volume of 300 m3 per hour must be circulated with an electrolyte movement of 20 volumes per hour.

FIG. 5 shows a device in which the electrolyte circulation is brought about by blow-in tubes which are arranged under the electrodes. The electrolyte arrives from the electrolysis cell in overflow boxes which are arranged on both sides of this electrolysis unit and is returned from there to the injection tubes. In the upper part of FIG. 5, a side view of an electrolysis unit with an electrolysis cell and two overflow boxes and blow-in tubes is shown below the electrodes. In the lower part of Fig. 5 is a view of the same device from above. If it is possible for reasons of space, it is advantageous to arrange a buffer tank (not shown) next to the electrolysis unit, into which the electrolyte is led from the overflow boxes and from there back to the electrodes via the blow-in tubes.

In addition to electrolyte depletion, current density is critical in the process of the present invention. In most electrolysis processes for the extraction or electrorefining of copper, in which work is usually carried out with a constant current density over the entire electrolysis zone, the most advantageous current density is determined from an economic point of view. However, the aim of the present invention is not the most cost-effective deposition of copper, but the cost-effective recovery of the purest possible ethylenediaminetetraacetic acid, which can be returned to the electroless copper plating bath for the circuit board process. In conventional electrolysis processes for copper production, the anodic and cathodic current densities for copper electrolytes of comparable concentration are around 200 A / m ² and in exceptional cases also around 300 A / m ² . Because of the electrochemical decomposition of ethylenediaminetetraacetic acid, these relatively high current densities cannot be used for these current density values in the context of the present invention. Experiments have shown that for the recovery of a pure ethylenediaminetetraacetic acid in the process according to the invention a maximum anodic current density of i + = 100 A / m ^{2 must} not be exceeded.

An electrolysis system for the method according to the invention is designed, for example, for a maximum current I _max of 6000 A, but it is only operated with a current of at most about 5400 A. The system contains 36 copper cathodes with an active total area of 1: f of 77.1 m ² and 38 stainless steel anodes with an active total area of ₁ : f of 88.9 m2. The maximum current densities are then i- _max = 70 A / m ² and i _{+ max} = 60.7 A / m ² , the anodic current density s being the cathodic current density, so when the anode is covered with oxygen, which is not quite sufficient is suppressed, the potential limit at which the decomposition of the EDTA begins is not exceeded. From the numerical values it follows that the electrolysis plant is only operated with about 60% of the maximum permissible anodic current densities. As a result, the probability that disruptive degradation products of ethylenediaminetetraacetic acid are formed by anodic oxidation is even lower.

However, in order to avoid the formation of disruptive degradation products entirely, it is most advantageous not only to work with constant current densities but also with current densities which are reduced continuously or in stages during the electrolysis according to the electrolysis characteristic curve (left-hand side of FIG. 4).

In the following table, column 1 shows the electrolysis time, divided into hours, and column 2 shows the decrease in the average cathodic current yield Ti - in percent as a function of the electrolysis time (column 1). These experiments were carried out at a constant anodic current density i ₊ of 100 A / m ² . Column 3 shows the proposed reduction in the anodic current density i ₊ in A / m ² and column 4 shows the current intensity I to be set for this purpose in A. Column 5 shows the mean cathodic current yields η given for the anodic current densities specified in column 3. A significant improvement over the values in column 2 (constant anodic current density) can be seen.

The table shows that in the first four hours at a constant anodic current density i ₊ of 100 A / m ^2, the mean cathodic current yield η is about 80%. Between the fourth and fifth hour, the current yield drops to about 37% at the same current density. After about 12 hours, the average cathodic current yield has dropped to a value below 1%, ie almost all of the electrical energy is no longer wasted on copper deposition but on water decomposition and for undesirable side reactions. Calculated from the table over ten hours, with a constant anodic current density of i ₊ = 100 A / m ^2, an average cathodic

Current efficiency η of 44%.

• EQ = Σl· t = 40500 A/h.

Dies entspricht einer Stromausbeute, berechnet über zehn Stunden von π = 69 %, wobei der Berechnung ein mittlerer Kupfergehalt von 2,2 g/I zugrunde liegt.The average cathodic current yield becomes significantly better if the anodic current density is reduced with increasing electrolysis time (column 3). According to Table 1, the total amount of electrical energy is:

• EQ = Σl · t = 40500 A / h.

This corresponds to a current yield, calculated over ten hours, of π = 69%, the calculation being based on an average copper content of 2.2 g / I.

berechnen. Es errechnet sich eine Dauer t von 10 Stunden.The electrolysis time can be determined from the experimental data in Table 1 according to Faraday's law on the basis of the following initial values

to calculate. A duration t of 10 hours is calculated.

According to these calculations, it can be assumed that the electrolysis can be stopped after about ten to twelve hours if the current densities are reduced, which means that the electrolysis time is halved compared to the previous recovery electrolysis of copper, which uses a constant anodic current density . The shorter electrolysis time also means that fewer EDTA decomposition products are formed.

Figure 6A shows the decrease in copper content in the copper plating bath during the first four hours of electrolysis. 6B shows the decrease in the copper content of the bath from the fifth to the twelfth hour of the electrolysis, in each case with a constant anodic current density i ₊ of 100 A / m ² . If the anodic current density is reduced during the electrolysis, an even better average cathodic current yield and a further reduction in the electrolysis time are obtained.

Claims (12)

1. Process for regenerating an electroless copper plating bath containing a complexing agent, comprising the following steps:

1. continuously or intermittently withdrawing the bath solution containing the complexing agent from the electroless plating bath;

2. reducing the copper content in the withdrawn bath solution to a value below 20 mg by electrolysis;

3. acidifying the bath solution thus obtained by precipitating the complexing agent and recovering same;

4. dissolving the recovered complexing agent in an alkaline electrolyte solution, and

5. returning the solution to the electroless copper plating bath.

2. Process for regenerating an electroless copper plating bath according to claim 1, characterized in that the complexing agent is ethylenediamine tetraacetic acid, potassium sodium tartrate, ethylenediamine tetraamine, triethanolamine or diethanolamine.

3. Process according to claim 2, characterized in that the complexing agent is ethylenediamine tetraacetic acid.

4. Process according to claim 1, characterized in that the electrolysis for reducing the copper content of the bath is effected at a high internal bath circulation rate.

5. Process according to claim 4, characterized in that the electrolysis is carried out at a bath circulation rate of about 10 to 50 volumes/h.

6. Process according to claim 5, characterized in that the electrolysis is carried out at a bath circulation rate of 20 volumes/h.

7. Process according to claim 1, characterized in that the pH value is kept constant during electrolysis.

8. Process according to claim 1, characterized in that an anodic current density i+ of 100 A/m² is not exceeded during electrolysis.

9. Process according to claim 1, characterized in that the anodic current density during electrolysis is reduced according to the electrolysis characteristic or in steps.

10. Process according to claims 1 to 3, characterized in that, after removal of the copper ions, the ethylenediamine tetraacetic acid is precipitated by acidification of the bath solution to a pH value below 2.0.

11. Process according to claim 10, characterized in that the precipitated EDTA is purified by being dissolved in sodium hydroxide and by being reprecipitated with H₂S0₄.

12. Process according to any one or a combination of the claims 1 to 11, characterized in that the purified EDTA is dissolved in sodium hydroxide solution and fed directly to the chemical copper plating bath, or that, using copper sulphate solution, a preliminary mixture is prepared which is fed to the copper plating bath.

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