DK151233B - PROCEDURE TO AVOID UNDESIRABLE COPPER TRANSFER ON EQUIPMENT SURFACES APPLIED IN AIRLESS PLATING BATH - Google Patents

Description

in 151233

BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to a method for avoiding unwanted copper deposits from powerless copper plating baths on metallic plating aids, such as containers and holding racks which are in contact with the deposition solution, 5 and whose surfaces as well as a model electrode which is also in contact with the irrigation solution, connected to a power source, whereby said surfaces are initially, to a large extent, completely resistant to a copper deposit.

10 Powerless metal depositing bath solutions, e.g. copperless plating baths are well known in the art dealing with metallization of metallic and non-metallic surfaces. Such bath solutions are distinguished by the fact that from them it is possible to deposit metallic coatings of arbitrary layer thickness without the need to supply electrons from an external power source for this purpose.

After a metal layer has deposited on the surface to be metallized, an autocatalytic process develops which continues as long as a sufficient amount of metal ions is present in the solution.

In the following, the metallization of plastics in general and the manufacture of printed circuits in particular are discussed.

For powerless metallization processes of the kind mentioned for 25 commercial purposes, relatively large containers are used. The items to be metallized, immersed or immersed in the copper deposition solution contained in the containers, and for this purpose the items are conveniently secured in frames or racks. It has been found that where the containers and holding racks consist of plastic or glass, the copper particles formed during the separation reaction are deposited on the surfaces of these containers and holding racks and adhere to them. These copper particles deposited in this way act as germs for additional copper deposits, so that most of the surface or surfaces of the objects that come into contact with the bath solution are covered with electrically separated copper, which results in additional consumption of baths. -chemical materials. In addition, it is necessary to continually interrupt the copper pitting process in order to remove the deposited copper coating on the holding racks and containers by etching. This etching process is undesirable, firstly because the deposition process has to be interrupted, secondly, large quantities of expensive etching solution are consumed, and thirdly, the resulting acidic copper solutions must be treated for the recovery of the copper.

A further disadvantage is that the etching results in powerful attacks on the non-metallic vessels, racks, etc. and considerably shortens their service life.

Because of their substantially greater resistance to etching solutions and thus longer service life, it is preferred to use metal containers and holding racks rather than glass or plastic materials. However, such vessels and equipment are even more exposed to the same disadvantages of unwanted copper deposits. The metals which are particularly suitable for the production of metallization apparatus, such as stainless steel and the like, are catalytically active against oxidation of the reducing agent (s) present in the bath solution, thus causing a spontaneous separation of copper particles which in turn catalytically further copper separation, so that on all surfaces which come into contact with the 30 electrically working copper plating bath solution, a copper deposit is formed. This is particularly to the detriment of the holding racks, whose holding clamps after copper plating processes can only be opened with difficulty.

It is admittedly known to make metallic surfaces, e.g.

35 on metallization tanks, holding racks or other objects, used in the currentless metallization / resistant for a time against the formation of metal deposits, using certain chemicals, e.g. by the surfaces being passivated by a treatment with nitric acid. However, such treatments provide only transient protection, and after a few hours, this surface protection is "worn off", so such a method cannot be used in practice in metal plating baths.

U.S. Patent No. 3,424,660 discloses a metallization tank with metallic surfaces which, upon application of a potential, can be protected against metal deposits, especially of nickel. The value of the protection potential used is shown by the current density / voltage curve. The current density is set to a maximum of 10 ^ A / cm ^.

15 in DE Publication No. 2,639,247 discloses that metallization tanks and holding racks made of metals such as cobalt and nickel can be made resistant to unwanted metal deposits by applying a voltage 2 which causes a current density of at least 4 mA / dm.

In Japanese Patent Specification No. 54-36577 of November 9, 1979, it has also been proposed that a metal vessel, e.g. of chromium-nickel steel can be made resistant to chemical coating if a positive electrical potential is applied to the surface of the vessel during the deposition operation.

In practice, the methods described above have not proved satisfactory for use in powerless copper-plating baths. While the bath is in operation, the necessary chemicals must be replenished, but this does not prevent local fluctuations in the bath's concentration, and in addition, this process also entails a risk of contamination of the bath solution. In addition, copper germs are formed, which in the solution grows to copper particles. Such copper particles or such copper dust as well as debris formed in the bath solution come into mechanical and electrical contact with the tank walls and holding racks, thereby resulting in a strong electrical current in these metal objects, resulting in the described methods of applied potential collapse and fall to a value below the oxidant potential of the reducing agent; thus, the desired purpose can no longer be achieved. The use of such counter potentials in copper plating baths requires a current that is at least two decimal sizes above what has been described so far in the literature in order for the stainless steel surfaces to be resistant to unwanted copper deposits. This is believed to be due to the significantly more pronounced catalytic activity of the copper with respect to steel in oxidation of the reducing agent, which is equivalent to the formation of electrons.

However, although the surfaces of the metallization devices, by the methods of the prior art, are resistant to metal deposits during the initial phase of the electroless metal plating, they will arise during the further course of the metallization process, rendering these methods unsuitable for mass production purposes.

A further problem often arises when copper plating objects, e.g. printed circuits immersed in the copper-plating solution retained in holding racks, and this problem consists in depositing copper especially on the edges of the support plate in areas that have not been sensitized for powerless metal deposition. When these unwanted copper layers come into contact with the holding rack, a sharp increase in power consumption occurs, which causes a reduction in voltage, possibly below the minimum value needed to keep the surfaces resistant to unwanted copper deposits. The power supply sources used in the methods of the prior art 35 provide a maximum current of 1 A, from which it will be seen, that this problem was not at all aware of the development of the aforementioned methods.

It is an object of the invention to provide a method of the kind initially provided, whereby the metallic surfaces of the tank, holding racks and additional metallization devices can be made resistant to unwanted copper deposits, not only at the beginning of the copper deposition process, but for a longer period of time.

The stated object is achieved by a method which according to the invention is characterized in that it is used as a power source whose output voltage is set to hold a value which ensures a potential which is at any moment more positive than the simultaneously measured. mixed potential as defined below, and which at a potential between -500 and +500 mV ensures a surface current density of between 0.1 and 4 mA / cm over the entire surface.

A first embodiment of the method according to the invention is characterized in that the potential of said surfaces as measured against a reference electrode is set between 20 -250 and -100 mV.

Another embodiment is peculiar in that the current is increased until the potential has reached a value sufficient to return to the deposited copper particles or germs in solution.

A third and preferred embodiment of the method according to the invention is characterized in that the potential between said surfaces and a reference electrode is measured, the current being adjusted as a function of the measured potential.

In practicing the method of the invention, the cost of producing printed circuits can be reduced by at least 30%, firstly because of savings in the consumption of bath chemicals as well as acids and alkalis for etching and subsequently neutralizing the metallization devices / and finally by the elimination of the cost 5 for the recovery of the chemicals from the etched solutions consumed as well as their treatment for compliance with the wastewater regulations.

In addition, the required work effort becomes substantially smaller and the production loss due to the hitherto necessary interruptions to the etching of the copper layers completely ceases.

Without in any way binding the invention to any theory, it is believed that the method of the invention is based on the following conditions.

The oxidation of the reducing agents present in the bath, such as e.g. formaldehyde, on catalytically active surfaces for such oxidation, releases electrons and thereby negatively charges the surfaces in question. The copper ions also found in the bath absorb these electrons and are reduced to metallic copper. If the copper ions kept in solution by suitable complexing agents come into contact with such negatively charged surfaces, a metallic copper deposit will be formed on these, at the same time reducing the negative potential of the surface. The potential that these two reactions form on the surface is referred to as "the mixed potential" or "the mixing potential".

The surface of steel, stainless steel or other metals suitable for tanks and holding racks is sufficiently catalytic to effect such an oxidation process, e.g. of formaldehyde so that copper deposits can be formed on these surfaces. If the surfaces in question are connected to a circuit, it is not only possible to compensate for the negative charge built up by the release of the electrons, but it is also possible to build a surface layer that is less or not catalytically active. for oxidation of reducing agents. As a result of this measure, no copper deposition occurs on these surfaces.

During the streamless metallization in mass production, metallic deposits are formed on all catalytically acting surfaces that come into contact with the metallization bath.

10 Statistically, a certain number of copper germs which, on the surface of the object to be metallized, are reduced to copper (0) or copper (I), will not be incorporated into the lattice, but will again end up in the bath solution. By accumulating such copper atoms, copper particles are formed, which in turn become catalytically active, thereby causing additional copper ions to be neutralized on their surface and deposited as metal. In addition, particles and debris, as well as concentration increases that may occur when the solution is depleted, cause the formation of 20 copper germs in the bath solution. Copper germs or particles in the bath solution tend to settle on metallic surfaces, such as e.g. the walls of the treatment tank or the holding devices or they form a precipitate on the bottom of the tank.

Such metallic copper deposition on the surfaces of the metallization devices causes them to behave in the same way as the articles to be copper plated, ie. that the above mentioned undesirable copper deposits occur.

The catalytic activity and thus the ability to oxidize on the reducing agent present in the bath is considerably greater on the copper surfaces than e.g. on the stainless steel surface and therefore require relatively few electrons produced on the copper particles, e.g. on the bottom of the tank, a significantly higher compensation current than that required for the passivation of the stainless steel surface.

Failure to follow the method of the invention will result in a substantially more negative potential on the metallic surfaces and hence a deposition of a copper layer thereon.

On the other hand, if, according to the invention, the power supply to the metallic surface is dimensioned so as to completely compensate for the resulting negative charge and thus maintain a corresponding positive potential on this surface to prevent copper deposition on the surfaces, then both the surfaces as well as the walls of the tank and the holding device with the copper particles touching them are catalytically inactive so that no copper is deposited on them. The result of the method 15 according to the invention are copper-free surfaces of the tank and the holding rack; the copper particles are only loosely adhered to these, which makes it possible to remove this precipitated copper in a simple and effortless manner.

According to the invention, the power source is set in such a way that on the tank walls and holding devices which come into contact with the bath solution it produces a potential sufficiently more positive than the mixing or separation potential, so that a copper separation on these surfaces is avoided. In addition, the power source must supply a current strong enough to maintain a catalytically inert surface, not only on stainless steel but also on the precipitated copper which contacts the surface of the tank and holding rack.

Thus, the invention provides a method 30 for electrically working copper plating baths and for depositing copper from these baths on support plates which have been sensitized to copper separation, using metallic plating aids and wherein the bath solution is brought into contact with at least one model electrode. the metallic plating aids as well as the model electrode or electrodes being connected to a power source, so that on the surface of said auxiliaries there is a voltage so much more positive than the mixing potential of the copper plating bath used, so that the said surfaces are mostly or completely made resistant to an undesirable metal deposit. The current source is arranged so that in its adjustable voltage range it can supply the necessary current to compensate for the electrons released by the oxidant's oxidation on the metal surfaces, and the resulting negative charge before and after the formation of copper particles on these surfaces. In this way, the surfaces of the copper particles and the other metal parts of the auxiliaries become almost or completely inactive against the deposition of metallic copper.

As an example of practicing the method of the invention in the electroless deposition of copper from a solution with which the surfaces of the plating aids also come into contact can be mentioned a process comprising the following steps: 1. First, the potential of the metallic surfaces of the plating aids is applied. which is sufficiently more positive than the mixing potential of the copper-less copper deposition solution to render the auxiliary metallic surfaces substantially resistant to copper-free copper deposition.

2. From each solution, copper is deposited without charge on a suitable substrate or stored in the tank.

3. During the copper deposition or during the storage of the solution, a metallic surface is maintained which is sufficiently more positive than the mixing potential of the solution to render the metallic surfaces resistant to undesirable copper deposition.

The described process can be applied to metallic equipment of any kind used in powerless copper deposition operations, or to the storage of powerless copper deposition solutions, including vessels, supports for the workpieces, pipelines, or any other part of the equipment which is in contact with the deposition solution.

In practice, the process described above can be carried out in the following manner: tank walls / holding racks, etc., with a power source and supplied with a current sufficient to build on their surface a positive potential that renders the surface inactive against the deposition of metallic copper.

10 Then a copperless deposition is made in the areas to be copper plated. During this deposition process, the current supplied to the non-metallized surfaces is kept constant so that the potential formed on these surfaces is sufficiently large to prevent a copper deposit on these surfaces.

The current density on the surfaces which do not need to be copper plated is preferably between 10 and 4 mA / cm.

The surfaces not to be copper plated are connected to the power source through at least one cathode placed in the bath solution. When the power is turned on, the power circuit is connected through the bath solution. The current is set to a value such that the potential built up on the surfaces which do not need to be copper plated is greater than the mixing potential of the bath solution, which prevents the deposition of copper 25 on these surfaces. During the deposition process, the power supply is regulated so that the resistance of the surfaces associated with the power source to metal deposits remains assured.

The term "mixing potential" or "mixed potential", as used herein, refers to the electrical potential at which copper begins to deposit from powerless copper plating baths on catalytically active or metal-sensitized surfaces. In other words, this term denotes the potential between a metal blank on which copper is deposited wirelessly and a standard reference electrode, both of which are in solution. Several methods for measuring the mixing potential are known, and one of them will be described below.

In general, copper deposition baths of the kind that can be used according to the invention have a mixing potential between -500 and -800 mV as measured relative to a standard silver chloride / silver electrode and between -550 and -850 mV as measured relative to a standard calomel electrode, in both cases measured by the working temperature of the deposition solution.

In practicing the method according to the invention, voltages between -500 and +500 mV are required - preferably they are between -250 and -100 mV as measured against a reference electrode - to keep the surfaces which are not to be metallized free of unwanted copper deposits. These surface potentials are sufficient to passivate all of the metal surfaces that are in contact with the bath liquid to a copper deposit, ie also any precipitated copper as well as copper already present on the peripheral areas of the circuit boards.

Advantageously, the principles of the invention can be used to keep metallic surfaces almost or completely free of unwanted copper deposits. This is especially true for holding racks for copper-plated plates that are immersed in the bath solution. These holding racks are connected to a power source, e.g. a rectifier designed to deliver the above-mentioned potential at currents up to e.g. 200 A, while the second output terminal of the power source is directly connected to a cathode located in the bath liquid. The applied current is sufficient to produce a passivating electrical potential on the surface of the holding device. Copper current is then precipitated loosely on the support plates (circuit boards) fixed in the holding racks, while the racks are supplied with a current sufficient to passivate them against the copper deposit.

While it is possible to use the same power source for both the tank walls and additional surfaces that do not need to be copper plated, it is recommended to use a separate power source for each part of the equipment to passivate all parts against unwanted copper deposits.

The above-mentioned technique can also be used to prevent copper deposition in areas of the support plate itself (the circuit board) where no copper deposition is desired. This is especially important for printed circuits produced by the so-called "additive" method. By this method, the circuit boards that are cut, provided with a mask and sensitized to powerless metal deposition, may exhibit unprotected areas at the edges.

In the subsequent metallization, copper can be deposited at both these edges and on the adjacent surface areas, and the layer thickness may be as great as at the desired conductor features. The result is a complete metallization of the peripheral area. Usually, the copper-plated edge areas that do not belong to the conductor pattern are cut off and discarded after the metallization. However, this copper plating of the peripheral regions can be avoided by the method according to the invention if electrical contact is established between the peripheral regions of the plate and the holding device and a sufficient current is maintained so that both the retaining means and the peripheral regions of the plate are kept substantially or completely free of copper.

30 Possibly separated copper, e.g. has settled on the bottom of the tank, can be easily removed by interrupting the deposition process, emptying the tank, and removing the precipitated copper, e.g. by brushing, sweeping or vacuuming. Such cleaning can also be performed without interrupting the deposition process, which the precipitated copper is aspirated.

13 151233

It should be strongly emphasized that, contrary to the known methods, in the process according to the invention no adherent precipitated copper is formed on the passivated surfaces, even if the deposition process 5 lasts very long. The metallic copper precipitated in the course of the process of the invention does not adhere to the surface and can, as mentioned above, be easily removed without the need to use etchants or similar powerful detergents. In practice of the method according to the invention, it will usually occur that a thin copper deposit is also formed on the cathode or cathodes which can be avoided completely or at least for the most part by placing a membrane between the cathode and the electroless copper solution. electric current to flow between the cathode and the deposition solution but which is impervious to copper ions. If the bath contains a copper ion complexing agent, such as e.g. amino acids with which the copper forms negative complexes, a cationic ion exchange membrane is used.

If the copper-formed complexes are positive, an anionic ion exchange membrane is used. If alkanolamines serve as complexing agents, then the copper-formed complexes will be neutral, and it is therefore possible to use either an anionic or cationic ion exchange membrane.

The method according to the invention can also be used for deposition tanks or storage containers containing the bath solution and whose walls consist of base metals, such as e.g. steel, iron, nickel, cobalt, titanium, tantalum, chromium or the like and - if desired - also copper. Similarly, the method can also be applied to other plating parts made of such metals.

The pH value of the streamless copper depositing bath solution is usually at 10, preferably at 11 or more.

14 151233

The invention will be explained in more detail below with reference to the drawing, in which fig. 1 is a schematic representation of a plant which can be used in the practice of the invention, comprising a coating tank with metallic container walls, coating solution, power source, electrodes, substrate to be coated, and a carrier; FIG. Fig. 2 shows in more detail an apparatus for automatic control which is useful in the practice of the invention; Fig. 3 is a graph of the current as a function of the voltage of stainless steel in a copper-free copper deposition solution; 4 is a graph of the current as a function of the voltage on copper in a solution of copper-free copper deposition of the same kind as in FIG. Third

In FIG. 1, a coating tank 2, the container walls of which are made of steel, preferably stainless steel, or other suitable electrically conductive material, contains powerless copper deposition solution 4. A metal electrode 6 is immersed in the deposition solution 4 and electrically connected to a negative terminal 8 of a DC power supply unit 10.

The tank surface 12 is electrically connected through a variable resistor 16 to a positive terminal 18 on the power supply 10. An mV meter 20 is also connected to the container wall 12 and to a standard reference electrode 22. A workpiece 24 is supported by a metal stand 26 .

The frame 26 is electrically connected to the surface 12a of the tank 2 and is immersed in the deposition solution 4.

The workpiece 24 is electrically insulated from the frame 26 by an insulator 27.

Preferably before the deposition solution 4 is made effective, e.g. by adding reducing agent or by increasing the pH or temperature, the surfaces 12a and 35b of the tank 2 are supplied with a voltage more positive than the expected mixed potential of the active deposition solution by adjusting the resistance 16 as needed. The composition or condition of the deposition solution 4 is adjusted by known means to start powerless coating, e.g. by adding a reducing agent, increasing the pH or increasing the temperature. The workpiece 24 is immersed in the deposition solution 4 and the coating begins. The electrical potential of the surface 12 relative to the reference electrode 22 is monitored during the coating by following the mV meter 20, and this potential is more positively maintained than the mixed potential of the deposition solution 4. This potential can be controlled manually as in the embodiment of FIG. 1 or automatically, as shown in FIG. 2nd

In FIG. 2, a 220 V AC line 28 is connected to 15 a DC power supply unit 30 capable of generating a current of e.g. 200 A at 7 V. A negative terminal 32 on the power supply unit 30 is electrically connected through a line 34 to electrodes 36 which are immersed in the metal tank 38. The tank 38 contains deposition 20 solution 40 and is grounded through an earth line 42.

A positive terminal 44 of the power supply unit 30 is electrically connected through a line 46 to throughput transistors 48 which is connected in parallel to and powered by a Darlington power transistor 50. Each of the throughput transducers 48 preferably has an output capacity of 50 A. Darlington the power transistor 50 is preferably set to a gain of approx. 10,000: 1st

The through transistors 48 are connected through an electrical conduit 52 to a measurement shunt 54 and further through an electrical conduit 56 to the tank 38. The measurement shunt 54 is connected through a conduit 58 to an ammeter 60 which measures the current from the transistor transistors 48 over the measurement shaft 54. 62 with a capacity of preferably 2 microfarads is connected between the electrical conduit 34 and the measurement shunt to reduce electrical background noise.

16 151233

An electrical line 64 exits the tank 38 and is connected to a positive input alarm 74 on a voltage amplifier 68.

An electrical line 70 passes from a reference electrode 72 and is connected to a negative input terminal 66 on the amplifier 68.

5 Amplifier 68 is set to a 10: 1 gain. The reference electrode 72 is an ordinary silver / silver chloride electrode or similar reference electrode in electrolytic connection with the deposition solution 40 in the tank 38.

Amplifier 68 is connected through electrical line 76 to negative terminal 78 on control amplifier 80. Output voltage from amplifier 68 to amplifier 80 is measured over a voltmeter 82 connected through line 84 to line 76. A positive input terminal 86 on control amplifier 80 is connected through a conduit 88 to a potentiometer 10 and to a FET switch 92. The potentiometer 90 has a maximum setting range from preferably 3 V positive to 2 V negative.

Electrical wires 94 and 96 go from terminals 98 and 100, respectively, on the measurement shunt 54 to a voltage amplifier 102, 20, the line 94 being connected to a positive input terminal 104 / and the line 96 being connected to a negative input terminal 106 on the amplifier 102. The output voltage from the amplifier 102 passes through a line 108 to a positive input terminal 110 of a control amplifier 112. The amplifier 112 is set 25 to a gain of 20: 1. A negative input terminal 114 of the control amplifier 112 is connected to the arm of a potentiometer 116. An electrical wire 118 passes from the output of the amplifier 112 to the FET coupler 92.

Inserted into the circuit is a capacitor 120 having a capacity of preferably 1 microfarad and a resistor 122 having a resistance value of preferably 1 ohms for the purpose of reducing background noise.

17 151233

To prevent overheating, the through transistors 48, power transistors 50, capacitors 120 and 62, resistor 122 and ammeter 60 are arranged in a heat protection jacket 124 indicated by dashed lines and cooled by a fan 126 connected to an alternating current line 128 of 110V. The heat protection cap 124 is made of aluminum or other heat absorbing material.

The method according to the invention is practiced in the following manner, with reference to FIG. 2. Alternating 10 current from the 220 V line 28 is converted to direct current in the DC power supply unit 30. Negative potential from the power supply unit 30 is supplied to the electrodes 36 in the tank 38. The electrodes are thus made cathodic. Positive potential from the power supply unit 30 is transmitted through the power transistor 50, the through transistors 48, the line 52, the measurement shunt 54 and the line 56 to the tank 38. The tank 38 is thus made anodic. The current through the measurement shunt 54 is monitored using the ammeter 60.

The silver / silver chloride reference electrode 72 is immersed in tank 38 and held in conjunction with the deposition solution 40 in the usual manner by an intermediate porous membrane. Thus, by connecting the reference electrode 72 and the tank 38 to each terminal of the amplifier 68 in the manner shown, the potential of the walls of the tank 38 is continuously monitored and further controlled in the following manner:

If the voltage from the amplifier 68 to the control amplifier 80 is predominantly positive, the amplifier 80 will tend to output a positive voltage.

On the other hand, if the voltage of the amplifier 68 to 30 of the amplifier 80 is predominantly negative, the amplifier 80 will tend to output a negative voltage. A positive voltage to the power transistor 50 causes it to generate a current. A negative voltage to the power transistor 50 causes it to shut down 35 and stop substantially all current. When the potential 18 or the voltage of the tank 38 during the deposition operation becomes less negative, i.e. more positively, relative to the reference electrode 72, the amplifier 68 supplies a positive voltage to the control amplifier 80, which in turn supplies a negative voltage to the power transistor 50. When adjusting the potentiometer 90, the positive output voltage of the control amplifier 80 is adjusted as required. the total output power of the amplifier 80 such that the desired current to the tank 38 10 is obtained, namely the current needed to maintain a potential of the tank 38 which is more positive than the mixed potential of the solution 40.

Too much current to the tank 38 is prevented by the voltage amplifier 102 and the control amplifier 112. The potential or voltage across the measurement shunt 54 is directly proportional to the current from the power transistor 50 and the through transistors 48. This voltage is amplified in the amplifier 102 and further in the control amplifier 112.

If the amplified voltage of the amplifier 112 to 20 of the control electrode in the FET coupler 92 rises above the threshold, the FET coupler 92 is terminated, thereby reducing the voltage from the potentiometer 90 to the set gain of the amplifier 80, thereby balancing the system.

The output of amplifier 102 is balanced by potentiometer 116 which establishes the set gain of amplifier 112. Adjustment of potentiometer 116 determines the maximum current allowed before the set gain of control amplifier 80 is reduced.

The function of amplifiers 102 and 112 is to limit the maximum current supplied to the tank and cathodes to protect the entire system.

In the aforementioned manner, the voltage applied to the tank 38 is maintained more positively than the known mixed potential of the deposition solution 40, so that substantially no metal is deposited on the walls of the tank 38.

In the particular procedure described above, metallic racks can be used to support the substrates to be coated and these racks can be made resistant to powerless copper deposition using the principles described. In this case, it is desirable to use a separate control circuit to supply the racks with power. If the substrates to be coated in the racks to be coated are copper-borne plates, a large power supply will be required to maintain the passivating electrical potential of the racks and the copper-bored plates. On the other hand, if it is desirable to coat the entire substrate, or if the copper bores on the plate 15 form part of or are in connection with the circuit pattern, it is preferable to insulate the substrate from the rack by interposing substantially electrically non-conductive material. see fig. First

Pig. Figures 3 and 4 show the current as a function of the voltage 20 for copper and stainless steel in a currentless copper deposition solution of the composition of Example 1.

Positive currents are oxidizing currents and negative currents are reducing, ie. depositing streams. At point "B" in FIG. 4 (copper electrode), there is no resulting current, 25 and this potential is known as the mixed potential of the deposition solution. In the "A" range, more copper ions than the reducing agent in the solution, here formaldehyde, are oxidized so that there is a resulting negative (depositing) stream. In region 30 "c", more formaldehyde is oxidized than copper ions are reduced, so that there is a resulting positive (oxidizing) current. In region "D", a membrane is formed on the surface of the copper electrode. This membrane is non-catalytic to the oxidation of formaldehyde. The maximum current required for 35 passivation has been determined to be 2 mA per mA. cm. Reduction of copper ions does not occur at potentials that are more positive than approx. -450 mV relative to the reference electrode, or 250 mV more positive than the mixed potential. Area "E", extending from 5 ca. -425 to -225 mV relative to the reference electrode is called the passivating region. In this range, the potential is anodic to reduce copper ions, and the electrode surface is non-catalytic for the oxidation of formaldehyde, leaving only a small current. Since the flow in this range is about the same for the solution without formaldehyde as for the solution therewith, it is assumed that the flow in this range is essentially not caused by the oxidation of formaldehyde. The current in region "F" is due to oxidation and partial dissolution of the electrode surface. Area "G" is another passivation area.

In addition to region "G", several components may be oxidized, such as OH ions, ethylenediaminetetraacetic acid (EDTA), copper or formaldehyde.

FIG. 3 shows that a stainless steel electrode is relatively passive from -500 to +500 mV. At potentials more negative than -500 mV, copper begins to settle on the stainless

steel surface that changes its characteristics. At -325 mV

the current density is 40 times less for stainless steel than 2 for copper, 0.02 versus 0.80 mA per meter. cm. Stainless steel is very slow to begin deposition in a powerless copper deposition bath. This is because it exhibits relatively little catalytic activity for the formaldehyde oxidation.

However, once the deposition has begun, it proceeds fairly rapidly as the copper deposition on the stainless steel surface provides high catalytic surface areas for oxidation of the reducing agent and thus the generation of electrons.

A potential of approx. -325 mV against the saturated calomel electrode is best for passivation of both stainless steel 35 and copper because it is in the center of the copper passivation area and the current density of stainless steel at this potential is very low.

15123¾ 21

The passivation area can be slightly shifted by changes in pH. The displacement is in the same direction as the mixed potential of the solution at pH changes and is of similar magnitude. Therefore, in one embodiment of the invention, a mixed potential probe has been used as the reference electrode in question.

In the measurements in FIG. 3 and 4, the potential values were measured with a Princetown Applied Research polarographic analyzer model 174A, and the reference for all measurements was a 10 saturated calomel electrode. The current was monitored while the potential was scanned under atmospheric air and reproduced on an X / Y printer.

To measure the mixed potential, a clean copper surface is placed in the streamless copper deposition solution, whereby 15 metal will begin to deposit on the copper surface. It should take 3 to 4 minutes to achieve a steady state. The copper surface is connected to one terminal of a large impedance millivolt meter, e.g. as used in pH meters. A reference immersed reference electrode is connected to the second terminal of the millivolt meter. The difference in potential between the copper surface and the reference electrode is measured to obtain the mixed potential of the copper deposition solution.

The invention is illustrated by the following examples which are not intended to be limiting.

Example 1

A 1.5 mm thick epoxy glass laminate is prepared in a known manner to produce a printed circuit conductor pattern by electroless copper deposition.

The laminate thus prepared is ready for immersion in a streamless copper deposition solution with e.g. the following composition: 22 151233

CuSO4.5H2O 10 g / l formaldehyde 4 ml / l wetting agent 0.2 g / l tetra-sodium salt 5 of EDTA 35 g / l sodium hydroxide (NaOH) to pH 11.7 (measured at 25 ° C) sodium cyanide (NaCN) 0.005 g / l 10 water by 1 1

operating temperature 72 ° C

The copper deposition solution in this example has a mixed potential of -630 + 20 mV measured relative to a silver / silver chloride electrode.

15 All components of said deposition solution except formaldehyde are mixed together in a stainless steel vessel. A stainless steel cathode is immersed in the solution and connected to the negative terminal of a variable DC rectifier with a maximum capacity of 8 V and 20 200 A. A silver / silver chloride reference electrode is immersed in the deposition solution and connected to one side of a millivolt meter. The other side of the millivolt meter is connected to a wall in the stainless steel vessel.

The electrical potential of the vessel wall relative to the reference electrode is adjusted to -200 mV by regulating the rectifier. The streamless copper deposition solution is made effective by adding the formaldehyde. The epoxy glass ~ laminate which has been pre-treated. as described and carried by a stainless steel frame, immersed in the 30 powerless copper deposition solution. Copper begins to settle on the laminate powerlessly. After 10 hours or after a copper deposit of 20 μιη has been obtained, the laminate is taken out of the deposition solution. It is noted that during the powerless coating reaction, substantially no copper is deposited on the surface of the stainless steel vessel or the stainless steel frame in contact with the powerless copper deposition solution.

Example 2 In this example, only two electrodes are used, i. no reference electrode.

A powerless copper deposition solution of the same composition as in Example 1 is placed in a vessel with stainless steel container walls. The tub has a capacity of 8000 1 2 10 and an internal surface area of approx. 60 m.

The rectifier is adjusted to achieve a potential of 0.45 V between the stainless steel cathode immersed in the deposition solution and the stainless steel walls in the tub. After this adjustment, it is noted that the walls of the tub have a potential of from -300 to -400 mV relative to the silver / silver chloride reference electrode. After this initial measurement, the reference electrode is disconnected and removed. The current required to obtain 0.45 V between the vessel walls and the steel cathode is 0.5 A corresponding to a current density of 10 2 20 mA per meter. cm.

Six stainless steel racks each containing 300 1860 cm substrate plates are placed in the copper-free copper deposition solution. They are removed after powerless formation of the predetermined conductor pattern, e.g. at intervals of 18-22 hours, 25 and replaced with new substrate plates to be coated. During the first 24 hours of the coating operation, where coated substrates are removed and new substrate plates are introduced into the deposition solution, it is noted that a precipitation comprising metallic copper is formed in the solution. Some 30 of these secretions come into contact with the surface of the tub. The current needed to maintain 0.45 V between the vessel surfaces and the steel cathode riser. After several days of operation, it is noted that the current required to maintain 0.45 V rises and falls in the range from 2-100 A, as additional metallic copper is separated, comes into contact with the vessel surfaces and is passivated.

At the end of e.g. for one week, the coating operation is interrupted. Separated copper in contact with the inner surfaces of the tub consists of passivated, non-stick particles which are easily removed by sweeping with a brush or by vacuuming.

Example 3 In this example, the method described in Examples 1 and 2 is followed except that the stainless steel racks are also connected to another suitable rectifier and another stainless steel electrode placed in the deposition solution and the potential is adjusted and maintained at about. 0.4 to 0.5 V measured between the rack and the other electrode, thereby rendering the surfaces of the racks and in some cases the abrasions of the plates coated with copper and in contact with the rack surface, impermeable to powerless metal deposition.

Claims (4)

151233 1. A method of avoiding unwanted copper deposits from powerless copper plating baths on metallic plating aids such as containers and holding racks which are in contact with the deposition solution and whose surfaces, as well as a model electrode which is also in contact with the deposition solution, are electrically conductive power source, whereby said surfaces are at first to a large extent completely resistant to a copper deposit, characterized in that such a power source is used whose output voltage is set to hold a value which ensures a potential which at any time is more positive than the simultaneously measured mixed potential 15 as defined in the specification and which at a potential between -500 and +500 mV ensures a surface current density of 2 between 0.1 and 4 mA / cm over the entire surface. Method according to claim 1, characterized in that the potential of said surfaces as measured against 20 a reference electrode is set between -250 and -100 mV. Process according to claim 1, characterized in that the current is increased until the potential has reached a value sufficient to again bring into solution deposited copper particles or germs. Method according to one or more of claims 1 to 3, characterized in that the potential between said surfaces and a reference electrode is measured, the current being adjusted as a function of the measured potential.