CH646732A5 - METHOD FOR PASSIVATING METAL SURFACES TO AVOID UNWANTED COPPER DEPOSITS FROM CURRENTLY WORKING COPPER BLADES. - Google Patents

Description

The present invention is to provide a method of the type mentioned in the preamble of claim 1, in which the metal surfaces of the tanks, holding frames and other metallization devices coming into contact with the copper bath solution not only at the beginning of the copper-plating process, but also over longer periods of time, that is also When storing the bath solution, unwanted copper deposits are made resistant.

According to the invention, the method has the features stated in the characterizing part of patent claim 1.

It is an advantage of the method according to the invention that copper separation baths can be operated on a mass production level using this method without undesired copper layers being deposited on the metallization devices coming into contact with the bath solution and having to be removed from them by etching.

Another advantage of the method according to the invention is that the copper precipitates, insofar as they are formed during the deposition process, can be easily removed from the device parts coming into contact with the solution, for example by brushing, wiping or vacuuming or the like, without interrupting the deposition process should be.

Finally, there is a further advantage in that undesired deposition of copper on the edges of the carrier plates can be avoided in the production of printed circuits.

The advantages mentioned above and other advantages are described below.

It should also be mentioned that the method according to the invention reduces the costs for the production of printed circuits by at least 30%, essentially by saving bath chemicals

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as well as acids and alkalis for the etching and subsequent neutralization of the metallization devices, and finally through the elimination of the costs for the recovery of chemicals from the used etching solutions and their processing in accordance with the wastewater regulations.

In addition, the workload is significantly reduced and the loss of production due to the previously required interruptions for etching the copper layers is completely eliminated.

The oxidation of reducing agents in the bath, such as formaldehyde, on surfaces that have a catalytic effect releases electrons and thus negatively charges the corresponding surfaces. The copper ions also in the bath absorb these electrons and are reduced to metallic copper. If the copper ions held in solution by suitable complexing agents come into contact with such negatively charged surfaces, a metallic copper precipitate is formed on them, while the negative potential of the surface is reduced at the same time. The potential that forms on the surface due to the two reactions is also referred to as the mixed potential.

The surface of steel or stainless steel or other metals suitable for the manufacture of tanks and holding frames is sufficiently catalytic to cause such an oxidation process, for example of formaldehyde, that copper deposits can form on these surfaces.

If the surfaces in question are connected to a circuit, it is not only possible to compensate for the negative charge which is formed by the release of the electrons, but also to form a surface layer which is less or not at all catalytic for the oxidation of the reducing agent . As a result of this measure, there is no copper deposition on those surfaces.

With electroless metallization in mass production, metallic deposits are deposited on all catalytically active surfaces that come into contact with the metallization bath.

Statistically, a certain number of copper nuclei, which are reduced to copper (O) or copper (I) on the surface of the object to be metallized, are not built into the grid, but return to the bath solution. Copper particles form from the agglomeration of such copper atoms, which in turn are catalytically active and thus have the effect that further copper ions neutralize on their surface and have a metallic deposit. In addition, dirt particles and increased concentrations, which can occur when the solution is refreshed, cause the formation of copper nuclei in the bath solution. Copper nuclei or particles in the bath solution tend to

to adhere to metallic surfaces, such as on the walls of the separation tanks or on the holding devices, or they settle in the form of a precisite on the bottom of the separation tanks.

Such a metallic copper deposit on the metallic surfaces of the metallization devices has the consequence that they behave like the objects to be coppered themselves; the undesirable copper deposits described above occur.

The catalytic activity and thus the ability to have an oxidizing effect on the reducing agent present in the bath is considerably greater on the copper surfaces than, for example, on the surface of stainless steel; the comparatively few electrons that are generated on the copper particles - for example on the tank bottom - require a much higher compensation current than would be required for the passivation of a surface made of stainless steel. If the teaching according to the invention is not followed, this results in a significantly more negative potential of the metallic surfaces and thus the deposition of a copper layer thereon. If - in accordance with the teaching according to the present invention - the power supply of the metallic surfaces is dimensioned such that these completely compensate for the negative charge that arises and thus a corresponding positive potential is maintained on them in order to prevent copper deposition on the surfaces mentioned, so It is achieved that the surfaces of the tank walls as well as the holding devices and the copper particles that are in contact with them are catalytically inactive, so that no copper is deposited on them. The result of the method according to the invention is a copper-free surface of the tanks and holding frames; the copper particles only adhere loosely to them, which makes removal of this copper precipitate easy and effortless.

According to the present invention, the current source is dimensioned such that it causes a potential on the tank walls and holding devices that come into contact with the bath solution that is sufficiently positive than the mixing or deposition potential, and thus copper deposition on these surfaces is avoided. Furthermore, the power source is said to provide a current that is strong enough not only to maintain a catalytically inactive surface on stainless steel, but also on the copper precipitate that is in contact with the tank and support frame surfaces.

According to the present invention, a method for electroless copper plating baths and for the deposition of copper from these baths on support plates which are sensitive to the deposition of copper using a metallic plating device was developed, according to which the bath solution is in contact with at least one counter electrode brought; and the surfaces of the metallic plating devices and the counterelectrode (s) are connected to a current source, so that there is a voltage on the surfaces of the devices which is correspondingly more positive than the mixed potential of the copper plating bath used, to a large extent or completely against said surfaces to make an unwanted metal deposit resistant. The current source is designed in such a way that its adjustable voltage range supplies the current required to compensate for the electrons released by the oxidation of the reducing agent on the metal surfaces and the resulting negative charge before and after the formation of copper particles on these surfaces. In this way, the surface of the copper particles and that of the other metal parts of the device become approximately or completely inactive against the deposition of metallic copper.

One embodiment of the invention is a method for electroless metal deposition, in which the metallic surfaces of the plating device are in contact with the copper deposition solution and which includes the following process steps:

(1) First, a potential is applied to the metallic surfaces of the device which is sufficiently positive compared to the mixed potential of the deposition solution so as to make these surfaces resistant to the electroless copper plating;

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(2) Then copper is electrolessly deposited from the bath solution on a suitable substrate, or the solution is stored in the tank;

(3) During the copper deposition or during the storage of the solution, a potential is maintained on the metallic surfaces which is sufficiently positive compared to the mixed potential of the solution, so as to make the metallic surfaces resistant to undesired copper deposition thereon.

The described method can be used for any type of metallic devices for electroless copper plating, regardless of whether they are used in the metallization process or only for storing the bath solution, and includes the tank walls as well as the support or holding devices and all installations that come into contact with the bath solution.

More specifically, the method described above is carried out as follows:

First, the surfaces that are not to be plated, such as tank walls, holding frames, etc., are connected to a current source and supplied with a current which is sufficient to form a positive potential on their surface, which renders them inactive against the deposition of metallic copper. Then electroless copper is deposited on the areas to be copper-plated. During the deposition process, the current supplied to the surfaces not to be metallized is always set in such a way that

that the potential developing on these surfaces is sufficiently large to prevent copper deposition on these surfaces. The current density is preferably between 1 (H and 4 milliamperes / cm 2 of surface not to be coppered.

To illustrate the method according to the invention, the surfaces not to be copper-plated are connected to the power source via at least one cathode in the bath solution. If the power is switched on, the circuit is closed by the bath solution. The current intensity is set so that the potential that forms on the surfaces that are not to be copper-coated is greater than the mixed potential of the bath solution, which prevents copper deposits on those surfaces. The power supply is regulated during the deposition process in such a way that the surfaces connected to the power source are resistant to metal deposits.

The term “mixed potential”, as used in this description, denotes the potential at which copper begins to deposit from electroless copper plating baths on catalytically active or metal-sensitized surfaces. In other words, it denotes the potential between a metal support on which copper is electrolessly deposited and a standard reference electrode, both of which are in the bath solution. Methods for measuring the mixed potential are known in the art; one of them is described below immediately before the examples.

In general, copper plating baths as are useful in the present invention have a mixed potential between -500 and -800 millivolts as measured against a standard chlorine silver / silver electrode each at the working temperature of the plating solution.

When carrying out the method according to the invention, voltages between higher than -500 and +500 millivolts are required - usually they lie between -300 and

-100 millivolts, measured against a reference electrode - to keep the surfaces that are not to be metallized free of unwanted copper deposits. These surface potentials are sufficient to passivate all metal surfaces in contact with the bath liquid against copper deposition, i.e. any copper precipitate and copper that is already on the edges of the carrier plates.

The basic idea of the present invention is successfully applied in order to keep metallic surfaces almost or completely free of undesired copper deposits; in particular holding racks for plates to be copper-plated, which are immersed in the bath solution. These holding racks are connected to a current source, for example a rectifier, which is dimensioned in such a way that it supplies the above-mentioned potential at currents of, for example, up to 200 amperes, while the other output of the current source is connected directly to a cathode which is in of the bath liquid. The current supplied is sufficient to generate a passivating electrical potential on the surfaces of the holding devices. Then copper is electrolessly deposited on the carrier plates fastened in the holding racks, while the racks are supplied with a current which is sufficient to passivate them against the copper deposition. Although it is entirely possible to use the same power source for the tank walls and other surfaces that are not to be copper-plated, a separate power source is recommended for each part of the device in order to passivate it against unwanted copper deposits.

The method described above can also be used to prevent copper deposition in the regions of the carrier plate itself which are not to be copper-plated. This is particularly important for printed circuits that are manufactured using the so-called “additive” process. It can happen that the cut, masked and sensitized for the electroless metal deposition carrier plates have unprotected areas at their edges. In the subsequent metallization, copper can also deposit on these edges and on the adjacent surface areas and reach the same layer thickness as the copper deposited on the conductor tracks. The result is complete edge metallization. Usually, the copper-plated edges that do not correspond to the conductor pattern are cut off and discarded after the metallization. However, this edge copper plating can be avoided by the method according to the invention if an electrical contact is established between the plate edge and the holding device and a corresponding current is maintained, so that both the holder and the plate edge remain essentially or completely free of copper.

In the event that a copper precipitate forms and, for example, settles on the bottom of the tank, it can easily be removed by briefly interrupting the deposition process, emptying the tank and removing the precipitate by brushing, wiping or vacuuming or similar processes. Such cleaning can also be carried out without interrupting the deposition process if the precipitate is suctioned off.

It must be expressly emphasized that - in contrast to the methods already known in the art - no adherent copper deposit forms on the passivated surfaces in the method according to the present invention, even if the deposition process takes a very long time. The metallic copper produced in the course of the process according to the invention does not adhere firmly

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on the surfaces and can therefore be easily removed, as described above, without the need for etching agents or similar, strong cleaning agents.

When the method according to the invention is carried out, a thin copper deposit will generally form on the cathode (s), which can be avoided completely or at least for the most part if a membrane is provided between the cathode and the copper deposition solution, which allows the current to flow, for copper ions but is impermeable. If the bath contains a complexing agent for copper ions such as amino acids, with which the copper forms negative complexes, a cation exchange membrane is used. If the complexes formed with copper are positive, an anion exchange membrane is used. If alkanolamines serve as complexing agents, the complexes formed with copper are neutral; so either an anion or a cation exchange membrane can be used.

The method according to the invention can also be used for the deposition or storage container containing the bath solution, the walls of which consist of non-noble metals, such as Steel, iron, nickel, cobalt, titanium, tantalum, chrome or the like and, if desired, also of copper. Similarly, the method can be used with other plating parts made from such metals.

The pH of the electroless copper plating bath solution is usually 10, and preferably 11 or above.

1 shows a simplified representation of a deposition device as it is to be used for the method according to the invention, consisting of the metal container with the bath solution, the power source, the electrodes, the plate to be metallized and the holding devices.

2 shows the device in detail with automatic bath control, which has proven to be particularly favorable together with the method according to the invention.

Fig. 3 is a graphical representation and shows the dependence of the current on the potential (voltage) for surfaces made of stainless steel and an electroless copper plating solution.

Fig. 4 is a graphical representation and shows the dependence of the current on the potential (voltage) for surfaces made of copper in electroless copper plating baths. The copper bath solution has the same composition as in FIG. 3.

In Fig. 1, the bath vessel 2, the walls of which are made of steel, preferably stainless steel, or of another suitable conductive material, contains the electroless copper plating solution 4. The metal electrode 6 is immersed in the bath solution 4 and thus an electrically conductive connection made for the negative output of the DC power source 10. The surfaces 12 of the tank 2 are electrically connected via the variable resistor 16 to the positive output 18 of the direct current source 10. The millivolt meter 20 is also connected to the tank wall 12 and the standard reference electrode 22. The workpiece 24 is held in the frame 26. The holding device 26 is in electrical contact with the surface 12a of tank 2 and hangs in the deposition bath solution 4. The workpiece 24 is electrically insulated from the holding device 26 by the insulator 27.

Preferably, before starting the bath work, which can be initiated, for example, by adding the reducing agent or by increasing the pH or the temperature, a potential which is more positive than the expected mixed potential of the bath solution 4 is applied to the surfaces 12a and 12b of the tank 2 placed by adjusting the resistor 16. According to a further embodiment of the invention, this voltage can be chosen to be so strongly positive that copper particles formed dissolve again. Bath composition and bath conditions are then e.g. adjusted by adding the reducing agent or by increasing the pH or the temperature and thus initiating the metal deposition. The workpiece 24 is immersed in the bath solution 4 and the deposition process begins. The electrical potential of the surface 12 with respect to the reference electrode 22 is checked during the deposition process by means of the millivolt meter 20 and is always regulated so that it is more positive than the mixed potential of the solution 4. The potential can be regulated either manually (Fig. 1) or automatically (Fig. 2).

Fig. 2 shows a 200 V AC line 28 that powers DC source 30; at 7 V, for example, this supplies a current of 200 amperes. The negative output 32 of the current source 30 is electrically connected through the line 34 to the electrodes 36 which hang in the metal container 38. The tank 38 contains the bath solution 40 and is connected to earth potential by means of an earth line 42. The positive output 44 of the current source 30 is electrically connected through the line 46 to the transistors 48, which are connected in parallel and are supplied by the Darlington high-current transistor 50. Each transistor 48 preferably has an output of 50 amps. The Darlington high-current transistor 50 is preferably designed for a gain factor of 10,000: 1.

The transistors 48 are connected to the tank 38 via the electrical line 52, the shunt resistor 54 and the line 56. The shunt resistor 54 is connected via line 58 to the standard ammeter 60, which measures the current from the transistors 48 through the shunt resistor 54. Capacitance 62 is preferably 2 microfarads and is connected to shunt resistor 54 via line 34 to reduce background electrical noise.

The electrical line 64 connects the tank 38 to the positive output 74 of the voltage amplifier 68. The electrical line 70 connects the standard reference electrode 72 to the negative output 66 of the amplifier 68. The amplifier 68 has a gain factor of 10: 1. The reference electrode 72 is one of the usual silver / chlorosilver electrodes (or an equivalent electrode) and is in conductive communication with the bath solution 40 in the tank 38 via an intermediate liquid.

The amplifier 68 is connected to the negative output 78 of the control amplifier 80 via the electrical line 76. The voltage supplied by amplifier 68 to amplifier 80 is measured by voltmeter 82, which is connected to line 74 via line 84. The positive output 86 of the control amplifier 80 is connected via the line 88 to the potentiometer 90 and the FET switch 92. Potentiometer 90 preferably has a scale of +3 V to -2 V.

The electrical lines 94 and 96 connect the outputs 98 and 100 of the shunt resistor 54 to the amplifier 102. The line 94 is connected to the positive input 104 of the amplifier 102. Line 96 connects the negative input 106 of amplifier 102. The voltage supplied by amplifier 102 is fed through line 108 to positive input 110 of control amplifier 112. The amplifier 112 has a gain factor of 20: 1. The negative input 114 of the control amplifier 112 is connected to the potentiometer 116. The electrical line 118 runs from the amplifier 112 to the FET switch 92.

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The capacitance 120, preferably 1 microfarad, and the resistor 112 of about 1 ohm are included in the circuit to reduce the background noise.

To avoid overheating, transistors 48, power amplifier 50, capacitors 120 and 62, resistor 122 and ammeter 60 are housed in a cooling block 124 (shown as a dashed line) and are driven by fan 126 (110 VAC) line 128) cooled. The cooling block is preferably made of aluminum or another common heat absorbing material.

In practice, the method according to the invention is carried out as follows (FIG. 2):

The 220 VAC from line 28 is fed to DC source 30 and converted to DC. The negative potential of the current source 30 is applied to the electrodes 36 in the tank 38; the electrodes 36 are thus cathodic. The positive potential of the current source 30 is 54 through the power transistor 50, the transistors 48, the line 52 to the shunt resistor 54 and by means of the line 56 to the tank 38; the tank 38 is therefore anodic. The current across the shunt resistor 54 is controlled using the ammeter 60.

The silver / chlorine silver reference electrode 72 is hung in the bath vessel 38 and brought into contact with the bath solution 40. The contact is established in a known manner with the interposition of a porous membrane. By connecting the electrode 72 and the tank 38 to the opposite poles of the amplifier 68, as previously described, the voltage on the tank walls is continuously monitored and regulated as follows: If the voltage from the amplifier 68 to the control amplifier 80 is predominantly positive, the amplifier 80 becomes specify a positive voltage. On the other hand, if the voltage from the amplifier to the control amplifier 80 is predominantly negative, the amplifier 80 will also deliver a negative voltage. If the transistor 50 is supplied with a positive voltage, a current flow occurs. A negative voltage at transistor 50 causes it to switch off and practically no current flows at all. If the potential at the vessel wall 38 becomes less negative in comparison with the reference electrode 72 during the deposition process, the amplifier 68 applies a positive voltage to the amplifier 80, which in turn outputs a negative voltage to the power current transistor 50. By setting the voltmeter 90, the output positive voltage at the control amplifier 80 is regulated accordingly in order to set the total output power of the amplifier 80 in such a way that the desired current flow to the tank 38 is achieved and thus a potential is established on the tank wall 38 which is more positive than that Mixed potential of the bath solution is 40.

Excess current flow to the tank 38 is prevented by the voltage amplifier 102 and the control amplifier 112. The potential (voltage) at the shunt resistor 54 is directly proportional to the current flow from the power amplifier 50 and the transistors 48. This voltage is raised in the amplifier 102 and further amplified by the control amplifier 112. When the amplified voltage from amplifier 112 exceeds the limit, FET switch 92 opens, divides the output voltage at potentiometer 90 and resets the set point at amplifier 80, bringing the entire system back into balance.

The output power of amplifier 102 is compensated for by the output power of potentiometer 116, which controls the set point on amplifier 112. The regulation of 116 determines the maximum current that is allowed to flow before the setting point of the control amplifier 80 has to be withdrawn. The function of amplifiers 102 and 112 is to limit the maximum current that may be applied to vessel walls 38 and the cathode so as to protect the entire system.

As previously described, the voltage across the tank 38 is maintained at a value that is more positive than the mixed potential of the bath solution 40; consequently almost no copper deposits on the tank walls.

In the process described above, metallic holding frames can be used for the plates to be copper-plated. These racks can also be made resistant to unwanted metal deposits using the previously described method. In this case, a separate circuit is preferably used to supply the racks with the required positive voltage. If the plates fastened for coppering in the frames have copper edges, the power supply must be chosen larger in order to maintain the potential of the frame surfaces and the plate edges sufficiently positive. If, on the other hand, the entire plate surface is to be copper-plated, or if the plates are to be provided with copper edges, it is necessary to isolate the plates from the holding frame by interposing an insulating material (cf. FIG. 1).

3 and 4 show the current as a function of the voltage for copper surfaces and those made of stainless steel in an electroless copper plating bath which has the composition given in Example 1. Positive currents are oxidizing currents, and negative currents are reducing, for example metal-depositing currents. At point "B" in Fig. 4 (copper electrode) there is no current flow; this potential is called the mixed potential. In area “A”, more copper ions are reduced than there are reducing agents in the solution. Here the formaldehyde is oxidized, so that a negative deposition current is created. In the "C" area, more formaldehyde is oxidized than copper ions are reduced, so that a positive or oxidizing current is generated. In the “D” area, a film is formed on the surface of the copper electrode that is not catalytically active for the formaldehyde oxidation. The maximum current for passivating the surface has been found to be 4 milliamperes / cm2. The reduction of copper ions no longer takes place at voltages above -450 mV, measured against the reference electrode. This value is equivalent to voltages that are more than 250 mV above the mixed potential. The range “E” between -425 and -225 mV, measured against the reference electrode, is called the “passivation range”. In this area the surface potential is too anodic to reduce copper ions and the electrode surface is not catalytic for the oxidation of formaldehyde, so that very little current flows. Since the current flow in the "E" area is the same for bath solutions with and without the addition of formaldehyde, it is assumed that the current flow in this area is not caused by the oxidation of the formaldehyde. The current flow in the “F” area is based on the oxidation and partial dissolution of the electrode surface. Area «G» is the second «passivation area». Then various bath components such as OH ions, EDTA,

Oxidized copper or formaldehyde.

3 shows the conditions when using a stainless steel electrode which is relatively passive from -500 to +400 mV. At a potential that is more negative than -500 mV, copper begins to deposit on the electrode surface, which changes its surface properties. At -325 mV the current density at rost5

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free steel 40 times less than copper (0.020 vs. 0.80 mA per cm2). When using an electrode made of stainless steel, the copper deposition is initiated very slowly, which is due to its poor catalytic activity for the oxidation of formaldehyde. However, once the deposition has started, it progresses very quickly because the copper deposits that form on the steel surface have a very high catalytic activity, i.e. a large number of electrons are released.

A potential of -.325 mV, measured against a calomel electrode, is preferably used for the passivation of surfaces made of stainless steel and copper, since this lies in the middle of the passivation zone for copper and the current density for stainless steel is very low at this potential .

The passivation area can shift slightly with a varying pH value, namely in accordance with the shift in the mixed potential in the event of pH value changes and also in a similar order of magnitude. In one embodiment of the present invention, a mixed potential probe is therefore used as the reference electrode.

The measured values shown in FIGS. 3 and 4 were determined with the aid of a “Polarographic Analyzer, Model 174A” (Prince-Town Applied Research) and a saturated calomel electrode as the reference electrode for all measurements. The current was monitored while the voltage was sampled in an air atmosphere and plotted on an X / Y scheme.

To measure the mixing potential, a clean copper surface is immersed in an electroless copper separating bath solution; Copper begins to deposit on the surface. A stable state is reached after 3 to 4 minutes. The surface is connected to an output of a high-ohmic voltage measuring device, to the other output of the measuring device a standard reference electrode is connected, which is also in the bath solution. The difference between the potential of the copper surface and the reference electrode is measured in order to determine the mixed potential of the deposition solution.

The following examples further illustrate the present invention.

example 1

An epoxy glass laminate 1.6 mm thick is pretreated in a known manner for electroless metal deposition to produce a printed circuit.

The plate pretreated in this way is immersed in a copper deposition solution of the following composition:

CUS04-5H20 10g / l

Formaldehyde 4 ml / 1

Wetting agent 0.2 g / 1

EDTA tetrasodium salt 35 g / 1

Sodium hydroxide (NaOH) to adjust the pH to 11.7 (at 25 ° C)

Sodium cyanide (NaCN) 0.005 g / 1

fill up with water

Operating temperature 72 ° C

The copper bath solution according to this example has a mixed potential of -630 ± 20 mV, measured against a standard silver / chlorine silver electrode.

All bath components of the solution, except the formaldehyde, are placed in a stainless steel container and mixed. A stainless steel cathode is immersed in the bath solution and connected to the negative output of a variable DC power source. The rectifier has a maximum capacity of 8 V and 200 amps. A standard silver / chlorine silver reference electrode is also immersed in the bath solution and connected to the one output of a millivolt meter; the other exit is connected to the stainless steel tank wall.

The electrical potential at the bath tank, measured against a reference electrode, is set to -200 mV by regulating the rectifier. The copper deposition is started by adding the formaldehyde. The plate prepared as described above is attached to a stainless steel frame and immersed in the bath solution. The copper deposition on the plate begins. After 10 hours, or after the copper has reached a layer thickness of 20 µm, the plate is removed from the bath solution. There was no significant copper deposition on the tank walls or the holding frame, both made of stainless steel.

Example 2

This example describes an embodiment of the invention in which two electrodes but no reference electrode are used.

An electroless copper plating bath solution of the same composition as in Example 1 is placed in a stainless steel container. The container has a capacity of 8000 liters and an inner surface of 60 m2.

The rectifier is set so that there is a potential of 0.45 V between the stainless steel cathode, which is in the bath solution, and the tank walls. It can then be determined that the tank walls have a potential of -300 to -400 mV, measured against a silver / chlorine-silver reference electrode. After this measurement, the reference electrode is removed from the bath solution. The current required to maintain the potential of 0.45 V between the vessel wall and the cathode is 0.5 ampere, which corresponds to a current density of 10 ^ milliampere / cm2.

1800 support plates, each with a surface area of 0.2 m2, are fastened in 6 stainless steel frames, each with 300 pieces, and immersed in an electroless copper separating bath. After a copper layer of the desired thickness has deposited on the conductor pattern (approx. 18-22 hours), the finished plates are removed from the bath and an equal number of carrier plates are attached to the 6 racks and immersed in the bath solution. After the first 24 hours of bath work, a precipitate of metallic copper particles is observed, some of which come into contact with the vessel walls. The current required to maintain the voltage of 0.45 V between the wall of the vessel and the counter electrode increases. During the following days of bath work, it is observed that this current rises and falls in a range between 2 to 100 amperes, due to further copper particles that come into contact with the vessel walls and are passivated.

After e.g. The separation process is interrupted for one week. The copper precipitate on the inner wall of the tank consists of passivated, non-adherent copper particles and can be easily removed by brushing or vacuuming.

Example 3

In this example, the same procedure is used as in Examples 1 and 2, with the difference that the stainless steel holding frames also use a second power source and a second suitable rectifier

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be bound so that a second counter electrode is immersed in the bath solution and the potential is set to 0.4 to 0.5 V and held at this value (measured between the holding frames and the second electrode). So

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the surfaces of the holding frames and, in some cases, the edges of the carrier plates are passivated against unwanted metal deposition.

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Claims (4)

646 732 PATENT CLAIMS 1.Process for passivating metal surfaces to avoid unwanted copper deposits from an electroless copper-depositing bath solution when storing the bath solution and / or when electrolessly depositing copper from the bath solution on support plates sensitized to this deposition, in which process plating devices with the metal surfaces are used , which are in contact with or brought into contact with the bath solution and which, in addition to a counter electrode, are electrically connected to a power source in order to make the metal surfaces at least approximately completely resistant to copper precipitation, characterized in that by means of the power source the potential at the metal surfaces is set and held more positively than the mixed potential of the copper bath solution, and that a controllable current is generated by means of the current source, which is sufficient over the entire range, by which at least e to compensate for any free electrons present in the bath solution on the metal surfaces both before and after the formation of a precipitate of metallic copper particles, such that both the metal surfaces and the surfaces of the copper particles are at least approximated during the entire storage or copper deposition process are kept completely free of unwanted copper deposits. 2nd 5 10th IS 20th 25th 30th 35 40 45 50 55 60 65 2. The method according to claim 1, characterized in that firstly a current is generated between the counterelectrode and the metal surfaces, for example that of a bath container or other metallic devices, which gives the metal surfaces a potential which is sufficiently positive compared to the mixed potential of the copper bath solution, in order to make the metal surfaces at least essentially inactive for the oxidation of the reducing agent present in the bath solution and thus resistant to the electroless copper plating, and for the current to be set in this way during storage of the bath solution and during operation for electroless metal deposition on objects intended for this purpose, or adjusts itself in such a way that the metal surfaces remain resistant to electroless copper deposition and copper particles deposited on them from the bath solution for electroless copper deposition on the same or on surfaces in electrical contact therewith and be kept in this condition. 3. The method according to claim 2, characterized in that the potential resulting from the current flow is sufficient to bring deposited copper particles or nuclei back into solution. 4. The method according to any one of claims 1 to 3, characterized in that the counter electrode is surrounded with a membrane which prevents the complex-bound copper ions present in the bath solution from reaching the counter electrode. 5. The method according to any one of claims 1 to 4, characterized in that the current intensity is set as a function of the potential measured between a reference electrode and the metal surfaces. 6. The method according to any one of claims 1 to 5, characterized in that the mixed potential of the bath solution, measured against a silver / chlorine silver electrode, is between -500 mV and -800 mV, and that an electrical potential is maintained on the metal surfaces, which is more positive than the measured mixed potential of the bath solution and is in the range from -500 mV to +500 mV, measured against a silver / chlorine-silver electrode. 7. The method according to claim 6, characterized in that that the potential at the metal surfaces is in the range from -300 mV to -100 mV, measured against a reference electrode, but is preferably at least -250 mV. 8. The method according to claim 1 or 2, characterized in that the current is between 10-4 and 4 mA / cm2. 9. The method according to claim 1, characterized in that a mains rectifier is used as the current source, the output voltage of which is set to and maintained at a value which brings about the required potential on the metal surfaces. 10. The method according to claim 1, characterized in that a plating device is used, which consists of a bath container with metallic walls and a wholly or partially made of metal holding device for the support plates to be copper-plated. 11. The method according to any one of claims 1 to 10, characterized in that the plating device is made of steel or stainless steel. 12. The method according to claims 1 to 11, characterized in that each individual part of the plating device is connected to a separate power source. Electroless metal depositing bath solutions, such as electroless copper baths, are well known in the art of metallizing metallic and non-metallic surfaces. Such bath solutions are characterized in that they can be used to deposit metal deposits of any layer thickness without supplying electrons from an external power source. After a layer of metal has deposited on the surface to be metallized, an autocatalytic process develops that continues as long as there are sufficient metal ions in the solution. In particular, the metallization of plastics in general, and specifically the production of printed circuits, should be mentioned here. In electroless metallization processes of the type mentioned for commercial purposes, relatively large containers are used. The workpieces to be metallized are immersed in the copper deposition solution in these containers; for this purpose, they are expediently held in frames or racks. It has been observed that if the containers and holding racks are made of plastic or glass, the copper particles which form during the deposition reaction are deposited on the surfaces of these containers and holding racks and adhere firmly to them. These copper particles, which are deposited in this way, act as nuclei for further copper deposits, so that most or all of the surfaces of the objects coming into contact with the bath solution are covered with electrolessly deposited copper, which means an increased consumption of bath chemicals. In addition, the copper-plating process must be interrupted again and again in order to free the holding frames and containers from the deposited copper coating by etching. This etching process is undesirable because, firstly, the deposition process has to be interrupted, secondly, large amounts of expensive etching solution have to be consumed, and thirdly, the acidic copper solutions formed have to be processed in order to recover the copper. In addition, non-metallic containers and holding frames are severely attacked by the repeated etching and their service life is shortened considerably. 3rd 646732 Due to their much greater resistance to etching solutions and therefore longer service life, containers and holding frames made of metal are preferable to those made of glass or plastic. However, such metallic tanks and racks have the previously described shortcomings of undesirable copper deposits to an even greater extent. The metals, such as stainless steel and the like, which are particularly suitable for the production of metallization devices, are catalytically active for the oxidation of the reducing agent or agents present in the bath solution and thus spontaneously cause the deposition of copper particles, which in turn cause further copper depositions catalytically, so that all forms a copper deposit with the electroless copper depositing bath solution. This has a particularly disadvantageous effect on the holding frames, the fastening clamps of which are difficult to open after the coppering process. It is now known to temporarily make metallic surfaces, for example of metallization tanks, holding racks or other objects used in electroless metallization with certain chemicals, resistant to the formation of metal deposits thereon, if they are passivated, for example, by treatment with nitric acid. However, such treatments only provide temporary protection; This surface protection is removed within a few hours, so that this method cannot be used for practical use in metallization baths. US Pat. No. 3,424,660 describes a metallization tank with metallic surfaces which can be protected against metal deposits, in particular against nickel deposits, by applying a potential. The value of this protection potential results from the current density / voltage curve. The current density is set to not more than 10 ~ 4 amps / cm2. German Offenlegungsschrift 2,639,247 describes 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 to them that has a current density of at least 4 milliamps / dm2. From Japanese Patent Laid-Open No. 54-36577 of November 9, 1979 is known to make a tank made of stainless steel resistant to unwanted metal deposits by applying a positive electrical voltage to them during the currentless metal deposition. In practice, the methods described above have not proven themselves in electroless copper plating baths. The necessary chemicals must be added during bath work. With this bath supplement, local fluctuations in the bath concentration cannot be avoided; furthermore there is also a risk of contamination of the bath solution during this process. In addition, copper nuclei form, which grow into copper particles within the bath solution. Such copper particles or copper dust as well as dirt, which have formed in the bath solution, come into mechanical and electrical contact with the tank walls and holding frames and thereby cause a high electrical current in these metal objects, which results in the breakdown of the potentials applied in accordance with the techniques described above has, so that they drop to a value which is below the oxidation potential of the reducing agent; the desired purpose can no longer be fulfilled. The use of such counter potentials for copper plating baths requires a current that is at least two orders of magnitude higher than that previously described in the literature in order to make the stainless steel surfaces resistant to undesirable copper deposits. It is believed that this is due to the much more pronounced catalytic activity of copper compared to steel with regard to the oxidation of the reducing agent, which is equivalent to the formation of electrons. Although according to the methods belonging to the prior art, the surfaces of the metallization devices are resistant to the metal deposition during the initial phase of the electroless metallization, however, this occurs in the further course of the metallization process, which makes these methods unusable for mass production. Another problem frequently arises when coppering objects, for example printed circuits, which are immersed in the coppering solution in holding racks and consists in the fact that copper is deposited, particularly at the edges of the carrier plates, in areas which are not sensitive to electroless metal deposition. If these undesirable copper layers come into contact with the holding frame, the current requirement increases sharply, which may result in a drop in the voltage below the minimum value required to maintain the surface's resistance to undesired copper deposits. The power supply sources used in the prior art methods deliver a maximum current of AI, from which it can be seen that the problem described above was not yet recognized by the inventors of those methods.