NL8002515A - METHOD FOR PASSIVATING METALLIC SURFACES IN CURRENT COPPER SALES. - Google Patents

Description

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Method for passivation of metallic surfaces and electroless copper deposition.

Electroless metal deposition baths, h.v. electroless copper solutions are known and suitable for depositing metal on non-metallic and metallic surfaces. Such solutions have the property of being able to deposit a metal in almost any desired thickness, without the need to supply electrons from an external current source. After a metal deposit is formed electrolessly on the surface of the article, the electroless plating process becomes autocatalytic, i.e. the metal deposit on the surface continues as long as the solution is replenished and maintained.

Special mention is made of the use of electroless metallization procedures in the coating of plastics, in particular in the manufacture of printed circuit boards.

When electroless coating operations of the aforementioned type are performed on a commercial scale, large coating vessels are usually used. The parts or objects to be coated are immersed in the copper deposition solution in the coating vessel. The parts or objects are generally supported on racks or frames immersed in the solution. It has been found in practice that if the coating vessel and the support racks are constructed of plastic material or glass, the copper particles formed in the solution during the electroless copper coating will stick to the surfaces of this installation. The attached copper particles provide places for further electroless copper deposition and growth. Finally, most or almost all of the surface of such an installation is covered with a deposit of electroless copper.

This process uses up the components of the deposition bath.

Furthermore, the coating treatment must be interrupted from time to time to empty the tank and etch the copper off the surfaces of the installation. This etching procedure is disadvantageous because treatment has to be stopped and large amounts of expensive etching, recovery and waste treatment chemicals are required.

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It is a further drawback that the etching procedure weakens the structure of the non-metallic coating vessels, racks and the like and shortens the useful life.

Use of coating vessels and other metallic coating installations would be desirable because of their greater durability and normally greater availability. However, in certain aspects such vessels and installations are even more subject to the aforementioned drawbacks. In particular, metals generally suitable for the construction of tanks and other coating plants, e.g. steel, including stainless steel, catalytically active with respect to oxidizing the reducing agent present in the electroless copper plating solutions, easily initiating the formation of an incipient copper deposit on its surfaces, which in turn causes continuous copper deposits on the metallic surfaces in contact with the electroless deposition solution are formed. This problem is particularly acute when adjustable metal racks, i.e., containing metal fasteners, are used because these fasteners are coated with an electroless metal deposit, making them difficult to release.

Coating vessels made of metallic support walls, as well as other metallic coating installations such as racks, plumbing and the like, are known to be made temporarily resistant to electroless metal deposition by chemical pretreatment, e.g. a nitric acid solution, which inactivates the respective catalytic oxidation surfaces of the reducing agent contained in the bath. However, such chemical treatments appear to lose their effect within a few hours of operation and are therefore impractical to use in a production method.

U.S. Pat. No. 3 ^ 2 ^. 660 discloses that coating vessels with metallic support walls can be protected from electroless metal deposition, especially nickel gate linings, by applying a potential thereon of a value corresponding to the resting potential or protection potential range of the current density / potential curve. The current density 800 2 5 15 3 _4 2 is set to no more than about 10 A / cm.

German Offenlegungsschrift 2,639.2 ^ 7 describes that cladding tanks or racks made of a metal, such as cobalt or nickel, can be made resistant to electroless metal deposits, such as an electroless copper deposit, by charging them with a current density of at least 2 least ^ mA / dm.

Japanese Patent Publication No. 5-365777 has also proposed making a metal coating vessel, such as chromium-nickel steel, resistant to chemical coating by applying a positive electrical potential to the surface of the vessel during treatment.

In commercial practice, the aforementioned procedures have not been satisfactory when used for electroless copper plating methods. As the desired electroless copper plating reaction 3 proceeds on the surfaces to be coated, the plating chemicals in the solution consumed must be replenished. This addition usually causes local fluctuations in the concentrations of the chemicals, as well as the introduction of further impurities in the solution. Copper sites are formed which grow into copper particles in the bulk of the solution. Such copper particles or dust as well as dirt in the deposition solution and discrete particles of deposited copper formed in the deposition solution come into contact with the tank and other installation surfaces. Such copper deposits, in mechanical and electrical contact with said surfaces, cause a high current to flow through the metallic plating device, thus decreasing the electrical potential applied to such an installation and falling below the value necessary is for inhibiting oxidation of the reducing agent so that it becomes unusable to keep said surfaces free of copper deposits formed electrolessly thereon. An equivalent area of metallic copper requires at least two orders of magnitude more current than stainless steel to resist electroless copper deposition.

This is believed to be due to the markedly higher catalytic activity of copper - compared to steel - to oxidize the reducing agent and thus the number of electrons formed.

Therefore, although the surfaces of known installations used are resistant to electroless deposition during the initial stages of treatment, after a short period of time, when such methods are used commercially, the surface resistance of the installation is lost. Thus, the known methods cannot be successfully adapted to commercial use.

An additional problem often encountered in the manufacture of articles to be coated which are held in coating racks, e.g. printed circuit boards, it occurs that in such procedures copper layers, which are not part of the circuit pattern itself, are sometimes formed on the edges of the insulating substrate. If such copper edges come into contact with the metal racks supporting the substrate, the power requirement will increase dramatically, reducing or decreasing the electrical potential applied across the rack so that it finally drops below the minimum potential , which is required to avoid coating when applying known procedures and regulations. The potentiostats of the known methods are capable of maximum currents of no more than 1 Amp, indicating a lack of knowledge or understanding of the problem found in larger scale coating, as indicated in a commercial application.

It is a main object of the invention to provide a method for electroless copper deposition, in which a metallic coating device in contact with the deposition solution is initially rendered resistant to electroless copper deposits and during prolonged periods of the coating treatment in a stable condition is maintained.

It is another object of the invention in commercial deposition processes to allow the use of metallic deposition plants for extended periods of time without building up copper deposits which must be removed by etching.

It is a further object of the invention to provide electroless copper deposition processes in which copper deposits in contact with the coating plant can be easily removed, such as by brushing, wiping, vacuum drawing, without the need for the coating treatment be shut down.

It is another object of the invention to provide improved methods for manufacturing printed circuit boards, avoiding adherent electroless copper deposits on the surfaces of the installation.

It is yet a further object of the invention to provide a method of forming printed circuit boards in which an undesired build-up of electroless copper or the edges of the panels to be coated is prevented. *

The aforementioned objects, as well as other objects which will become apparent from the description, can be achieved according to the method of the invention to be described now.

In particular, the invention achieves a reduction in coating costs of up to $ 30 or more, which is primarily due to the savings in coating or deposition chemicals and acids and neutralizing bases required to periodically etching the copper off the coating tanks and other surfaces of the installation and recovering valuable components from the etchant used and the waste treatment of the latter. Additional savings can be achieved by avoiding labor costs and lost production time required by such etching, recovery and waste treatment procedures.

Without limiting the invention in any way, it is believed to be based on the following inventive views and observations:

The oxidation of suitable reducing agents, which are present in electroless copper plating baths, eg formaldehyde, on surfaces which are catalytically active for such oxidation reactions produces electrons, thereby negatively charging such a surface; the copper metal coating from the copper ion-containing solutions consumes electrons. Copper ions which remain in solution by suitable complexing agents and which come into contact with such a charged surface are therefore deposited as metallic copper, thereby reducing the negative charge of the respective surface leading to an electroless deposition of a 6 copper layer on the surface. The potential of the surface created by the two reactions is known as the "mixed potential".

The surface of steel used for tanks or racks, as well as stainless steel and other suitable metals, is sufficiently catalytic with respect to the oxidation of e.g. formaldehyde to be sensitized to the electroless formation of copper deposits.

By making the respective metal surfaces part of an electrical circuit comprising a source of electricity, it is possible not only to compensate the charge formed on said metal surface due to the catalytic oxidation of the reducing agent, but also to compensate the surface provided with a layer of reduced or substantially absent catalytic activity for the oxidation of said reducing agents. The result of this procedure is that no copper deposits are formed on said surface (or surfaces).

In practical use, electrically formed copper deposits are formed on all surfaces or objects that are sensitive to such deposits and are immersed in the electroless copper solution.

Statistically, a certain number of copper ions on the surface, from the object to be coated to copper (0) or copper (I), are not included in the surface deposition network, but float back into the bulk of the solutions. Agglomerations form 25 particles which are themselves catalytically active and thus give rise to copper deposition on the surface thereof. Furthermore, due to impurities and the local increase in the concentration of certain chemicals due to the replenishment of the deposition bath solution, there is also a certain tendency to form copper sites and particles in the mass of the deposition bath solution. Copper sites or particles present in the bath solution adhere to metal surfaces, e.g. the tank fenders, racks and other deposition equipment while also depositing such particles on the bottom of the coating vessel.

35 In the emergence of such. precipitation in contact with the metallic surfaces of the coating plant, begin 800 2 5 15 * 1 7.

these surfaces take on copper deposits, and after a short period of time they behave ill in the same manner as metallic surfaces in contact with electroless copper plating baths, and thus generally produce the above-described undesirable results.

The catalytic activity and thus the oxidation efficiency of the reducing agent on copper surfaces is considerably higher than the catalytic activity of e.g. stainless steel, so that for the electrons formed on relatively few copper particles in contact with e.g. the bottom surface of the coating vessel, 3 compensating currents from the above source of electricity are required, which are drastically greater than the currents required to pass the stainless steel surface. Except if one follows the instructions of the invention, this leads to a change in the potential of the metallic surface to such more negative values that formation of an electroless deposit on said surfaces is caused. However, in accordance with the present invention, if the supply of electricity to the metal surfaces is dimensioned to provide the current necessary to effectively compensate for the electrons formed on the copper surfaces to maintain a surface potential which is sufficiently positive to suppress copper cladding on said surfaces and preferably to provide a catalytically inactive surface coating on both the metal walls of the cladding vessel and the installation, as well as on the surface of. the copper particles in contact therewith, no coating will take place on the walls or copper particles in contact with these walls.

This leads to a coating installation, which remains free of copper deposits, whereby the deposited copper particles are prevented from depositing on the respective metal surfaces, and thus these deposits can be easily removed.

According to the invention, the supply of electricity, also referred to as energy supply, is dimensioned such that the potential of the coating installation surfaces is thereby maintained at a value which is sufficiently more positive with respect to the mixed or coating potential that electroless coating action reactions on such metal walls are prevented; at which

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8 further provides a current sufficient to form catalytic inactive surface layers not only on e.g. the walls of stainless steel tanks, the surfaces of an installation, etc., but also on the surfaces of deposited copper and copper in contact with such surfaces.

According to the invention, there is provided a method of maintaining electroless copper plating solutions and electroless plating of copper from such solutions on substrates sensitive to electroless plating of copper, wherein surfaces of the metallic plating plant are in contact with said solution, which method comprises contacting said solution with at least one counter electrode; and connecting the metallic fence coating installation and the counter electrode (s) to a source of electricity such that the surface of the coating installation acquires a potential sufficiently more positive than the mixed potential of the electroless copper plating bath solution around the surface practically or fully resistant to electroless metal deposition, which source of electricity has provision for setting and maintaining said potential in the catchment area necessary to compensate for the electrons formed by the oxidation of the solution Reductant present when the solution is brought into contact with said metallic surfaces of said fencing installation, before and after copper particles have settled or deposited on the surfaces and thus substantially or completely the surfaces of said copper particles and of said coating installationcatalytically inactive are kept insensitive to electroless metal deposits.

In one embodiment of the invention 3, a method for electroless deposition of copper from an electroless copper plating solution on a substrate or for maintaining such solutions is provided, wherein surfaces of a metallic plating plant are in contact with said solution, which method includes:; 5 1} initially applying a potential sufficiently positive to the metallic installation over the 800 2 5 15 * 9 mixed potential of the electroless copper solution to make the surfaces of the installation substantially resistant to electroless copper -deposition; 2) electroless deposition of copper on the substrate from the electroless copper solution or storage of the solution; and 3) while electroless copper is deposited on the substrate or the solution is stored, maintain on the metallic surfaces of a potential which is sufficiently more positive than the mixed potential to render the surfaces resistant to electroless copper deposition.

The disclosed method can be applied to any type of metallic installation, used for electroless copper deposition treatments or for storing electroless copper cladding solutions, including coating vessels, racks supporting the substrate to be coated, lead work or other installation parts in contact with the coating solution .

The aforementioned method is more particularly carried out by initially through the metallic installation, e.g. run a cladding vessel, rack, etc., a current sufficient to generate an electrical potential on the surfaces of said installation that is positive enough to withstand adherent electroless copper; and electrolessly depositing copper on the substrate to be coated; as well as as the deposition reaction proceeds to supply the aforementioned current at a level sufficient to maintain a desired, sufficiently positive electrical potential across the surfaces of the metallic device to impede electroless coating. Preferably, a current becomes -U. . 2 m apply the area between 10 and 4 mA / cm surface, which must be made resistant to electroless deposition.

By way of illustration, the procedure is performed by providing at least one cathode in the electroless copper plating solution which is electrically connected to the surfaces of the plating plant through a source of electricity. Thus, a current is supplied to the chain closed by the stern bath solution and an electric potential is created on the surface of the installation which is sufficiently more positive than the mixed potential of the plating bath solution to form an adhesive copper coating on the surfaces. hinder the installation. This supplied current is regulated during the electroless copper deposition to the electric potential on the surfaces of the coating. 5 keep the installation at a desired level, sufficiently positive to keep the surfaces, as well as the surfaces of the copper particles deposited on the said surfaces, virtually inactive and insensitive to electroless deposits.

The term "mixed potential" refers to that electric potential where copper begins to precipitate electrolessly from an electroless copper plating solution on the sensitive surface in contact therewith. In other words, it is the electric potential, measured between a suitable metallic substrate, which is electrolessly coated with copper and a standard reference electrode in electrical connection with the substrate to be coated. Procedures for measuring the mixed potential of electroless copper plating solutions are known. Such a procedure is indicated in the description below.

Electroless copper deposition solutions useful for the invention are generally characterized by a mixed potential within the range between -500 and -800 m.V. to a standard silver-silver chloride reference electrode and in the range between -550 and -850 mV relative to a saturated kalomol reference electrode, measured at the operating temperature of the plating bath solution.

When conducting electroless copper plating procedures according to the invention, typically electrical potentials of between -500 and +500 mV and usually between -300 and -100 mV, relative to the reference electrode, are applied and maintained on the metallic support walls of the coating vessel and / or any other metallic coating installation in contact with the bath solution. Such potentials are sufficient to cover the surfaces of the installation as well as any amount of copper already in contact with it, e.g. to passivate deposited copper, copper edges on the coated panels and the like.

The principles of the invention are advantageously used to render substantially insensitive to metallic racks supporting the substrate to be coated in the coating solution all or all of electroless copper plating. In particular, such a rack is electrically connected to a clamp of a power source, e.g.

A rectifier, sized around the aforementioned defined potential in a wide range, e.g. up to 200 Amps with the other clamp of the power source connected to a cathode suspended in the deposition solution. A current sufficient to apply a passivating electric potential to the rack is supplied, copper is electrolessly deposited on the substrate supported in the rack and during deposition the current is adjusted to the rack in its passivated state, insensitive to electroless copper deposits. Although the same power source as for the coating vessel or other coating installation can be used, it is preferable to use a separate power source for each part of the installation to be desensitized and maintained.

The aforementioned technique can additionally be applied to the electroless deposition of copper on undesired areas of it.

20 coating substrate, to avoid. Particularly for printed circuit boards employing the additive manufacturing technique, in some cases the insulating panels pre-cut, masked and sensitized leave an exposed sensitized edge on the sides of the circuit board. panel and the adjacent panel surface. When the panel is contacted with the deposition solution, together with the copper deposition in the desired areas, copper begins to deposit on the sensitized panel edges and the adjacent sensitized surface, in the same thickness as on the unprotected areas of the panel, that correspond to the desired chain pattern. As a result, a continuous rim of copper is formed on the panel. Typically, this copper edge, which is not part of the chain pattern itself, is cut from the rest of the panel and drained. The build-up of such a copper edge can be prevented by contacting an edge 35 of the panel with a metallic surface of the rack and supplying and maintaining sufficient current to the rack to cover both the rack and the copper edge. of the panel practically insensitive to electroless deposition.

Metallization vessels, as well as other metallic deposition plants, using the method of the invention can process any copper precipitate that can adhere to the surface, e.g. Copper particles that have fallen on the hull of the stern casing are cleaned by temporarily stopping the sterning treatment, emptying and brushing the coating bath, or vacuuming the copper deposits. Such a cleaning stage can also be used during the stern coating treatment without the vessel having to be emptied, for instance by drawing a vacuum. It should be noted that, contrary to the known methods in the method of the invention, copper will not adhere to the passivated installation surfaces even after prolonged operation and such copper can be easily removed by the aforementioned cleaning procedure, without the need for etching or any other intensive chemical or mechanical cleaning procedure.

In the practice of the method of the invention, it will usually be found that a thin layer of electroless copper deposits is formed on the cathode or cathodes. Copper deposition on the cathode surfaces can be completely or at least partially prevented by placing a membrane between the cathode and the electroless copper deposition solution, which allows electrical conduction between the cathode and the deposition solution, but prevents passage of copper ions from a deposition solution,

Ion exchange membranes, whether anionic or cationic, can be used for this purpose. The choice of the particular ion exchange membrane will depend on the specific copper ion complexing agent used in the bath. In both cases where the complexed copper has a negative charge, such as when the complexing agent is of the amino acid type, cation exchange membranes are used. In those cases where the complexed copper has a positive charge, anion exchange membranes are used. If the complexed copper is neutral, such as in the case where alkanolamine complex formers are used in the bath, either an anionic or cationic exchange membrane can be used.

The method of the invention is useful for coating and storage vessels, in which the support walls or the surfaces of the walls are made of non-precious metals, such as steel, iron, nickel, cobalt, titanium, tantalum, chromium, and the like and copper if desired. Similarly, the method can be used to render other types of metal-made coating plants resistant to adherent electroless copper deposits.

0 The pH of the electroless copper deposition solution is usually at least 10 and preferably 11 or higher,

Fig. 1 illustrates a simplified coating system that can be used with the invention, comprising a coating tank with metallic support walls, a coating solution, an I5 power source, electrodes, the substrate to be coated and the support member.

Fig. 2 shows a more detailed system adapted for automatic control useful for practicing the invention; Fig. 3 is a graph showing current as a function of the potential (voltage) for stainless steel in an electroless copper plating solution; Figure U is a graph showing current as a function of the potential (voltage) for copper in an electroless copper plating solution of the same composition as in Figure 3.

In Figure 1, a coating tank 2, the support walls of which are made of steel, preferably stainless steel, or another suitable electrically conductive material, contains an electroless copper-coating solution b. Metal electrode 6 is immersed in solution Ij and electrically connected to the negative terminal 8 of a DC power supply unit 10. Surface 12 of tank 2 is electrically connected via a variable resistor 16 to a positive terminal 18 of the unit 10. A millivolt meter 20 is also connected to the support wall 12 and to the standard reference electrode 22. The object 2b to be processed is supported on a metal rack 26. Rack 26, 35 in electrical contact with surface 12a of tank 2, is suspended in the coating solution b. The object 2b to be treated is electric

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14 isolated from rack 26 by the non-conductor (insulator) 27. Before starting the operation of the coating solution b (eg by adding a reducing agent or by increasing the pH or temperature), preferably a potential which is more positive then the expected mixed potential of the operating coating solution applied to the surfaces 12a and 12b of tank 2 by adjusting resistor 16 as needed. The composition of coating, solution k and conditions are adjusted by known means to start the electroless coating (eg, by adding the reducing agent, increasing the pH or temperature). The object 2k is immersed in the coating solution b and the coating begins.

The electrical potential of surface 12 with respect to the reference electrode 22 is followed during coating by reading milli-voltmeter 20 and this potential is kept more positive than the mixed potential of the coating solution. controlled manually, as in the embodiment of Figure 1 or automatically as illustrated in Figure 2.

In Figure 2, a 200 V AC power line 28 extends to the DC power supply 31, e.g. is capable of a flow of 200 Mg. at 7 V. The negative terminal 32 of the power supply unit 30 is electrically connected by lead 3¼ to electrodes 36 suspended in the metal tank 38. Tank 3S contains coating solution ^ 0 and is grounded by wire b2. The positive terminal kb of the supply unit 30 is electrically connected through line h6 25 and passes transistors 1 * 8, which are parallel and driven by a Darlington power transistor 50. Each of the pass transistors U8 preferably has an output capacitance of 50 Mp . The Darlington power transistor 50 is preferably set to a gain of about 10,000: 1. Pass transistors H8 are connected by electrical line 52, meter shunt 5 ^ and electrical line 56 to tank 38. Meter shunt 5 ^ is connected by line 5Ö with standard vaporizer 60, which measures the current from the forward transistors U8 over the meter shunt 5 ^. Capacitor 62, preferably having a capacity of 2 microfarads, is connected across the electrical lead 3 ^ and the 35 meter shunt 5b to reduce the electrical background noise.

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Electrical line 64 extends from tank 38 and is connected to the positive terminal jk of a voltage amplifier 66. Electrical line TO extends from the standard reference electrode 72 and is connected to the negative terminal 66 of amplifier 68. Amplifier 68 is set to a gain of 10: 1. Reference electrode 72 is a conventional silver / silver chloride electrode or equivalent reference electrode, in a salt bridge connection with the coating solution 40 in tank 38.

Amplifier 68 is connected by electrical line 78 to the negative terminal 78 of the control amplifier 80. The voltage output from amplifier 68 to amplifier 80 is measured by standard voltmeter 82 connected through line 8½ to lead 74. The positive terminal 86 of the control amplifier 80 is connected by line 88 to potentiometer (set point) 90 and to the FET switch 92. The potentiometer 90 preferably has a maximum possible setting of 3 V positive to 2 V negative. Electrical leads 94 and 96 extend from terminals 98 and 100, respectively, from the meter shunt 54 to the voltage amplifier 102. Lead 94 is connected to the positive input terminal 104 of amplifier 102. Lead 96 is connected to the negative input terminal 106 of amplifier 102. The voltage output of amplifier 102 goes via line 108 to the positive input terminal 110 of control amplifier 112. Amplifier 112 is set to a gain of 20: 1. The negative input terminal 114 of control amplifier 112 is connected to potentiometer (set point) 116 Electrical line 118 passes through amplifier 112 to FET switch 92.

Capacitor 120, preferably with a capacity of 1 microfarad, and resistor 112, preferably with a 1 ohm resistor, are chained to reduce the background noise.

To prevent overheating, 30 pass transistors 48, power transistor 50, capacitors 120 and 62, resistor 122 and ammeter 60 are preferably placed on heat reservoir 124 (indicated by dotted lines) and cooled by a fan 126 connected to a 110 V AC line 128. Reservoir 124 is made of aluminum or other standard material, which bears heat absorber.

In practice, the method of the invention 16 is carried out as follows: According to Figure 2, an alternating current of 220 V from line 28 is converted into a direct current in the DC power supply unit 30. The negative potential from the supply unit 31 is applied to the electrodes 36 in tank 38. Thus, electrodes 36 are made cathodic. A positive potential from supply unit 30 is driven through power transistor 50, pass transistor 48, line. 52, meter shunt 5¾ and line 56 to tank 38.

Tank 38 is thus made anodic. The current across the meter shunt 5 ^ is monitored using ammeter 60.

A silver / silver chloride reference electrode 72 is suspended in tank 38 and conventionally maintained in communication with the coating solution U0 through a porous intermediate membrane. Thus, by connecting the reference silver / silver chloride electrode 72 and tank 38 to the opposite terminals of amplifier 68 in the manner shown, the potential (voltage) of the walls of tank 38 is continuously monitored and additionally controlled as follows:

If the voltage from amplifier 68 to control amplifier 80 is predominantly positive, amplifier 80 tends to deliver a positive voltage. Conversely, if the voltage from amplifier 68 to control amplifier 80 is substantially negative, amplifier 80 tends to output a negative voltage. A positive voltage to power transistor 50 causes the last current to flow.

A negative voltage to power transistor 50 causes the latter to shut off and virtually all current to stop flowing.

During the coating, when the potential (voltage) of tank 38 becomes less negative (ie, more positive) relative to the reference electrode 72, amplifier 68 applies a positive voltage to control amplifier 80, which in turn applies a negative voltage 30 applies to power transistor 50. By setting the set point, the positive voltage output of control amplifier 80 is controlled as needed to set the total output of amplifier 80 and to provide the desired current flow to the tank 38 as needed to maintain the potential (voltage) of the tank 38 at the set potential, is more positive than to maintain the mixed potential of solution Ho.

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It is avoided that excess current flows to tank 38 by means of voltage amplifier 102 and control amplifier 112. The potential (voltage) across the met shunt 5I * is directly proportional to the current flowing out of the power transistor 50 and the pass transistors 1 * 8. This voltage is increased in amplifier 102 and further amplified in control amplifier 112. If the amplified voltage from amplifier 112 to the gate of PET 92 rises above the cutoff point, the FET switch 92 opens, dividing the output of potentiometer 90 and the setpoint of amplifier 80 is lowered, returning the system to its equilibrium state.

The output of amplifier 102 is balanced by the output of potentiometer 116, which establishes the setpoint of amplifier 112. Setting 116 determines the maximum current allowed before the setpoint of control amplifier 805 is lowered. The function of amplifiers 102 and 112 is to limit the maximum current supplied to the tank and cathodes to protect the overall system.

In the aforementioned manner, the voltage applied to tank 38 is kept more positive than the known mixed potential: 0 of the sterngear solution 1 * 0; as a result, there is virtually no metal deposit on the walls of tank 38.

In the specific procedure described above, metallic racks can be used to support the substrates to be coated and the like, and such racks can be rendered resistant to electroless copper deposition according to the principles described herein. In such a case, it is desirable to use a separate control circuit for supplying power to the racks. If the substrates to be coated on the racks are copper-edged panels, greater power will be required to maintain the passivating electric potential on the racks and the copper edges on the panel. On the other hand, if it is desired to coat the entire substrate or if the copper edges on the panel form part of or are connected to the chain pattern, it is preferable to isolate the substrate from the rack by inserting a substantially electrical non-conductive material (see figure 1).

18

Figures 3 and 4 show the current as a function of the voltage for copper and stainless steel in an electroless plating solution with the composition of Example I. Positive currents are oxidizing currents and negative currents are reducing, i.e., coating currents. At point "B" in Figure 4 (copper electrode}, there is no net current flow, this potential is known as the mixed potential of the deposition solution. In region "A", more copper ions are reduced than reducing agent is present in the solution; here formaldehyde oxidized to produce a net negative (coating) -10 flow. In region "C", more formaldehyde is oxidized than copper ions, so that there is a net positive (oxidizing) flow here. In region "D", a film on the surface of the copper electrodes This film is non-catalytic with respect to the oxidation of formaldehyde The maximum current required for passivation is set at 1+ mA / cm The copper ion reduction does not occur at potentials that are more positive than about -450 mV with respect to the reference electrode or 250 mV more positive than the mixed potential. Region "E", which ranges from about -425 to -225 mlS with respect to d The reference electrode is called the passive spring region. In this range, the potential is too anodic to reduce copper ions and the electrode surface is non-catalytic with respect to the oxidation of formaldehyde, so that little current flows. Since the flow in this range for the solution without and for the formaldehyde solution is approximately equal, it is believed that the flow flowing in this range is practically not caused by the oxidation of formaldehyde. The current flowing in region "F" is caused by the oxidation and partial dissolution of the electrode surface. Region, rG "is a second passivation region. Outside area "G", various components can be oxidized, OH ions, EDTA, copper or formaldehyde.

Figure 3 shows that a stainless steel electrode is relatively passive from -500 to +400 mV. At potentials that are more negative than -500 mV, copper begins to deposit on the stainless steel surface, which changes its properties. At -325 mV, the current density is 40 times less for stainless steel than for copper (0.020 mVs; 0.80 mA / cm ^}.

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Stainless steel is very slow in introducing the coating into an electroless copper plating bath. This is because it shows a relatively poor catalytic activity for the oxidation of formaldehyde. Once the coating has started, go. However, this progresses rapidly, since the copper deposit formed on the stainless steel surface provides surface areas of high catalytic activity for the oxidation of the reducing agent, and thus the formation of electrons.

A potential of about -325 mV (relative to the saturated calomel electrode) is best for passivating as much stainless steel as copper since it is in the center of the copper passivation region and the stainless steel current density at this potential is very low.

The passivation range may shift slightly with pH changes. The shift is in the same direction as the mixed potential of the solution changes with the pH and the same magnitude. Therefore, in one embodiment of the invention, a mixed potential probe is used as the respective reference electrode.

For the representations in Figures 3 and 4, the 4 potential values measured with a model 1T ^ A Polarograph Analyzer (Princetown Applied Research) and the reference for all measurements required a saturated calomel electrode. The flow is monitored as the potential is scanned under an air atmosphere and recorded on an X / Y recorder.

When the mixed potential is measured, a clean copper surface is placed in the electroless copper deposition solution; metal then begins to deposit on the copper surface.

3-5 minutes are given to allow a steady state to be reached. The copper surface is connected to a clamp of a high impedance millivolt meter, as used with standard pH meters.

A standard reference electrode immersed in the bath is connected to the terminal of said millivolt meter. The difference in potential between the copper surface and the reference electrode is measured to determine the mixed potential of the copper deposition solution.

20

The invention is now illustrated for the following examples, which are not intended to be limiting.

Example I

A 1.6 mm thick epoxy glass laminate is formed in a known manner to produce a printed circuit conductor network using electroless copper deposition.

The laminate thus obtained is ready for immersion in an electroless copper deposition solution of e.g. the following composition: 10 CuSOi ^ HgO 10 g / 1 formaldehyde 1+ ml / 1 humectant 0.2 g / 1 tetra sodium salt of EDTA 35 g / 1 sodium hydroxide (UaOH) to pH 11.7 (measured at 25 ° C) sodium cyanide (NaCN) 0.005 g / l water to volume operating temperature 72 ° C.

The copper deposition solution of this example has a mixed potential of -630 + 20 mV, measured with respect to the standard silver / silver chloride electrode.

All the ingredients of the aforementioned deposit solution except for the formaldehyde are mixed in a stainless steel coating 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 200 A. A standard silver / silver chloride reference electrode is immersed in the deposition solution and with one side of the milli-voltmeter connected. The other side of the millivolt meter is connected to a wall of the stainless steel vessel.

The electrical potential on the coating vessel wall with respect to the reference electrode is set at -200 mV by controlling the rectifier. The electroless copper plating solution is started by adding formaldehyde. The epoxy glass laminate pretreated as described and supported on a stainless steel rack is immersed in the electroless copper plating 800 2 5 15 21 solution. Copper begins to deposit electrolessly on the laminate.

33a 10 hours or after a copper deposition of 20 µm, the laminate is removed from the coating solution. It is observed that during the electroless coating reaction virtually no copper has been electrolessly deposited on the surface of the stainless steel vessel or stainless steel rack, in contact with the electroless copper deposition solution.

Example II

This example illustrates an embodiment of the invention in which two electrodes are used, ie working without a reference electrode. An electroless copper plating solution of the same composition as in Example 1 is placed in a vessel with stainless steel support walls. The vessel has a volume 2 of 8000 l and an internal surface of about 60 m. The rectifier is adjusted to obtain a potential of 0.1 * 5 V between the stainless steel cathode immersed in the coating solution and the stainless steel walls of the vessel. After this setting, it is observed that the walls of the vessel have a potential of -300 to -1 * 00 mV with respect to the silver / silver chloride reference and ie-20 electrode. 33a this initial measurement, the reference electrode is disconnected and removed. The initial current required to provide 0.1 * 5 V between the vessel walls and the steel cathode is 0.5 Amp, equivalent to -1 * 2 a current density of 10 mA / cm.

Six stainless steel racks, containing 300 suhstraat panels 2

25 each containing 0.18 m / rack are placed in the electroless copper plating solution. They are removed after the preset conductor pattern has been electrolessly formed, e.g. at intervals of 6-10 pm and replaced with new substrate to be coated. During the first 21 hours of the coating treatments, as coated substrates are removed and new substrate panels are introduced into the coating solution, a metallic copper-containing precipitate is observed to form in the solution. Part of this precipitate comes into contact with the surfaces of the coating vessel. The current required to maintain 0.1 * 5 V between the surfaces of the vessel and the steel cathode increases. During the next several working days, it is observed that the current required to supply 0.1 * 5 V.

800 2 5 15 22 rises and falls in the region of 2-100 Amp as further metallic copper precipitates and contacts the surfaces of the vessel and is passivated. At the end of h.v. the sterngear treatment is interrupted for a week. The copper precipitate, in contact with the interior surfaces of the vessel, consists of passivated, non-adhesive particles, which are easily removed by brushing or vacuum drawing.

Example III

In this example, the procedure as described in Examples I and II is followed except that the stainless steel racks are also connected to a second suitable rectifier and a second stainless steel electrode placed in the plating bath solution and the potential is set and maintained at approximately 0.00 - 0.5 V, measured between the rack and the second electrode, thus making the surfaces of the racks and in some cases the edges of the panels, which are coated with copper and in contact with the rack surface be made insensitive to electroless metal deposits.

800 2 5 15

Claims (11)

Method for maintaining electroless copper clad solutions and electroless deposition of copper from such solutions on substrates sensitive to electroless cladding of copper, surfaces of the metallic cladding installation being in contact with said solution characterized in that said solution is contacted with at least one counter electrode; and the metallic coating installation and the counter electrode (s) are connected to a source of electricity such that a potential is supplied to the surface of the coating installation which is sufficiently more positive than the mixed potential of said electroless copper cladding solution, in order to render these surfaces virtually completely resistant to electroless metal deposition; which source of electricity has provision for setting and maintaining said potential -J5 over the current region necessary to compensate for the electrons formed by oxidation of the reducing agent contained in the coating solution upon contact with the metallic surfaces of the coating plant and after copper particles have settled or deposited on these surfaces, thus making the surfaces of the said copper particles and of the coating plant substantially completely catalytically inactive and insensitive to electroless deposits. 2. A method according to claim 1 for electroless deposition of copper from a deposition solution with a known mixed 2? potential on a substrate sensitive to this deposition or to maintain such solutions in a metallic installation whose surfaces contact the said solution, characterized in that 1. an electric current is initially allowed flowing between the metallic surfaces of the coating installation in contact with the solution and a counter electrode, which current is sufficiently large to provide an electric potential on said installation surfaces which is sufficiently more positive than 80 0 2 5 15 2k the mixed potential of the solution to make the installation surfaces substantially completely "resistant to the formation of an adhering electroless copper deposit; 2. electroless copper is deposited on the substrate from the electroless copper plating solution or stored, and 3. electroless deposition of copper on the substrate or storing the solution, the supplied adjusts the current to the surfaces of the installation such that an electric potential is maintained which is sufficiently more positive than said mixed potential to withstand electroless copper deposition on the surfaces. A method according to claim -1 or 2, characterized in that the electroless copper deposition solution has a mixed potential in the range between -500 and -800 mV relative to a standard silver / silver chloride reference electrode and in the range between - 550 and -850 mV relative to a saturated calomel reference electrode. h. Method according to claim 1 or 2, characterized in that the electric potential applied and maintained on the surfaces of the metallic installation is in the range between -500 and +500 mY with respect to the reference electrode, and more is positive than the mixed potential. Method according to claim k, characterized in that the electric potential is in the range between -3Q0 and -1Q0 mV with respect to the reference electrode. A method according to claim 1 or 2, characterized in that the electric potential applied and maintained on the surfaces of the metallic installation is at least about 250 mY more positive than the mixed potential. The method according to claim 2, characterized in that 2 is the current in the range between 10 to k mA / cm area to be rendered resistant to electroless copper deposition. 8. A method according to claim 1, characterized in that the source of electricity is a power rectifier, the output voltage of which is set and maintained at a value selected for achieving the aforesaid potential on the surfaces of the output. 5 15 25 pound installation within the selected operating range of the supplied current. Method according to claim 8, characterized in that the power rectifier is the rectifier described in Figure 2. Method according to claims 1-8, characterized in that the coating installation comprises a coating vessel with metallic support walls, in which the deposition solution is maintained. 11. A method according to claims 1-9, characterized in that the coating installation comprises a rack supporting the substrate 10 to be coated, which rack is made entirely or partly of metal. Method according to claims 1 to 10, characterized in that the coating installation is made of steel. Method according to claim 12, characterized in that the coating installation is made of stainless steel. 15 1U. Method according to claims 1 - 13, characterized in that each separate part of the coating installation is connected to a separate source of electricity. 800 2 5 15