SE453925B - PROCEDURE TO AVOID UNUSED COPPER PROPOSALS BY POWERLESS COPPER PLATING - Google Patents

Description

453 925 10 15 20 30 35 2 availability. However, such vessels and equipment are in some respects largely subject to the same kind of disadvantages. Metals which are generally suitable for the construction of tanks and other coating equipment, for example steel, including stainless steel, are catalytically active for oxidation of the reducing agents present in the electroless copper plating solutions and thus initiate the formation of incipient copper coating on its surfaces, which coating in turn causes copper coatings to form continuously on the metal surfaces in contact with the electroless plating solutions. This problem is particularly acute when adjustable metal scaffolds, for example comprising metal fasteners, are used, since the fasteners are coated with electrolessly formed metal deposits, which makes it difficult to remove them.

It is known that cladding vessels of metallic retaining walls, as well as other cladding equipment, eg scaffolding, conductor components and the like, can be made Lemporarily resistant to electroless metal deposition by pretreatment with chemicals, eg nitric acid solutions, which renders the susceptible surfaces inactive for catalytic oxidation of the reducing agent present in the plating bath. However, these chemicals all tend to be nibbled within hours of operation and are therefore completely impractical for use in a manufacturing process.

US-PS 3 HZH 660 discloses that cladding vessels with metallic walls can be protected against electroless metal deposition, especially nickel coatings, by applying to them a potential having a value corresponding to the resting potential or protection potential range of the current density / potential curve. The current density is set to a maximum of approx. 10_u A / cm2.

German Offenlegungsschrift 26 39 2U7 describes that cladding tanks and scaffolding, made of a metal, eg cobalt or nickel, can be made resistant to electroless metal deposition, eg electroless copper plating, by charging with a current density of at least H mA / dm2.

It has also been proposed, in Japanese Patent Publication No. 54-56577, that a metal coating vessel, for example of 10 15 20 30 35 453 925 in 3 chromium nickel steels, may be made resistant to chemical plating if a positive electrical potential is applied to the surface of the coating vessel. during plating.

In commercial use, the methods described above are not satisfactory when applied to electroless copper plating. When the desired electroless copper plating reaction on the surface to be plated continues, spent plating chemicals in the solution must be renewed. The addition of chemicals usually causes local fluctuations in the concentration of the chemicals and also the addition of impurities to the solution. In addition, copper sprouts, which grow into copper particles, are formed in the solution.

These copper particles or dust as well as dirt in the plating solution and discrete particles of precipitated copper formed in the plating solution come into contact with the surfaces of the tank and other equipment. These copper precipitates in mechanical and electrical contact with the surfaces cause the flow of large current through the metallic plating equipment and cause, in the known methods, that the applied electrical potential on the equipment decreases and falls below that required to prevent oxidation. of the reducing agent, which known methods are thus unsuitable for keeping the surfaces free of electrolessly deposited copper coatings. An equivalent area of metallic copper requires at least two ten powers more current than stainless steel to withstand electroless copper plating.

It is assumed that this is due to significantly higher catalytic activity for copper, relative to steel, for oxidation of the reducing agent and thus the number of electrons generated.

Although the surfaces of the equipment in known processes are resistant to electroless deposition during the initial stage of the coating process, the surfaces of the equipment lose their resistance after a short time when attempts are made to carry out sensing. Thus, known methods have not been successfully adapted to industrial processes. industrial scale. _ application. 453 925 10 15 20 h) 01 30 35 Ä Additional problems often arise in the manufacture of objects which are to be plated while they are held in positions, for example printed printed circuit boards in particular. In such manufacturing processes, layers of copper, which do not form part of the circuit pattern itself, are sometimes formed on the edges of the insulating substrate. If these copper edges are in contact with the scaffold which supports the substrate, the need for current increases sharply, which reduces the electrical potential applied to the scaffold so that it ultimately falls below the minimum potential required to prevent plating when using known Methods and theories: The potentiometers used in known methods can withstand at most currents of one ampere, which indicates a lack of knowledge or understanding of the problems when plating on a large scale-adjacent problems, for example on an industrial scale.

It is an object of the present invention to provide a method for electroless copper deposition, wherein metallic plating equipment in contact with the coating solution is initially made resistant to electroless copper deposits and kept in a resistant state for a long time during the coating process.

It is another object of the present invention to enable the use of metallic coating equipment for long periods of time in industrial scale coating processes without building up copper coatings which must be removed by etching.

It is another object of the present invention to provide electroless copper plating processes in which copper deposits in contact with the coating equipment can be easily removed, for example by brushing, scrubbing, with vacuum and the like, substantially completely or completely without interrupting the plating operation.

It is another object of the present invention to provide a method of making printed circuit boards which avoids adhesive electrolessly deposited copper coatings on the surfaces of the equipment.

It is another object of the present invention to provide a method of making printed circuit boards in which undesired build-up of electrolessly deposited copper on edges and boundary areas of cards being plated is prevented.

These objects, as well as other objects which appear from the following detailed description, are achieved by the present method, which is now described.

It is noteworthy that the application of the present invention can lead to a reduction in plating costs of up to 30% or more, mainly due to the savings of plating chemicals and acids and neutralizing bases required to periodically etch. remove copper from surfaces of cladding tanks and other equipment and for recycling valuable components from used etching solution and waste treatment thereof.

Additional savings are made by avoiding labor costs and lost production time, which are associated with etching, recycling and waste treatment.

Without intending to limit the invention, some theories and observations are set forth which are believed to be crucial to the invention.

The oxidation of suitable reducing agents, which are present in electroless baths for copper plating, for example formaldehyde, on surfaces catalytically active for such oxidation processes generates electrons, which gives these surfaces a negative charge; plating of copper metal from solutions containing copper ions consumes electrons.

Copper ions, which are kept in solution by suitable complexing agents and come into contact with these charged surfaces, are therefore deposited as metallic copper, thus reducing the negative charge of the susceptible surface and leading to electroless deposition of a copper layer on the surface. The potential of the surface obtained by the two reactions is known as "mixing potential".

The surface of steel as well as stainless steel and other suitable materials, which are used for the tank and scaffolding, are sufficiently catalytic for oxidation of, for example, formaldehyde, to be made susceptible to electroless formation of copper deposits. By making the respective metal surface or metal surfaces part of an electrical circuit, which comprises a current source, it is not only possible to compensate for the charge formed on the metal surface as a result of the catalytic oxidation. of the reducing agent, but also to provide the surface with a layer of reduced or substantially non-existent catalytic activity for oxidation of the reducing agent. The result of such a process is that no copper deposit is formed on the surface or surfaces.

In practical application, electrically formed copper deposits are formed on all surfaces or objects which are susceptible to such deposition and are dipped in the electrolessly operating copper solution.

Statistically, a certain number of copper ions, reduced to copper (O) or copper (I) on the surface of the object being plated, will not be incorporated into the lattice of the coating but will flow back to the solution. Agglomeration produces pairs, which in themselves are catalytically active and thus subject to copper deposition on their surface. In addition, and caused by dirt and by local increase in the concentration of certain chemicals due to renewal of the plating bath solution, there is also a certain tendency for the formation of copper sprouts and particles in the interior of the solution of the bath solution. Copper sprouts and particles present in the bath solution adhere to metal surfaces, such as tank walls, scaffolding and other plating equipment, as well as forming a precipitate of particles on the bottom of the coating vessel.

When these precipitates are in contact with the metal surfaces of the coating equipment, these surfaces begin to receive copper coatings and after a short time they will behave in the same way as metal surfaces in contact with electroless copper plating baths, which generally leads to the previously stated, unwanted result.

Catalytic activity and thus the ability to oxidize the reducing agent is significantly higher on copper surfaces than the catalytic activity of, for example, stainless steel, which leads to electrons generated on relatively few copper particles in contact with, for example, the bottom of the coating vessel. 20 25 30 35 453 925 7 requires compensating currents from the current source, which currents are much higher than the currents required to passivate the stainless steel surfaces. Failure to follow the instructions given here will lead to a change in the potential of the metal surface to a sufficiently more negative value, so that an electroless coating forms on the surfaces.

According to the present invention, the supply of current to the metal surfaces is so dimensioned that it is possible to provide the required current for effectively compensating the electrons generated on the copper surfaces to maintain a surface potential sufficiently positive to suppress copper deposition on the surfaces. , and preferably provide a catalytically inert surface layer on both the metal walls of the coating vessel and the equipment and on the surface of copper particles in contact therewith, whereby no plating on walls or copper particles in contact with the walls takes place. This leads to the plating equipment being kept free of copper deposits and to the precipitated copper particles being prevented from returning to the respective metal surface, so that simple removal of the precipitates is possible.

The power source, also referred to as the power source, is according to the present invention dimensioned so that it holds the potential of the surfaces of the plating equipment at a value sufficiently more positive with respect to the mixing or plating potential to prevent electroless plating reactions from occurring on these metal walls; and also to supply a suitable current for the formation of catalytically inert layers not only on, for example stainless steel tank walls, equipment, but also on the surfaces of copper deposited on and in contact with these surfaces.

According to the present invention there is provided a method of accommodating solutions for electroless copper deposition and electrolessly depositing copper from these solutions on substrates which are susceptible to electroless plating of copper, the surfaces of the metallic equipment being in contact with the solution, which method comprises bringing the solution in contact with at least one counter electrode, connecting the metallic plating equipment and the counter electrode (s) to a current source, so that the plating equipment is provided with a potential sufficiently more positive than the mixed potential of the electroless copper plating bath solution to make surface substantially or completely resistant to electroless deposition of metal, which current source comprises means for regulating and keeping the potential within the range of current necessary to compensate for the electrons generated by oxidation of the reducing agent present in the plating solution. , upon contact with the metallic surfaces of the plating equipment before or after copper particles have settled or precipitated on the surfaces, the surfaces of the copper particles and of the plating equipment being made and kept substantially or completely catalytically inactive and not susceptible to electrolessly formed metal deposits. In one embodiment of the present invention there is provided a method of electrolessly depositing copper from a electrolessly operating copper plating solution on a substrate or containing these solutions, wherein surfaces of metallic plating equipment are in contact with the solution, which method comprises one (1) first applies a sufficiently more positive potential than the mixed potential of the electrolessly operating plating solution to the metal equipment to make the surfaces of the equipment substantially resistant to electroless copper deposition; (2) electrolessly deposits copper on the substrate from the electrolessly operating copper solution or stores the solution; and (3) during electroless deposition of copper on a substrate or storage of the solution on the metal surfaces holds a potential sufficiently more positive than the mixed potential to resist electroless deposition of copper.

The described method can be applied to all kinds of metallic equipment used in electroless copper plating, or storage of electrolessly operating copper plating solutions, which include plating vessels, scaffolds supporting the substrate to be plated, wires and other equipment details in contact with the plating solution.

The method described above is applied in the practical practice, in particular by applying to the metal equipment, for example plating vessels, scaffolding, a current sufficient to obtain an electrical potential of the surfaces of the equipment, which potential is sufficiently positive to withstand electroless copper deposition; and electrolessly deposits copper on the substrate, which is plated; and as the deposition progresses, the current supplies in an amount sufficient to maintain a desired, sufficiently positive electrical potential on the surfaces of the metallic equipment to resist electroless plating. Preferably, a current is applied in the range between 10- "and H mA / cm2 of the surface area, which is to be made resistant to electroless coating.

For example, the present process is carried out by providing at least one cathode in the electroless copper plating solution and which is in electrical connection with the surfaces of the plating equipment via a current source. Then a current is applied to the circuit, which is closed by the plating solution to form an electric potential on the surface of the equipment, which potential is sufficiently more positive than the mixed potential of the plating bath solution, to resist the formation of an adhesive copper coating on the surface of the equipment. This applied current is regulated during the electroless copper deposition to keep the electrical potential of the equipment surfaces at desired levels, which are sufficiently positive, to render the surfaces as well as the surfaces of copper particles deposited on these surfaces substantially inactive and not susceptible to electrolytically formed deposits.

The term "mixing potential", as used herein, is intended to denote the electrical potential at which copper begins to be deposited without current from a non-current copper plating solution on a susceptible surface with which it is in contact. In other words, it is the electrical potential, measured between a suitable metallic substrate, which is electrolessly coated with copper, and a standard reference electrode in electrical connection with the substrate, which is a method of measuring the mixing potential of electroless copper plating solutions. Such a method is described immediately before the examples. 10 15 25 30 35 455 925 10 In general, electroless copper plating solutions which can be used in the present process are characterized, for example, by a mixing potential in the range between -500 and -800 mV relative to a standard silver / silver chloride electrode and in the range between -550 and -850 mV relative to a saturated calomel electrode, measured at operating temperature of the bath solution.

In carrying out the present electroless copper plating process, electrical potentials of -500 to +500 mV and usually of -300 to -100 mV relative to the reference electrode are typically applied and maintained to the walls of the coating vessel and / or other metallic plating equipment in contact with the bath solution. These potentials are sufficient to passivate the surfaces of the equipment as well as the copper that may be in contact with them, such as precipitated copper, copper stripes on the cards being plated, and the like.

The principles of the present invention are used to advantage to make mainly or completely metallic scaffolds which support substrates plated in the plating solution non-susceptible to electroless copper deposition. In particular, these racks are electrically connected to a connection of a current source, for example a rectifier, dimensioned to achieve the previously defined potential within a large current range, for example up to 200 A, and the second connection of the current source is connected to a cathode immersed in the solution. A current sufficient to constitute a passivating electrical potential on the scaffold is applied, copper is deposited without current on the substrate supported by the scaffold, and during deposition the current is regulated to keep the scaffold in its passivated state and is not receptive. for electrolessly formed copper off- For be- there are settlements. Although the same power source may be used the laying vessel or other plating equipment, it is preferred to use a separate power source for each piece of equipment to be made and kept non-receptive; The described technique can also be used to prevent electroless deposition of copper on unwanted areas of the substrate, which are plated. Especially in the manufacture of printed printed circuit boards using additive techniques, in some cases the insulating boards, which have been pre-cut to size, are provided with a mask and sensitized, with an exposed sensitized stripe. on the edge of the card and the adjacent card surface. When the card is in contact with the plating solution, copper begins to precipitate, together with copper coating on desired areas, on the edges of the sensitized card and sensitized surface adjacent thereto to the same thickness as on the exposed areas of the card, thus forming a continuous strip of cups on the card. In typical cases, the cut corresponds to the desired circuit pattern. pes this copper strip, which is not part of the circuit pattern itself, away from the rest of the card and discarded. The build-up of this copper edge can be prevented by bringing the edge of the card into contact with a metal surface of the scaffold and by applying suitable current and holding on the scaffold to make both the scaffold and the copper edge of the card at least substantially impervious to electroless deposition.

Plating vessels as well as other metallic plating equipment used in the present process can be cleaned of any deposited copper deposits which may adhere to the surface, such as copper particles which have fallen to the bottom of the coating vessel, by temporarily interrupting the plating, emptying the plating vessel and removal of the copper precipitate by brushing, scrubbing or by means of negative pressure. Such a cleaning can also be used during plating without emptying the plating vessel, for example by means of a vacuum. It should be noted that copper according to the present process, in contrast to the prior art, does not adhere to the surfaces of the passiviscred equipment even after long operating time, and this copper can be easily removed by the above-mentioned cleaning methods and without the need for etching or other aggressive chemical or mechanical purification methods. In carrying out the present process, it is usually found that a thin layer of electrolessly deposited copper is deposited on the cathode or cathodes. Copper coating on the cathode surfaces can be completely or partially avoided by arranging a membrane between the cathode and the electrolessly operating copper plating solution, which allows electrical conduction between the cathode and the plating solution but prevents the passage of copper ions from the plating solution.

Ion exchange membranes, either anionic or cationic, can be used for this purpose. The choice of the special ion exchange membrane depends on the special complexing agent used in the bath for copper ions. In cases where the complexed copper has a negative charge, as is the case when the complexing agent is of the amino acid type, cation exchange membranes can be used. In cases where the complexed copper has a positive charge, anion exchange membranes are used. If the complexed copper is neutral, such as when alkanolamine complexing agents are used in the bath, either an anion or cation exchange membrane can be used.

The present method is effective in the use of plating and storage vessels in which the walls or surfaces of the walls are in contact with the bath and consist of non-precious metals, for example steel, iron, nickel, cobalt, titanium, tantalum, chromium or the like. and if desired, copper. The process can be used to make other types of plating equipment of these metals resistant to adhesive electrolessly formed copper coatings.

The pH of the electroless plating solution is usually at least 10 and preferably 11 or higher.

Pig. 1 shows a simplified plating system which can be used to practice the present invention, and comprises a plating tank with metallic walls, plating solution, current source, electrodes, substrate to be coated, and supporting means.

Pig. 2 shows a more detailed system, adapted for automatic control, which system is useful in the practical practice of the present method.

Pig. 3 graphically shows the current as a function of the potential (voltage) for stainless steel in a solution for electroless copper deposition.

Pig. H graphically shows the current as a function of the potential (voltage) for copper in a solution for depositing copper copper with the same composition as in Fig. 3. The plating tank 2 in Fig. 1, whose walls are in front made of steel, preferably of stainless steel, or other suitable electrically conductive material, contains a current-free working copper plating solution H. A metal electrode 6 is immersed in the plating solution H and electrically connected to the negative connection 8 by a direct current source 10. variable resistor 16 to the positive terminal 18 of the power source 10.

The surface 12 of the tank 2 is electrically connected via a One millivoltmeter 20 is also connected to the wall 12 and to a standard reference electrode 22. The workpiece 2H is supported in a metal stand 26. The stand 26, in electrical contact with the surface 12a of the tank 2, is immersed in the plating solution U. The workpiece 2H is electrically insulated from the stand 26 by a non-conductor (insulator) 27.

Before the operation of the plating solution is started (eg by adding reducing agent, or by raising the pH or temperature), a potential, which is more positive than the expected mixed potential of the working plating solution, is applied to surfaces 12a and 12b of the tank 2 by position as required by the resistor 16. The composition and condition of the plating solution 4 are set in a known manner for starting the electroless plating (eg by adding reducing agent, raising the pH, raising the temperature). The workpiece 2H is dipped in the plating solution H and the plating begins. The electrical potential of the surface 12 with respect to the reference electrode 22 is sensed during plating by observing the millivoltmeter 20, and this potential is kept more positive than the mixed potential of the plating solution H. This potential can be controlled manually, as in the embodiment of FIG. 1 or automatically, as shown in Fig. 2.

In Fig. 2, a 200 V AC line 28 goes to a DC source 30, at 7 V. The negative terminal 32 of the current source 30 which can, for example, generate a current of 200 A is electrically connected via a line 3H to electrodes 36 which are immersed in a metal tank 38. The tank 38 contains plating solution 40 and is grounded via line H2. The positive terminal HU of the current source 30 is electrically connected via line 10 to 455 453 925 14 96 to pass transistors 48, which are connected in parallel and driven by a Darlington power transistor 50. Each pass transistor 48 preferably has a output capacity of 50A. The Darlington power transistor 50 is preferably set to a gain of about 10,000: 1.

The pass transistors H8 are connected via the electrical line 52, the measuring shunt SN and the electrical line Sßanshnnatí fl tank 38. The measuring shunt SU is connected via the line 58 to a standard ammeter 60, which measures the current from the pass transistors over the measuring shunt 5H. The capacitor 62, preferably with a capacitance of 2 UF, is connected via the electrical line 3H and the measuring shunt SH for reduction of electrical background noise.

The electrical lead BH emanates from the tank 38 and is connected to the positive terminal 7H of the voltage amplifier 68. An electrical lead 70 extends from the standard reference electrode 72 and is connected to the negative terminal 66 of the amplifier 68. The amplifier is set to a 10: 1 gain. The reference electrode 72 is a conventional silver / silver chloride electrode which is connected via a salt bridge to the plating solution H0 in the tank 38. the amplifier 68 is connected via an electrical wire 76 to the negative connection terminal 78 of the control amplifier 80. The output voltage from the amplifier 68 to the amplifier 80 is measured over a standard voltmeter 82, connected with the line BH to the line 7H. The positive connection terminal 86 of the control amplifier 80 is connected with the line 88 to potentiometer (set point) 90, and to PET switch 92. The potentiometer preferably has a possible maximum setting of + 3V to -2V.

The electrical leads 9h and 96 run from the terminals 98 and 100, respectively, of a measuring shunt SH to a voltage amplifier 102. The lead 94 is connected to the positive terminal 10H of the amplifier 102. The lead 96 is connected to the negative terminal 106 of the amplifier 102. The output voltage from the amplifier 102 goes through the line 108 to the positive terminal 110 of the control amplifier 112¿ 10 15 20 25 30 35 453 925 15 The amplifier 112 is set to a gain of 20: 1.

The negative terminal 11% of the amplifier 112 is connected to the potentiometer (set point) 116. The electrical line 118 goes from the amplifier 102 to the FET switch 92.

A capacitor, preferably with a capacitance of 1 uF, and a resistor, preferably with a resistance of 1 ingår are included in the circuit for reducing the background noise.

To prevent overheating, the pass transistors H8, the power transistors 50, the capacitors 120 and 62, the resistor 122 and the ammeter 60 are arranged on a heat receiver 12H (marked with dashed lines) and cooled with a fan 126, connected to a 110V AC line 128: The heat recipient 124 is made of aluminum or other standard material that absorbs heat.

In practice, the present invention is carried out as follows: Fig. 2 shows that an alternating current of 220V in a line 28 is transferred to direct current in a direct current power amplifier 30.

Negative potential from the power amplifier 30 is applied to electrodes 30 in the tank 38. The electrodes 36 are thus cathodic. Positive potential from the power amplifier 30 passes through the power transistor 50, the pass transistor H8, the line 52 the measuring shunt SU and the line 56 to the tank 38. The tank 38 is thus anodic. The current across the measuring shunt 54 is measured using an ammeter 60.

A silver / silver chloride electrode 72 (reference electrode) is immersed in the tank 38 and is kept in communication with the plating solution in the usual manner by means of an intermediate porous membrane. By connecting the silver / silver chloride electrode 72 and the tank 38 to opposite connections of the amplifier 68, in the manner shown, the potential (voltage) of the walls 38 of the tank 38 is continuously registered and is further controlled as follows: Um the voltage from the amplifier 68 for controlling the amplifier 80 is predominantly positive, the amplifier 80 will emit a positive voltage. On the other hand, if the voltage from the amplifier 68 for controlling the amplifier 80 is predominantly negative, the amplifier 80 will emit a negative voltage. A positive voltage to the power transistor 50 leads to the latter generating a current flow. A negative voltage to the power transistor 50 leads to the switch-off of the latter and essentially that current ceases to flow. During plating, when the potential (voltage) of the tank 38 becomes less negative (ie, more positive) relative to the reference electrode 72, the amplifier 68 provides a positive voltage to the control amplifier 80, which in turn provides a negative voltage to the power transistor 50. By controlling the set point 90, the positive output voltage from the control amplifier 80 is regulated as needed to set the total output signal from the amplifier 80 and to obtain the desired current flow to the tank 38 required to hold the potential (voltage). ) of tank 38 on the setpoint potential, which is more negative than the mixed potential of solution H0.

Excess current is prevented from flowing to the tank 38 by means of the voltage amplifier 102 and the control amplifier 112.

The potential (voltage) across the measuring shunt SU is directly proportional to the current flow from the power transistor 50 and the pass transistors H8. This voltage is amplified in amplifier 102 and further amplified in control amplifier 112. If the amplified voltage from amplifier 112 to the gate of FET 92 rises above the cut-off point, the FET switch 92 opens, reducing the voltage dividing the output of potentiometer 90, amplifier 80. the system returns to equilibrium.

Amplifier 10? output signal is balanced by the output signal of the potentiometer 116, which determines the set point of the amplifier 112. Changing 116 determines the maximum allowable current before reducing the control point 80's set point. The function of amplifiers 102 and 112 is to limit the maximum current supplied to the tank and electrodes to protect the entire system.

In the manner described, the voltage applied to the tank 38 is kept more positive than the known mixed potential of the plating solution H0; as a result, substantially no metal is deposited on the 38 walls of the tank. In the process described, metallic scaffolds can be used to support the substrates to be plated, and these scaffolds can be made resistant to electroless copper plating using the methods described above. the principles. In these cases, it is desirable to use a separate control circuit for supplying power to the racks. If the substrates supported by the racks which are plated are short with copper edges, a larger power source is required to maintain the passivating electrical potential of the racks and the copper edges of the board. If, on the other hand, it is desirable to plate the entire substrate or if the copper edges of the card form part of or are connected to the circuit pattern, it is desirable that the substrate be insulated from the stand by placement between them of a substantially electrically non-conductive material (see Fig. 1).

Pig. 3 and R show the current as a function of the voltage for copper and stainless steel in an electroless copper plating solution with the composition given in Example 1. Positive currents are oxidizing currents and negative currents are reducing currents, ie plating currents. At point "B" in Fig. 4 üagmar electrode, area = 0.341 cng) there is no net current flow, this potential is known as the mixing potential of the plating solution.In area "A" more copper ions are reduced than the present amount of reducing agent, so here formaldchyd In the region "C", more formaldehyde is oxidized than copper ions are reduced, so that there is a positive (oxidizing) net current here. film on the surface of the copper electrode This film is non-catalytic for the oxidation of formaldehyde The maximum current required for passivation has been set to H mA / cm2 Reduction of copper ions does not occur at potentials that are more positive than about -H50 mV relative to the reference electrode or 250 mV more positive than the mixed potential The range "E", which ranges from about -H25 to -225 mV relative to the reference electrode, is called the passivation interval. In this interval the potential is too anodic for reduction of copper ions and the electrode surface are non-catalytic for oxidation of formaldehyde, so there is a small current flow here. Since the current in this region is as large for the solution without formaldehyde as it is for the solution with formaldehyde, it is assumed that the current flowing in this region is not mainly due to oxidation of formaldehyde.

The current flowing in the range "F" depends on oxidation and partial dissolution of the electrode surface. The "G" area is a second passivation area. Beyond the range "G", several constituents can be oxidized, OH-, EDTA, Cu or HCHO.

Pig. 3 shows that a stainless steel electrode (area = 0.472 cm2) is relatively passive from -500 to +400 mV. At more negative potentials than -500 mV, copper deposition on the stainless steel surface begins, whereby the properties of the electrode change. At -325 mV, the current density H0 is less for stainless steel than for copper (0.020 vs. 0.80 mA / cmz). Stainless steel is very slow in its initiation of plating in an electrolessly operating copper plating bath. This is because it shows relatively poor catalytic activity for the oxidation of formaldehyde. However, when plating is initiated, it occurs rather rapidly, since on the surface of stainless steel formed copper coating gives the surface high catalytic activity for oxidation of the reducing agent and thus generation of electrons.

A potential of about -325 mV (against saturated calomel electrode) is best for passivation of both stainless steel and the range of 'for stainless steel copper, as it is in the middle of copper passivation and the current density is very low at this potential.

The passivation interval can be shifted slightly with pH changes. The displacement takes place in the same direction as the change in the mixed potential of the solution with the pH change and is of the same order of magnitude. In one embodiment of the invention, therefore, a mixed potential probe is used as the respective reference electrode.

For the preparation of Figs. 3 and U, a polarographic analyzer model 17HA (Princetown Applied Research) was used to measure the potential values and the reference electrode for all measurements was a saturated calom electrode. The current was measured during the sweep of the potential under an air atmosphere and recorded on an X / Y printer. sweep speed 2 mv / S. To measure the mixing potential, a clean copper surface was placed in the electroless copper plating solution. Metal began to deposit on the copper surface. You waited for 3-H minutes to set a steady state. The copper surface was connected to a connection terminal of a millivolt meter with high impedance, eg one used in standard pH meters. A standard reference electrode, immersed in the bath, was connected to the second terminal of the millivolt meter. To determine the mixed potential of the copper plating solution, the potential difference between the copper surface and the reference electrode was measured.

The invention is illustrated by the following examples, which are not intended to limit its scope. In Example 1, an epoxy glass laminate having a thickness of 1.6 mm was manufactured in the usual manner for the production of a printed printed circuit board using electroless copper deposition.

The prepared laminate was immersed in an electroless copper plating solution with, for example, the composition: CuSOu-5H 2 O 10 g / l formaldehyde H ml / l wetting agent 0.2 g / l tetrasodium salt of EDTA 35 g / l light film hyapoxy (Neon) to pH 11.7 (measured -f at 2s ° c> sodium cyanide (NaCN) 0.005 g / l water to volume working temperature 72oC The copper plating solution according to this example has a mixing potential of -630 t silver / silver chloride electrode 20 mV, measured against a standard All components of the specified plating solution except formaldehyde A stainless steel cathode was dipped in the solution and connected to the negative terminal of a variable DC rectifier with a maximum capacity of 8V and ZUOA A standard silver / silver chloride electrode was immersed in the plating solution and a connection of a millivoltmeter. The other end of the voltmeter was connected to a wall by men. "10 15 20 25 30 35 453 925 20 The electrical potential p on the wall of the plating vessel relative to the reference electrode was set to -200 mV control of the rectifier. The electroless copper plating solution was started by adding the formaldehyde.

The epoxy glass laminate, which had been pretreated as described and supported in a stainless steel stand, was immersed in the electroless copper plating solution. Electroless deposition of copper on the laminate begins. After 10 hours, or when a coating of 20 μm has been obtained, the laminate was removed from the plating solution. It is observed that substantially no copper has been electrolessly deposited on the stainless steel vessel surface and the stainless steel surface of the scaffold in contact with the electroless copper plating solution during the electroless plating reaction.

Example 2 This example illustrates an embodiment of the invention using only two electrodes, ie without a reference electrode.

An electroless copper plating solution with the same composition as in Example 1 was placed in a vessel with stainless steel walls. The vessel has a capacity of 8000 l and an internal surface area of about 60 m2. The rectifier was set to obtain a potential of 0.1 H5 V between the stainless steel cathode immersed in the plating solution and the stainless steel vessel walls. After this setting, it is observed that the vessel walls have a potential of -300 to -H00 mV relative to the reference electrode (Ag / AgCl). After this measurement, the reference electrode was disconnected and removed. The initial current required to obtain 0.5 H5 V between the vessel walls and the steel cathode A / cm.

Six stainless steel racks, each up to 0.5A, which is equivalent to a current density of 10_u bar 300 backing boards with an area of about 0.18 m2, were placed in the electroless copper plating solution. They were removed after electroless formation of a predefined wiring pattern, for example at intervals of 18 - 22 hours, and replaced with new substrates. During the first 24 hours of plating, when plated substrates were removed and new cards were inserted into 10 15 20 453 925; 21 plating solution, it was observed that a precipitate of metallic copper is formed in the solution. Parts of the precipitate come into contact with the surface of the plating vessel. The current required to maintain 0, H5 V between the vessel surfaces and the steel cathode increases. During the following operating days, it was observed that the current required to maintain 0, H5 V increases and decreases in the range from 2 to 100A when more precipitation of metallic copper comes in contact with the walls of the vessel and is passivated.

At the end, for example after one week, the plating process is interrupted. The copper precipitate, which is in contact with the inner surfaces of the vessel, consists of passivated non-adherent particles, which are easily removed by brushing with a brush or by means of a vacuum.

Example 3 In this example, the procedure described in Example 1 or 2 is followed except that the stainless steel racks were also connected to a second, suitable rectifier and a second stainless steel electrode was placed in the plating bath solution and the potential was adjusted to and maintained at approx. 0, H - 0.5 V measured between the stand and the other electrode, making the surfaces of the stands and in some cases the edges of the cards, plated with copper and in contact with the surface of the stand, non-susceptible to electroless metal deposition.

Claims (7)

10 15 20 25 30 35 453 925 22 PATENT REQUIREMENTS A method for avoiding undesired copper deposits from electrolessly plating copper plating solutions and for electrolessly depositing copper from such a solution on a substrate susceptible to electroless copper plating, wherein surfaces of metallic plating equipment are in contact with the solution, these surfaces being connected to a counter electrode a current source, so that these surfaces are made completely or substantially resistant to electroless copper deposition, characterized in that said current source is a rectifier, the output voltage of which is set to and maintained at a value selected so that on the metallic equipment the applied and held potential is more positive than the mixed potential of the copper solution measured at the same time, which is in the range between -500 and -800 mV in relation to a standard silver / silver chloride reference electrode, and wherein the applied and held the potential is more positive than -500 mV, and is in particular between -500 and +500 mV relative to refer single electrode. 2. A method according to claim 1, characterized in that the electric potential is in the range between -300 and -100 mV relative to the reference electrode. 3. A method according to claim 1, characterized in that the electrical potential applied and held on the surfaces of the metallic equipment is at least 250 mV more positive than the mixed potential. 4. A method according to claim 1, characterized in that the current is in the range 10-4 - 4 mA / cm 2 surface area, which is to be made resistant to electroless copper deposition. 5. A method according to any one of claims 1 - 4, characterized in that the equipment comprises a plating vessel with metallic walls, in which the plating solution is held, and a completely or partially metal scaffold carrying the substrate to be plated. Method according to one of Claims 1 to 5, characterized in that the plating equipment consists of steel, in particular stainless steel. 7. A method according to claims 1 - 6, characterized in that each separate piece of the plating equipment is connected to a separate power source. and F: