GB2052560A - Anodic Passivation of Electroless Plating Equipment - Google Patents

Abstract

The metallic surfaces of plating equipment in use during electroless copper deposition, such as vessels 2, racks 26 supporting substrates 24 to be plated, plumbing are rendered substantially resistant to electroless copper deposition for extended periods by initially imposing on such equipment surfaces an electrical potential via power supply 10 more positive than the mixed potential of the electroless copper deposition solution and sufficiently positive to resist electroless deposition. The workpiece substrate 24 is insulated from rack 26 by non-conductor 27. The electrical potential supplied by electrode 6 and the vessel walls 12a, 12b, is monitored during the electroless plating using reference electrode 22. <IMAGE>

Description

SPECIFICATION Passivation of Metallic Equipment Surfaces in Electroless Copper Deposition Processes Electroless metal deposition baths, e.g., electroless copper solutions, are known in the art as useful for providing metal deposits on nonmetallic and metallic surfaces. Such solutions are characterized by the capacity to deposit metal in virtually any desired thickness without the need for supplying electrons from an external source of current. After a metal deposit is formed electrolessly on the surface of the article, the electroless plating process becomes autocatalytic, i.e., continues to deposit metal on said surface so long as the solution is replenished and maintained.

Special mention is made of the use of electroless metallizing procedures in the plating of plastics generally, and in the manufacture of printed circuit boards particularly.

When electroless plating operations of the foregoing type are practiced on a commercial scale, typically, large plating vessels are employed. The parts or articles to be plated are immersed in the copper deposition solution in the plating vessel. The parts or articles are, in general, supported on racks or frames immersed in the deposition solution. It has been found, in practice, that if the plating vessel and support racks are constructed of plastic materials or glass, copper particles forming in the electroless copper plating solution during the plating reactions become attached to such equipment surfaces. The attached copper particles provide sites for further electroless copper deposition and growth.

Eventually, most or substantially all of such equipment surfaces become covered with a deposit of electroless copper. This process uses up plating bath constituents. Furthermore, the plating operation must be interrupted from time to time to empty the tank and etch away the copper from the equipment surfaces. This etching procedure is disadvantageous in that the plating operation must be stopped, and large amounts of expensive etching, reclamation and waste treatment chemicals are required.

A further disadvantage is that the etching procedure weakens the structure of the nonmetallic plating vessels, racks, etc. and shortens their useful life.

The use of plating vessels and other plating equipment made of metallic material would be desirable because of greater durability and normally greater availability. Such vessels and equipment are however, and, in certain respects, even to a higher degree subject to source of the same disadvantages. Specifically, metals generally suitable for the construction of tanks and other plating equipment. e.g., steel including stainless steel, are catalytically active to the oxidation of the reducing agent present in the electroless copper plating solutions thus readily initiating the formation of an initial copper deposit on its surfaces which, in turn, causes copper deposits to continuously form on metallic surfaces in contact with the electroless deposition solution.This problem is particularly acute where adjustable metal racks, i.e., containing metal fasteners, are used because the fasteners become coated with an electrolessly formed metal deposit thus making difficult their release.

It is known in the art that plating vessels made of metallic retaining walls, as well as other metallic plating equipment such as racks, plumbing and the like, can be rendered temporarily resistant to electroless metal deposition by pretreatment with chemical, e.g., nitric acid solutions, thus rendering the respective surfaces inactive for the catalytic oxidation of the reducing agent present in the plating bath. Such chemical treatments tend, however, to wear off within hours of operation and are, therefore, completely impractical for use in a manufacturing operation.

U.S. patent 3 424 660 disclosed that plating vessels having metallic retaining walls can be protected against electroless metal deposition, particularly nickel platings, by imposing a potential thereon at a value corresponding to the rest potential or the protection potential range on the current density/potential curve. The current density is adjusted to not more than about 10-4 amperes per cm2.

German Offenlegungsschrift 26 39 247 discloses that plating tanks and racks made of a metal such as cobalt or nickel can be rendered resistant to electroless metal deposition, such as electroless copper deposition, by charging with a current density of at least 4 milliamperes per dm2.

It has also been proposed, in Japanese Patent Publication 54-36577 dated November 9,1 979, that a metal plating vessel, such as of chromiumnickel steel, can be rendered resistant to chemical plating of a positive electrical potential is applied to the plating vessel surface during the plating operation.

In commercial practice, procedures such as the foregoing have not proved satisfactory when applied to electroless copper plating processes.

As the desired electroless copper deposition reaction on surfaces to be plated proceeds, plating chemicals used up in the solution must be replenished. The chemical replenishment usually causes local fluctuations in the concentrations of chemicals and further the introduction of impurities into the solution. Furthermore, copper sites growing into copper particles form in the bulk of the solution. Such copper particles or dust as well as dirt in the deposition solution and discrete particles of precipitated copper formed in the deposition solution come into contact with the tank and other equipment surfaces.Such copper precipitates in mechanical and electrical contact with said surfaces cause the flow of high current through said metallic plating equipment thus, in the prior art processes, causing the electrical potential imposed on such equipment to dscay and fall below that needed to inhibit the oxidation of the reducing agent and thus unsuitable for keeping said surfaces free of copper deposits electrolessly formed on them. An equivalent area of metallic copper requires at least two orders of magnitude more current than stainless steel to resist electroless copper deposition.

It is assumed, that this is caused by the substantially higher catalytic activity of copper if compared to steel - for the oxidation of the reducing agent and, thus, number of electrons generated.

Thus, although the equipment surfaces in prior art processes are resistant to electroless deposition during the initial stages of the plating operation, when attempts are made to operate such prior art processes on a commercial scale, after a brief period of time the equipment surfaces lose their resistance. Thus, prior art procedures have not been successfully adapted to commercial use.

An additional problem is often encountered in the manufacture of articles to be plated while held in plating racks, e.g., printed circuit boards, in particular. In such manufacturing procedures, sometimes layers of copper which are not part of the circuit pattern itself are formed on the borders of the insulating substrate. If such copper borders contact the metal racks supporting the substrate, the demand for current is drastically increased, thus diminishing or decaying the electrical potential applied to the rack such that it eventually falls below the minimum potential required to avoid plating, when using the prior art procedures and teachings.The potentiostats of the prior art processes are capable of maximum currents of no greater than one ampere, indicating a lack of knowledge or understanding of the problems encountered in plating on a larger scale, such as in commercial operation.

It is an object of this invention to provide a process for electroless copper deposition in which metallic plating equipment in contact with the deposition solution is rendered initially resistant to electroless copper deposits and maintained in a resistant state for extended periods of the plating operation.

It is another object of this invention to enable use in commercial scale deposition processes of metallic plating equipment for extended periods without the build-up of copper deposits which must be removed by etching.

It is another object of this invention to provide electroless copper deposition processes in which copper precipitates in contact with the plating equipment can be easily removed, as by brushing, sweeping, vacuuming, and the like, substantially or completely without shutting down of the plating operation.

It is another object of this invention to provide improved methods of printed circuit board manufacture in which adherent electroless copper deposits on equipment surfaces are avoided.

It is another object of this invention to provide a method for the production of printed circuit boards in which undesired build-ups of electroless copper on the edges and borders of panels being plated is prevented.

The foregoing objects as well as additional objects which will be clear from the following description are achieved by the process of the invention now described.

Notably, the practice of this invention can result in a reduction in the cost of plating of up to 30% or more, due principally to savings on plating chemicals and acids and neutralizing bases needed to periodically etch copper away from plating tanks and other equipment surfaces and for the reclamation of valuable constituents from the used etchant and waste treatment of the same.

Additional savings are achieved due to avoidance of labor costs and lost production time necessitated by such etching, reclamation and waste treatment procedures.

Without the intent to limit the invention in any way, it is believed that it is based on the following inventive concepts and observations: The oxidation of suitable reducing agents, present in electroless copper plating baths, e.g., formaldehyde, on surfaces catalytically active for such oxidation reaction produces electrons thus charging such surface negatively; plating of copper metal out of solutions comprising copper ions uses up electrons.

Copper ions; kept in solutions by suitable complexing agents and coming in contact with such charged surface are, therefore, plated out as metallic copper thus reducing the negative charge of the respective surface and resulting in electroless deposition of a copper layer on said surface. The potential of the surface resulting from 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 to the oxidation of, e.g., formaldehyde, to be rendered receptive to the electroless formation of copper deposits.

By making the respective metal surface or surfaces part of an electric circuit comprising a source of electricity it is not only possible to compensate for the charge formed on said metal surface as the result of the catalytic oxidation of the reducing agent, but also to provide the surface with a layer of reduced, or substantially nonexistant, catalytic activity for the oxidation of said reducing agent. The result of such procedure is, that no copper deposit forms on said surface or surfaces.

In practical use, electrically formed copper deposits are produced on all surfaces or articles receptive to such deposition and immersed in the eiectroless copper solution.

Statistically, a certain number of copper ions reduced to copper(0) or copper(l) on the surface of the article to be plated is not incorporated into the lattice of the surface deposit, but floats back into the bulk of the solutions. Agglomerations form particles which, in themselves, are catalytically active and thus subject to copper deposition on their surface. Furthermore, and caused by dirt and by the local increase of concentration of certain chemicals as the results of replenishing of the plating bath solution, there is also a certain tendency to form copper sites and particles in the bulk of the plating bath solution.

Copper sites or particles present in the bath solution attach themselves to metal surfaces, e.g., tank walls, racks and other plating equipment as well as forming a precipitate of such particles at the bottom of the plating vessel.

With the occurrence of such precipitation in contact with the metallic surfaces of plating equipment these surfaces start accepting copper deposits and after a short period of time behave in the same manner as metal surfaces exposed to electroless copper plating baths and in general do thus cause the above described, undesirable results.

Cataiytic activity and thus efficiency of the oxidation of the reducing agent on copper surfaces is substantially higher than the catalytic activity of, e.g., stainless steel, thus causing that the electrons generated on relatively few copper particles being in contact with, e.g., the bottom surface of the plating vessel, demand compensating currents, from the source of electricity mentioned above, which are drastically higher than the currents needed for passivation of the stainless steel surface. Except if one follows the teachings of the present invention, this results in a change of the potential of the metallic surface to sufficiently more negative values for causing the formation of an electroless deposit on said surfaces.If, however, in accordance to the teachings of the present invention, the supply of electricity to the metal surfaces is dimensioned in such way that it is able to provide the current necessary for efficiently compensating for the electrons generated on said copper surfaces to maintain a surface potential sufficiently positive for suppressing the copper plating on the said surfaces and, preferably, providing a catalytically inactive surface layer on both the metal walls of the plating vessel and equipment as well as the surface of copper particles in contact with them, no plating on walls or copper particles in contact with said walls occurs. This results in the plating equipment to be kept free of copper deposits and the copper particles precipitated being kept from backing to the respective metal surface thus allowing easy removal of said precipitates.

The supply of electricity, also referred to herein as power supply, is dimensioned, in accordance with this invention, in such way that it maintains the potential of the plating equipment surfaces at a value sufficiently more positive with respect to the mixed- or plating-potential for inhibiting the electroless plating reactions from occurring on such metal walls; and, furthermore, to supply a current adequate for forming catalytically inactive surface layers not only on, e.g., stainless steel tank walls, equipment surfaces etc., but also on the surfaces of copper precipitated and in contact with such surfaces.

According to this invention, there is provided a method for containing electroless copper deposition solutions and for electrolessly depositing copper from such solutions on substrates receptive to electroless plating of copper in which surfaces of metallic plating equipment are in contact with said solution, said method comprising contacting the said solution with at least one counter-electrode; and connecting the metallic plating equipment and said counter-electrode(s) to a source of electricity so that the surface of the plating equipment is provided with a potential sufficiently more positive than the mixed potential of said electroless copper plating bath solution to render said surface substantially or completely resistant to electroless metal deposition, said source of electricity having means for adjusting and maintaining said potential over the range of current necessary for compensating the electrons generated by the oxidation of the reducing agent present in the said so!ution when contacting said metallic surfaces of said plating equipment prior to and after copper particles have been settling or precipitating on said surfaces thus rendering and maintaining the surfaces of said copper particles and of said plating equipment substantially or completely catalytically inactive and not receptive to electrolessly formed metal deposits.

In one of the embodiments of this invention, there is provided a method for electrolessly depositing copper from an electroless copper plating solution on a substrate, or for containing such solutions, in which surfaces of metallic plating equipment are in contact with the said solution, said method comprising (1) initially imposing on the metallic equipment a potential sufficiently more positive than the mixed potential of the electroless copper solution to render the equipment surfaces substantially resistant to electroless copper deposition; (2) electrolessly depositing copper on said substrate from said electroless copper solution or storing said solution; and (3) while electrolessly depositing copper on said substrate or storing said solution, maintaining on the metallic surfaces a potential sufficiently more positive than said mixed potential to resist electroless copper deposition.

The described process can be applied to any type of metallic equipment used in electroless copper deposition operations or for the storing of electroless copper plating solutions, including plating vessels, racks supporting the substrate being plated, plumbing or any other piece of equipment in contact with the said plating solution.

The foregoing process is practiced more specifically by initially imposing on the metallic equipment, e.g., plating vessel, rack, etc., a current sufficient to establish an electrical potential on the surfaces of said equipment sufficiently positive to resist adherent electroless copper formation; and electrolessly depositing copper on the substrate being plated; and, as the deposition reaction proceeds, provide the aforesaid current at a level sufficient to maintain a desired, sufficiently positive electrical potential on the metallic equipment surfaces to resist electroless plating. Preferably, a current in the range between 10-4 and 4 milliamperes per cm2 of surface area to be rendered resistant to electroless deposition is applied.

By way of illustration, the procedure is carried out by providing at least one cathode in the electroless copper deposition solution and via a source of electricity in electrical connection with the surfaces of the plating equipment. A current is thus applied to the circuit closed by the plating bath solution to create an electrical potential on the equipment surface sufficiently more positive than the mixed potential of the plating bath solution to resist formation of an adherent copper deposit on said equipment surfaces.This applied current is regulated during the electroless copper deposition to maintain the electrical potential on the surfaces of the plating equipment at desired levels sufficiently positive for rendering said surfaces as well as the surfaces of copper particles precipitated on said surfaces substantially inactive and non-receptive to electrolessly formed deposits.

The expression "mixed potential" as used herein is intended to mean that electrical potential at which copper begins to deposit electrolessly from an electroless copper plating solution onto a receptive surface with which it is in contact. Stated another way, it is the electrical potential measured between a suitable, metallic substrate being electrolessly plated with copper and a standard reference electrode in electrical connection with the substrate being plated.

Procedures for measuring the mixed potential of electroless copper plating solutions are known in the art. One such procedure is described in the text hereinbelow immediately preceding the examples.

In general, electroless copper deposition solutions useful in the present invention are e.g., characterized by a mixed potential within the range between -500 and -800 millivolts relative to a standard silver-silver chloride reference electrode, and in the range between550 and -850 millivolts relative to a saturated calomel reference electrode, measured at the operating temperature of the plating bath solution.

Typically, in carrying out electroless copper deposition procedures according to this invention, electrical potentials in the range between -500 and +500 millivolts, and more usually between -300 and -100 millivolts, relative to the reference electrode, are imposed and maintained on the metallic retaining walls of the plating vessel and/or on any other metallic plating equipment in contact with the bath solution. Such potentials are sufficient to passivate equipment surfaces, as well as any copper already in contact therewith, e.g., precipitated copper, copper borders on panels being plated, and the like.

The principles of this invention are advantageously employed to substantially or completely render non-receptive to electroless copper deposition, metallic racks supporting the substrate being plated in the deposition solution.

Specifically, such rack is electrically connected to a terminal of a current source, e.g., a rectifier, dimensioned to provide the above defined potential over a wide range of current, e.g., up to 200 ampere, and the other terminal of the current source being connected to a cathode suspended in the deposition solution. A current sufficient to establish a passivating electrical potential on the rack is supplied, copper is electrolessly deposited on the substrate supported on the rack, and during deposition the current is adjusted to maintain the rack in its passivated state and not receptive to electrolessly formed copper deposits.

Although the same current source as for the plating vessel or other plating equipment may be used, it is preferred to use a separate source of electricity for each piece of equipment to be rendered and maintained non-receptive.

The foregoing technique can be used, moreover, to prevent the electroless deposition of copper on undesired areas of the substrate to be plated. With particular reference to printed circuit board manufacture using additive techniques, in some cases the insuiating panels which have been pre-cut to size, masked and sensitized, are left with an exposed sensitized border on the edges of the panel and the adjacent panel surface.

When the panel is exposed to the deposition solution, together with copper deposition in the desired areas, copper begins to plate out on the sensitized panel edges and sensitized surface adjacent thereto to the same thickness as on the exposed areas of the panel corresponding to the desired circuit pattern. As a result, a continuous border of copper forms on the panel. In the typical case, this copper border, which is not part of the circuit pattern itself, is cut off from the rest of the panel and discarded. The build-up of this copper border can be prevented by contacting an edge of the panel with a metallic surface of the rack and supplying and maintaining adequate current to the rack to render both the rack and the copper border of the panel at least substantially nonreceptive to electroless deposition.

Plating vessels as well as other metallic plating equipment used in the process of this invention can be cleaned of any copper precipitate which may cling to the surface, e.g., of copper particles.

dropped to the bottom of the plating vessel, by temporarily stopping the plating operation, emptying the plating vessel and brushing, sweeping or vacuuming the copper precipitates away. Such cleaning may also be employed during the plating operation without emptying the plating vessel, as by vacuuming. It should be noted that, in contrast to the prior art, in the process of this invention copper does not adhere to the passivated equipment surfaces even after prolonged operation, and such copper can be easily removed by the foregoing cleaning procedures and without the need for etching or other harsh chemical or mechanical cleaning procedures.

In conducting the process of this invention, it will normally be found that a thin layer of electroless copper deposits on the cathode or cathodes. Copper deposition on the cathode surfaces can be completely or at least partially avoided by interposing between the cathode and electroless copper deposition solution a membrane permitting electrical conductivity between the cathode and deposition solution but preventing passage-through of copper ions from a deposition solution.

Ion exchange membranes, either anionic or cationic, can be employed for this purpose.

Selection of the particular ion exchange membrane will depend on the specific copper ion complexing agent employed in the bath. In cases where the complexed copper possesses a negative charge, as where the complexing agent is of the amino acid type, cation exchange membranes are employed. In those cases where the complexed copper possesses a positive charge, anion exchange membranes are employed. If the complexed copper is neutral, as in the case when alkanolamine complexing agents are used in the bath, either an anion or cation exchange membrane may be employed.

The process of the present invention is effective for use with plating and storing vessels in which the retaining walls or the surfaces of said walls are made of nonnoble metals such as steel, iron, nickel, cobalt, titanium, tantalum, chromium or the like and, if so desired, copper. Similarly, the process can be used to render resistant to adherent electrolessly formed copper deposits other types of plating equipment made of such metals, The pH of the electroiess copper deposition solution is usually at least 10, and preferably 11 or above.

Fig. 1 illustrates a simplified plating system which can be used to practice the present invention, comprising a plating tank with metallic retaining walls, plating solution, power source, electrodes, substrate to be plated and support means.

Fig. 2 shows a more detailed system, adapted for automatic control, which is useful in the practice of this invention.

Fig. 3 is a graph showing the current as a function of potential (voltage) for stainless steel in an electroless copper deposition solution.

Fig. 4 is a graph showing the current as a function of potential (voltage) for copper in an electroless copper deposition solution of the same formulation as in Fig. 3.

With respect to Fig. 1, plating tank 2, the retaining walls of which are made of steel, preferably stainless steel, or other suitable electrically conductive material, contains electroless copper plating solution 4. Metal electrode 6 is immersed in plating solution 4 and electrically connected to negative terminal 8 of direct current power supply unit 1 0. Surface 12 of tank 2 is electrically connected through variable resistor 16, to positive terminal 1 8 of power supply unit 10. Millivoltmeter 20 is also connected to retaining wall 12 and to standard reference electrode 22. Workpiece 24 is supported on metal rack 26. Rack 26, in electrical contact with surface 1 2a of tank 2, is suspended in plating solution 4.Workpiece 24 is electrically insulated from rack 26 by non-conductor (insulator) 27.

Preferably, before starting the operation of the plating solution 4 (e.g., by adding reducing agent, or by raising pH or temperature), a potential more positive than the expected mixed potential of the operating plating solution is applied to surfaces 1 2a and 1 2b of tank 2 by adjusting resistor 16 as needed. Plating solution 4 formulation and conditions are adjusted by known means to start electroless plating (e.g., by adding the reducing agent, raising the pH, raising the temperature).

Workpiece 24 is immersed in plating solution 4, and plating begins. The electrical potential of surface 12 with respect to reference electrode 22 is monitored during plating by observing millivoltmeter 20, and this potential is maintained more positive than the mixed potential of plating solution 4. This potential can be regulated manually, as in the embodiment shown in Fig. 1, or automatically, as illustrated in Fig. 2.

With reference to Fig. 2, 200 V alternating current line 28 extends to direct current supply unit 30, which is, e.gX, capable of generating 200 amperes of current at 7 volts. Negative terminal 32 of power supply unit 30 is electrically connected by line 34 to electrodes 36, which are suspended in metal tank 38. Tank 38 contains plating solution 40 and is grounded by grounding wire 42. Positive terminal 44 of power supply unit 30 is electrically connected by line 46 to pass transistors 48, which are in parallel and driven by Darlington power transistor 50. Each of pass transistors 48 preferably has an output capacity of fifty amperes. Darlington power transistor 50 is preferably set for a gain of about 10000:1.

Pass transistors 48 are connected by electrical line 52, meter shunt 54, and electrical line 56 to tank 38. Meter shunt 54 is connected by line 58 to standard ammeter 60, which measures the current from pass transistors 48 across meter shunt 54. Capacitor 62, preferably having a capacitance of 2 microfarads, is connected across electrical line 34 and meter shunt 54 to reduce background electrical noise.

Electrical line 64 extends from tank 38, and is connected to positive terminal 74 of voltage amplifier 68. Electrical line 70 extends from standard reference electrode 72, and is connected to negative terminal 66 of amplifier 68. Amplifier 68 is set for a gain of 10:1. Reference electrode 72 is a conventional silver/silver chloride electrode, or equivalent reference electrode, in salt bridge communication with plating solution 40 in tank 38.

Amplifier 68 is connected by electrical line 76 to negative terminal 78 of control amplifier 80.

The voltage output from amplifier 68 to amplifier 80 is measured across standard voltmeter 82, connected by line 84 to line 74. Positive terminal 86 of control amplifier 80 is connected by line 88 to potentiometer (set point) 90, and to FET switch 92. Potentiometer 90 preferably has a maximum possible setting of from 3 V positive to 2 V negative.

Electrical lines 94 and 96 extend from terminals 98 and 100, respectively, of meter shunt 54, to voltage amplifier 102. Line 94 is connected to positive input terminal 104 of amplifier 102. Line 96 is connected to negative input terminal 1 06 of amplifier 102. The voltage output from amplifier 102 goes through line 108 to positive input terminal 110 of control amplifier 112. Amplifier 112 is set for a gain of 20:1.

Negative input terminal 114 of control amplifier 112 is connected to potentiometer (set point) 116. Electrical line 118 runs from amplifier 112 to FET switch 92.

Capacitor 120, preferably having a capacitance of one microfarad, and resistor 112 preferably having a resistance of 1 ohm, are included in the circuit for purposes of background noise reduction.

Preferably, to prevent overheating, pass transistors 48, power transistor 50, capacitors 120 and 62, resistor 122 and ammeter 60 are situated on heat sink 124 (indicated by dotted lines), and cooled by fan 126 connected to 110 V alternating current line 128. Heat sink 124 is made of aluminum or other standard heat absorptive material.

In practice, the process of the present invention is carried out as follows: With reference to Fig. 2, alternating current from 220 V line 28 is converted to direct current in D.C. power supply unit 30. Negative potential from power supply unit 30 is applied to electrodes 36 in tank 38. Electrodes 36 are thus made cathodic. Positive potential from power supply unit 30 is sent through power transistor 50, pass transistor 48, line 52, meter shunt 54, and line 56 to tank 38. Tank 38 is thus then made anodic. The current across meter shunt 54 is monitored by use of ammeter 60.

A silver/silver chloride reference electrode 72 is suspended in tank 38 and kept in communication with plating solution 40 in conventional manner by means of a porous membrane therebetween.

By connecting 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 thus continuously monitored, and moreover, controlled as follows: If the voltage from amplifier 68 to control amplifier 80 is predominantly positive, amplifier 80 tends to turn out a positive voltage. If, on the other hand, the voltage from amplifier 68 to control amplifier 80 is predominantly negative, amplifier 80 tends to turn out a negative voltage.

A positive voltage to power transistor 50 causes the latter to generate current flow. A negative voltage to power transistor 50 causes the latter to shuf off and substantially all current flow to cease.

During plating, as the potential (voltage) of tank 38 become less negative (i.e., more positive) with respect to reference electrode 72, amplifier 68 applies a positive voltage to control amplifier 80, which, in turn, applies a negative voltage to power transistor 50. By adjusting set point 90, the positive voltage output of control amplifier 80 is regulated as needed to establish the overall output of amplifier 80 and to achieve the desired current flow to tank 38 as needed to maintain the potential (voltage) of tank 38 to the set potential which is more positive than the mixed potential of solution 40.

Excess current is prevented from flowing to tank 38 by means of voltage amplifier 102 and control amplifier 112. The potential (voltage) across meter shunt 54 is directly proportional to the current flow from power transistor 50 and pass transistors 48. This voltage is boosted 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, FET switch 92 opens, voltage dividing the output potentiometer 90 reducing the set point of amplifier 80, thus returning the system to equilibrium.

The output of amplifier 102 is balanced by the output of potentiometer 11 6 which establishes the set point of amplifier 112. Adjusting 11 6 determines the maximum current allowed before reducing the set point of control amplifier 80. The function of amplifiers 102 and 112 is to limit the maximum current applied to the tank and cathodes to protect the total system.

In the foregoing manner, the voltage applied to tank 38 is maintained more positive than the known mixed potential of plating solution 40; in consequence, substantially no metal deposits on the walls of tank 38.

In the specific procedure described above, metallic racks can be used to support the substrates being plated and such racks can be rendered resistant to electroless copper deposition using the principles described. In such case, it is desirable to employ a separate control circuit for supplying current to the racks. If the substrates supported on the racks and being plated are panels having copper borders, a larger current supply will be required to hold the passivating electrical potential on the racks and copper borders on the panel. If, on the other hand, it is desired to plate the entire substrate or if the copper borders on the panel form part of or are in interconnection with the circuit pattern, it is preferred to insulate the substrate from the rack by interposition of a substantially electrically nonconductive material. (See Fig. 1).

Figs. 3 and 4 show the current as a function of the voltage for copper and stainless steel in an electroless copper deposition solution having the composition of Example 1. Positive currents are oxidizing currents and negative currents are reducing, i.e., plating, currents. At point "B" in Fig.

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 being reduced than reducing agent present in the solution, here formaldehyde is being oxidized, so there is a net negative (plating) current. In region "C", more formaldehyde is being oxidized than copper ions reduced, so there is a net positive (oxidizing) current. In region "D", a film forms on the surface of the copper electrode. This film is non-catalytic to the oxidation of formaldehyde. The maximum current required to passivate has been established to be 4 milliamps per cm2. Copper ion reduction does not occur at potentials more positive than about450 millivolts (mV) relative to the reference electrode, or 250 millivolts more positive than the mixed potential.Region "E", extending from about425 to -225 mV relative to the reference electrode is called the passivation range. In this region, the potential is too anodic to reduce copper ions and the electrode surface is non-catalytic to the oxidation of formaldehyde, so there is little current flow. Since the current in this region is about the same for the solution without formaldehyde as for the solution with it, it is assumed that the current flowing in this region is substantially not caused by the oxidation of formaldehyde. The current flowing in region "F" is due to the oxidation and partial dissolution of the electrode surface. Region "G" is a second passivation region. Beyond region "G" several constituents may be oxidized, OH ions, EDTA, copper or formaldehyde.

Fig. 3 shows that a stainless steel electrode is relatively passive from -500 to +400 mV At potentials more negative than -500 mV copper begins to plate on the stainless steel surface, altering its characteristics. At -325 mV the current density is 40 times less for stainless steel than for copper (.020 vs. 0,80 mA per cm2).

Stainless steel is very slow to initiate plating in an electroless copper plating bath. This is because it displays relatively poor catalytic activity for the oxidation of formaldehyde. However, once plating is initiated, it proceeds rather 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, generation of electrons.

A potential of about325 mV (versus the saturated calomel electrode) is best for passivation of both stainless steel and copper since it is in the middle of the copper passivation range 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 solution mixed potential with pH changes, and is of similar magnitude. In one embodiment of this invention, a mixed potential probe is therefore employed as the respective reference electrode.

For the representations in Figs. 3 and 4, the potential values were measured using a model 1 74A Polarographic Analyzer (Princetown Applied Research) and the reference for all measurements was a saturated calomel electrode. The current was monitored as the potential was scanned under an air atmosphere and recorded an an X/Y recorder.

To measure the mixed potential, a clean copper surface is placed in the electroless copper deposition solution; metal will begin to deposit on the copper surface. Allow 3 to 4 minutes to permit a steady state to be attained. Connect the copper surface to one terminal of a high impedance millivoitmeter, such as used in standard pH-meters. Connect a standard reference electrode, immersed in the bath, to the other terminal of said millivoltmeter. Measure the difference in potential between the copper surface and the reference electrode to establish the mixed potential of the copper deposition solution.

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

Example 1 An epoxy-glass laminate having a thickness of 1,6 mm is prepared in known ways for the manufacture of a printed circuit conductor network employing electroless copper deposition.

The thus prepared laminate is ready for immersion in an electroless copper deposition solution of, e.g., the following composition: CuS04.5H20 lOg/I formaldehyde 4 ml/l wetting agent 0.2 g/l tetra-sodium salt of EDTA 35 g/l sodium hydroxide (NaOH) to pH 11.7 (measured at 250C) sodium cyanide (NaCN) 0,005 g/l water to volume operating temperature 720C The copper deposition solution of this example has a mixed potential of -630+20 mV, measured with reference to a standard silver/silver chloride electrode.

All of the ingredients of the foregoing deposition solution except the formaldehyde are mixed together in a stainless steel plating vessel.

A stainless steel cathode is immersed in the solution and connected to the negative terminal of a variable D.C. rectifier having a maximum capacity of 8 V and 200 A. A standard siiver/silver chloride reference electrode is immersed in the deposition solution and connected to one side of a millivoitmeter. The other side of the millivoltmeter is connected to a wall of the stainless steel vessel.

The electrical potential on the plating vessel wall relative to the reference electrode is adjusted to -200 mV by regulating the rectifier. The eiectroless copper plating solution is started up by adding the formaldehyde. The epoxy-glass laminate, which has been pretreated as described and is supported on a stainless steel rack, is immersed in the electroless copper plating solution. Copper begins to deposit electrolessly on the laminate. After 10 hours, or after a copper deposit of 20 ym has been achieved, the laminate is taken from the deposition solution.It is observed that during the electroless plating reaction, substantially no copper has been electrolessly deposited on the stainless steel vessel surface or stainless steel rack in contact with the electroless copper deposition solution.

Example 2 This example illustrates an embodiment of this invention using two electrodes only, i.e., operated without a reference electrode.

An electroless copper plating solution having the same composition as in Example 1 is placed in a vessel having stainless steel retaining walls. The vessel has a capacity of 8000 liters and an internal surface area of about 60 m2.

The rectifier is adjusted to obtain a potential of 0,45 V between the stainless steel cathode immersed in the plating solution and the stainless steel walls of the vessel. After this adjustment, it is observed that the walls of the vessel have a potential of from -300 to -400 mV, relative to the silver/silver chloride reference electrode. After this initial measurement, the reference electrode is disconnected and removed. The initial current needed to achieve 0,45 V between the vessel walls and steel cathode is 0,5 ampere, equivalent to a current density of 10-4 milliamperes per cm2.

Six stainless steel racks, containing 300 substrate panels of 2 sq ft each per rack, are placed in the electroless copper plating solution.

They are removed after electrolessly forming the predefined conductor pattern, e.g., at intervals of 18 to 22 hours, and replaced with fresh substrate panels to be plated. During the first 24 hours of plating operation, as plated substrates are removed and new substrate panels are introduced into the plating solution, it is observed that a precipitate comprising metallic copper forms in the solution. Some of this precipitate comes into contact with the surfaces of the plating vessel.

The current needed to maintain 0,45 V between the vessel surfaces and the steel cathode rises.

Over the next several days of operation, it is observed that the current required to maintain 0,45 V rises and falls in the range of from 2 to 100 amperes as further metallic copper precipitates, comes into contact with the vessel surfaces and is passivated.

At the end of, e.g., one week, the plating operation is interrupted. The copper precipitate in contact with the interior surfaces of the vessel consists of passivated non-adherent particles which are easily removed by sweeping with a brush or by vacuuming.

Example 3 In this example, the procedure as described in Examples 1 or 2 is followed except that the stainless steel racks, too, are connected to a second, suitable rectifier and a second stainless steel electrode placed in the plating bath solution and the potential is adjusted and maintained at, approximately, 0,4 to 05 V, measured between the rack and the second electrode thus rendering non-receptive to electroless metal deposition the surfaces of the racks and in certain cases the borders of the panels plated up with copper and in contact with the rack surface.

Claims (14)

Claims

1. A method for containing electroless copper deposition solutions and for electrolessly depositing copper from such solutions on substrates receptive to the electroless plating of copper in which surfaces of metallic plating equipment are in contact with said solution, comprising the steps of contacting the said solution with at least one counter-electrode; and connecting the metallic plating equipment and said counter-electrode(s) to a source of electricity so that the surface of the plating equipment is provided with a potential sufficiently more positive than the mixed potential of said electroless plating solution to render said surfaces substantially or completely resistant to electroless metal deposition; said source of electricity having means for adjusting and maintaining said potential over the range of current necessary for compensating the electrons generated by the oxidation of the reducing agent present in the said plating solution when contacting said metallic surfaces of said plating equipment prior and after copper particles have been settling or precipitating on said surfaces, thus rendering and maintaining the surfaces of said copper particles and of said plating equipment substantially or completely catalytically inactive and not receptive to electrolessly formed deposits.

2. The method of claim 1 for electrolessly depositing copper from a deposition solution of known mixed potential on a substrate sensitive to said deposition or for containing such solutions in which surfaces metallic equipment are in contact with the said solution, comprising the steps of (1) initially providing an electrical current to flow between said metallic surfaces of plating equipment in contact with the solution and a counter-electrode, said current being sufficient to provide an electrical potential on said equipment surfaces sufficiently more positive than the mixed potential of the solution to render the equipment surfaces substantially or completely resistant to formation of an adherent electroless copper deposit; (2) electrolessly depositing copper on said substrate from said electroless copper plating solution of storing said solution; and (3) while electrolessly depositing copper on said substrate or storing said solution, adjusting the applied current to said equipment surfaces to maintain an electrical potential sufficiently more positive than said mixed potential to resist electroless copper deposition thereon.

3. The method of claims 1 or 2, in which the electroless copper deposition solution has a mixed potential in the range between500 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.

4. The method of claims 1 or 2, in which the electrical potential imposed and maintained in said metallic equipment surfaces is in the range between500 and +500 mV relative to said reference electrode, and more positive than said mixed potential.

5. The method of claim 4, in which said electrical potential is in the range between -300 and -100 mV relative to said reference electrode.

6. The method of claims 1 or 2, in which the electrical potential imposed and maintained on said metallic equipment surfaces is at least about 250 mV more positive than said mixed potential.

7. The method of claim 2, in which said current is in the range between 10-4 to 4 milliamperes per cm2 of surface area to be rendered resistant to said electroless copper deposition.

8. The method of claim 1 wherein the said source of electricity is a power rectifier having its output voltage adjusted to and maintained at a value selected for achieving said potential and the surfaces of said plating equipment within the selected operational range of current supplied.

9. The method of claim 8 wherein the power rectifier is the rectifier shown in Fig. 2 and described with respect to this Figure.

10. The method of one or more of claims 1 to 8 in which said plating equipment comprises a plating vessel having metallic retaining walls in which the deposition solution is contained.

11. The method of one of more of claims 1 to 9 in which said plating equipment comprises a rack supporting the substrate to be plated, said rack being constructed in whole or partly of metal.

12. The method of one or more of claims 1 to 11 in which said plating equipment is made of steel.

13. The method of claim 12 in which said plating equipment is made of stainless steel.

14. The method of one or more of claims 1 to 13, wherein each separate piece of said plating equipment is connected to a separate source of electricity.