CA1093911A

CA1093911A - Electroless copper plating

Info

Publication number: CA1093911A
Application number: CA272,215A
Authority: CA
Inventors: George A. Butter; John F. Mccormack; John D. Williamson; Rudolph J. Zeblisky
Original assignee: Kollmorgen Technologies Corp
Current assignee: Kollmorgen Technologies Corp
Priority date: 1976-04-08
Filing date: 1977-02-21
Publication date: 1981-01-20
Also published as: IL51772A0; CH633585A5; IT1115850B; DE2715850C2; AT351884B; FR2347453B1; AU2357377A; IL51772A; AU509685B2; SE431351B; ATA245477A; DK158977A; NL7703878A; JPS5932542B2; DE2715850A1; JPS52123335A; SE7703989L; GB1529151A; FR2347453A1

Abstract

For: ELECTROLESS COPPER PLATING

Abstract of the Disclosure.- Maintaining copper ion concentration and controlling the increasing concentration of salt-forming anions in electroless copper plating of articles by contacting all or a portion of the copper deposition solution after interruption of plating in an enriching operation to replenish its depleted copper content by using a selected copper-containing substance (e.g., cupric oxide or basic copper sulfate) having either a small or no content of salt-forming anions to minimize the undesirable build-up of salts, and resuming plating with the enriched solution or portion thereof.

Description

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Field of the Invention.- The present invention i~
concerned with electroles9 deposition of copper and alloys thereof onto suitable substrate9, more particularly with improved control of the concentrations of the constituents of .':1 . ~-'.
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Background of the Invention - Numerous procedures have been suggested for the chemical or electroless plating of various articles with a thin layer of copper by immersion of the articles in an electroless copper deposition solution. For example, good results have been obtained in plating many types of synthetic resins for use as printed circuit boards, etc., by using the methods described in Zeblisky et al, U.S. Patent No. 3,095,309.
The plating formulations described therein may be summarized as:
Soluble cupric salt (CUSO4) ....... 0.5 g. to saturation, e.g., 0.002 - 0.15 mol or more Alkali metal hydroxide (NaOH)...... sufficient for pH 10.5 - 14 Formaldehyde (40~ aqueous) ........ 5 - 100 ml., e.g., 0.06 -1.3 mol Complexing agent (Na4EDTA)* ....... 0.5 - 2.5 mols per mol of Cu *Tetrasodium salt of ethylene diamine tetra-acetic acid Soluble inorganic cyanide (NaCN)... ..Ø001 -3.0 g., et.g., 0.00002 - 0.06 mol Water.............................. sufficient for 1 liter ., .

From a standpoint of materials consumed and the product and by-products, the basic electroless copper plating reaction in the foregoing type of deposition baths is:

CuSO4 + 2 HCHO + 4 NaOH -~ Cu; + H2 + 2 NaOOCH + Na2S04 + 2H20.

jk/~b - 109391i 1 Thus, ignoring the minor effect of a side reaction of the

2 Cannizæaro type, for each equivalent weight (31.77) of product ~ in the form of copper deposited on the surfaces of the articles 4 being plated, two equivalents of sodium salts accumulate in the plating bath as by-products in a weight ratio approximating 6 51 parts of sodium sulfate and 49 parts of sodium formate. It 7 will be appreciated by those skilled in the art that these 8 weight ratios will be altered somewhat when other cupric salts 4 (e.g., the chloride or carbonate) are employed and also when other alkali metal hydroxides (e.g., potassium or lithium) 11 are used. The by-product salt concentrations in the bath 12 continue to increase as more and more copper is plated onto 13 articles. If this is allowed to continue, excessive salt 14 concentrations develop and these have a detrimental effect lS on the copper metal being deposited, especially in regard to 16 its ductility. In commercial operations of a prolonged or 17 continuous nature, such undesirable build-up of salts has been 18 kept under control by draining off 20 to 65% of the deposition l9 bath during a day's operation depending mainly on the plating rate and replenishing the bath with water and solutions of the 21 aforementioned constituents of the bath.

23 While practically all of the copper content of such 24 bath effluents is generally recovered as scrap copper, and sometimes a substantial part of the complexing agent (e.g~, 26 65 - 90%) also, by the plating firm or a salvage firm; extra 27 processing is required and the recovered material may be of 28 lower quality than the original substances. Moreover, there 29 is still a large volu~e of the treated bath effluent to be disposed of with proper regard for protecting the environment : ~
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~1 1093 1 from this array of pollutants in the form of a strong solution 2 of salts containing small amounts of copper ions and other

3 toxic materials.

The present invention is primarily directed at 6 improved control of the concentration of the reactants and 7 by-products of such electroless metallization and their recovery;
8 and it provides signficant environmental and operating cost advantages. Among these are more efficient utili7ation of raw materials, reclaiming or recovering valuable bath components 11 from the deposition bath effluent in greater proportion and/or 12 in a more readily usable form or state, and discharging much 13 less by-product or waste materials and pollutants from the 14 process. In addition, the new method is unusually flexible and versatile in respect to the sources of components of the 16 deposition bath and the different ways by which they may be 17 recovered or reclaimed. A fuller understanding of these and 18 other benefits of the improved process will be apparent from 19 the detailed disclosure hereinafter.

21 Summary of the Invention.- According to the present 22 invention, in its broadest aspects, there is provided a process 23 for maintaining the concentration of copper ions within a 24 predetermined operating range and controlling the increasing concentration of salt-forming anions in an operating electroless 26 copper deposition bath in a deposition vessel which compries:
27 a) depositing metallic copper or an alloy 28 thereof onto catalytic surfaces of a substrate from an electro-29 less copper deposition bath comprising an aqueous alkaline medium, copper ions, a complexing agent for copper ions, and a _ 4 _ ~ .

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---~ 10~39~1 lA reducing agent for copper ions until the copper content in said 1 bath is substantially depleted;
2 b) interrupting deposition of copper by said 3 bath or a portion thereof;

4 c) at least intermittently contacting all or a portion of said bath during interruption for a sufficient 6 time with a copper-containing substance wherein any salt-forming 7 anion content is substantially less than one equivalent weight 8 per equivalent weight of copper therein and the copper contents 9 of said substance is capable of forming a copper complex with said complexing agent to thereby enrich said bath or portion 11 thereof by substantially replenishing the content of copper 12 removed from said bath by said deposition on the substrate; and 13 d) resuming deposition of copper from said 14 enriched bath or portion thereof on a catalytic surface of a substrate.
16 A number of methods can be used to carry out the 17 interrupting step (b): for example, the operation of the 18 bath can be interrupted by removing the workpieces from the 19 deposition bath; alternatively, all or part of the bath can be withdrawn, e.g, by pumping, from the bath deposition vessel.
21 In addition, the temperature of the bath can be lowered suffi-22 ! ciently to stop its operation' in this connection, it has been 23 found that lowering the temperature to between about 10 and 24 35 below the operating temperature of the bath will readily accomplish this. Ideally, the temperature will be lowered 26 to 40 to 60C. during this step. A ~urther way of interrupting 27 the bath operation is to allow the reducing agent content to 28 become depleted, e.g., to allow the formaldehyde content to 29 deplete temporarily. On the other hand, the operation of the bath can be interrupted by using oxidizing agents such as ,., ,,,., , . I
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1 hydrogen peroxide, or to bubble air into the bath, which 2 depletes the reducing agent and interrupts its operation.
3 Finally, stabilizers for the bath, such as cyanide, and 4 sulfide-containing compounds can be added and these will temporarily interrupt operation of the bath.
7 According to a preferred feature of this invention, 8 there is provided a process for maintaining the concentration 9 of copper ions within a predetermined operating range and controlling the increasing concentration of salt-forming anions 11 in an operating electroless copper deposition bath in a deposi-12 tion vessel, which includes the steps of:
13 a) depositing metallic copper or certain 14 alloys thereof onto catalytic surfaces of a substrate from an electroless copper deposition bath comprising an aqueous alka-16 line medium, copper ions, a complexing agent for copper ions, 17 and a reducing agent for copper ions;
18 b) at least intermittently withdrawing all or 19 a portion of said bath into contact for a sufficient time with a copper-containing substance wherein any salt-forming anion 21 content is substantially less than one equivalent weight per 22 equivalent weight of copper therein and the copper content of 23 said substance is capable of forming a copper complex with said 24 complexing agent to thereby enrich said withdrawn bath or a portion thereof by substantially replenishing the content of 26 copper removed from said bath by said deposition on the su-27 strate; and 28 c) at least intermittently returning said 29 enriched bath or portion thereof to said deposition vessel.

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lQ~3911 Other aspects of the invention involve combinations of the aforementioned procedure with one or more of such features as employing designated classes of components of the deposition bath and preferred species thereof; as well as using specified tech-niques in the enriching operation, and especially the substances employed therein. Still other features, objects and advantages of this novel process will be apparent to those skilled in the art upon consideration of the detailed disclosure which follows.

Detailed Description of the Invention - The present method provides superior results in the additive metallization or plating of copper onto catalytic substrate surfaces from electro-less copper deposition baths. It also encompasses electroless codeposition of minor amounts of other suitable metals, especially nickel and cobalt, along with tne copper in plating substrates with alloys containing major proportions of copper.

Catalytic substrates and the preparation of their sur-faces are well known in the art. Many are described in the cited Zeblisky et al patent, including stainless steel and other metals receptive to the "chemical plating" of copper; resinous or ceramic articles having cuprous oxide or other catalytic particles em-bedded and dispersed therein, and other resinous articles. Cata-lytic substrates are also disclosed in U.S. Patent No. 3,560,257.
In brief, the surface preparation may typically involve carefully cleaning and scouring a metal substrate or abrading the surface in the case of a plastic article containing an embedded ik/ ~

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catalyst; and for conventional resin articles in general, clean-2 ing then sensitizing the surface with a solution of stannous 3 chloride followed by a seeding solution of palladium chloride 4 may be used.

6 In providing better control of the concentrations 7 of copper ions and salt-forming anions, the instant process 8 may be employed in conjunction with certain prior art electro-9 less copper deposition baths as starting formulations provided that replenishment of these baths is performed according to 11 the enriching procedure described herein. For instance, aqueous12 deposition solutions initially containing a complexing agent 13 for copper, an alkali, a reducing agent for copper, and a 14 water soluble cupric salt, such as the sulfate, chloride, nitrate, acetate, etc., are suitable, and these are exemplified 16 in the Zeblisky et al patent which also discloses suitable 17 proportions. Such salts introduce into the starting bath 18 one chemical equivalent weight o~ the salt-forming anion for each 19 equivalent weight of copper therein. The copper ion content of the electrcless deposition bath may range from about 0.1 21 gram per liter up to saturation; but concentrations of about 22 0.1 to 5 g./l. are generally preferred, and excellent results 23 are obtainable with amounts of 2.30 to 2.55 g./l.

A wide variety of compounds are suitable complexing 26 agents for the bath provided that they are capable of forming 27 complexes or chelates with copper and dissolving at least one 28 of the water insoluble, copper-containing substances employed 29 in the enriching ste? as well as being compatible with other constituents of the bath under operating conditions. Among .- ',., ~.,,,,',,,,, .' l ~ .:
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these agents are amines and polyamines containing alkylol and/or carboxyl groups, and alkali metal salts of such compounds. A few of the many possible examples include ethylene diamine tetra-acetic acid (EDTA), N,N,N',N'-tetrakis(2-hydroxypropyl) ethylene diamine (Quadrol or THPED), nitrilo-triacetic acid (NTA), hydroxyethyl ethylene diamine triacetic acid (HEDTA), cyclohexylene diamine tetra-acetic acid (CDTA), diethylene triamine penta-acetic acid (DTPA), and mono-, di-, tri-, etc. sodium or potassium salts of those compounds. Still other suitable complexing agents include Rochelle Salt and the compounds described in Lukes U.S. Patent Nos.
2,996,408 and 3,075,856, Agens U.S. Patent No. 3,075,855, Wein U.S.
Patent No. 3,093,509 and Atkinson U.S. Patent No. 3,119,709. Lukes is concerned with alkanolamine organic acids as exemplified by N-(2-hydroxyethyl)-N-carboxy methyl methylamine, N,N-(2-hydroxy-ethyl) glycine and N- 2-hydroxyethyl)N, N ' N- tricarboxy methyl ehtylene diamine. Agens discloses alkanolamines, such as N-methyl-diethanolamine, triethanolamine and triisoproanolamine as well as tetraalkaneammonium hydroxides like tetraethylammonium hydroxide.
Wein described gluconic acid compoundsA The Atkinson patent is concerned with hydroxyalkyl derivatives and with organic acid derivatives of ethylene diamine, as exemplified by the afore-mentioned EDTA.

The amount of complexer charged to the deposition bath may range from 1 to 20 moles or more per atomic weight or gram-atom of copper in the initial copper-containing sub-stance in the starting bath. However, the complexing agent is preferably in substantial excess (e.g., 50 - 200~ or more) g _ . .. : :, ,. : :

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1 above the quantity required for complexing all of copper in 2 the initial solution. Such an excess ensures that there will 3 be an ample amount of free or available complexing agent 4 capable of dissolving a copper-containing substance in the enrichment step. In general, about 1.5 to 7 moles of the com-6 plexing agent are desirably charged to the starting bath for 7 each gram-atom of copper present. In the case of EDTA, abou~
8 20 to 30 g./l. of ~D~A in excess of ~he mole equivalent of copper 9 is usually preferred for a bath similar to that specified hereinafter.

12 Other conventional components of the electroless 13 copper deposition bath include aqueous formaldehyde (e.g., 14 about 1 to 100 ml. of 37% formalin per liter of total bath) as a reducing agent which is usally preferred over other 16 known reducers, including sodium borohydride, aminoboranes, 17 etc.; a small quantity of sodium or po~assium cyanide as a 18 stabilizer against bath decomposition; sodium or other alkali 19 metal hydroxide to provide an alkaline pH of preferably about 2~ 11 to 14, and aboutO.0001 to 1% of a wetting agent of the types 24 stable at high ph v~lues .. , . , .
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PC-127B ~ 109, 9il 1 For illustration, good results have been obtained in 2 practicing the present process in starting with an electroless 3 copper deposition bath that was originally prepared in the 4 following sequence of additions.
Liters 6 Deionized water 4000 7 *Aqueous solution of 540 g.ll. of 8 Na4EDTA 2H2O
9 **Aqueous solution of 250 g.ll. of CuSO4 5H2O 180 11 180 g. of sodium cyanide dissolved ~ 12 in sufficient water(180 grams) 1 13 Aqueous solution of 750 g.ll. of l 14 NaOH -- to pH 11.8 7.5 ¦ ~ 15 20% Gafa ~ RE-610 surfactant 6 ~l 16 37% Aqueous formaldehyde 20 1 17 *25 g.ll. in excess of copper complexing requirement.
1 18 **2.5 g./l. copper ions in overall deposition bath.
!; 19 j 20 In the electroless plating of copper onto catalytic 21 substrate surfaces in such a bath at a temperature of typically 22 65 - 75C., there is, of course, a gradual decrease in the 23 concentration of copper ions in the solution and a correspond-24 ing increase in the concentration of free or excess complexing agents, and the copper removed as plated metal must eventually 26 be replaced.

. 28 The procedure for enriching such depleted solutions 29 by replenishing their contents of dissolved copper is an important feature of the processing combination of the present . _11- , .

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~ ~ 3 1033~3:i1 l 1 invention. Also, it is a versatile procedure that can be 2 carried out by several different techniques and also may 3 employ a number of different copper-containing substances from 4 different sources, including scrap and waste materials, and with selective control of the build-up of salt-forming anions 6 in the deposition bath to which the enriched solution is added.
7 In accomplishing this, all or a portion of the deposition 8 solution during temporary interruptio~ of operation is enriched 9 by intimate contact with the selected copper-containing sub-stance, filtered when necessary, and then the plating operation 11 is resumed.

14 The present process utilizes novel enriching agents, namely, copper-charged exchange resins, cupric oxide, 16 cupric hydroxide, basic cupric oulfate - CuS04-3Cu(OH)2, basic 17 cupric chloride - CuC12~3Cu(OH)2 or CuC12-Cu(OH)2, basic 18 cupric carbonate - CuCO3-Cu(OH)2, and cupric oxychloride -19 CuC12-2CuO. Despite considerable differences in some of the physical and chemical properties of these substances, they 21 have a number of common properties that are significant in 22 respect to the instant invention. All contain copper; and 23 any salt-forming anion content is far less than one equivalent 24 weight per equivalent weight of copper (31.77) therein. For example, some of them are devoid of salt-forming anions (e~g., 26 cupric oxide) while others have a low content thereof (e.g., 27 basic cupric sulfate). Further, all of them are insoluble in 28 water at neutral and high pH values; but all are capable of 29 forming soluble complexes with free complexing agents for copper in alkaline acueous media.

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1 The cupric hydroxide, oxychloride and basic copper salts 2 are readily soluble in the bath or portion thereof selected for 3 enrichment at its operating temperature and pH, since it has 4 a substantial content of excess or available complexing agent,

5 e.g., 20-30 g./l. Accordingly, no difficulties are encountered in adding the copper compound in measured amounts in either 7 particulate form or as an aqueous slurry to the depleted bath 8 liquor in a tank of measured volume while maintaining 9 thorough agitation therein. For illustration, the complexing of other basic cupric salts is believed to take place similarly 11 to that shown for basic cupric sulfate in the following equation:
12 4 Na4EDTA + CuS04-3Cu(OH)2-~4 CuNa2EDTA + Na2S04 + 6 NaOH.
13 In this complexing reaction, the formation of a relatively 14 large amount of sodium hydroxide and particularly the small amount of by-product sodium sulfate are significant. After 16 the enriched solution or portion is ready for use in an 17 electroless deposition bath, its copper complex and sodium 18 hydroxide eventually participate in an electroless plating 19 reaction along the following lines.
20 CuNa2EDTA + 2 HCHO +4NaOH ~ Cu~ + H2 2 H O + .
21 Na4EDTA + 2 NaOOCH
22 ~'hile the foregoing procedure does not reduce the formation 23 of sodium formate in the plating reaction, there is a marked 24 reduction of 75% in the formation of sodium sulfate in the complexing reaction in comparison with employing normal cupric 26 sulfate as the source of copper. That reduction is important, 27 for it permits reducing by about 47% the volume of bath effluent 28 which is discarded to prevent an excessive build-up of salt-29 forming anions in the deposition bath while still maintaining the quality of the copper plating. Moreover, there are ' " , ' ' , ' . ' "

-~ 1 10~3911 PC-127B l 1 ¦ indications that relatively high concentrations of sodium sulfate 2 ¦ in the bath tend to degrade the properties of the copper being 3 ¦ deposited considerably more than similar concentrations of the 4 ¦ formate salt. Also, the formation of sodium hydroxide in ¦ complexing copper in the instant enrichment step is advantageous

6 ¦ in meeting a substantial part of the alkali requirement of the

7 ¦ plating step, for none is produced in complexing copper salts

8 ¦ normally used, e.g., copper sulfate.

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10 ¦ It is possible to reduce by 60% the flow of bath

11 ¦ effluent necessary for restraining salt build-up in the bath,

12 ¦ so that the bath effluent amounts to only 40% of that required

13 ¦ for a similar deposition bath made with cupric sulfate. This

14 ¦ result is obtainable according to the present invention with

15 ¦ several copper-containing substances which may be employed as

16 ¦ sources of copper for replenishil~g that consumed in the bath.

17 ¦ These materials include cupric oxide and cupric hydroxide as

18 ¦ well as exchange resins charged with copper ions or

19 1 copper complexes with the various complexing agents mentioned

20 ¦ hereinbefore.
211 .-22 ¦ Suitable exchange resins may be charged with 23 ¦ copper ions in known manner by the passage of aqueous solutions 24 ¦ of compounds containing copper through a single column or 25 ¦ series of columns having beds of particulate resin therein to 26 ¦ charge copper ions on the resin particles. Thereafter, the 27 ¦ electroless metal deposition bath can be passed through the 28 ¦ bed or beds of resin for replenishment by éxtracting copper-29 ¦ containing material or the resin can be placed on the depoeltion 30 ¦ vessel. With the resin in an external circuit, the sodium ! .

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-- 10~3911 or other alkali metal ions can be exchanged for copper in theresin beds, thereby regenerating the ion exchange resin for repeated use.

Cation exchange resins in general are suitable for use in the present process, but those operative over a broad acidity range of pH 2 to 14 are usually preferred. Excellent results have been obtained with polystyrene resins, particularly those with a -SO3 ionic group. Also, many other resins useful in the present process are listed together with their characteristics in "Ion Exchange" by F. Helfferich, McGraw-Hill Book Co., New York, New York (1962), Pages 574-577.

In many instances, cupric oxide is the preferred en-riching agent, and it may be obtained by the treatment of certain spent etching solutions as described later at low cost and a suit-ably high purity. It dissolves in forming a complex with the available complexing agent in the deposition bath to be enriched as may be illustrated by the equation:
CuO + Na4EDTA + H2O ~ CuNa2EDTA + 2 NaOH.
It will be noted that this type of complexing reaction does not involve any undesired formation of salts. ~nother advantage lies in the production of an even greater proportion of sodium hydroxide than in the case of the basic copper salt. The eventual electro-less plating reaction of the enriched bath or portion thereof is the same as hereinbefore with the formate salt providing the necessary ionizable anion.

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~(~939:11 1 However, some problems are encountered in dissolving 2 cupric oxide in the enriching operation as it goes into solution 3 rather slowly even with vigorous agitation. Also, the un-4 dissolved oxide particles catalyze the deposition thereon of electroless copper from a solution having relatively high 6 plating activity. These difficulties can be overcome with 7 appropriate procedures. The plating activity may be reduced 8 to eliminate or at least minimize such unwanted copper deposi-9 tion by a number of expedients, including reducing the pH of the bath liquor being enriched, lowering the 11 concentration of reducing agent therein or cooling the solution 12 about 10 to 35C. below the electroless plating temperature 13 which is usually between about 65 and 75C. but may be sub-14 stantially lower in some instances. In general, cooling appears to be most feasible for commercial operations, and it may 16 often be accomplished without cooling equipment. In tests 17 with samples withdrawn from an electroless copper plating bath 18 containing 30.6 g./l. of excess or available Na4EDTA as the 19 complexing agent and subjected at lower temperatures to vigorous agitation with excess amounts of cupric oxide, it was

21 found that 78% of the available complexer reacted in 20

22 minutes and 99% in 30 minutes at 40C. while 95% reacted in 10

23 miw tes at 54C. all without appreciable copper deposition.

24 Accordingly, complexing or enriching temperatures of about 40 to 60 are suitable and 54 to 58 is regarded as the optimum 26 range for this particular deposition solution which is typically 27 maintained at 65 - 75^C. in the electroless plating operation.
28 ¦ Preferred temperatures for complexing cupric oxide with 29 ¦ solutions containing different complexing agents or other com-30 ¦ ponents are best determined by similar experiments.

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~093911 1 Plated substrates of high quality have been obtained 2 with the process of the present invention employing the initial 3 bath composition listed hereinbefore both with intermittent 4 and with continuous regeneration of the solution for enrich-ment with cupric oxide for substantial periods of time.
6 For illustration, in operating a large production tank in 7 electroless plating under heavy loading and a high plating 8 rate of 0.14 mils per hour, 35% of the bath volume was circulated 9 hourly through an external enrichment system with 95% of the available complexing agent reacting with the copper oxide. -12 A multiple tank arrangment may desirably be employed 13 in such a system with a s~ream of the deposition solution being 14 passed to a holding tank for cooling to about 55C. while it is being collected for a half hour prior to being pumped into one 16 or two larger dissolving or mixing tanks which operate in alter-17 nating cycles. The bottom of that mixing tank already contains 18 a substantial residue from its last cycle in the form of cupric 19 oxide slurried in the solution, and more of the oxide is added while this tank is being filled with the liquor from the holding 21 tank. Meanwhile, the agitator is operating from the start of 22 filling until 10 minutes after the tank is full; then the un-23 dissolved cupric oxide particles are allowed to settle for 24 the balance of the one hour residence time. In the middle of this period, the other mix tank starts discharging its upper 26 clear solution through a skimmer or other suitable decanting 27 device and, if necessary, through an appropriate filter or other 28 suitable solids separation device for removing any solid 29 particles larger than about 2 microns. The clear solution, enriched in copper, then is delivered to a replenisher holding , .~ ,, , :
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-`-` 1093911 tank from which it is returned to the electroless plating tank or perhaps passed to a similar plating tank in battery operations.

As mentioned before, exchange resins in general are suit-able for use in the present process, but those operative over a broad acidity range of pH 2 to 14 are usually preferred. Excellent results have been obtained with carboxylic resins, and particularly those containing chelating functionality. See, for example, U.S.
2,993,782 and 2,980,607. These are available from commercial sources, e.g., from Rohm and Haas Company, Philadelphia, Pa. 19105, under the name Amberlite XE-318.

As indicated earlier, one of the principal concerns of the present invention is to minimize the quantity of effluent which must be discarded to prevent the build-up of excessive concentra-tions of salt-forming anions in the deposition bath, and the in-stant process allows that discharge to be reduced from about 47 to 60% in volume. For economy in operations, it is still permis-sible to have substantial contents of salt-forming ions, as exem-plified by about 14 to 18 grams per liter of sodium sulfate and about 68 g./l. of sodium formate in the electroless copper deposi-tion bath when basic copper sulfate is employed as the enrichingagent, and even more sodium formate may be tolerated when no other salt-forming anion is present. Such control is obtainable by monitoring the deposition bath continuously or at suitable inter-vals with analyses for the formate ion and any other salt-forming anion as well as by the specific gravity of the bath. Suitable specific gravity ranges IO 391i l ll PC-127B l 1 ¦ may be determinedby reference to those analyses for each 2 ¦ particular ba~h composition, for some variation is to be 3 ¦ expected when different copper-containing substances are used 4 ¦ as the enriching agent. While this bath effluent may be ¦ discarded as waste, especially in view of its considerably 6 ¦ reduced volume, it is usually preferable to reclaim the two 7 ¦ most valuable co~ponents thereof, for these may be recovered 8 ¦ in a state that is pure enough for use in the instant process.
9 I .
¦ With a number of the complexing agents described 11 ¦ hereinbefore, this reclamation of the bath effluent or discharge 12 ¦ may be accomplished by a two-stage treatment. The first stage 13 ¦ may involve removing the copper as metal either by electro-14 ¦ plating it onto copper anodes or by adding an excess of both ¦ formaldehyde and sodium hydroxide to the effluent to precipitate 16 ¦ the copper as a powder; and, in the case of EDTA and similar 17 ¦ complexers) thereafter treating the resulting liquid or filtrate 18 ¦ with sulfuric or another mineral acid to a pH of 1.8 - 3.0 19 ¦ in the second stage to precipitate the complexing agent. After 20 ¦ filtering out the complexer, the liquid is ;iscarded. The 21 ¦ copper can be converted into cupric oxide or into a basic cupric 22 ¦ sulfate or chloride or oxychloride in known manner. The solid 23 ¦ EDTA complexing agent can be readily dissolved in aqueous 24 ¦ sodium hydroxide and returned to the process by introduction

25 ¦ into either the enriching operation or a~ the electroless

26 ¦ deposition bath. Recoveries typically amount to more than

27 ¦ 99% of the copper and 90% or more of the complexing agent.
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¦ In addition to the electroless copper plating 2 ¦ of articles in their entirety, for decorative or protective 3 ¦ purposes, the present process is particularly suitable for the 4 ¦ production of printed circuits employing known masking ¦ techniques. Further, such electrolessly plated substrates may 6 ¦ thereafter be electroplated with copper or other desired metals.
7 l 8 ¦ While the present invention has been described in 9 ¦ connection with a limited number of embodiments for the purpose lO ¦ of full and detailed disclosure, it will be apparent to those 11 ¦ skilled in the art that many other modifications and variations 12 ¦ of the procedure are within ~he purview of the invention.
13 ¦ Accordingly, this invention should not be construed as limited 14 ¦ in any particulars except as may be recited in the appended 15 claimt or required b the prior art.

~267 281 .

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Claims

Claims:

1. A method for maintaining the concentration of copper ions within a predetermined operating range and controlling the increasing concentration of salt-forming anions in an operating electroless copper deposition bath in a deposi-tion vessel which comprises:
a) depositing metallic copper or an alloy thereof onto catalytic surfaces of a substrate from an electro-less copper deposition bath comprising an aqueous alkaline medium, copper ions, a complexing agent for copper ions, and a reducing agent for copper ions until the copper content in said bath is substantially depleted;
b) interrupting deposition of copper by said bath or a portion thereof;
c) at least intermittently contacting all or a portion of said bath during interruption for a sufficient time with a copper-containing substance wherein any salt-forming anion content is substantially less than one equivalent weight per equivalent weight of copper therein and the copper content of said substance is capable of forming a copper complex with said complexing agent to thereby enrich said bath or portion thereof by substantially replenishing the content of copper removed from said bath by said deposition on the substrate; and d) resuming deposition of copper from said enriched bath or portion thereof on a catalytic surface of a substrate.

2. A method according to Claim 1 wherein said interrupting step (b) is carried out by removing workpieces having catalytic surfaces from said deposition vessel.

3. A method according to Claim 1 wherein said interrupting step (b) is carried out by withdrawing all or a portion of said bath from said deposition vessel.

4. A method according to Claim 1 wherein said interrupting step (b) is carried out by lowering the temperature of said bath between about 10 and 35°C. below its operation temperature.

5. A method according to Claim 1 wherein said interrupting step (b) comprises allowing the reducing agent content of said bath to become temporarily depleted.

6. A method according to Claim 1 wherein said interrupting step (b) comprises adding an oxidizing agent in an amount sufficient to deplete the amount of reducing agent in said deposition bath.

7. A method according to Claim 1 wherein said interrupting step (b) comprises adding a stabilizer to said deposition bath.

8. A method according to Claim 1 wherein said copper-containing substance is of the group consisting of copper, cupric hydroxide, cupric oxide, cupric oxychloride and the basic carbonate, basic chloride and basic sulfate salts of copper.

9. A method as defined in Claim 1 wherein said reducing agent comprises formaldehyde.

10. A method as defined in Claim 1 wherein said copper-containing substance comprises cupric oxide.

11. A method as defined in Claim 10 wherein said reducing agent comprises formaldehyde.

12. A method as defined in Claim 10 which further comprises separating any cupric oxide particles large enough to catalyze the electroless plating of copper thereon prior to resuming electroless deposition.

13. A method as defined in Claim 12 which comprises filtering or decanting to separate said particles.

14. A method as defined in Claim 10 wherein said bath contains a concentration of available complexing agent in excess of that required for complexing said cupric oxide.

15. A method as defined in Claim 1 wherein said copper-containing substance comprises a copper-charged exchange resin.

16. A method as defined in Claim 15 wherein said resin is a cation exchange resin capable of operating at pH
values between 2 and 14.

17. A method as defined in Claim 16 wherein said resin is a polystyrene resin.

18. A method as defined in Claim 15 wherein said resin contains chelating functionality.

19. A method as defined in Claim 1 wherein said complexing agent is of the group consisting of ethylene diamine tetra-acetic acid, hydroxyethyl ethylene diamine triacetic acid, diethylene triamine penta-acetic acid, nitrilo-triacetic acid, cyclohexylene diamine tetra-acetic acid, N,N,N',N'-tetrakis (2-hydroxypropyl) ethylene diamine and alkali metal salts of said acids.

20. A method as defined in Claim 1 wherein said copper-containing substance is cupric oxide and said complexing agent is N,N,N',N'-tetrakis (2-hydroxypropyl) ethylene diamine.

21. A method as defined in Claim 1 wherein said copper-containing substance is cupric oxide and said complexing agent is of salt of ethylene diamine tetra-acetic acid.

22. A method as defined in Claim 21 wherein said reducing agent comprises formaldehyde.

23. A method as defined in Claim 1 which further comprises maintaining the temperature bath during interruption of deposition between about 10 and 35°C. below the operating temperature of said deposition bath during a substantial period of contact with said copper-containing substance.

24. A method as defined in Claim 1 which further comprises maintaining the temperature during interruption of deposition between about 40 and 60°C. during a substantial period of contact with said copper-containing substance.

25. A method as defined in Claim 1 which comprises continuously withdrawing a portion of said bath from said deposition vessel and continuously returning said enriched portion to said bath at a rate sufficient to maintain a sub-stantially constant concentration of copper ions in said bath.

26. A method for maintaining the concentration of copper ions within a predetermined operating range and controlling the increasing concentration of salt-forming anions in an operating electroless copper deposition bath in a deposi-tion vessel which comprises:
a) depositing metallic copper or an alloy thereof onto catalytic surfaces of a substrate from an electro-less copper deposition bath comprising an aqueous alkaline medium, copper ions, a complexing agent for copper ions, and a reducing agent for copper ions;
b) at least intermittently withdrawing all or a portion of said bath into contact for a sufficient time with a copper-containing substance wherein any salt-forming anion content is substantially less than one equivalent weight per equivalent weight of copper therein and the copper content of said substance is capable of forming a copper complex with said complexing agent to thereby enrich said withdrawn bath or portion thereof by substantially replenishing the content of copper removed from said bath by said deposition on the substrate; and d) at least intermittently returning said enriched bath or portion thereof to said deposition vessel.