CA1183101A

CA1183101A - Passivation of metallic equipment surfaces in electroless copper deposition processes

Info

Publication number: CA1183101A
Application number: CA000350888A
Authority: CA
Inventors: Rudolph J. Zeblisky
Original assignee: Kollmorgen Technologies Corp
Current assignee: Kollmorgen Technologies Corp
Priority date: 1979-04-30
Filing date: 1980-04-29
Publication date: 1985-02-26
Also published as: DK151233C; NL189769B; CH646732A5; DE3016994C2; DK151233B; AU5767780A; DE3016994A1; GB2052560A; DK185080A; IT1128151B; SE453925B; AT369037B; NL8002515A; IT8048537A0; FR2455641B1; ATA230980A; FR2455641A1; GB2052560B; SE8003204L

Abstract

ABSTRACT OF THE DISCLOSURE

The metallic surfaces of plating equipment in use during electroless copper deposition, such as vessels, racks supporting substrates being plated, plumbing, and the like are rendered substantially resistant to electroless copper deposition for extended periods by initially imposing on such equipment surfaces an electrical potential more positive than the mixed potential of the electroless copper solution and sufficiently positive to resist electroless deposition, electro-lessly depositing copper on the substrate being plated, and while depositing such copper maintaining on the equipment surface a potential sufficiently positive to resist the formation of adherent electroless copper deposits.

Description

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BACXGROUND OF THE_ ` Electroless ~etal deposition baths, e.g., electroless copper solutions, are known in the art as useful for providing metal deposits on non-metallic and metallic surfaces. Such solutions are characterized by the capacity to deposit or plate metal in virtually any desired thiclcness without the need for supplying electrons from an external source of current. After an electroless metal deposit is formed on the surface of the 1'' ~'"-' ' '" '''" ' " i' ' ' , ; .`, ,~ .i;";,~
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l article, the ~lect~oless solution becomes autocatalytic, l.e.,

2 continues to deposi~ metal so long as ~he solution is rèplenished

3 and main~ainedO
Special men~lon is m3de of the use of elec~role~s 6 metall~zing procedures in the plating of plastics generally, 7 and in the manufacture of printed circui~ boards particularly.
8 In tne plating of plastics, copper is electrolessly deposited 9 on the surface of an areicle to produce a metalli ed plastic article for use, e.g. f in the auto industry as grills or ll decorati~e stripping, in the toy industry as metallized plastic 12 miniatures, and in home products as door knobs and the like.
`13 14 In the manufacture of printed circuit boards, in one type of procedure a metallic or sensitized non-metallic sub-16 st_atum is provided with copper foil or a thin layer of electro-17 lessly deposited copper, Selected areas of the copper corre-18 sponding to the desired circuit pattern are then masked. The un-~j/7L
lg masked copper is then ~ up, as by electroplating. The mask~
inO material is removed from the copper~ The thin layer of back-21 ground copper is next etched away, In another type of procedure, 22 the surface of the substratum is sensitized for electroless 23 deposition, areas not corresponding to the circuit pattern are 24 masked, and copper is deposited on the unmasked sensitized areas by electroless and/or electr~plating techniques until the desired 26 copper thickness is attained, ~7 28 When electroless plating operations of the foregoing 29 type are practiced on a commercial scale, typically, large plating vessels are employed, . . ~
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595 187A , .' .
1 The parts or ar~icles to be plated are im~ersed in the electro-2 less copper deposition solution in the plating vessel. The 3 parts or articles are, moreover, supported on racks or fra~e~
immersed in the deposition solution. It has been found, ~n practice, that if the plating vessel and support racks are 6 constructed of a non-metal, e.g., plastic or glass, copp~r 7 ! particles for~ing in the electroless copper solution during 8 il the electroless plating reaction become attached to such g l, equipment surfaces. The attached copper particles provide , sites for further electroless copper deposition and growth.
11 ' If this pheno~.enon is permitted to proceed unimpeded, eventually 12 , most or substantially all of such equipment surfaces become 13 covered with a deposit of electroless copper. This weakens 14 the deposition soluti.on. Furthermore, the plating operation , must be interrupted to empty the tank and etch away the copper 16 I from the non-metallic equipment surfaces. This etching pro- ¦
17 ii cedure, though necessary to recapture the copper and avoid 18 , waste, is nevertheless disadvantageous in that the plating 19 operation must be stopped, etching chemicals must be stored l and handled, etc., and large amounts of expensive etching 21 1I chemicals are often required. Thus, additional costs are 22 1 imposed due to interruptions in the plating operation, and 23 ll because of the labor involved in the etching process. A further 24 I.l disadvantage is that this etching weakens the structure of the 25 I non-metallic plating vessels, racks, etc. and shortens their I
26 1~ useful life.

28 The use of plating vessels and other plating equipment 29 made of metal].ic ~laterial is desirable because of greater durability and normally greater availability, These are never-theless subject to m~y of the same disadvclntages just described.
Specifically, copper deposits form continuously and catalytically on the It~tallic equiF~.ent surfaces in contac-t. with the electroless deposition solution, and plating occurs even faster than ~ those cases where non-metallic ecluipment is employed. This problem is particularly acute where adjustable m~tal racks, i.e~, containing me-tal fas-teners, are used because the ~asteners become coa~ecl with an electroless metal deposit making di.fficult the~r re].ease.
It is kncwn in the art -that plating vessels made of metallic retain mg walls, as well as other metallic plating equipmænt such as racks, plumbing and the like, can be rendered temporarily resis~ant to electroless metal deposition by pretreabr,en-t with chemicals, e~g., nitric acid solutions. Such chemical treat-ments ten to wear off within hours of operation, however.
Conse~en-tly, ~he platinq proceclure must be interrupted to emp-~y the plating vessel and renew the chemical treat~ent.
Klein et al, in U.S. 3,424,660, disclose -that plating vessels having metallic retaining walls can be pro-tected against electroless metal deposition, particularly electroless nicke-, by impQSing a p~tential thereon at a value corresponcling to the rest potential or the protec-tion potential range on the ~rrent densi~y~-poten-tial curve. The current density is adjusted to not ~ore than about 10 4 amperes per sc~are centir.e-ter.
German OffenlecJungsschrift 2639247 of Bol~lolder et al, dated February 2, 1978 discloses that plating tanks and racks made of a metal such as cobal-t or nickel can be rendered resistant to electroless met,al deposi-tion, such ~b/!`

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595-187A . .-l as elec~roless copper deposition, by charging wlth a curren~
2 denslty of a~ lease 4 milliamperes per square decimeter. .-.
3 . ~ ..

4 It has also been proposPd, in Japan Patent Publication S ~4-36577, dated November 9, 1979, tha~ a metal plating ~essel, such as of chromiu~-nickel steel, can be rendered resistant 7 I to chemical plating i a positive elertrical potential i5 applied 8 1 to the plating vessel surface during the plating operation~.

10 I In commercial practice, procedures such as the foregoing ll I ha~e not proved satisfactory when applied to electroless copper 12 I plating processes, As the electroless copper deposi~ion reaction 13 ~ proceeds, plating chemicals in the solution must be replenished, 14 1 T.he chemical replenishment usually causes local fluctuations in lS I the concentrations of chemicals and the introduction of impurit-16¦1 ies into the solution, Then too, dirt or dust gradually - -171¦ accumulates in the deposition solution, As 2 result, discrete 18 I particles of precipitated copper form in the deposition solution l9 I and come into contact with the equipment surfaces, Such copper precipitates draw high current through the metallic plating equip 21 ~ent in prior art processes, causing the electrical potential - :
22 imposed on such equip~ent to decay and fall `below that needed to 23 resist electroless deposits. An equivalent area of metallic 24 copper requlres at least two orders of magnitude more current than stainless steel to resist electroless copper deposttion, 26 Thus, although the equipment surfaces ln prior art processes 27 are reslstant to electroless depositiori during the initial 28 stages o the plating operation, when attempts are made to 29 operate such prior art processes on a commercial scale,-with .

replenishment and unavoidable copper precipitation fror~ the ' - 5 -. ..' '~ . ,' ''.
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l bulk solution~ af~er a brief period the equlpment suriaces ~.
2 iose their resistance: Thus~ prior art procedures have not be~n ..
3 succes~iully adapted ~o comm rcial u~e. .
4 -.

5 An additional problem is often encountered in the manu- .. -

6 facture of printed circuit boards, in parlticular. In such manu- ..

7 facturing procedures, sometimes layers of copper whic~ ar~ not

8 part of the circuit pattern it.self are formed on the borders of

9¦ the insulating substratum. If such copper borders contact the lO1 metal racks supporting the substratum, the current is drastically ..
ll¦ increased, thus diminishing or decaying the electrical poten~ial 121 applied to the rack such that i~ eventually falls below t~e l31¦ minimum potential required to avoid plating. The prior art pro-14j cedures have failed to avoid this problem also. The potentio.
` 15 stats of the prior art processes are capable of ma~imum currentE
16 of no ~reater than one ampere, indicating a lack of knowledge or 17 understanding of the problems encountered in plating on a larger 18 scale, such as in commercial operations, ....
19 . ~.'.'.. "
20 Ii is an object of this invention to provide a process ..
21 for electroless copper deposition in which metallic platin~
22 equipment in contact with the deposition solution is rendered 23 initially resistant to electroless copper deposits and maintained 24 in a resistant state for extended periods of the plating opera-25 tion.
26 .
27 It is another ob~ect of this invention to enable use 28 in commercial scale deposition processes of metallic plating 29 equlpment for longer periods without the build-up of copper ....
30 deposit~ which mus~ be removed by e~chingt ..
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595-187A . , ¦
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i It is ~nother object of this invention to providc 2 electro1ess copper deposition processes in wh~ch copper p~e-3 cipitates in contact with the plating equipment ca~ be easll~
4 remo~ed, as by brushing, sweeping, vacuuming, a~d the like, without the stopping or shutting Aown of the plating operat~on, S .
7 It is another object of this invention to pro~ide 8 improved methods of printed circuit board manufacture in which ~s ~ . . 9~ ¦~adh~en~ electroless copper deposits on equipment surfaces are lO ¦ avoided, ' 11 I
12 ¦ It is another object of this invention to pro~ide a 13 I method for the production of printed circuit boards in which 14l~ undesired build-ups of electroless copper on the edges and borders of panels being plated is prevented, 16 I .
17 ¦ The foregoing objects as well as addttional objects 18 I which will be clear from the following description are achieved 19 by the process of the invention now described.

21 ~ Notably, the practice of this invent~on can result 22 in a reduction in the cost of plating of up to 30 percent or 23 more, due principaliy to savings on plating chemicals and acids 24 and neutralizing bases needed to periodically etch copper away from plating ~anks and other equipment surfaces. Additional 26 savings are achieved due to avoidance of labor costs and lost 27 productlon time necessitated by such etchlng procedu~es, 29 / .
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1 DESCRIPTlON OF THE lNVENTI~~ - ...
2 . . .
3 According to this invention, there is provided an i~prove ~
4 ment in a method for electrolessly depositing copper fro~ an .-5 electroles~ coppe~ deposition solution on a substrate, in which ..
metalllc equipmen~ is in con~act w~th the solution, The improve- ....
7 ment comprises: .
8 ...
9 (1) initially imposing on the metalllc A.~.
equipment a potential more positive than the 11¦ mlxed potential of the electroless copper .-.
12¦ solution and sufficiently positive to render ~.
13¦ the equipment surface substantially resistant 14l to electroless copper depositi~n;
. .
16 (2) electrolessly depositing copper on 17 said substrate from said electroless copper ...
18 solution; and . ....
19 .
(3) while electrolessly depositing ...... -21 copper on said substrate, maintaining on 22 the metallic equipment surface a potential : :
23 more positive than said mixed potential and :
24 sufficiently positive to resist electroless copper deposition, 26 . .
27 The described process can be applied to any type of . 28 metallic equipment used in electroless copper deposition opera- ..
29 tions, including plating vessels, racks supporting the sub- .
strate being plated, p:Lumbing or any other piece o equipment ..... . . . ..

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1 in contact ~-lth the plating solution, 3 The foregoing process is practiced more spec~fically by 4 initially imposing on the metallic equip~ent surface, e,g,, plating vessel, rack, etc., a eurrent su~ficlent to establish 6 an electrical potential sufficiently pos~tive to resist adherent 7 electroless copper formation, electrolessly depositing copper 8¦ an the substrate being plated, and adjusting the aforesaid 9~ applied current as the deposition reaction proceeds to maintain 101 a sufficiently positive electrical potential on the metallic ~ equipment surface, i.e., positive enough to resist electroless 12,l plating. Preferably, a current in the range between 10 4 and 4 13~l milliamperes per square centimeter of surface area to be render-14 l¦ ed resistant to electroless deposition is used.

16 I By way of illustration, the procedure is carried out by 17jl providing at least one cathode in the electroless copper deposi-18 l¦ tion solution and in electrical connection with the metallic 19,l equipment surfaces, A current is applied in the electrical 2QIl connection between the cathode and metallic equipment surface 21 ! sufficient to create an electrical potential on the equipment 22 surface more positive than the mixed potential of the deposition 23 solution and positive enough to resist formation of an adherent 24 electroless copper deposit. This applied current is regulated during the electroless copper deposition to maintain t~e electric 26 al potential on the metallic equipment surface sufficiently 27 positive, 28 .
29 The terminology "mi~ed potential" herein is intended to mean that electrical potential at which copper begins to . ,' , ,_9_ _ .' ~ . .
- - . , . , ~

595-187~
,., 1 deposit electrolessly fro~ an electroless copper solution onto 2 a receptive surface with which it is in contact. Stated another ..
3 way, it is the electrical potential measured between a substrate 4 being plated with electroless copper and a standard reference S electrode in electrical connection with the substrate bein~
6 plated. Procedures for measuring ~he mixed potential of electro-7 less copper solution are known in the art. One such procedure 8 is described in the text hereinbelow immediately preceding the examples.
11 In general, electroless copper deposition solutions ..
12 useful in this invention are chzracterized by a mixed potential 13 within the range between -500 and -800 millivolts relative to a 14 standard silver-silver chloride reference electrode, and in the lS range between -550 and -850 millivolts relative to a saturated 16 calomel reference electrode, at the operating temperature of the :~
17 deposition solution, 18 .
19 Typically, in carrying out electroless copper deposition procedures according to this invention, electrical potentials 21 in the range between -500 and +500 millivolts, and more usually 22 between -300 and -100 millivolts, relative to the reerence 23 electrode, are imposed and maintained on the metallic retaining 24 walls of the plating vessel and/or on any other metallic .~ ao Jler~oJs ~oror/vc~
plating equipment in contact with the bath. Suchlcurrents 26 sufficient to passivate equipment surfaces, as well as any 27 copper already in contact therewith, e,g,, precipitated copper, 28 copper borders on panels being plated, and the like, r"
.29 r:
Uslng the principles described, this invention can be ' . 1 .
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595-187A ~, 1 advantageously employed to passivate, l.e., render resist~n~
2 to electroless copper, me~allic racks ~upporting the substrate 3 being plated in the deposition solution. Speci~ically, the ~ack 4 is electrically connected to a terminal of a current source, e.g., a standard 200 ampere rectifier, and the other terminal of the 6 current source is connected to a cathode suspended in the deposi-7 tion solution. A current sufficient ~o ,establish a passivating 8 electrical potential on the rack is supplied, copper is electro-9 ~ lessly deposited on the substratum supported on the rack, and during deposition the current is adjusted to ~aintain the rack 11 in a passivated state, Although ~he rack can be passivated usin~
12~ the same current source as for the plating vessel or other l3¦l platin~ equipment, it is preferred ~o use a separate power source 14 l¦ for each piece of equipment being passivated, 16ll The foregoing technique can be used, moreover, to prevan 17 1¦ the electroless deposition of copper on undesired areas of the 18 ¦¦ substratum. With particular reference to prlnted circuit board 19~1 manufacture using additive technia,ues, in some cases the insulat-ing panels which have been pre-cut to size, ~lasked and sensitized 21 are left with an exposed sensitized border on the edges of the 22 panel and the adjacent panel surface. When the panel is exposed 231 to the deposition solution, copper begins to plate out on the 24 sensitized panel edges and sensitized surface adjacent thereto to the same thickness as on the exposed areas of the panel corre-26 sponding to the desired circuit pattern, As a result, a con~
27 tinuous border of copper forms on the panel, In the typical case 28 this copper border, which is not part of the circuit pattern it-29 self, is cut off ~rom the rest of the panel and discarded, The build-up of this copper border can be prevented by contacting 331~
595-187A "

1 an edge of the panel with a metalllc surface of the rack and 2 supplying and mainr.aining enough current to the rack to passivate 3 both the rack and th~ copper border on the panel.

Plating vessels as well as other metallic plating equip-6 ment used ln the process of this invention can be cleaned of 7 any copper precipitate which may cling to the surface, by 8 I temporarily stopping ~he plating operation, emptying the plating 9 ¦ vessel and brushing, sweeping or vacuuming the copper away.
lO ! Such cleaning may also be employed during the plating operation 11 without emptying the plating vessel, as by vacuuming. It should 12 ¦ be noted that, in contrast to the prior art, in the process of 13 ~ this invention copper does not adhere to the passivated equip-14 ment surfaces even after prolonged operation, and such copper can be easily removed by the foregoing cleaning procedures and withou 16 the need for etching or other harsh chemical or mechanical clean-17 ing procedures.
18 .
19 In conducting the process of this invention, it will normally be found that a thin layer of electroless copper deposit 21 on the cathode or cathode~. Copper deposition on the cathode 22 surfaces can be ~ompletely or zt least partially avoided by inter 23 posing between the cathode and electroless copper deposition 24 solution a membrane permitting electrical conductivity between the cathode and deposition solution but preventing passage throu 26 o~ copper ions from a deposition solution.
27 ~ .
28 Ion exchange membranes, either anionic or cationic, can 29 be employed for this purpose, Selection o~ the particular ion exchange membrane will depend on the specific copper ion com-.' . .
.... . ~ . .. .: ............... -. . . .

595-18`7A
. ....
1 plexing agent e~ployed in the bath. In cases where complexed ..
2 copper possesses a negativ~ charge, as where the complexing ,~
3 agent is of the amino acid type, cation exchange mem~ranes are ....
4 employed An example of such a membrane ~.s DuPont's Nafion. ID .~..
those cases where the complexed copper possesses a positive 6 charge, anion exchange membranes are employed, An example of 7 such a membrane is RAI Research Inc.'s Permion 1020. If the 3 complexed copver is neutral, as in the case when alkanolamine complexing agents are employed in the bath, either an anion or - ...
cationic exchange membrane may be employed.
,11 ....
12 The process of this invention is effective for use with 13 plating vessels in which the retaining walls are made of non-14 noble metals such as steel, iron, nickel, cobalt, copper, titan- -ium, tantalum, chro~iu~ or the like. Similarly, the process can 16 be used to render resistant ~o adherent electroless copper 17 deposits other types of plating equipment made of such metals. ..
18 ..
19 Elec~roless copper deposition solutions useful in this ..
invention comprise, in general9 copper ion, a complexing agent ,, .
21 for the copper, a reducing agent or agents, and a suitable 22 solvent, e,g,, water. The copper ion may be supplied in the form 23 of a copper salt, preferably water soluble, e,g,, copper sulfate, 24 copper chloride, copper nitrate, or the like, .
26 The reducing agent is usually formaldehyde or a formal-27 dehyde precursor, such as paraformaldehyde, trioxane and form-28 aldehyde bisulfite, or for~aldehyde dertvative.
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Complexing agents for copper ions are well known, In 13 ~ , ;
. ~ ,tr'f~J~ ~f~rk. ' ~l~3~

gelleral, these are selec-ted from amollg Rochelle sal-ts, ethylene-diaminetetLaacetic acid (EDTA) or salts thereof, diethylene-diamineE~ntaa.ce-tic acid, ni-trilotriacetic acidr alka~ola~ines, gluconic acid, and the like. These are con~lercially available or can be prepared by follGwing procedures described in the literature.
Other ingredients, such AS stabilizers, ductility pro~
moters, wetting agents, and the like, can also be included in -the electroless copper deposition bath. The wetting agent is preferably employed ~n small amounts, e.g., less than 5 grams per li-ter.
Ca~,mercially available e.~amples are Pluronic* P85, BAS~-Wyandotte Corp., a block co~olymer of ethylene oxide and propylene o~ide, and Gafac* RE 610, GAF Corp., an anionic phosphate es-ter.
Special mention is made of the electroless copper deposition solu-tions described in copending Canadian application Serial No. 331,706, filed July 12, 1979. Ihese provide unusually fast rates of deposition with achievement of copper deposits of good quali~y which do not flake off from the substratum.
The pH of the copper deposi-tion solution is usually at least 10, and preferably 11 or above. For pH adjustment, an alkali metal hydroxide, e.g., po-t~ssium hydroxide, sodi~n hydroxide, or -the like, is typically employed.
me respec-tive concentrations of the various ingredients in the electroless copper deposition solutions are subject to wide variation within the follcwing preferred ranges:

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, 31Cl l Copper ~al~ 0.002 ~o 1.20 ~ol 2 Reducing agent 0.03 to 3 ~013 3 Comple~ing a~ent 0.05 to 20 ~i~es the 4 mols of copper p~ ad~ustor sufficient to give a pH
of lO.0 to 14.0, pr~fer-ably 11.0 to 14.0, measured 6 25t~coom temperature, e.g., 8 ¦ ~ates su~ficient to make one liter The copper plating solutions may be compounded within 11 more narrow li~its as follows:
~2 13 ¦ Soluble cupric salt, I preferably cupric 14 I sulfate or cupric I chloride 0.002 to 0.4 mol Formaldehyde or 16 formaldehyde precursor 0.06 to 0.50 mol 17 Cupric ion complexin~ 0.5 to 2.0 times the 18 ¦ agent, as described. mols of copper 191¦ Alkali metal hydroxide, sufficient to give pH
preferably sodium 11.2 to 13.9, at room hydroxide temperature, e.g., 25~C.
21 Water sufficient to make one liter 23 Concentrated versions of the foregoing can be manufactur 24 ed for subsequent dllution to the operating solutions described herein.

27 In the t.ypical case, the substratum to be metallized, 28 if initially non-receptive to electroless deposition, is treated 29 conventionally to re~nder it receptive to electroless copper from an electroless copper deposition solution, Wlth reference . ," , ,~
~ "'-" ' ' , ' ' I ' .,~ .

to the plating of plas-tics in general and the n-!anuEaeture oE
printed circuit boards in particular, this is done by con-tacting -the surface areas to receive the de~osit Wi h a sensi-tizing~sc~ed:ing solu-tion, Eor example, stannous chloride (SnC12), followed by dilute palladium chloride (PdC12), or alternati.vely, by eontaeting with a dilute acidic solution preparel from palladium chloride and exeess stannous chloride.
In the manufacture of printed circuit boards using addi-ti.ve or semi-additive proeedures, to obtain enhaneed adhes;~on of -the eleetroless metal layer to the substratum a layer of adhesive whieh ineludes finely divided partieles ol an oxidizable or degradable rubber is provided on the surface o~ the insulating substratum. The adherent resinous layer is norn~lly present in a thiekness, when dry, of about 20 to 30 microns-or more. Such resinous layers ean comprise, for example, an eFx~ resin~ a pheno'formaldehyde condensation preduet, a nitrile rubber in a eurable eomposition, ;melanine, polyaerylie, polyester, natural rubber or curable polystyrene resin, and the li~e, applied to the insulatina substra~m in a partially hardened sta-te, as ~y transfer e~ating, ~n~ ~hen c~red.
The adherent resinous layer is preferably pretrea-tea with a stro~
oxi`dizing agent suc.~ as ehrc~osulf.urie aeid, i~e., Cr~VI) in su'.:Eurie aeid, or a permanganate solution. Such artieles are deseribed in U.S.
Patent 3,625,758.
On the other hand, the insulating subs-tratum ean be eatalytie throughtout i.ts entire mkass to the deposition of eleetroless metal, sueh as by a unifol~ dispersion therein of a eatalytie ~iller, e.g., as shcwn in U.S. Patent 3,62~185.

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When through holes are drilled or punched in the insulating substratum Eor purposes of forming circu:it interconnections, the walls of the throuyh holes, containing exposed ca-talytic ma-terial, do not need to be sensitized.
Followin~ the foregoing pretreatments, -the sub-stratum to be plated is immersed in or otherwise exposed to the electroless copper deposition bath and plating is carried ou-t in accordance with the process of the invention.

BRIEF DESCRIPTION OF TE~E DRAWINGS
.
Fig. 1 illustrates a simplified plating system which can be used to practice this invention, comprising a ~;ating tank with metallic retaining walls, plating solution, power source, electrodes, substratum -to be plated and support means.
Fig. 2 shows a more detailed system, adapted for automa-tic 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 poten-tial (voltage~ for copper in an electroless copper deposition solution of the same formulation as in Fig. 3.

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595-1~7 1 DETAILED DESCRIPTION OF THE DRA~ING~
3 With respect to Fig. 1, plating tank 2, the retaining 4 walls of which are made oi steel, preferably stainless steel, or other suitable electrically conductive material, contain~
6 electroless copper plating solution 4. Metal probe 6 is 7 immersed in platin~ solution 4 and electrically connected to 8 nègative terminal 8 of direct current power supply unit 10.
9 Surface 12 of tank 2 is electrically connected through variable 10l¦ reslstor 16, to positive terminal 18 of power supply unit 10.
11¦ Retaining wall 12 of tank 2 is also connected to millivoltmeter 12¦ 20 and to standard reference electrode 22. Reference electrode 13 22 is in communication with plating solution 4. Workpiece 24.
14 ! is suppor~ed on metal rack 26. Rack 26, in electrical contact 15 ¦ with surface 12a of tank 2, is suspended in plating solution 4.
16 Workpiece 24 is electrically insulated from rack 26 by non-17 conductor (insulator) 27.

19 In practice, an electrolytic couple comprlsing suriace 12 (12a and 12b) of tank 2 and probe 6 ls formed by connecting 21 ¦ these to the opposite terminals of power supply unit 10, as 22 shown. Preferably before starting plating solution 4 (e.g., 23 by adding reducing agent, or by raising pH or temperature), 24 a potential more positive than the expected mixed potential of the started plating solution is applied to surfaces 12a 26 and 12b of tank 2 by adjusting resistor 16 as needed. Plating 27 so].ution 4 fol~ulation and conditions are adjusted by known .
28 means to start electroless plating (e.g,, by adding the reducing 29 agent, raising the pH, raising the temperature), Workpiece 24 3~ is lmmersed in plating solution 4, and plating begins, The 3:1131 ~ ;

$95-187A
1 electrical pOT ential of surface 12 wlth respect to reerenc2 2 electrode 22 is monitored durin~ plating by observing mllll-3 voltmeter 20, and this potential is maintained morc positiYe than the ~ixed potential of plating solution 4, The electrlcal potential of the plating equipment surfaces can be regulated 6 manually, as in the embodiment shown in Fig, 1, or automatically, 7 as desired. A system in which the electrical potent~al is 8 controlled auto~atically is illustrated in Flg, 2.
Wi.th reference to Fig, 2, 220-volt alternating current ll! line 28 extends to direct current supply unit 30, which is 12 capable of generating one hundred amperes of current at seven 13 volts. Negative terminal 32 of power supply unit 30 is 14 electrically connected by line 34 to metal probes 36, which lS are suspended in metal tank 38. Tank 38 contains plating 16¦ solutlon 40 and is grounded by grounding wire 42, Positive 17¦ terminal 44 of power supply unit 30 is electrically connected 18¦ by line 46 to pass transistors 48, which are in parallel lg~ and driven by Darlington power transistor 50. Each of pass transistors 48 preferably has an output capaclty of fifty 21 amperes. Darlington power transistor 50 is preferably set 22 for a gain of about ten thousand:one.

24 Pass transistors 48 are connected by electrical line 52, meter shunt 54, and electrical line 56 to tank 26 38, Meter shunt 54 -Ls connected by line 58 to standard 27 ammeter 60, which measures the current from pass trans~stors .
~8 48 across meter shunt 54, Capacitor 62, preferably having a 29 capacltance o~ two m.Lcrofarads, ls connected across electrical llne 34 and meter shult 54 to reduce background electrical nolse, .
~ 19 ~
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59S-187A ...
. , . ~:.",' 1 Electrical line 64 extends from cank 38, and is conneete~ ..
2 to positive terminal 74 of voltage amplifler 68. Electrical l$ne .-..
3 70 e~tends from s~andard reference electrode 72, and is connected ..-to negative terminal 66 of amplifier 68~ Amplifler 68 is set for ~.....
a gain of ten:one. Reference electrode 72 is a conventional 6 silver/silver chloride electrode, or equivalent reference 7 electrode, in salt bridge communication with plating solution ...
8 40 in tank 38.
9 .. ' Amplifier 68 is connected by electrical line 76 to 11 negative terminal 7~ of control amplifier 80. The voltage o~tput 12 I from amplifier 68 to amplifier 80 is measured across standard 131l voltmeter 82, connected by line 84 to line 76. Positive terminal 141 86 of control amplifier 80 is connected by line 88 to potentio-meter (set point~ 90, and to FET switch 92, Potentiometer 90 16 preferably has a maximum possible setting of from 3 volts 17 positive to 2 volts negative. ...
. .
1~ ,,,','.' 19 Electrical lines 94 and 96 extend from terminals 98 and 100, respectively, of meter shunt 54, to voltage amplifier 102.
21 Line 94 ls connected to positive input terminal 104 of amplifier 22 102. Line 96 is connected to negative input terminal 106 of 23 amplifier 102. The voltage output from amplifier 102 goes through 24 line 108 to positive input terminal 110 of control amplifier 112.
Amplifier 112 is set for a gain of twenty:one, Negative input 26 terminal 114 of control amplifier 112 is connected to potentio 27 meter (set point) 116. Electrical line ll8 runs from amplifier 28 112 to FET switch 92.

Capacitor 120, preferably having a capacitance of one ..

. ,' ' .
' "'' ' I -' f ' . ..
595-187A .

l microfarad, and resis~or 122 preferably having a resistance of one ohm, are included in the circuit for purposes of background ...
3 noise reducti~n. ....
4 ........................................................................ ...
Preferably, to pre~ent overheating, pass transistors 6 48, power transistor 50, capacitors 120 and 62, resistor 122 .
7 and ammeter 60 are situated on heat sink 124 (indicated by 8 dotted lines), and cooled by fan 126 connected to llO-volt .....
9 alternating curren~ line l28. Heat sink 124 is made of aluminum lO or other standard heat absorptive material. ..
.11 . ".
1~ In practice, the process of this invention is carried 13 out as i.ollows: .
14 .
With reference to ~ig, 2, alterna~ing current from 220-16 volt line 28 is converted to direct current in D.C. power supply ..
17 unit 30. Negative potential from power supply unit 30 is applied ....
181 to electrodes 36 in tank 38. Electrodes 36 are thus made .-lgll cathodic. Positive potential from power supply unit 30 is sent ....
20 through power transistor 50, pass transistors 48, line 52, meter .
21 shunt 54, and line 56 to tank 38. Tank 38 is thus then made 22 anodic. The current across meter shunt 54 is monitored by use 23 of ammeter 60, 24 . . ..
A silver/silver chloride reference electrode 72 is 26 suspended in tank 38 and kept in communication with plating 27 solution 40 in conventional manner by means of a porous membraDe 28 therebetween. By cormecting reference electrode 72 and tank 38 . .
2g to the opposite termi.nals of amplifier 68, in the manner shown, .
30 the potential (voltage) o~ the walls of tank 38 is thus contin- ..
,,' ..
- 21 ~

. ' , .
. , . .. . . , _ .

IL~LE33~
5~5-187~

1 uously monitored, ~nd moreover, controlled as follows:

3 If the voltage from amplifier 68 to control amplifies 4 80 is predominantly positive, amplifie:r 80 tends to turn out a positive voltage If, on the other hand, the volta~e from ampli 6 fier 68 to control amplifier 80 i5 predominantly negative, ampli-7 fier 80 tends to turn out a negative voltage. A positive ~oltage 8 to power transistor 50 causes the latter to generate current 9 flow. A negative voltage to power transistor 50 causes thP
latter to shut off and substantially all current flow to cease.
11 During plating, as the potential (voltage) of tank 38 becomes less 12 negative (i.e.~ more positive? with respect to referenoe 13 electrode 72, amplifier 68 applies a positive voltage to contr.ol 14 amplifier 80, which, in turn, applies a negative voltage to power transistor 50. By adjusting set point 90, the positive 16 voltage output of control ~mplifier 80 is regulated as needed 17 to establish the overall output of amplifier 80 and to achieve 18 the desired current flow to tank 38 as needed to maintain the 19 potential (voltage) of tank 38 to the set potential which is more positive than the mixed potential of solution 40, 22 Excess current is prevented from flowing to tank 38 by 23 means of voltage amplifier 102 and control amplifier 112, The 24 potential (voltage) across meter shunt 54 ls directly proportion-al to the current flow from power transistor 50 and pass trans-26 istors 48. This voltage is boosted in amplifier 102 and further 27 amplified in control amplifier 112, If the amplified voltage 28 from amplifier 112 to the gate of FET 92 rises above the cut-29 off point, FET switch 92 opens, voltage dlviding the output of potentiometer 90 rleducing the set point of amplifier 80, thus .' '. .
~ . ~ . .

3~

,....
1 re~urning the system to equilibrium.
3 The output of amplifier 102 is balanced by the output 4 of potentiometer 116 which establishes the set point of amplifier S 112. Adjusting 116 determines the maximum current allowed before 6 reducing the set poin~ of control amplifier 80. The function 7 of amplifiers 102 and 112 is to limit the maxi~um current 8 applled to the tank and cathodes to protect the total system. -g .,~
In the foregoing manner, the voltage applied to tank 38 11 is maintained more positive than the known mixed potential of 12 ¦ plating solution 40, and substantially no metal deposits on the -13ll walls of tank 3B.

151l In the specific procedure described abave, metallic 161 racks can be used to support the substrates being plated and 17 such racks can be rendered resistant to electroless copper 18 deposition using the principles described. In such cases, it l9i is desirable to employ a separate control circuit for supplying current to the racks. If the substrates supported on the racks 21 and being plated are panels having copper borders, a larger ~ :
22 current supply will be required to hold the passivating electrical 231 potential on the racks (and copper borders on the panel). If, 24 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 26 interconnection with the circuit pattern, it is preferred to 27 insulate the substrate from the rack by interposition of a 28 substantially electrically non-conductive ~aterial (See Fig, 1), 2~ The metal racks as weLl as any metal fasteners will, of course, still be resistant to electroless plating.

l~ 1163101 1 1 5~187A
.. ....
1 Figs. 3 and 4 show the current as a function of the 2 voltage for copper and staInless steel in an electroless 3 solution having the same compositions of Example l. Positive _ 4 currents are oxidizing cu-rents and negative currents are reducing, i,e., plating, currents, At point l'B" in Fig. 4 6 (copper electrode), ~here is no net current flow, so this 7 ¦ potential is the mixed potential of the deposition solution.
8 I In region "A" J more copper ions are being reduced than 9 formaldehyde is being oxidi~ed, so there is a net negative (plating) current, In region "C", more formaldehyde is being 11 oxidized than copper ions reduced, so there is a net positive 12 (oxidizing) current. In region "D", a film forms on the -~
13 surface of the copper electrode that is non-catalytic to the .
14 oxidation of formaldehyde, so the current decreases. The maximum current required to passivate is 4 milliamps per square 16 centimeter. Copper ion reduction does not occur at potentials 17 more positive than about -450 millivolts (mV) relative to the 18 reference electrode, or 250 millivolts more positive than the 19 mixed potential. Re~ion "E", extending from about ^425 to -225 mV, is the passlvation range. In this reglon, the potential 21 is too anodic to reduce copper ions and the electrode surface 22 is non-catalytic to the oxidation of formaldehyde, so there is 23 little current flow. Since the current in this region is about 24 the same for the solution without formaldehyde as for the solution with it, most of the current flowing in this region 26 is not due to the oxidation of formaldehyde, Some copper 27 oxidation may occur in this region. The current flowing in 28 region "~" is due to the oxidation and partial dissolution 29 o the electrode ~urface, Region "G" is a second passlvation region. Beyond region "G" several things could oxldl~e - -. . .

- 24 _ .' , .
.. . ~ ...... , r .

11833L01 ~ ! ' 59~-187~ ' 1 OH lonc, EDTA, copper or formaldehyde, '-' 2 3 Fig. 3 shows that a stainless steel electrode i~
relatively passlve from -500 eO ~400 m~. At potentials mose .:
negative than -500 mV copper would begin to plate on the 6 stainless steel, altering its characteristics, At -325 mV
7 the-current density is 40 ~imes less for stainless steel 8 than for copper (.020 vs. .80 mA per cm2). Stainless s~eel 9 is very slow to initiate plating in an electroless copper

10 ¦ bath. This is because it is an extremely poor catalyst for

11 I the oxidation of formaldehyde. However, once plating ~ h&~

12 it can proceed rapidly because the copper can then catalyze :-'

13 the formaldehyde oxidation.

14 A potential of about -325 mV (versus the saturated 16 calomel electrode) is bes't for passivation of both stainless 17 steel and copper since it is in the middle of the copper 18 passivation range and the stainless steel current density is 19 very low, .'-21 The passivation range can shift slightly as the pH
22 changes. The shift is in ehe same directlon as the solution 23 mixed potential with pH changes, and is of similar magnitude.
24 Thus, a mixed potential probe for the reference electrode could work very well, 27 In Figs, 3 and 4, the values for potentials were 28 measured using a model 174A Polarographic Analyzer (Princeton 29 Applied Research) and the reference for all runs was a saturated calomel ele'ctrode. l'he current was monitored as the potential ' -'25'~ ' .,'. .' ,. . .' 1~ 11S31~

5~5-1~7 . . ,,.,.'.
1 was scanned under an air at~osphere and recsrded on 2 P~R X-~ ~-2 Record~r, 4 MEA_UREM~`NT OF_MIXED PC?TENT~A~ - ._ S "
6 Place a clean copper surface in the electroless metal 7 deposition solution - metal will begin to deposit on the ..
8 copper surface, Allow 3-4 minutes to permit a steady state 9 to be attained. Connect the copper surface to one terminal of a high impedance millivoltmeter, such as a standard pH^meter, Connect a standard reference electrode immersed in the bath ...
12 to the other terminal of the high impedance millivoltmeter.
I .....
13¦ Measure the difference in potential (voltage) between the 14¦ copper surface and the reference electrode, This is the m~xed

15¦ potentiàl of the copper deposition solution, i,e,, the potential

16~ at which copper deposits from the solution, I .:' 19 ~
20¦ The invention is illustrated in the following examples, 21¦ which are not intended to be limiting. ~ :

23¦ EXAMPLE 1 241 .
25¦ An article useful in the manufacture of printed circuit 26 ¦ boards is prepared for metallization in accordance with this 28 ¦ invention as followsl-~, Z9 ¦ (a) Take a clean epoxy-glass laminate (SYNTHANE
30 ¦ FB 620, Taylor Corporation), having a thickness of 1,6 milli-~r~ ~e ~,~k ~ 26 -l ' .
.. .... . , ' ''' . ., .~. ~

I ~ 3~ I ~
` . ` ~, 59~t-187A
..
1 meters; coat the top and bottom surfaces with the following 2 composition:

4 Methyl ethyl ketone415 grams ..
Cellosolve acetz~e2,375 grams 6 Nitrile rubber liquid 590 grams Nitrile rubber, in lumps 350 grams 8 I Oil soluble phenol resin, --9 I thermal-setting350 grams I ! Epoxy resin ~epichloro-10 I hydrin) d~rivative400 grams -11~, SiO2, finely divided300 grams 12 ¦l 8utyl carbitol1,830 gra~s --13l1 Viscosity about 600 cps.
¦l at 22C.

; 16 ll Apply enough adhesive to achieve an adhesive layer thickness,

17 l¦ when dry, of 25 micrometers. -

18 ¦!
Il . ,.-19ll (b) Air dry for several minutes, and heat at 160C.
20 I fo- 30-90 minutes to harden adhesive layer.

22 (c) Drill laminate with multiple through holes, each 23 hole having a diameter of one millimeter.

(d) Promote bonding sites by contacting adhesive 26 layer for 15 minutes with a so~ution having the following 27 composition:

/
. .

~ t~dc ~ k . 27 ~
.' .
, 1, . . .. ~ ,- ,. - - . . .. .".

Chromic acid (.CrO3) 100 grams Concentxa-ted sulfuric acid (H2SO~) 500 grams Wa-ter (to make) 1,000 milliliters (e) Rinse briefly wit:h water.

(.) Neutralize surface by im~ersing ~or several minutes ln an aqueous solution comprising 1% sodium meta-bisulfite (Na2S205)..
; (.g) Immerse workpiece for several mi~nutes in cold running water.
.(h.) Sensitize adhesive surface and through holes to electroless copper deposition by contact~ng for 5 minutes with O~YTRON* Ac-tivator 316, a palladium chloride/--tin chloride sensitizing solution commercially available from Sel-Rex Co. of OMF Corp., Nutley, New Jersey.
Rinse sensitized surface b iefly with water.
(j) Contact for 5 minutes with an aqueous solution comprising 5% fluoboric acid by volume, to remove excess tin salt.
(k) Rinse briefly with water.
fil ~ Air dry.

*trade mark mab/~

~1 11831~1 1 595-1~7A

- 1 (m) Selectively mask areas of adhesive layer not 2 corresponding to desired circuit patter~s with RIST0N ~ , a dry 3 fil~ acrylate photoresist available from DuPont. The through 4 holes and areas of the adhesive surface corresponding to the desired circuit pattern should be left unmasked and exposed.

7 The laminate is now ready for electroless metal 8 deposition in accordance with this invention as follows:

10 ¦ The electroless copper deposition solution has the 11 following composition and is operated at 72C,:
12 ~
13 CuS0 '5H 010 grams/liter 14 formaldehyde4 milliliterslliter 15 I wetting ag~nt 0.2 gram/liter 16 I tetra-sodium salt of EDTA35 grams/liter 17 sodium hydroxide (NaOH) to pH 11.7 (as measured at 1& 25 C.

19 sodium cyanide (NaCN) 0.005 gram~liter water to volume 21 ~
22 The copper deposition solution has a mixed potential 23 of -630 + 20 millivolts, measured with reerence to a standard 24 silver/silver chloride electrode.
.
26 All of the ingredients of the foregoing deposition 27 solution except ~he ormaldehyde are mixed together in a stain- .
28 less steel plating vessel. A stai.nless steel cathode ls 29 i.mmersed in the solution and connected to the negative terminal of a variable d.c. rectifier having a maximum capacity . ' - 29 ~

~ ! ~ .

I ~ 31~ I ~
.' . ~ ?
595-187~ .
, . , ..........
1 of 8 volts, 130 amperes. A standard silver/silver chl~rid2 2 reference electrode is i~mersed in the deposition solution ~ , 3 and connected to one side of a millivoltmeter outside the , solution, The other side of the millivoltmeter is connected 5 to a wall of the stainless steel ves~el, .-7 The electrical potential on the plating vessel wall 8 relative to the reference electrode is adjusted to -200 milli-9 volts by regulating the rectifier, The electroless copper 10 solution is started up by adding the formaldehyde. The époxy- -.
glass laminate, which has been pretreated and sensitized as .-.
12 ¦I described and is supported on a stainless steel rack, is 13l¦ immersed in the electroless copper sol~ution. Copper begins to.
4 ¦I deposit electrolessly on the laminate. After 10 hours or/ 2 copper depos~t of 20 micrometers has been achieved, the laminate 16 is taken from the deposition solution. It is observed that ,,-,.
17 during the electroless plating reaction, substantially no ::
18 electroless copper has been deposited on the stainless steel :::
19 vessel surfaces or stainless steel rack in contact with the electroless copper deposition solution.
21 I ~ `
22 EXAMPI.E 2 24 The procedure of Example 1 is repeated, except that an electroless copper deposition solution having the following 26 composition is used: ' 27 / ' :
28 / , ' . ~, 29 / ':
30 / ' . ~

¦ ilb31(~

5g5-18 . ....

l Cupric chloride 9.~5 grams/liter 2 Tetrasodium ~alt 3 of EDTA35 grams/liter Wetting agent 0.2 gram/liter S Sodium cyanide (NaCN) 0.004 gramJliter 6 Formaldehyde (37%)3.3 ~lilliliters/liter 7 Sodi~lm hydroxide (NaOH~ to pH 11.6 (as measured at 8¦ Water to volume -gjl Temperature 72~C.
1011 Mixed potential -610 millivolts ,...
12 1I Substantially the sa~e results are ob~ained.
13 l~
14¦~ EXAMPLE 3 15 1l 16 ¦¦ This exa~ple illustrated the use of a two electrode 17 ll system, i.e., operated without a reference electrode in a 18 I process according to this invent~on.
lg I '.'.. '' An electroless copper solution having the same 21 composition as in Fxample l is placed in a vessel having 22 stainless steel retaining walls. The vessel has a capacity 23 of 8,000 liters and an internal surface area of about 60 square 24 meters.
26 , The rectifier is adjusted to obtain a potential o~
27 O. 45 volt between the stainless steel cathode immersed in the 28 deposition solution alld the stainless steel walls of the vessel.
29 After this ad~ustment, it is observed that the walls of the vessel have a potential of from -300 to -400 millivolts, 1. . ",.. ,........... .. ... \ . . .,,,: ~

~ L83~LQ~L ~ r . ~ '.!
595-187A :

1 relatlve to the silver/silver chloride reference electrode.
2 After this initial measurement, the reference electrode is 3 disoonnected and removed, Tne initial current needed to achieve 4 0.45 ~olt between the vessel walls and steel cathode is 0.5 _ S ampere, equivalent to a current density of 10 4 milliamperes per square centimeter, 8 ~ Six stainless steel racks, contalning 300 substrates 9 per rack, are placed in the electroless copper solution. These ~
are removed at intervals of 18 to 22 hours, to rPpla~e the 11 plated substrates with fresh substrates to be plated. During 12 the first 24 hours of plating operation, as plated substrates 13 are removed and new substrates are introduced into the solution, 14 it is observed that a precipitate comprislng metallic copper forms in the solution. Some of t.his precipitate comes into 16 contact wi~h the surfaces of the plating vessel. The current 17 needed to maintaln 0.45 volt between the vessel surfaces and 18 steel cathode rises. Over the next several days of opera~ion, 19 it is observed that the current required to maintain 0.45 volt rises and falls in the range of from 2 to 100 amperes as further .
21 metallic copper precipitates, co~es into contact with the 22 vessel surfaces and is passivated, 24 At the end of about one week, the plating operation is interrupted. The copper precipi~ate in contact with the 26 interior surfaces of the vessel consists of passivated non-27 adherent particles which are easily removed by sweeping with 28 a brush.
29 . .
The invention in its broadest aspects is not limited ~ 32 .' .'. I .
,,,, . ..... - ~ .. ~ ~

~ 1183101. 1 ~

595-187A . ` ., '.' ,, .1.

1 to the speci~ic steps, procedures and materials described, It ..
2 is to be understood, therefore, that departures ~ay be mad~
3 from the speclfic embodiments described ~hile keeping within ...
4 the scope of the accompanying claims and without departing .. ~.
from the principles of the invention or without sacrificing 6 its chief advantages.
7 / .
8 / ~
9 I ' ..
10 / .........
.11 / I ',',.'".
12 / ~ I
13 / ~:
14 / .
15 / .
16 I j .

19 / - I .

20 I . ..

22 / .~
~' / - . . : ' 25 / .
26 / . .
27 / ' .
28 I .
29 / . ...
30 I ~ . ...

'I .1'' ,

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. In a method for electrolessly depositing copper from an electroless copper deposition solution of known mixed potential on at least one substrate sensitive to such deposition, the improvement for rendering the metallic surfaces of plating equipment in contact with the solution substantially resistant to electroless copper deposition, comprising:
(1) initially imposing on said metallic plating equipment surfaces an electrical potential more positive than the mixed potential of the electroless copper solution but not substantially more positive than required to render said equipment surfaces substantially resistant to electroless copper deposition;
(2) electrolessly depositing copper on said substrate from said electroless copper solution; and (3) while electrolessly depositing copper on said substrate, monitoring and automatically adjusting said more positive electrical potention so as to maintain said more positive electrical potential sufficiently positive to resist electroless copper deposition on said equipment surfaces.

2, The method of Claim 1, in which the electroless copper solution has a mixed potential in the range between -500 and -800 millivolts relative to a standard silver-silver chloride reference electrode and in the range between -550 and -850 millivolts relative to a saturated calomel reference electrode.

3. The method of Claim 2, in which the electrical potential imposed and maintained on said metallic equipment surfaces is in the range between -500 and +500 millivolts relative to said reference electrode, and more positive than said mixed potential.

4. The method of Claim 3, in which said electrical potential is in the range between -300 and -100 millivolts, relative to said reference electrode.

5. The method of Claim 2, in which the electrical potential imposed and maintained on said metallic equipment surfaces is at least about 250 millivolts more positive than said mixed potential.

6. The method of Claim 1, in which said plating equipment comprises a plating vessel having metallic retaining walls in which the deposition solution is contained.

7. The method of Claim 1, in which said plating equipment comprises a rack supporting the substrate being plated in the deposition solution, said rack being con-structed wholly or in part of metal.

8. The methods of claims 6 or 7, in which said equipment metal is steel.

9. The methods of claims 6 or 7, in which said equipment is stainless steel.

10. A method for electrolessly depositing copper from a deposition solution of known mixed potential on a substrate sensitive to said deposition, comprising:
(l) initially imposing an electrical current on the metallic surfaces of plating equipment in contact with the solution, said current being sufficient to provide an electrical potential on said equipment surfaces more positive than the mixed potential of the solution but not substantially more positive than required to render the equipment surfaces substantially resistant to formation of an adherent electroless copper deposit;
(2) electrolessly depositing copper on said substrate from said electroless copper solution; and (3) while electrolessly depositing copper on said substrate, monitoring and automatically adjusting said current imposed on said equipment surfaces so as to maintain such electrical potential more positive than said mixed potential but not substantially more positive than required to resist electroless copper deposition on said equipment surfaces.

11. The method of claim 10, in which the electrical potential imposed and maintained on said metallic equipment surfaces is in the range between -500 and +500 millivolts, relative to a standard silver-silver chloride reference electrode or saturated calomel reference electrode, and more positive than said mixed potential.

12. The method of claim 11, in which said electrical potential is in the range between -300 and -100 millivolts, relative to said reference electrode.

13. The method of claim 11, in which said electrical potential is at least 250 millivolts more positive than said mixed potential.

14. The method of claim 10, in which said current is in the range between 10-4 to 4 milliamps per square centimeter of surface area to he rendered resistant to said electroless copper deposition.

15. The method of claim 10, in which said equipment surfaces are comprised of stainless steel.

16. The method of claim 10, in which said plating equipment comprises a plating vessel having stainless steel retaining walls, at least one rack made wholly or in part of stainless steel, or both of the foregoing.

17. The method of claim 10, in which the electroless copper deposition solution comprises copper ion, a complexing agent for copper ion and a reducing agent.

18. The method of claim 10, in which the substrate being plated comprises an insulating material useful in the manufacture of printed circuit boards.

19. In a method for the production of printed circuit boards by depositing of copper on a substrate from a solution comprising copper ion, a complexing agent for copper ion and formaldehyde, while said solution is contained in a vessel having stainless steel retaining walls and said substrate is supported in said solution on a rack comprising stainless steel, the improvement comprising:

(1) providing at least one cathode in the electro-less copper solution and in electrical connection with said retaining walls and rack;
(2) initially imposing a current in said electrical connection, said current being sufficient to establish an electrical potential on said retaining walls and rack more positive than the mixed potential of said electroless copper solution but not substantially more positive than required to resist formation thereon of an electroless copper deposit from said solution;
(3) electrolessly depositing copper on said substrate from said electroless copper solution; and (4) while electrolessly depositing copper on said substrate, regulating said current imposed on said electrical connection so as to maintain said electrical potential more positive than said mixed potential but not substantially more positive than required to resist formation of an electroless copper deposit on said electrical connection.

20. The process of claim 19, in which the electroless copper solution has a mixed potential in the range between -500 and -800 millivolts relative to a standard silver-silver chloride reference electrode, and in the range between -550 and -850 millivolts relative to a saturated calomel reference electrode.

21. The method of claim 20, in which the electrical potential imposed and maintained on the retaining walls and rack is in the range between -500 and +500 millivolts, relative to said reference electrode.

22. The method of claim 21, in which said electrical potential is in the range between -300 and -100 millivolts, relative to said reference electrode.

23. The method of claim 21, in which said electrical potential is at least 250 millivolts more positive than said mixed potential.

24. The method of claim 19, in which said current is in the range between 10-4 to 4 milliamps per square centimeter of surface area to be rendered resistant to said electroless copper deposition.

25. A method for rendering a steel surface in contact with metallic copper substantially resistant to electroless copper deposition from a solution in contact therewith, comprising:
(1) initially imposing an electrical current on said steel surface sufficient to provide an electrical potential thereon more positive than the mixed potential of said solution but not substantially more positive than required to render said surface substantially resistant to electroless copper deposition from said solution;
(2) electrolessly depositing copper from said solution on a substrate immersed therein; and (3) while said electroless copper deposition is taking place, monitoring and automatically adjusting said current imposed on said steel surface so as to maintain the electrical potential thereon more positive than said mixed potential but not substantially more positive than required to resist formation thereon of an adherent electroless copper deposit.