CA2193007C - Process for extending the life of a displacement plating bath - Google Patents

Abstract

A stabilized spray displacement tin plating process of copper printed circuit innerlayers is disclosed for the manufacture of multilayer printed circuit boards. During prolonged use of spray tin plating, the plating solution becomes saturated with cupurous thiourea complex which precipitates interfering with spray nozzles and other mechanical components. The tin plating solution is stabilized by removing portions from the reservoir before saturation is reached, selectively precipitating the thiourea complex, and returning the remaining solution back to the reservoir. The precipitated cupurous thiourea complex is redissolved in acid and either disposed of by conventional waste treatment or the acid solution can be electrowinned to reclaim copper and oxidize thiourea to form a more acceptable acid solution for waste treatment.
Preferably,the thiourea is isolated from the anode during electrowinning, so that the acid thiourea solution formed may be used to partially replenish the tin plating solution in the reservoir. By such periodic removal of the complex, the plating bath is stabilized and its useful life is extended dramatically.

Description

2l3~aa7 PROCESS FOR EXTENDING THE LIFE OF
A DISPLACEMENT PLATING BATH
This application is a divisional of Canadian Patent Application Serial No. 2,083,196, filed November 18, 1992.
B~f ~t7~t~UND OF T17~ ~NVENTION
This invention relates to the chemical displacement plating. More particularLy, this invention relates to plating of tin on copper, copper alloys, and other metals by l0 chemical displacement using a spray or cascade application process. Still more particularly, this invention relates to the use of such chemical displacement plating in the manuf acture of printed circuit boards .
Coatings of tin typically have been applied to surfaces 15 of copper and copper based alloys by a particular mode of displacement plating , i . e ., immersion plating techni~ues such as disclosed in U.S. Patent 2,891,871, U.S. Patent 3, 303, 029 and U. S . Patent 4, 715, 894 . ( "Displacement"
plating is also known as "replacement" plating and the terms 20 are intended to be synonymous herein. ) In the disclosed immersion tin plating techniques, a bath is prepared containing an aqueous solution of a tin(II) salt, an acid, and thiourea or a thiourea derivative as essential ingredients. In the immersion tin plating process, an 25 article bearing a copper surf ace , e . g ., a copper clad printed circuit board, is immersed in the plating ~ath for a period of time during which the surf ace copper metal is oxidized to copper(I) ion and complexed with the thiourea and is replaced at the surface by the COn-;ULL~lltly reduced 30 tin metal from the tin(II) ion. After displacement plating has been completed to a desired thickness, the article is removed from the bath and is rinsed to remove residual plating solution. During the plating process the concentration of copper(I) thiourea complex in the immersion 35 bath increases. Likewise, some aerial oxidation of tin(~I) ion leads to increased tin(IV) ion concentration during the life of the plating bath. However, the concentrations of _ _ 21g3007 copper(I) complex and tin(IV) ion rapidly equilibrate due to the substantial drag-out of the plating solution with the plated article and the subsequent bath repl~n; ~hr-nt. The presence of tin( IV) ion in tin displacement plating is 5 undesirable since it reduces the ef f iciency of the plating bath. Immersion plating baths typically have a very small 6urface-to-volume ratia which minimizes aerial oxidation and typically the equilibrium concentration of tin~IV) ion is within acceptable limits. Nevertheless, when plated surface 10 thickness is critical, as in some printed circuit board applications, undesirable aerial oxidation during removal of the article from the immersion bath can result in streaks of non-uniform thickness in the plated surface.
Chemical displacement plating has been used in the 15 manufacture of printed circuit boards (PCB's) and particularly multilayer printed circuit boards. Printed circuit boards comprise a non-conducting or dielectric such as a f iberglass/epoxy sheet which is clad with a metal conductive layer such ~s copper on either one or both 20 surfaces. The metal l~yer on the PCB before processing typically is a continuous layer of copper which may be interrupted by a pattern of plated through holes or vias linking both surfaces of the board. During processing selected portions of the copper layer are removed to form a 25 raised copper circuit image pattern of the PCB. Multilayer PCB's are typically constructed by interleaving imaged cnn~ t i ve layers such as one containing copper with dielectric adhesive layers such as a partially cured B-stage resin, i.e., a prepreg, into a multilayer sandwich which is 30 then bonded together by applying heat and pressure.
Production of these types of printed circuit boards are described in "Printed Circuits Handbook", Third Edition, edited by C.F. Coombs, Jr., McGraw-Hill, 1988. Since a conductive layer with a smooth copper surface does not bond 35 well to the prepreg, several copper surface treatments have been developed to increase the bond strength between the layers of the multilayer PCB sandwich.
One such copper surface treatment is the use of immersion tin and tin alloys as a bonding medium for 2193~7 multilayer circuits as disclosed by Holtzman et al ., U. S .
Patent 4,715,894. In the dlsclosed process an immersion tin composition is disclosed containing both thiourea compounds and urea compounds to displacement plate the copper surface 5 of each PCB with tin by the immersion process prior to laminating them to form a multilayer board. Although bond strength of multilayer PCB's prepared by this immersion process was improved, the production efficiency of multilayer PCB's is linited by the batch process wherein 10 substantial quantities of plating bath i5 lost through drag-out of the solution with each PCB processed. Moreover, the PCB's made by this immersion process are susceptible to defects due to streaking described supra.
Innerlayer bonding of multilayer PCB's has been further 15 improved by the process disclosed in P;~11A-1;nn, U.S. Patent 5,073,456 and in a publication in "Printed Circuit Fabrication", Vol. 13, No. 5, Pages 46-60, May 1990 by K.H.
Dietz, J.V. Palladino and A.C. Vitale, entitled MULTILAYER
BONDING: CURRENT TECHNOLOGY AND A NEW ALTERNATIVE. The 20 in-line process disclosed includes a spray displacement tin plating step followed by a post-treatment step with a silane bonding mixture of a ureido silane and a disilyl crosslinking agent. In particular, PCB's are fed by conveyor through a series of treatment and rinse stations in which 25 the PCB's are sequentially cleaned, microetched, spray tin displacement plated, post-treated with the silane bonding mixture and dried. The PCB's prepared by this spray tin displacement plating system are substantially free of streak defects observed in the immersion batch process and the 30 multilayer PCB's prepared therefrom demonstrate improved resistance to A~ m; n~tion during typical high temperature soldering operations. During the plating process the plating solution is sprayed onto the PCB and the excess solution is recovered and returned to the plating bath sump 35 with minimal drag-out to succeeding rinse stations.
Although improved multilayer PCB's have been obtained by the disclosed process, it has now been observed that the activity of the plating bath solution declines during use due to the accumulation of tin(IV) ioD formed by aerial oxidation of tin(II) during the spray application step.
Concurrently, the concentration of copper ( I ) thiourea complex increases in the recirculated plating solution until its solubility limit is surpassed and crystalline complex is 5 precipitated which clogs the spray nozzles and interferes with the mechanical components of the plating system. In order to take full advantage of the benefits of the spray tin displacement plating process, there is a need to stabilize the activity of the tin plating bath and eliminate 10 the insoluble copper(I) thiourea complex precipitate and thereby extend the life of the plating bath with minimal replPn;chrPnt.
SI~IMARY OF TTT~ INvENTIoN
The life of the displacement plating bath has been 15 extended by the present invention which is a process for displacement plating a substrate metal surface with an other metal comprising the steps:
(a) providing a reservoir of aqueous plating solution comprising;
(i) a metal ion of a free metal, which is present in its lowest oxidation state wherein the free metal is different from the metal of the substrate surf ace;
(ii) a complexing agent;
( iii ) an acid; and (iv) the free metal;
(b) directing a stream of the aqueous plating solution onto the substrate metal surface; whereby a portioll of the metal ions of ( i ) are oxidized to ions in a higher oxidation state, and another portion of the metal ions of ( i ) are reduced to free metal, wherein said reduced free metal displaces surface substrate metal which is oxidized to an ion and complexed with the complexing agent to form a substrate ion complex dissolved in the reacted aqueous displacement plating solution at the surface of the substrate metal; and (c) returning the plating solution to the reservoir whereby at least a portion of the metal ions present in their higher oxidation state are reacted with the free metal _ . , ... . . _ _ . .. . . ... ... . _ ... _ . _ . . _ . . _ _ _ ... .

, ,,~,, , 2193007 ( iv) to form metal ions present in their lowest oxidation state to replenish the aqueous plating solution.
The life of the displ ~c ~ plating bath has been further extended and formation of complex precipitate 5 therein prevented by another ~ L of the present invention which is a process for displacement plating a substrate metal surface with an other metal comprising the steps:
(a) providing a reservoir of aqueous plating solution 10 comprising;
(i) a metal ion of a free metal, which is present in its lowest oxidation state wherein the free metal is different from the metal of the substrate surface;
(ii) a complexing agent; and (iii) an acid:
(b) directing a stream of the aqueous plating solution onto the substrate metal surface; whereby a portion of the metal ions of ( i ) are oxidized to ions in a higher oxidation 20 state, and another portion of the metal ions of (i) are reduced to free metal, wherein said reduced free metal :pla~ q surface substrate metal which is oxidized to an ion and complexed with the complexing agent to form a ~uL:,~L~te metal ion complex dissolved in the reacted aqueous 25 plating solution at the surface of the substrate metal; and (c) returning the plating solution to the reservoir;
(d) repeating steps (b) and (c) for a series of substrate metal surfaces whereby the uu~c~ LLd~ion of the ~iub:~LLate metal ion complex in the plating solution reaches 30 a high level which is below the ~:u~.c~:l.LLdtion at which the ion complex precipitate is formed; and when the cul~c~ LL~ltion of the substrate metal ion complex in the plating solution reaches the high level, ( e ) withdrawing a portion of the volume of the aqueous 35 plating solution from the reservoir;
( f ) cooling the withdrawn portion to a temperature at which the substrate ion complex is insoluble and the complexing agent is soluble so that the substrate ion complex precipitates from the solution;

. ~ 2193~Q7 (g) removinq the substrate metal ion complex precipitate from the solution;
(h) returning the solution to the reservoir; and ( j) repeating steps (e) through (h~ a sufficient number 5 of times until the concentration of the substrate metal ion complex in the plating solution drops to a predetPrm; ned low level. The activity of the plating solution may be maintained by adding sufficient complexing agent to compensate for its removal as a substrate metal ion complex.
In still another ' 'i---,t of this invention, the substrate metal or its alloy is reclaimed from its ion complex and the freed complexing agent is recycled into the displacement plating solution by the added process wherein the following steps are added:
(k) dissolving the removed substrate ion complex precipitate in an aqueous acid solution to form a redissolved substrate ion complex solution;
( 1 ) electroplating the substrate metal onto a cathode from the rediEsolved substrate ion complex solution contained in an electrolytic cell having the cathode and an anode to reform the complexing agent in the aqueous acid solution, wherein the electrolytic cell is configured so that the reformed complexing agent is isolated from the anode; and (m) replenishing the aqueous plating solution of step (a) with the aqueous acid solution containing the reformed complexing agent.
In an alternate ~mho~ L of this invention the ~-ub:,Llclte metal is reclaimed from the ion complex and the resulting complexing agent is oxidized for ~ pos~l as an acid waste by an electrowinning process wherein the following steps are added:
(k) dissolving the removed substrate ion complex precipitate in an aqueous acid solution to form a redissolved substrate ion complex solution;
(n) electroplating the substrate metal onto a cathode of an electrolytic cell containing the cathode and an anode from the redissolved substrate ion complex solution contained therein to reform the complexing agent whereby the 21s3un7 reforming complexing agent is oxidized at the anode to form an acid solution of the oxidized components of the complexing agent; and (o) disposing of the acid solution of the oxidized 5, -n~nts of the complexing agent.
In yet another aspect, the present invention provides a process for displacement plating a substrate metal surface with an other metal comprising the steps:
(a) providing a reservoir of aqueous plating solution 10 comprising: - I
(i) a metal ion of a free metal, which is present in its lowest oxidation state wherein the free metal is different from the metal of the substrate surface;
(ii) a complexing agent; and (iii) an acid;
(b) directing a stream of the aqueous plating solution onto the substrate metal surface; whereby a portion of the metal ions of (i) is oxidized to ions in a higher oxidation state, and another portion of the metal ions of (i) is reduced to free metal, wherein said reduced free metal displaces surface substrate metal which is oxidized to an ion and complexed with the complexing agent to form a substrate metal ion complex dissolved in the reacted aqueous plating solution at the surface of the substrate metal; and (c~ returning the plating solution to the reservoir;
(d) repeating steps (b) and (c) for a series of substrate metal surfaces whereby the concentration of the substrate metal ion complex in the plating solution reaches a high level of about 80% to 9596 of its saturation concentration which is below the concentration at which the ion complex precipitate is formed; and when the concentration of the substrate metal ion complex in the plating solution reaches the high level, (e) withdrawing a portion of the volume of the aqueous plating solution from the reservoir;
( f ) cooling the withdrawn portion to a temperature at which the substrate ion complex is insoluble and the 7a complexing agent is soluble so that the substrate ion complex precipitates from the solution;
(g~ removing the substrate metal ion complex precipitate from the solution;
(h) returning the solution to the reservoir; and (i~ repeating steps (e) through (h) a sufficient number of times until the concentration of the substrate metal ion complex in the plating solution drops to a predetormin~d low level of about 80% or lower of the saturation concentration.
In yet another aspect, the present invention provides a process for displacement plating a substrate metal surface with an other metal comprising the steps:
(a) providing a reservoir of aqueous plating solution comprising:
(i) a free metal which is different from the metal of the substrate surface;
(ii) a metal ion of the free metal (i), which is present in its lowest oxidation state;
(iii) a complexing agent; and (iv) an acid;
(b) directing a stream of the aqueous plating solution onto the substrate metal surface; whereby a portion of the metal ions of (ii) is oxidized to ions in a higher oxidation state, and another portion of the metal ions of (ii) is reduced to free metal, wherein said reduced free metal displaces surface substrate metal which is oxidized to an ion and complexed with the complexing agent to form a substrate ion complex dissolved in the reacted aqueous displacement plating solution at the surface of the substrate metal; and (c) returning the plating solution to the reservoir whereby at least a portion of the metal ions present in their higher oxidation state is reacted with free metal (i) to form metal ions present in their lowest oxidation state to replenish the aqueous plating solution.

2l93ao7 7b DETArT Fn DEcrRTpTIoN OF TH~ INVENTION
The present invention is directed to the stAh; l; ZAtion and repl~n;chr~nt of displacement plating processes in which the plating solution is sprayed, cascaded, poured onto or 10 otherwise applied in the presence of air to the substrate surface to be plated. The present invention is also directed to the reduction of waste generated by such a plating process.
Displacement platil~g solutio~ns useful in this invention 15 include the immersion tin and tin alloy solutions disclosed in Holtzman et al ., U. S . Patent 4 , 715 , 894 . The displacement metal plating process does not employ an electric current but is based on an electrochemical displacement reaction.
The metal substrate that is to be plated generally is more 20 active (less noble) thall the metal salt that is dissolved in the coating composition or plating solution. Copper may be plated by tin solution even though copper is more noble than tin when the immersion coating composition is acidic and contains thiourea as a so-called complexing agent. It has 25 been theorized that the relative electrode potentials of tin and copper are reversed in the presence of thiourea under acidic conditions. Once the metal substrate is completely coated, it is no longer available to displace the metal ions in the ~; c~l A~ ~nt coating composition. Metal ions 30 contemplated for use in the present invention generally are simple cations of the metal salt, e.g., tin(II) and tin(IV) ions .
Displ Al- ~ tin plating solutions are particularly susceptible to aerial oxidation. Consequently, application 35 of such solutions typically has been limited to immersing or dipping substrates into the plating solution, thereby minimizing aerial oxidation of the plating bath. A spray displacement tin plating process for bonding multilayer printed circuit boards is disclosed in Palladino, U.S.

21g3007 Patent 5,073,456 and in the Dietz et al. publication in "Printed Circuit Fabrication", supra. Such an in-line spray process while having advantages over the batch immersion process, is particularly impacted by aerial oxidation and 5 build up of by-products in the plating solution. The present invention is particularly directed to minimi~in~ or eliminating such limiting effects on the spray displacement tin plating process.
The present invention will be described in the context 10 of a spray displ i~r~ L tin plating process, particularly for the manufacture of multilayer printed circuit boards.
The multilayer printed circuit board has alternating layers of dielectric material which support copper circui~ry (which may have interspaced other layers such as a copper 15 sheet which serves as a ground plane ) which are adhered to an insulating layer through intermediate layers. The circuit board has conductive through holes which form electrical paths across the entire thickness of the board.
In formation of multilayer circuit boards several dozen 2~ conductive and nonconductive layers can be employed. Also, for formation of multilayer circuit boards, it is necessary to drill holes and defects can occur due to (9~ nin~tion of layers in the areas immediately surrounding a hole. If a defect is present in one o~ the layers or if delamination 25 occurs, generally the entire board must be scrapped.
Therefore high quality in each of the steps of formation of the printed circuit board is essential for commercial production. One such step for forming high quality multilayer boards is the formation of defect free tin 30 plating over the copper circuitry of each constituent board.
A starting material is a dielectric layer which contains on one or opposite surfaces a ~ lin~ of copper.
This copper layer is of a thickness of at least 4 microns and more preferably 32 microns and it is used to form 35 conductive circuitry. Well known techniques can be employed to form such circuitry such as described in Coombs supra.
The composition of the dielectric layer is not critical provided it functions as an electrical insulator.
Preferably, a partially cured thermosetting polymer ~,"~, 21g~o7 composition is employed which is known in the art as prepreg or "B" stage resin.
After formation of the conductive circuitry, it is nPe-Pqq~ry to form a thin outer layer of tin thereon. The 5 circuitry of the printed circuit board typically is first cleaned and etched, such as disclosed in Palladino supra.
The cleaned and etched printed circuit board is then tin plated using a process for rl; c~ L plating a copper surface of a printed circuit board ~with tin or a tin alloy 10 comprising the steps: (a) providing a reservoir of aqueous displacement tin plating solution comprising;
(i) tin(II) ion, (ii) a thiourea, (iii) an acicl, and ~iv) free tin metal;
wherein the ratio of the surf ace area of the f ree tin metal to the volume of the aqueous displacement tin plating solution is at least 4 in2/gallon (6.8 cm2/liter); (b) spraying a stream of the aqueous displacement tin plating 20 solution from the reservoir onto the copper surface; whereby a portion of the tin(II) ion is aerially oxidized to a tin(IV) ion, and whereby the tin(II) ion is reduced to the free metal to displace surface copper which is oxidized to copper(I) ion and complexed with the thiourea to form a 25 copper(I) thiourea complex dissolved in the reacted aqueous displacement tin plating solution at the surface of the copper; and ( c ) returning the sprayed and reacted aqueous ~9; qpl ~t-, L tin plating solution to the reservoir so that the portion of the tin( IV) ion formed is reacted with the 30 surface of the free tin to form twice the portion of tin(II) ion, so that the aqueous displacement tin plating solution of step (a) is replenished with the portion of the tin(II) ion f ormed .
As the concentration of copper ( I ) thiourea in the 35 aqueous displacement tin plating solution approaches saturation, precipitation of the complex is prevented by the process of the present invention wherein, after a period of plating use during which the concentration of the copper(I) thiourea complex in the aqueous displacement tin plating ~3~07 , . ~.

solution reaches a high level which i8 below the concentration at which the copper ( I ) thiourea complex precipitate is formed, e.g., between 85 % and 95 % of the saturation concentration of the conplex, the following steps 5 are added to the process described supra: (d) withdrawing a portion of the volume, e.g., 5 96 to 20 % of the volume, of the aqueous displacement tin plating solution from the reservoir; (e) cooling the withdrawn portion to a t~ L~lLULt: at which the copper(I) thiourea complex is 10 insoluble and the thiourea is 601uble so that the copper(I) thiourea complex precipitates from the solution; (f) removing the co;?per(I) thiourea complex precipitate from the solution, e.g., by filtering, centrifuging or the like; (g) returning the solution to the reservoir; and ( h ) repeating 15 steps ~d) through (g) a sufficient number of times, e.g., three or four times, until the concentration of the copper~I) thiourea complex in the aqueous displacement tin plating solution drops to a predetermined low level , e . g ., about 80 % of the saturation concentration of the complex.
20 If n~ ry, additional thiourea may be added to replenish the plating solution . The precipitated copper ( I ) thiourea complex produced by this process may be disposed of using conventional waste-treatment processes such as that disclosed by l~ietz et al., supra. Since thiourea is an 25 objectionable waste, a waste-treatment process using IIYdLUY~ peroxide was developed for its treatment to reduce concentrations to less than 1 ppm.
The copper or its tin alloy may be ro~ (' from precipitated copper(I) thiourea complex and the freed 30 thiourea may be recycled to replenish the aqueous displacement tin plating solution by the added process of this invention wherein the following steps are added: (k) dissolving the removed copper(I) thiourea complex precipitate in an aqueous acid solution to form a 35 r~9icf:olved copper(I) thiourea complex solution wherein the acid is the same acid ( iii ) as in the aqueous displacement tin plating solution; ( 1 ) electroplating the copper onto a cathode from the redissolved copper(I) thiourea complex solution contained in an electrolytic cell having the _ _ _ _ . .

cathode and an anode, to reform the thiourea in the aqueous acid solution, wherein the electrolytic cell is configured so that the reformed thiourea is isolated from the anode , e . g ., by a salt bridge , semipermeable membrane , a 5 fritted glass divider or the like; and (m) replenishing the aqueous displacement tin plating solution of step ( a ) with the aqueous acid solution containing the ref ormed thiourea .
The spray displacement tin plating process described by these added steps is virtually a closed loop plating system 10 with a very long plating solution life. The displacement tin plating solution is substantially replenished by the quantity of free tin metal added, i.e., the tin surface area in the solution, and the strength of the acid solution used to dissolve the complex precipitate. The only substantial 15 by-product would be the electrowinned copper alloy tbronze) with little or no waste treatment needed for rinse water.
An alternate process to the ~iepQRAl of the copper(I) thiourea complex precipitate formed in step (f ) above is an electrowinning process of this invention wherein the 20 following steps are added after step (g): (k) dissolving the removed copper(I) thiourea complex precipitate in an aqueous acid solution to form a redissolved copper(I) thiourea complex solution; (n) electroplating the copper onto a cathode of an electrolytic cell containing the cathode and 25 an anode, from the redissolved copper(I) thiourea complex solution contained therein to reform the thiourea so that the reformed thiourea is oxidized at the anode to form an acid solution of the oxidized components of the thiourea;
and (o) ~licpocing of the acid solution of the oxidized 30 c~mr~rPnts of the thiourea. By this process, the lliRplAI ~ t tin plating solution is partially replenished by the quantity of free tin metal added, i.e., the tin sur~ace area in the solution and additional acid thiourea solution would be needed to complete the repl-~n; :l L.
35 Also by this process, the copper or copper alloy (bronze) is reclaimed from the copper ( I ) thiourea complex and the resulting thiourea is oxidized for disposal as an acid waste by conventional waste treatment procedures.
The aqueous displacement tin plating solution may 21g30D7 contain additional ~ ts such as urea, reducing agents, surfactants and the like as disclosed in Holtzman et al., supra and Palladino, supra. ~hen a tin alloy is to be plated, a salt of a second metal such as lead, is present in 5 the solution. In a preferred F~mhr~ r 1 the aqueous displ ~ ~ tin plating solution contains a thiourea compound, a tin(II) salt, a reducing agent, an acid and a urea compound.
The tin(II) salts of an inorganic (mineral) acid such 10 as the sulfur, phosphorus and halogen acids may be used or an organic acid may be used (e.g., tin(II) formate, tin(II) acetate and the like). Preferred are the tin(II) salts of the sulfur acids such as sulfuric acid and sulfamic acid.
Alkali metal stannates may also be used such as sodium or 15 potassium stannate and the known equivalents thereof. Where tin/lead alloy coatings are deposited, lead acetate may be used as the lead salt.
The thiourea compounds that are used may be either thiourea or the various art known derivatives, homologes, or 20 analogues thereof such as disclosed in columns 11 and 12 of Holtzman et al ., U . S . Patent 4, 715, 894, supra . Thiourea is pref erred .
Free tin metal may be present in the aqueous displacement tin plating solution in any form, e.g., 25 extruded tin, "mossy" tin, cast tin, and the like. Extruded tin, such as tin slabs conventionally used as electrolytic anodes or tin wire, is preferred since the amount needed to control stabilization of the solution is easily adjusted by removing or adding portions of the tin to achieve the 30 desired surface to volume ratio. The ratio of the tin surface to the volume of the aqueous displacement tin plating solution typically will be at least about 4 in2/gallon (6.8 cm2/liter) and preferably about 16 in /gallon ( 27 . 2 cm /liter) or greater. The acids that are 35 used may be organic acids or inorganic acids (mineral acids ) based on sulfur, phosphorus, the halogens, or the mixtures thereof, the sulfur based mineral acids being preferred such as sulfuric acid and sulfamic acid. Particularly preferred is the mixture of sulfuric acid and hypophosphorus acid.
. . _ _ _ _ . _ _ _ _ _ .

Q~7 Some of the organic acids that may be used comprise monocarboxylic or dicarboxylic acids having up to about six carbon atoms such as formic acid, acetic acid, malic acid, maleic acid, and the like.
It is preferred, if possible, not to use halogen acids or halogen salts since halide residues will be produced in the tin coating deposited. Halide salts interfere with electrical properties of the tin and may also act as corrosive materials in the coating.
The urea compound that may be used may be either urea or the various art known derivatives, homologes, or analogues thereof such as disclosed in columns 12 through 15 of Holtzman et al., U.S. Patent 4,715,894, supra. Urea is pref erred .
Chelating agents that may be used generally comprise the various classes of chelating agents and specif ic compounds disclosed in Kirk-Othmer, ~ncyclo~r~rq;a of ~hPmir~l TF~r hnnlo~y, Third Edition, Volume 5, pages 339-368.
Chelating agents that are especially preferred comprise ~mi nnc~rboxylic acids and hydroxycarboxylic acids . Some aminocarboxylic acids that may be used comprise ethylPn~r~ m;n-~tetraacetic acid, hydroxyethyl-ethylenediaminetriacetic acid, nitrilotriacetic acid, N-dihydroxyethylglycine r and ethylenebis ( hydroxy-phenylglycine). Hydroxy carboxylic acids that may be used comprise tartaric acid, citric acid, gluconic acid and 5-sulfosalicylic acid.
The various reducing agents that may be used are well known in the art and generally comprise organic aldehyde whether saturated or unsaturated, aliphatic or cyclic, having up to about ten carbon atoms. Lower alkyl aldehydes having up to about six carbon atoms may be employed in this respect such as formaldehyde, acetaldehyde, propionaldehyde, butyraldehyde, and the like. Especially preferred aldehydes comprise hydroxy aliphatic aldehydes such as glyceraldehyde, erythrose, threose, arabinose and the various position isomers thereof, and glucose and the various position isomers thereof. Glucose has been found to act to prevent oxidation of the metal salts to a higher oxidation state, . , , ... . . _ . _ . . .. .. _ .. .. . .. . _ _ _ _ _ _ _ ~ls~aa7 e.g., tin(II) ion to tin~IV) ion, but also as a chelating agent and is especially useful for these reasons.
The surfactants that may be used comprise any nonionic, anionic, cationic or amphoteric surfactant such as those 5 listed in Kirk-Othmer, ~nf~yclo~p~ of 'hPm;~-~l TP~hn~logy, Third Edition, Volume 22, pages 332-387. The nonionic surfactants are especially preferrec~.
The various components of the aSIueous displacement plating solution may be present at conventionally 10 established concentrations. Typically, the displacement plating solution will contain on a molar basis:
- about 1 to about 15 parts of the displ ~ ~ metal ion;
- about 10 to about 125 parts of a thiourea compound; and - about 1 to about 360 parts of an acid.
15 The solution may also Gontain on a molar basis;
- about 10 to about 125 parts of a urea compound;
- about 5 to about 40 parts of a chelating agent; and - about 5 to about 110 parts of a reducing agent.
The solution concentrations may, of course, vary ~lPrPn~; ng 20 on the particular plating application intended.
In the process for the manufacture of multilayer circuit boards, the tin coated copper circuitry of each c c ^~t circuit board is further treated to form a thin layer of an oxide, hydroxide or combination thereof on the 25 surf ace of the tin in order to improve the bonding to the interleaved dielectric layers. Preferably, the treated tin surface is further treated with a silane bonding mixture to further improve bonding of the cn~r~npnt layers of the multilayer circuit board during its manufacture and end use 30 life. The silane bonding mixture is a mixture of a ureido silane and a disilyl crosslinking agent which is disclosed in Palladino, U.S. PateL~It 5,073,456, supra.
In addition to its use in the manufacture of multilayer printed circuit boards described supra, the stabilized spray 35 displacement plating process of this invention may be used in other plating applications, e.g., as an etch resist in the manufacture of printed circuit boards. In the plate-and-etch method of circuit koard manufacture, a polymeric or resin resist image is first formed on a copper , ... _ ... .. _ . . .. .. _ ... .. , . . . _ _ _ _ _ _ _ _ ~ ~ 21~3~7 clad circuit board substrate and then a metal resistant to etchants is plated on the copper surface areas not protected by the polymer resist image to form a complimentary metal resist image. The polymer resist image is then stripped 5 from the copper surface and the uncovered copper not protected by the metal resist image is removed from the substrate by an etchant to form the printed circuit. The use of immersion tin coatings as an etch resist in the plate-and-etch process is disclosed in Holtzman et al ., U . S .
lO Patent 4,657,632, wherein an etch resist immersion tin composition is selectively applied to the metal layer to leave areas of coated and uncoated metal followed by etchin~
the metal not coated with the resist. In the disclosed process, immersion tin composition is applied as a 15 substantially pore f ree coating at thicknesses f rom about 0.08 to about 0.175 microns. Holtzman et al.,'632, further discloses that such immersion tin coatings ~vt r, deficiencies in conventional electroplated tin-lead resists during subsequent soldering operations. In the conventional 20 soldering operation, a solder mask is first applied to the printed circuit board to cover all board areas except where components are to be soldered thereto. Prior to the application of the solder mask the elect}olytically deposited tin-lead etch resist on the circuit is removed by 25 reflowing the deposit at elevated temp~L~LuLes and since the removal is not always uniform the circuit board sometimes has to be subjected to a leveling process. Such a leveling process comprises passing the board over a hot air knife, i . e ., a constricted elongated hot air jet . Holtzman et 30 al.,'632, disclose that when immersion tin coatings are used as the resist, the reflow and hot air leveling steps can be eliminated .
The stabilized spray displacement plating process of this invention and its equivalents described supra may be 35 used to produce superior etching resists for the plate-and-etch manuf acture of circuit boards . When used to produce an etching resist, the aqueous displacement plating solution of this invention, will contain a water soluble salt of the displacement metal ion present in its lowest oxidation . . , ~

2193~07 state. Such metal salts comprise those based on metals of group IVA; VB; VIB; VIIB; VIII; IB; IIB and IIIA of the Periodic Table of the Elements; the group IVA, VIII, IB, IIB, and IIIA metals being preferred; and the group IVA, VIII, and IB metals being especially preferred. Preferred metals which fall into this class are tin, lead, mercury, nickel, gold, silver, indium, germanium and palladium. The anions of these metal salts are the same as those def ined herein for the tin salts. Particularly preferred are tin and various combinations of tin and other metals such as tin-lead, tin-nickel, tin 1U'~ and the like.
Additionally, the metal salts as defined above and herein are typically employed in their lowest oxidation states, e.g., stannous tin(II): nickelous Ni(II); mercurous Hg(I);
aurous Au(I) and the like. In one ~ L it is preferred to employ tin in its lowest oxidation state whereas any of the other metal salts may be employed in any oxidation state. Various mixtures of these other metal salts may also be employed. Salts of germanium, lead, mercury, silver, indium, gold and palladium are especially suitable .
In the instance when a solder mask is to be applied to a printed circuit substrate, displacement plating of the copper printed circuit may be deferred until after the solder mask has been applied, or displacement plating may be repeated prior to the soldering operation. Such deferral or repetition can improve the solder wetability of the plated connection sites during assembly and soldering of, ~ npnts to the circuit.
The processes of this invention for stabilizing and extending the life of a spray displacement plating solution will now be illustrated by the following examples.
r le 1 Innerlayers for the manufacture of multilayer printed circuit boards were chenically cleaned, treated with a displacement tin composition and a silane bonding mixture in an in-line, conveyorized, spray treatment system such as disclosed in Palladino supra and in Dietz et al. supra.
The in-line spray system used to prepare the innerlayer .. . .. ..

21g730O7 panel surfaces had a conveyor speed of 4 feet (1.22 meters) per minute and contained the following process steps and conditions .
R i n ~e ( Sol uti on ~ Spray Conveyor Water Temp. Pressure Length ( cm ) E~n!d ( Deq . C ~ ( kgf /cm 1.Panel Feed ~ 58 - - -( Input) 2.AlkAl ;nP Cleaner 51 - 49 1.76 10 3 . Double CC Rinse~^' 51 - 16 1. 41 4 . Microetch 97 - 30 1. 76 5.Triple CC Rinse~b~ 76 15.1 LPM() 16 1. 41 6 . Displacement Tin 122 - 24 1. 76 Application 15 7 . Triple CC Rinse~ 76 15.1 LPM(') 43 1. 76 8.Air Knife Drying 33 - 41 9 . Silane Treatment 51 - 24 1. 76 10. Hot Air Dryer 76 - 54 11. Output Conveyor 104 (a) Double CC Rinse (The term "CC" means counter current. ) is a two stage rinse wherein the last stage is f ed by the acidic effluent of the Triple CC rinse of Step 5, the first stage is fed by the effluent of the last stage and the effluent of the first stage is discarded.
(b) Triple CC Rinse is a three stage rinse wherein the last stage is fed by a high ~auality water source, e.g., softened water, the second stage is fed by the effluent of the la6t stage, the f irst stage is f ed by the effluent of the second stage and the acidic effluent of the first stage is fed to the double CC rinse of Step 3.
(c) LPM is liters per minute.
(d) Triple CC Rinse is a three stage rinse wherein the last stage is fed by deionized water, the second stage is f ed by the ef f luent of the last stage, the f irst stage is f ed by the ef f luent of the second stage and the effluent of the first stage is discarded.

t~3Q~7 The A 1 kA l; nc~ cleaner used in the system was VersaCLEAN' 415 (Du Pont) and the microetch was SureETCH 550 (Du Pont) potassium peroxy monosulfate/sulfuric acid.
In Step 6 the displacement tin composition was formed 5 by mixing Solution A and Solution B of the following compositions:
Solul:ion A
D. I . Water ~ 200 ml Conc. H2S04 100 ml Hypophosphorus acid; (509~) 40 ml tin ( I I ) sul f ate 2 0 gms D . I . Water To 0 . 5 liter Solution B
Thiourea 60 gms Urea 40 gms D . I . Water To 0 . 5 liter Sufficient solution was prepared to adequately fill the system reservoir.
In Step 9 the silane ~ea, -nt solution was prepared by 20 adding 60 ml of glacial acetic acid to 151 liters (40 gallons) of D.I. (deionized) water. 0.83 % by sol~tion weight (1573 grams) of gamma-ureidopropyltriethoxysilane coupling agent in methanol (50 %) (A-1160 Union Carbide) and 0.17 9~ by solution weight (322 grams) of 25 1,2-bis(trimethoxysilyl)ethane was then added followed by sufficient deionized water to produce 189 liters (50 gallons ) of solution . The solution was then mixed by activating the recirculating system of the silane treatment spray module. The solution is allowed to mix for 15 to 20 30 minutes to insure complete hydrolysis of the or~An~ci l An~ to an organosilane-triol.
The concentration of tin ( II ) ion in the spray displacement tin solution was monitored during use by employing the following analytical ~- u~edu~
35 1. Withdraw 10 ml of the displacement tin solution from the reservoir of the spray system and dilute it to 100 ml with deionized water.
2 . Add 10 ml of a buffer solution prepared from 40 . 6 g potassium acetate, 10 ml glacial acetic acid, and 212 ml deionized water.

3. Adjust the solution pH to 4 with a 50% solution of sodium hydroxide and add 10 drops of 10 g/L methyl thymol blue indicator solution.
5 4. Titrate the solution with 0.05 M EDTA (etnylPnP~ m~nP-tetraacetic acid) solution to the end point which is a blue to yellow color shift, e.g., deep blue to lighter brownish-orange. The tin(II) ion concentration in grams per liter is equal to the ml of the EDTA solution used times 0.7, i.e., [Sn(II~) ] = 0 .7 X ml EDTA.
The freshly prepared displ 7~r~ t tin solution has a tin(II) lon concentration of about 11 g/L but during use in the spray plating process the tin(II) ion concentration and plating activity drops due to its removal as plated tin and 15 to aerial oxidation to tin(IV) ions. Normal replPn;~l -nt procedures could be employed to raise the tin(II) ion concentration but are ineffective in maintaining the plating efficiency at the high activity level needed for a commercial process. The activity level should be 20 sufficiently high so that plated boards are substantially defect free. Since there is minimum "drag-over", i.e., removal of plating solution along with tne board during transition into the rinse cycle of the system, aerial oxidation products accumulate in the reservoir. The 25 displacement tin solution typically is discarded when the tin ( II ) ion can no longer be maintained above 2 . 0 g/L.
The rate of aerial oxidation of tin(II) ion to tin(IV) ion is controlled by the amount of agitation created during the spray process and thereby is eguipment rl~p~n~l~nt. Using 30 existing equipment, the rate of tin(II) ion oxidation varies from 0 . 2 to 1. 0 g/L per hour during spray agitation .
A freshly prepared displacement plating solution was used until the tin(II) ion uul,c~ LLc-tion reached a level of about 5 g/L. At this point, 12 extruded tin slabs (of the 35 type conventionally used as electrolytic anodes ) having a total surface area of 12 square feet (1.11 square meters) were placed in the reservoir of the displacement tin solution and the concentration of tin(II) ion was monitored over a period of two months of normal plating use. After . ~ 2~83007 approximately two weeks, the concentration of tin(II) ion stabilized at about 8 . O g/L and during the two month period, the surfaces of the tin slabs were etched away. The innerlayers prepared during this two month period passed AOI
5 inspection immediately before they were layed-up in the manufacture of multilayer boards. During this time the multilayer boards produced continued to pass production qualification criteria, including thermal stress, humidity, pink ring, and adhesion testing criteria.
FY~mnle 2 When the spray displAc~ L plating process described in ExaDple 1 was used over a prolonged period of time, crystals formed in the plating bath reservoir which clogged the spray nozzles and interfered with the mechanical components of the conveyor system. At this jU~ ULe the plating bath was discarded and the system cleaned and charged with fresh plating solution. The crystals were isolated and determined to be a copper ( I ) thiourea complex .
The same displ A~'~ L tin plating process as described in Example l was carried out except that soluble copper(I) thiourea complex was removed from the plating bath using the following procedure:
When the concentration of the copper ( I ) thiourea complex in the plating bath solution reached about 80 % to 95 % of its saturation concentration;
1. 10 gallons (37.9 liters) of plating bath solution was removed from the reservoir, i.e., about 6.5 % of the reservoir volume.
2. The solution which was removed was cooled to 55F, +/- 3F (13C +/- 3C) and held at that temperature for about 10 minutes to selectively f orm copper ( I ) thiourea complex crystals.
3. The crystals formed were removed from the solution by pumping the solution through an in-line cartridge filter system which removes particles above 5 microns.
The supernatant solution was fed back into the plating bath reservoir.

4. Steps 1 through 3 were repeated 4 times using the same cooling/filtering system; (At this juncture of the .. _ . ... . , . ,, _ . ~ 2~ 7 process the concentration of the copper ( I ) thiourea complex in the plating bath had dropped to about 80% or lower. ) 5. 5 gallons (18.9 liters) of 10 % sulfuric acid solution at 115F +/- 5F (13C +/- 3C) was circulated through the cooling/filtering system to dissolve the crystals precipitated in Steps 1 through 4; and 6. The 10 % acid solution resulting from Step 5 was disposed of by a conventional waste treatment process for thiourea, copper and acld wastes.
By the removal of excess copper ( I ) thiourea complex using this process, the useful life of the displacement tin plating solution was more than doubled while all multilayer boards which were prepared with the plating solution passed 15 production qualification criteria.
EY~mnle 3 This example demonstrates that a dilute acid solution of thiourea can be ~,,v~d from the 10 % acid solution resulting from Step 5 of Example 2, which may then be used 20 to partially replenish the displacement tin plating solution .
A 10% sulfuric acid solution, containing the copper-thiourea sulfate salt, was placed in a two cell electrochemical cell with a fine glass frit separating the 25 two chambers. In one chamber was placed 30 cm of 1 mm Pt wire (working electrode) and a Ag/AgCl reference electrode, and in the second chamber, 30 cm of 1 mm Pt wire (counter electrode). The reference electrode provides a stable reference potential to insure the applied voltage does not 30 drift.
Bulk electrolysis (electrowinning) was conducted on the dilute acid solution containing the salt using an overvoltage of 200 mV. Once the current dropped to the background level the electrolysis was stopped. The thiourea 35 concentration in the acid solution dropped to about 75% of it's initial concentration as det~rm; n~d spectrophotometrically by the absorbance at 236 microns which is assigned to thiourea. The copper concentration was measured by atomic absorption analysis and dropped from 60 ppm to below 10 ppm.
This method would allow the recovery ( from the working electrode chamber) and reintroduction of the dilute acid/thiourea solution back into the displacement tin 5 plating reservoir, while recovering copper in a recyclable form. The dilution caused by the extra 5 gallons ( 18 . 9 liters ~ of dilute acid in which the thiourea remains dissolved, would c, ~ ~q~te for normal evaporative water loss from the plating solution.
EY~le 4 This example discloses an alternate process for disposing of the thiourea and copper wastes from the 10 %
acid solution resulting from Step 5 of Example 2.
A 10% sulfuric acid solution, containing the copper-15 thiourea sulfate salt, was placed in a single chamberelectrochemical cell. Into the chamber was placed 30 cm of 1 mm Pt wire as a working electrode, a Ag/AgCl reference electrode, and 30 cm of 1 mm Pt wire as a counter electrode. The reference electrode provides a stable 20 reference potential to insure the applied voltage does not drift .
Bulk electrolysis (electrowinning) was conducted on the dilute acid solution containing 3 . 0 g/L of the salt using an overvoltage of 200 mV. Once the current dropped to 25 the background level, the electrolysis was stopped. The thiourea concentration in the acid solution dropped to substantially zero as determined spectrophotometrically by the absence of absorbance at 236 microns which is assigned to thiourea. The copper concentration was measured by 30 atomic absorption analysis to be below 10 ppm. Placing the working and counter electrodes in the same chamber, and performing bulk electrolysis on the solution resulted in the substantially complete oxidation of the thiourea at the counter electrode. This process allows the removal of copper 35 in a recyclable form along with complete oxidation of thiourea to provide a solution which can be disposed of by conventional acid waste practices.

2193~07 ~;y le 5 This example demonstrates the ef f ects of varying the thiourea and copper thiourea complex levels upon the efficacy and quality of tin plating using an immersion tin 5 displacement plating bath.
A solution, termed "A", was ~Le~aled consisting of DI
water (lO0 ml), concentrated sulfuric acid (50 ml), 5096 hypophosphorus acid (20 ml), tin(IV) sulfate (16 g. ), and additional DI water (dilutionr to 250 ml) .
The following plating solutions were prepared using the "A" solution:
#l 1. 2 g thiourea 10 ml DI water lO ml "A" solution 15 #2 0 . 9 g thiourea lO ml DI water lo ml "A" solution #3 o . 6 g thiourea lO ml DI water 10 ml "A" solution #4 o . 3 g thiourea lO ml DI water 10 ml "A" solution #5 1. 2 g thiourea lO ml DI water 10 ml "A" solution 2 . o g copper~ thiourea complex (prepared in lab) #6 1. 2 g thiourea lO ml DI water lO ml "A" solution 2.0 g coppertI)-thiourea complex (isolated from used commercial immersion tin bath) Tne above solutions were prepared and tested for plating quality and efficacy. Strips of rolled-annealed copper were plated for 1 minute in a given bath. After rinsing and drying, the plated strips were ~ mi n~-l for quality and uni~ormity of the tin plating. The results _ _ _ _ _ . , . , . .. _ . _ _ _ ... . _ .

,,.,~ 21g~o7 presented in the table below indicate that thiourea level is critical for achieving high quality and uniformity in the plated tin. The thiourea level should be maintained at about 45 g/L or higher in order to achieve good tin plating 5 that is uniform. Furth~ Le, the results 7 LL,lte that the copper ( I ) -thiourea complex when present at high levels is deleterious toward the quality and uniformity of the plated tin.

10 Sample TU Level Cu-TU Complex LEVEL Observations (g/L) (g/L) #1 60 0 Good Tin Plate #2 45 0 Good Tin Plate #3 30 0 Poor, Very Thin Tin Plate #4 15 0 No Tin Plate #5 60 100 Poor, Thin Streaked Tin Plate #6 60 100 Poor, Thin Streaked Tin Plate * TU is thiourea E~ nnle 6 This example indicates feasibility of crystallization for removal of copper-thiourea complex, i . e., solubility at 25 a higher temperature (3~C) and insolubility at a lower t. ~LULe (ambient, albout 24C).
An immersion disrl~ L tin plating bath was prepared using 125 ml of "A" solution and 125 ml of "B" solution as defined in Example 1. The bath was used to plate 200 2 x 2 30 inches (5.1 x 5.1 cm) coupons of copper laminate individually with 90 second immersion time at ambient temperature. DI water was added periodically to make up for liquid loss due to drag-out . DI water ( 25 ml ) was added three times before plating of the 46th, 115th, and 150th 35 coupons. A white precipitate formed in the bath starting with the first addition of water ~nd increasing with each subsequent addition.

.t~ g3`~07 After completion of plating of the 200 coupons, the spent plating solution was filtered to collect the white precipitate. The precipitate was washed with water and air dried. A sample of the precipitate was analyzed using 5 conventional inductively coupled plasma analysis which indicated the precipitate to be copper-thiourea complex in which there are about 3 to 4 thiourea molecules complexed to one copper ion.
The effect of temperature on the solubility of copper-10 thiourea complex in the immersion tin plating solution wastested. A solution of the tin plating bath, prepared by mixing equal volumes of "A" and "B" solutions (as defined in Example 1 ), was heated on a hot plate. Some of the copper-thiourea complex was added, and the mixture was stirred as 15 it was being heated. The precipitate was observed to dissolve in the tin plating solution at 38C.
In this example, a total of 200 2 x 2 inches ( 5 .1 x 5 .1 cm) coupons of copper laminate were tin-coated. This represents a usage level of 168 surface square feet of 20 copper surface per gallon (4.12 square meters per liter) of plating solution.
FY~le 7 An immersion displacement tin plating bath was prepared using 282 ml of "A" solution and 282 ml of "8" solution as 25 defined in Example 1. ~he bath was used to plate 3 x 3 inches ( 7 . 6 x 7 . 6 cm) coupons of copper laminate at 25C with a 9o second immersion time. The coupons were immersed in a 10~ sulfuric acid predip solution prior to immersion in the tin bath. After removal from the tin plating bath, the 30 coupons were rinsed with DI water, dried, and then inspected for the quality of the plated tin.
A solid replenisher consisting of 11.7 g of tin(II) sulfate and 7.7 g of thiourea was prepared. After the processing of every 50 coupons, 2 . 73 g of the replenisher 35 was added to ~he tin plating bath with mixing and heating until it had dissolved. The processing of the next set of 50 coupons was then started. A total of 200 coupons were coated in this manner with shiny, uniform tin. After the coating of 200 coupons with the repl~ni qhr~nt, the bath 21g3007 volume was 445 ml.
The 250 ml portion of the partially spent tin plating bath above was used to coat a total of 400 2 x 2 inches ( 5 .1 x 5.1 cm) coupons of copper laminate with shiny, uniform 5 tin. After the coating of each group of 50 coupons, 1.21 g of the solid replenisher was added to the bath with mixing and heating as n.~c~ ry until the solids had dissolved. At this point, the coating of the next set of 50 coupons was then initiated.
By this repl~n;qr L process, a total of 200 3 x 3 inches (7.6 x 7.6 cm) coupons and 400 2 x 2 inches (5.1 x 5.1 cm) coupons were coated with shiny, uniform tin. This represents a c9 LLclted maximum usage level with repl~n; ~hr-nt of 503 surface square feet of copper surface 15 per gallon (12.3 square meters per liter) of plating solution . In contrast, the ~- IL L e~ ding maximum usage level in Example 6 without repl~n; ql L was 168 surface square feet per gallon (4.12 square meters per liter).
Thus, the solid replenisher of tin(II) sulfate and thiourea 20 was effective in greatly extending the usable bath life of the immersion displacement tin plating bath. It is likewise expected that the solid replenisher would be equally effective in replF~n;chinr the spray displ qr L plating bath such as described in Example 1. However, when the 25 spray displacement plating bath contains tin metal as in Example 1, only solid thiourea would be needed to replenish the bath.

Claims (36)

WHAT IS CLAIMED IS:

1. A process for displacement plating a substrate metal surface with an other metal comprising the steps:
(a) providing a reservoir of aqueous plating solution comprising;
(i) a metal ion of a free metal, which is present in its lowest oxidation state wherein the free metal is different from the metal of the substrate surface;
(ii) a complexing agent; and (iii) an acid;
(b) applying the aqueous plating solution onto the substrate metal surface; whereby a portion of the metal ions of (i) is oxidized to ions in a higher oxidation state, and another portion of the metal ions of (i) is reduced to free metal, wherein said reduced free metal displaces surface substrate metal which is oxidized to an ion and complexed with the complexing agent to form a substrate ion complex dissolved in the reacted aqueous displacement plating solution at the surface of the substrate metal; and (c) returning the plating solution to the reservoir;
characterized in that the reservoir of aqueous plating solution contains free metal (iv) which is the free metal of the metal ions present in their lowest oxidation state (i);
whereby at least a portion of the metal ions present in their higher oxidation state is reacted with free metal (iv) to form metal ions present in their lowest oxidation state to replenish the aqueous plating solution.

2. The process of claim 1, wherein the ratio of the surface area of the free metal (iv), to the volume of the aqueous plating solution is at least 4 in2/gallon (6.8 cm2/liter).

3. The process of claim 1, wherein the portion of the formed metal ions present in their lowest oxidation state is twice the portion of the metal ions present in their higher oxidation state which reacted with the free metal (iv).

4. The process of claim 1, wherein the substrate metal surface is copper or a copper alloy.

5. The process of claim 1, wherein the metal ion present in its lowest oxidation state (i) is a water soluble metal salt comprising a salt based on a free metal of group IIIA;
IVA; IB; IIB; VB; VIB; VIIB; and VIII of the Periodic Table of the Elements and mixtures thereof.

6. The process of claim 5, wherein the free metal is taken from the group consisting of tin, lead, mercury, nickel, gold, silver, indium, germanium, palladium, and mixtures thereof.

7. The process of claim 1, wherein the metal ion present in its lowest oxidation state (i) is tin(II).

8. The process of claim 7, wherein the aqueous plating solution contains one or more metal salts, the metal of the metal salt being selected from the group consisting of germanium, lead, mercury, silver, indium, gold and palladium, wherein the metal ion of the metal salt is present in any oxidation state.

9. The process of claim 8, wherein the metal of the metal salt is lead.

10. The process of claim 1, wherein the complexing agent (ii) is thiourea or a substituted thiourea.

11. The process of claim 1, wherein the acid (iii) is an inorganic acid based on sulfur, phosphorus, the halogens or mixtures thereof.

12. The process of claim 1, wherein the acid (iii) is an organic monocarboxylic or dicarboxylic acids having up to about six carbon atoms.

13. The process of claim 1, wherein the aqueous plating solution contains an additive selected from the group consisting of a urea compound, a reducing agent, a chelating agent, a surfactant, and mixtures thereof.

14. The process of claim 1, wherein the substrate metal surface is the surface of electrically conductive copper circuitry adhered to at least one surface of a dielectric layer support with the circuitry having a thickness of at least 4 microns and wherein the metal ion present in its lowest oxidation state (i) is tin(II) ion, the complexing agent (ii) is thiourea, the acid (iii) is sulfuric acid, the metal ion present in its higher oxidation state is tin(IV) ion, the complexed substrate metal ion is a copper(I) thiourea complex, and wherein the reservoir of aqueous plating solution contains a free metal (iv) which is tin.

15. A process for displacement plating a substrate metal surface with an other metal comprising the steps:
(a) providing a reservoir of aqueous plating solution comprising:
(i) a free metal which is different from the metal of the substrate surface;
(ii) a metal ion of the free metal (i), which is present in its lowest oxidation state;
(iii) a complexing agent; and (iv) an acid;
(b) directing a stream of the aqueous plating solution onto the substrate metal surface; whereby a portion of the metal ions of (ii) is oxidized to ions in a higher oxidation state, and another portion of the metal ions of (ii) is reduced to free metal, wherein said reduced free metal displaces surface substrate metal which is oxidized to an ion and complexed with the complexing agent to form a substrate ion complex dissolved in the reacted aqueous displacement plating solution at the surface of the substrate metal; and (c) returning the plating solution to the reservoir whereby at least a portion of the metal ions present in their higher oxidation state is reacted with free metal (i) to form metal ions present in their lowest oxidation state to replenish the aqueous plating solution.

16. The process of claim 15 wherein the ratio of the surface area of the free metal (i), to the volume of the aqueous plating solution is at least 4 in2/gallon (6.8 cm2/liter).

17. The process of claim 15 wherein in step (c), the portion of the formed metal ions present in their lowest oxidation state is twice the portion of the metal ions present in their higher oxidation state which reacted with the free metal (i).

18. The process of claim 15 wherein the substrate metal surface is copper or a copper alloy.

19. The process of claim 15 wherein the free metal (i) is a metal of group IVA; VB; VIB; VIIB; VIII; IB; IIB and IIIA
of the Periodic Table of the Elements.

20. The process of claim 19 wherein the free metal (i) is taken from the group consisting of tin, lead, mercury, nickel, gold, silver, indium, germanium, palladium, and mixtures thereof.

21. The process of claim 15 wherein the metal ion present in its lowest oxidation state (ii) is a water soluble metal salt comprising a salt based on metals of group IVA; VB;
VIB; VIIB; VIII; IB; IIB and IIIA of the Periodic Table of the Elements.

22. The process of claim 21 wherein the metal of the water soluble metal salt is taken from the group consisting of tin, lead, mercury, nickel, gold, silver, indium, germanium, palladium and mixtures thereof.

23. The process of claim 15 wherein the metal ion present in its lowest oxidation state (ii) is tin(II).

24. The process of claim 23 wherein the aqueous plating solution contains one or more metal salts, the metal of the metal salt being selected from the group consisting of germanium, lead, mercury, silver, indium, gold and palladium, wherein the metal ion of the metal salt is present in any oxidation state.

25. The process of claim 24 wherein the metal of the metal salt is lead.

26. The process of claim 15 wherein the complexing agent (iii) is thiourea or a substituted thiourea.

27. The process of claim 15 wherein the acid (iv) is an inorganic acid based on sulfur, phosphorus, the halogens or mixtures thereof.

28. The process of claim 27 wherein the acid (iv) is sulfuric acid or sulfamic acid.

29. The process of claim 27 wherein the acid (iv) is a mixture of sulfuric acid and hypophosphorous acid.

30. The process of claim 15 wherein the acid (iv) is an organic monocarboxylic or dicarboxylic acid having up to about six carbon atoms.

31. The process of claim 30 wherein the organic acid is taken from the group consisting of formic acid, acetic acid, malic acid, malefic acid, and mixtures thereof.

32. The process of claim 15 wherein the aqueous plating solution contains an additive selected from the group consisting of a urea compound, a reducing agent, a chelating agent, a surfactant, and mixtures thereof.

33. The process of claim 15 wherein the substrate metal surface is the surface of electrically conductive copper circuitry adhered to at least one surface of a dielectric layer support with the circuitry having a thickness of at least 4 microns.

34. The process of claim 33 wherein the free metal (i) is tin, the metal ion present in its lowest oxidation state (ii) is tin(II) ion, the complexing agent (iii) is thiourea, the acid (iv) is sulfuric acid, the metal ion present in its higher oxidation state is tin(IV) ion, and the complexed substrate metal ion is a copper(I) thiourea complex.

35. The process of claim 15 wherein the stream of the aqueous plating solution is directed onto the substrate metal surface as a spray of the solution.

36. The process of claim 15 wherein the stream of the aqueous plating solution is directed onto the substrate metal surface as a cascade of the solution.