The Use of Gold in AutocatalyticPlating ProcessesW. S. Rapson and T. GroenewaldResearch Organisation, The Chamber of Mines of South Africa, Johannesburg

Certain electroless gold plating procedures are autocatalytic innature, while gold can also be used as an activator or initiator in the

autocatalytic plating of plastics and of other non-conductingmaterials by metals such as nickel and copper. This paper describesthe scientific basis of autocatalytic plating, and reviews electrolessgold plating procedures. The effectiveness of gold as an activatorin the autocatalytic plating of plastics is also briefly surveyed.

The literature relating to so-called "electroless"plating is confusing, not only because it tends to belargely of an empirical nature, but also because theterm "electroless" has often been used to describeprocesses which are different in character. True"electroless" or autocatalytic plating should not beconfused either with electrochemical displacementdeposition, or with homogeneous chemical reductionprocesses, such as silvering. In the former, depositionis accompanied by dissolution of the basis metal,which must therefore be less noble than the metalbeing plated. It does not, therefore, occur on gold.Moreover, deposition ceases immediately access ofplating metal ions to the basis metal is not possible.Only thin platings can therefore be expected by thisprocedure. In the latter, deposition occurs indis-criminately over all objects in contact with thesolution. After initial deposition has occurred, how-ever, autocatalytic deposition may constitute the mainplating process.

Introduction of the Process

The first autocatalytic plating process was thatinvented by Brenner and Riddell (1) in 1946 for the"electroless" plating of nickel. Its distinguishingfeature was that deposition of nickel on steel wasachieved by immersion of the steel in an ammoniacalsolution containing a nickel salt together withhypophosphites. The formation of nickel wasconfined to the surface of the steel, and took placeas a result of a reduction of the nickel salt by thehypophosphite, which was heterogeneously catalysedby the metal surface, and specifically by the metalitself. This nickel plating process rapidly foundapplication in practice (2, 3, 4).

Since this discovery, similar autocatalytic platingprocesses have been developed for a number of

metals, both singly and in combination, and havebecome important as supplements or alternatives toelectroplating in a number of circumstances. Thusthey provide a means of depositing, on articles ofcomplex shape, uniformly thick coatings that arerelatively fine grained, pore free, and corrosionresistant.

In the case of articles made of an active metal,an initial deposit of the catalytically active metalbeing plated is formed by electrochemical displace-ment, after which autocatalytic deposition takes over.In other cases, a brief cathodic pulse may be requiredto deposit sufficient catalytic metal from the solutionto initiate the reaction. This can be achieved eitherby an external power source, or by contact (belowsolution level) with an active metal such as aluminiumwhich forms a local galvanic cell with the basis metal.

In the case of non-metallic and non-conductingarticles such as plastics, initiation of reaction isachieved by adsorbing a reducing agent on the sub-strate, and then immersing it in a solution of someother metal such as palladium or gold, which iseasily reduced and initiates the plating of the desiredmetal. This procedure, still imperfectly understood,is the basis of the considerable industry based todayupon the plating of plastics. The pre-treatment ofthe surface of the plastic by "sensitisation" and"activation" is an important step in the procedure ineach instance. Such pre-treatment must not onlypromote the adsorption of the reducing agent(usually a stannous salt) on the surface, and thedeposition of the activating palladium or goldcatalyst, but must also etch the plastic surface so thatthe metal plating is "keyed" to it. The pre-treatmentwill therefore vary not only from plastic to plasticbut may also vary in its effectiveness according to themethod of production of even one type of plastic.

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So far as gold is concerned, autocatalytic platingis therefore of interest from two points of view,namely:

(a) The development of processes for the auto-catalytic plating of gold. Although a variety ofprocesses has been described for the chemicaldeposition of thin gold coatings on basis metals,and although a number of proprietary prepara-tions are available for this purpose, the availableinformation relating to them indicates that theyare mostly not truly autocatalytic. In fact, aswe shall see, it is only relatively recently that thefirst truly autocatalytic processes for plating ofgold have been described.

(b) The possible development of the use of gold asan activator in the plating of plastics. Gold iseffective as an activator, but circumstances haveso far favoured the use of palladium activatorsin industrial practice.

As a preliminary to a discussion of these twomatters, it is appropriate to review briefly thescientific basis of autocatalytic plating.

The Scientific Background toAutocatalytic Plating

The basic reaction can be represented in the simplecase where the oxidation state of the metal in theplating solution is one, as

MM+ + Red —k M + Ox

catalyst

or more generally, in the case of metals in higheroxidation states n, by two half redox reactions

nRed ----- nOx + ne- (1)M*^+ + ne- • M (2)

where M is the metal being plated and Red representsa reductant, which for the sake of simplicity isrepresented as being transformed in the process intoOx, the oxidised state, by transfer of a single electron.

In practice the metal M is usually present in theplating solution in complexed form, and the con-centration of M°+ ions in the plating solution isdetermined by the stability constant of the complex,and the concentration of the complexant.

Redox reactions of the above type are often catalysedby metallic surfaces and it has been suggested (5)that the role of the metal in such instances is simplythat of a conductor of electrons, accepting theelectrons released in the first stage of the reactionfrom the reductant at one point on its surface, andreleasing them to the oxidant in the second stage atanother point thus:

nRed- nOx + ne--->

Solution ne- Metal

M< -+ Mn+ + ne-<

In brief the position in an autocatalytic platingreaction is that we have at the conducting metalsurface two separate redox reactions occurring, eachof which strives to establish its own equilibriumpotential on the metal.

Electrochemical theory (6-10) predicts that in asituation such as this a steady state will be establishedwith the metal at what is termed a steady state mixedpotential EMS„ the exact value of which will bedetermined by the shapes of the current density-potential and Tafel curves for the two electrodeprocesses which are involved.

The characteristics of this steady state mixedpotential are the following:

(a) both redox systems are removed from their owncharacteristic equilibrium potentials;

(b)net electrochemical reaction therefore occurs ineach redox system;

(c) the condition for the steady state is that the sumof the cathodic current densities is equal to thesum of the anodic current densities, for thesystem is an isolated one.

The situation is illustrated schematically inFigure 1, taken from Paunovic (9) of PhotocircuitsCorporation. This shows Tafel lines for a systemwith two different simultaneous electrochemicalreactions. At the steady state, in terms of (c) above

1M+1Red=1M+iRed• For large values of the anodic

or cathodic polarisation -1, however, iM can be

neglected in comparison with i M, and 1Red in

comparison with 'Red, and the condition for the

steady state potential reduces to iM =iR d The rateof deposition of metal is equal to the rate of oxidationof the reducing agent. As will be apparent fromFigure 1, therefore, if the current-potential functionsare known for the individual electrode processes,EM?, can be plotted and the rate of depositioncalculated from the value (idep) of i at this potential.

The rates of deposition at potentials away from thesteady state mixed potential can likewise be deduced—such potentials being achieved of course under theinfluence of an external potential source.

Factors Affecting AutocatalyticPlating Reactions

From the above it will be clear that the factorswhich affect the current-potential relationships forthe partial redox reactions involved will affect thevalues of EM, and idep. They will in fact determinewhether autocatalytic plating will occur, and if so,at what rate.

The main such factors are the equilibrium poten-tials for the partial redox reactions, their exchange

120

Fig. 1 Current—potentialcurves for a system withtwo different simultaneouselectrochemical reactions.(After Paunovic)

M 1M ' f (q )

1P

'xaed

eq

ed

,RPa ß(7J

log i deplog i

current densities and their transfer coefficients onthe metal in question, the temperature, the types ofovervoltage effects and the presence of additionagents.

Exchange Current Densities

The effect of exchange current density uponEM,, and idep is illustrated in Figure 2, after Paunovic(9). It is shown here how changes in the exchangecurrent density io of one of the redox reactions affectsthe values of EM,, and 'dep at a metal surface at whichtwo redox reactions are occurring.

The importance of this exchange current densityeffect is underlined by the work of Spiro and Ravnö(10) at Imperial College, London, on heterogeneouscatalysis in solution. These investigators drewattention in 1965 to the dependence of catalysis uponthe electrochemical reversibilities of the two redoxreactions involved in catalytic reactions of this typeand to the fact that these reversibilities could bemeasured quantitatively in terms of the correspondingexchange current densities at the metal surface.

In a study of the effects of platinum on a largenumber of oxidation-reduction reactions, they esti-

Fig. 2 The effect ofchanges in the exchangecurrent density (i.e. in thereversibility) of one ofthe redox reactions on themixed potential and therate of the auto-catalyticdeposition.(After Paunovic) logi1dep

log i2dep

log i3dep

log i

121

mated i o values from simple current-voltage curvesand from e.m.f. behaviour, and in this way wereable to confirm that catalytic effects occurred:

(i)when i s values were such as to indicate that thehalf redox reactions were reversible; and

(ii)when only one of the redox reactions wasreversible and the difference between thereduction potentials of the two redox reactionswas at least 0.5 V.

They analysed conditions in such reactions, andinterpreted their results by reference to three curvesof current density against electrode potential for threeindependent redox couples of platinum electrodes(Figure 3).

The slopes of these curves where they cross thezero current axis can be shown to be proportionalto the exchange current density of the couple con-cerned, so that PQR represents a fairly irreversiblecouple, whereas ABC and DEF correspond to re-versible couples. Where two reversible couplessuch as ABC and DEF are present together, thecurrent density-voltage curves are steep, and at themixture potential the anodic and cathodic currentsare large, so that a rapid heterogeneous reaction canbe expected. Where one couple is reversible (ABC orDEF) and the other irreversible (PQR), however, itwill be seen that the mixture potential is in each casemuch closer to the e.m.f. of the reversible couple(B or E), than to that of the irreversible one (Q).Moreover the current (ab or de) at the mixturepotential is smaller than that (st), but neverthelessappreciable. Heterogeneous reaction between twocouples can therefore be fast even if one is irreversible,and it can be shown that the reaction rate increasesthe further apart the e.m.f.s of the two couples.Finally, where both couples are irreversible it willbe apparent that the situation will be similar to thatwhere only one couple is irreversible, except that themixture potential will be now some distance awayfrom the e.m.f.s of the individual couples, and thecurrent density at the mixture potential will be smallunless the two e.m.f.s are far apart.

Comparison of the results of this analysis by Spiroand Ravnö with their experimental findings recordedabove, underlines the importance of the degree of thereversibility of the two couples, as reflected by theirexchange current densities, and of the magnitudeof the difference between their redox potentials,in determining whether or at what rate they willbring about autocatalytic reaction when in com-bination.

Concentration of Reactants

Exchange current density is dependent on theconcentrations of reactants, which affect the rate ofautocatalytic deposition accordingly. Thus for

Fig. 3 Schematic diagram of current density—voltage curves for two reversible couples (ABC,DEF) and one irreversible couple (PQR). (AfterSpiro and Ravnö)

reaction (1) (on page 120) this dependence is given bythe equation

is =nFKCox =nFKCRed (3)

where K and K are the rate constants for the forwardand reverse reactions and Co,, and CRed are theconcentrations of ox and Red respectively. Withchanges in io, the current-potential relationshipsi=f() for the reaction alters also (cf. Fig. 2).

Similar effects follow on changes in concentrationof the metal ions M"+ in the plating medium.

In practice other effects may be superimposedupon these. Thus it is a matter of experience that ifthe concentrations of metal ions or reductant aremade too high in the plating medium the rate of thehomogeneous redox reaction may become significant,and there is then a risk that metal may be precipitatedthroughout this medium and not solely on the surfaceto be plated. It is for this reason, as well as fromsolubility considerations generally, that a complexantis normally added. The metal ion concentration isthen controlled as already mentioned, by the stabilityof the complex ion which is formed, and the con-centration of the complexant—provided, of course,that the complex ion itself does not promote thehomogeneous redox reaction.

As a result of the use of complexants, othereffects may also arise. Thus in the autocatalyticdeposition of gold from cyanide solutions, usingborohydride as the reductant, the deposition rate

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increases with KAu(CN), concentrations to amaximum and subsequently decreases. This hasbeen interpreted (11) as due to competitive adsorp-tion of Au(CN)z and BH3OH- (the effective re-ductant) on the substrate surface prior to the redoxreaction.

TemperatureThe rate constants in equation (3) are subject to the

normal Arrhenius effect, and positive temperatureeffects on plating rates are therefore observed. Theseare most important in practice.

Addition AgentsThe use of complexing agents in autocatalytic

plating solutions may affect the deposition of metal,not only as a result of the control exercised over metalion concentrations, but also as a result of the adsorp-tion of the metal complex ion or the complexant onthe metal surface. An example of this has beenquoted above, and in general the rates of the electrodereactions can be envisaged as being affected as aresult of adsorption of complex ions, complexingagents or other additives in two ways. They are:

(a)A decrease in surface available for the electrodereactions. This results in an increase in actualcurrent densities and therefore in overvoltage.

(b)A new slow step—namely transport of reactantsthrough the adsorbed film—may be introducedinto the process.

This brings us to a consideration of overvoltageeffects.

Overvoltage EffectsRates of autocatalytic deposition as determined by

the current density are of course subject to limitationsin the rates of any one of the consecutive electrodeprocesses, such as charge transfer, mass transport ofreactants or products of reaction, and reactions whichprecede or follow charge transfer.

Figure 4, from Paunovic, illustrates how the rateof autocatalytic deposition and the mixed potentialare affected by such different limitations.

Effects of AgitationThe effects of agitation are of interest in relation

to these overvoltage effects, since agitation could beexpected to reduce the effects on reaction rates oflimitations in mass transfer rates.

In practice agitation of baths during autocatalyticplating is common practice, and is based upon theimprovements in uniformity of deposition and inplating rates achieved in electrolyte deposition. Theeffects of agitation upon these factors in autocatalyticplating, however, have been reported to be dependent(12) on concentrations, temperature, and the activa-

E E

a i b i

E E

C d

Fig. 4 Dependence of the rate of electroless deposi-tion and the mixed potential upon the type of over-voltage. (After Paunovic)(a) Control by anodic charge transfer reaction(b) Control by cathodic charge transfer reaction(c) Control by diffusion of cathodic (Ox) reactant(d) Control by diffusion of anodic (Red) reactant

tion energy of the chemical reaction involved, and thedeposition rate found to be either enhanced or im-paired depending on a number of parameters.

Side ReactionsWhere hypophosphites are used as the reducing

agent, deposition of phosphorus with the metal mayoccur. This is particularly significant in the auto-catalytic plating of nickel and cobalt using hypo-phosphites, where the product always containsphosphorus in amounts of 2.5 to 14 per cent. Scholderand Heckel (13) have noted that the phosphorus ispresent in the precipitate as phosphides and that thephosphorus content of the deposit increases withincreases in the acidity of the plating medium.

Similarly, boron may be deposited in small amountswith the metal when borohydrides are used asreducing agents.

In general, however, the most generally observedside reaction during autocatalytic plating is theevolution of hydrogen and the ratio hydrogen evolved/metal plated is a significant one in practice.

Changes in Bath CompositionApart from depletion of metal and reductant, the

most significant effect of bath usage on bath com-position is the increase in acidity caused by the acidgenerated during the reaction.

Buffering agents may therefore be added to theplating bath to counter this effect. Such buffering

123

agents may, however, have other effects in theprocess.

General Conclusion

In view of the number of factors which affect theprocess, and the complexity of their interactions, itis not surprising that much of the literature relatingto the formulation of plating baths has an empiricalflavour, and that the development of plating bathshas outstripped detailed understanding of thereaction mechanisms involved in them.

Electroless Plating of GoldA number of "electroless" gold plating solutions,

some of them proprietary, have been described and/orare on the market. They have been reviewed byOkinaka (14), who has stressed that, in a number ofinstances, it is by no means clear whether the pro-cesses are autocatalytic or not. At the one extremethere are solutions which clearly effect plating byelectrochemical displacement. They give rise tothin coatings only, and will not plate gold itself. Inbetween such solutions, however, and the boro-hydride baths which are truly autocatalytic, there are"electroless" gold baths which deposit gold on basismetals such as copper and nickel at fast rates and toconsiderable thicknesses. Nevertheless, they do notplate gold itself, so that these plating processescannot be truly autocatalytic.

Okinaka notes three possibilities which couldexplain the behaviour of such baths, namely:

(i)the reaction is an electrochemical displacementand produces a porous deposit,

(ii) the basis metal acts as a catalyst, but golddoes not, and the deposit is porous,

(iii) the reaction is indeed autocatalytic, but onlythe freshly deposited gold is active as a catalyst.

The studies of the processes in this category whichhave been reported do not, however, provide a basisfor testing these possibilities. That the latter pos

-sibility is a real one is indicated by a recent observa-tion by Groenewald (15) of a catalytic effect of goldwhich is operative only when the gold is freshlydeposited.

Types of Electroless Gold BathsThe following types of electroless gold baths are

discussed by Okinaka (14) and by Goldie (16):

Hypophosphite BathsThese include:

(i) The Swan and Gostin bath (17-20);

(ii)The Brookshire bath (21);

(iii) The Ezawa and Ito pure gold bath (22);

(iv) The Ezawa and Ito gold-nickel alloy bath (23).

Hydrazine BathsThese include:

(i) The Gostin and Swan bath (24);

(ii)The Luce bath (25).

The Diethylglycine Baths of Walton et al. (26, 27)These employ citric and/or tartaric acids in

addition to diethylglycine as reducing agents.

Other Baths

The ammonia baths of Schnable, McCormack andZeblinsky (28);

The thiourea bath of Oda and Hayashi (29);

The borohydride bath of Okinaka et al. (30-37);

The dimethylamine borane bath of McCormack(38);

The aldehyde-amine borane bath of Rich (39);

The early formaldehyde baths of Wein (40);

Baths using unstable organogold compounds (41).

Information relating to the composition of thevarious proprietary electroless gold solutions whichare marketed today is lacking.

Of the above baths, those based upon the use ofborohydrides have been investigated and reportedupon in greatest detail and more detailed referenceto them seems justified.

The Borohydride Baths of Okinaka et al.Bath compositions can be varied over a consider-

able range, but the following is typical: KAu(CN) 2

(0.03M), KCN (0.1M), KOH (0.2M), KBH 4

(0.2M). Such a bath, operated at 70°C, can giveplating rates of up to 6µm/h. (42).

Since the hydrolysis of the BH4 ion according tothe overall equation

BH,- + 2H 2O -• BO, + 4H,is catalysed by acids, an important role of the KOHin the bath is to stabilise it against decompositionduring storage. The deposition rate of the gold is,however, lower at the higher KOH concentrationsat all levels of gold concentration, so that a compro-mise has to be effected in the formulation of solutionsin practice.

The effect has its origin in the fact that thehydrolysis of the BH4- ion is progressive, and occursessentially in two steps (43-45):

BH,- + H 2O -^ BH 2OH- + H,BH 2OH- + H 2O -^ BO 2 + 3H 2

with the BH 2OH- being the species responsible for

124

the reduction. Its concentration during depositionis only 2 per cent or less of the initial BH4 con-centration. The efficiency of utilisation of KBH4

in the process is therefore -low.The deposition rate increases to a maximum and

subsequently decreases. This has been interpretedas due to competitive adsorption of Au(CN)ti andBH2OH- in the substrate surface prior to the redoxreaction.

In tune with this is the observation that smallamounts of Ni(CN)4 in the plating bath drasticallyreduce gold deposition rates, the effect being inter-preted as due to interference by this ion also withBH3OH- adsorption on the gold surface. Thismeans that gold deposition rates on nickel are low,since initial deposition of gold on nickel occurs bydisplacement, nickel going into solution, as Ni(CN)4 .

While the deposition rate is consistently higher atlower KCN concentrations, the presence of excessfree cyanide is reported as essential for bath stability.Presumably the KCN acts also as a buffer. In itsabsence metallic gold is liberated even at roomtemperature.

Above 85°C the baths decompose rapidly. Upto that temperature, however, deposition ratesapproximately double for each 10°C increase intemperature, the corresponding Arrhenius plotyielding a straight line.

The instability of the baths can be exploited torecover the gold from used baths. On boiling, ifnecessary with addition of extra KBH4, the gold isprecipitated and can be filtered off.

Deposition rates are not constant but decrease withtime, an effect which is in most instances a result ofhydrolysis of the borohydride. While this can becompensated for by increasing borohydride concen-trations, this also increases the rate at which thelatter is lost by hydrolysis.

A typical bath has been described as showing adecrease of 27 per cent in deposition rate afterstorage at room temperature for thirty-two days.

Stirring effects are difficult to predict in auto-catalytic plating reactions. In this instance, however,it was found that plating rates could be approximatelydoubled by stirring.

The useful life of a typical borohydride bath at theoperating temperature, when used without replenish-ment, is limited to four to five hours, because ofhydrolysis of the borohydride. The useful life can,however, be increased to twenty hours by replenish-ment, the limiting factors being apparently theeffects of increasing metaborate ion and cyanide ionconcentrations. Gold is preferably added in the formof AuCN.

In the case of both gold and borohydride additions,these are preferably made below 40°C.

Properties of Electroless Gold Plating fromBorohydride Solutions

Adhesion Excellent on metalsAppearance Matt yellowDensity Bulk gold (19.3 g/cma)Hardness Soft (Knoop 60-80)Porosity —Zero for deposits >1 .m on

uniform substratesPurity > 99.9%, —0,0001 % BResistivity Bulk gold (>0,03 ohm/sq

at 1 µm)T/C Bondability Excellent

The lack of any significant contamination with boronis to be noted.

Plating has been effected on gold, silver, copper,nickel, cobalt, iron, palladium, platinum, brass,Kovar, Permalloy and steel. Deposition also occurson non-conductors such as glass and plastics, ifthese are sensitised with SnCl 2 and activated withPdCl 2 .

Because of a tendency for nickel and copper todissolve initially by galvanic displacement, this typeof bath is apparently not suitable for deposition ofgold on very thin films of copper or nickel.

The properties of the deposit are summarisedin the table above (42).

Gold as an Activator in thePlating of Plastics

The sensitisation and activation processes asdeveloped and applied up to 1968 in the autocatalyticplating of plastics have been described by Goldie(16).

In brief, the position is that a number of metals,including palladium, silver, gold and platinum havebeen found to initiate the autocatalytic deposition ofother metals on plastics. Although at the presenttime palladium is used exclusively for this purpose,several authors (46-48) have nevertheless claimedgold to be technically superior to palladium inparticular applications, and one investigator (49),reporting on the size of the microdeposits from goldand palladium systems, found that gold gave by farthe finest deposits. The use of gold colloids for theactivation of ABS plastic has also been investigated(50).

Although there are references (46-49, 51-61) in thepatent literature to the use of gold as an activator forelectroless plating of plastics, it does not seem tohave enjoyed application in practice. The successfuldevelopment of processes based on its use couldtherefore well create an important new application forgold and might well stimulate advances in the platingof plastics.

125

References

1 A. Brenner and G. E. Riddell,51. Res. Nat. Bur. Stand.,1946,37,31

2 G. Gutzeit, Plating, 1959, 46, 1158, 1275, 1377; 1960,47,63

3 D. J. Levy, Proc. Am. Electroplaters Soc., 1963, 50, 294 K. M. Gorbunova and A. A. Nikiforova, "Physical

Principles of (Electroless) Nickel Plating", Acad. Sci.U.S.S.R., Inst. Phys. Chem., Moscow, 1960. Translatedfrom Russian by the Israel Program for ScientificTranslations, Jerusalem, 1963

5 M. Spiro, Y. Chem. Soc., 1960, 36786 C. Wagner and W. Traud, Z. Electrochem., 1938, 44,

3917 J. O'M. Bockris, "Modern Aspects of Electrochemistry",

ed. J. O'M. Bockris and B. E. Conway, Butterworth,London, 1954, 253

8 M. Stern and A. L. Leary, Y. Electrochem. Soc., 1957,104, 56

9 M. Paunovic, Plating, 1968, 55, 116110 M. Spiro and A. B. Ravnö, Y. Chem. Soc., 1965, 7811 Y. Okinaka, Y. Electrochem. Soc., 1973, 120, 73912 N. Feldstein and P. R. Amodio, Plating, 1969, 56, 124613 R. Scholder and H. Heckel, Z. Anorg. Allgem. Chem.,

1931, 198, 329; cf ref. 4, p. 714 Y. Okinaka, "Gold Plating Technology", ed. F. H.

Reid and W. Goldie, Electrochemical Publications, Ayr,Scotland, 1973, 82

15 T. Groenewald, J. Appl. Electrochem., 1975, 5, 7116 W. Goldie, Ed., "Metallic Coating of Plastics", Vol. I

and II, Electrochemical Publications, Ayr, Scotland,1969

17 S. D. Swan and E. L. Gostin, Metal Finish., 1961,59, 4, 52

18 J. L. Jostan and A. F. Bogenschutz, Plating, 1969, 56,399

19 Y. Tanabe and H. Matsubayashi, J. Metal Finish. Soc.Japan, 1970, 21, 335, 436

20 A. Brenner, "Modern Electroplating", ed. F. A.Lowenheim, 2nd Ed., Wiley & Sons, New York, 1963

21 R. R. Brookshire, U.S. Patent 2,976,181, 196122 T. Ezawa and H. Ito, Japan Patent 40-1081, 1965;

see ref. 1423 T. Ezawa and H. Ito, Japan Patent 40-25881, 1968; see

ref. 1424 E. L. Gostin and S. D. Swan, U.S. Patent 3,032,436,

196225 B. M. Luce, U.S. Patent 3,300,328, 1967; see ref. 1426 R. F. Walton, Y. Electrochem. Soc., 1961, 108, (8), 76727 D. S. Pokras, T. L. Sullens and R. F. Walton, U.S.

Patent 3,123,484, 196428 F. W. Schnable, J. F. McCormack and R. J. Zeblinsky,

U.S. Patent 3,468,676, 1969

29 T. Oda and K. Hayashi, U.S. Patent 3,506,462, 197030 R. Sard, Y. Okinaka and J. R. Rushton, Plating, 1971,

58, 89331 Y. Okinaka and C. Wolowodiuk, Plating, 1971, 58, 108032 Y. Okinaka, y. Electrochem. Soc., 1973, 120, 73933 Y. Okinaka, Plating, 1970, 57, 91434 Y. Okinaka, R. Sard, C. Wolowodiuk, W. H. Craft and

T. F. Retajezyk,,J. Electrochem. Soc., 1974, 121, 5635 Y. Okinaka, R. Sard, W. H. Craft and C. Wolowodiuk,

paper presented at Full Mtg. of Electrochem. Soc.,Miami Beach, 1972

36 R. Sard, J. Electrochem. Soc., 1970, 117, 937 R. Sard,.. Electrochem. Soc., 1970, 117, 115638 J. F. McCormack, U.S. Patent 3,589,916, 197139 D. W. Rich, Plating, 1971, 58; see ref. 14, p. 10040 S. Wein, Metal Finish., 1948, 46, 8, 58; cf U.S. Rept. PB

111,332, Office of Tech. Services, U.S. Dept. ofCommerce, 1953

41 C. Gibson, Nature, 1937, 140, 27942 N. Feldstein Plating, 1974, 61, 14643 J. A. Gardiner and J. W. Collat, J. Am. Chem. Soc.,

1964, 86, 316544 J. A. Gardiner and J. W. Collat, J. Am. Chem. Soc.,

1965, 87, 169245 J. A. Gardiner and J. W. Collat, Inorg. Chem., 1965, 4,

120846 E. B. Saubestre, U.S. Patent 2,872,359, 195947 H. Narcus, Proc. Am. Electroplaters Soc., 1956, 43,

15748 F. Pearlstein, Metal Finish., 1955, 53, 5949 M. Matsunaga, Y. Hagiuda and K. Ito, Metal Finish

1968, 66, 8050 Fulmer Res. Inst., Confidential Repts. to Chamber of

Mines of South Africa51 A. E. Bergstrom, German Offen. 1,915,252, 196952 I. C. Hepfer, U.S. Patent 3,370,974, 196853 W. H. Young, B. Csuthy and J. Guidess (Timex Corp.),

U.S. Patent 3,565,770, 197154 K. Heymann and G. Woldt (Schering A.G.), German

Offen. 1,949,278, 197155 H. P. K. Hentzschel (Texas Instruments Inc.), German

Offen. 1,949,754, 197056 D. G. Foulke and A. Simonian (Sel-Rex Corp.), U.S.

Patent 3,516,848, 197057 Sel-Rex Corp., British Patent 1,101,848, 196858 K. D. Baker (Silvercrown Ltd.), British Patent 1,193,823,

197059 N. B. Rajchenbaum (English Numbering Machines

Ltd.), British Patent 1,122,235, 197060 Schering A. G., French Patent 1,553,827, 196961 G. Okuno and C. Yamauchi, Dutch Appl. 66.10976,

1967

Gold Plating from a Thiosulphate BathSulphite electrolytes have gained considerable

ground by comparison with the more establishedcyanide solutions for gold plating over the last fewyears, but another new type of electrolyte, based on athiosulphate complex, has been developed for use in thegold plating of printed circuits in Poland. In a paperpresented to the recent annual conference of theInstitute of Metal Finishing in London, ProfessorTadeusz Zak, Director of the Institute of PrecisionMechanics in Warsaw, and two of his colleagues,Dr. Jan Socha and S. Safarzynski, gave details of thissolution and of the cathodic and anodic processesoccurring during deposition.

The bath finally formulated contained 20 g/1 goldin the form of a thiosulphate complex, 50 g/1 citricacid and 10 g/1 sodium tetraborate as buffer. Theeffects of temperature, pH and concentration of bothgold and the complexing agent were studied, andoptimum operating conditions were established. Anoperating temperature of 20°C with a pH of 9.0 to 10.0were found to be most suitable.

The bath is said to be highly stable, to give a depositof greater hardness than is obtained with a sulphiteelectrolyte, and to permit of a speed of deposition sometwo to five times greater than that usually associatedwith an acid cyanide bath.

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