Understanding Gold Plating

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U nderstanding Gold Plating

Peter WilkinsonEngelhard Lim ited, Cinderford, G loucestershire, United Kingdom

For e f fective operation, gold plating baths must require the mini-mum of attention in respect of replenishment and replacement,and must produce as little sub-standard or reject work as possible.To achieve this, an understanding of the processes involved andthe factors affecting them is advantageous. This article attempts to

provide such an understanding in a simple manner. Special atten-tion is given to the plating of gold from acid baths containing

monovalent gold complexes.

The use of gold as a contact material on connectors,switches and printed circuit boards (PCB's) is now well estab-lished. There is also an awa reness that only gold depos its fromacid (pH 3.5-5.0) gold(I) cyanide baths containing cobalt,nickel or iron as brightening agents have the appropriateproperties for a contact material. Gold deposits from: puregold(I) cyanide baths; from such baths containing organicmaterials as brightening agents; or from electrolytes containinggold in the form of the go ld(I) sulphite com plex, do not havethe s l id ing wear res i s tance which i s nece ssary for ma ke andbreak connections (1).

It was known for m any years that deposits from a cid gold(I)cyanide baths br ightened with cobal t , n ickel or i ron conta inedabou t 0.25 per cent of base m etal, but this did not explain whytheir de nsities (16.0-17.0 g/cm3) were so much lower than thedensity of pure gold de posits (19.3 g/cm3). The classical exper-iment of Munier (2), in which she dissolved the gold of thebrightened de posi ts inaqua regia vapour to reveal a polymernetwork , showed tha t depos i ts o f th i s type were m uch m orecom plex than was formerly thought.

In what follows, an earlier account (3) of the factors in-volved in the form ula t ion an d opera t ion of ac id g o ld p la tingbaths is brought up to date . Br ief acco unts are a lso inc luded ofthe e lec t rodeposi t ion o f gold f rom gold(II I) cyanide baths andfrom gold(I) sulphite baths.

The Formulation of Gold Electrolytes

Range of Electrolytes AvailableThere are o nly three com plexes of gold which are sui table

for com m ercia l use in gold p la t ing baths . By far the m ost im-portant are the cyanide and s ulphite complexes of gold(I) , ofwhich the potassium de rivatives are KAu (CN), and IC3Au(S03),

respectively.

Gold B ull.,1986, 19, (3)

Other gold(I) complexes, such as those with thiomalate andthiosulphate ha ve been con sidered , but i t is doubtful i f theywill ever find comm ercial application.

In terms of electricity consumed, gold(I) baths are moreefficient than gold(III) baths. Thus, assuming 100 per centeff iciency, 100 am p-min wil l deposi t 12.25 g of go ld from amo novalent go ld com plex, but only 4.07 g from a trivalent goldcomplex.

Never theless , deposi t ion f rom acid go ld(II I) cyanide bathshas m erit in the plating of ac id resistant substrates such as stain-l ess s tee ls , to which e lec t rodepos i ted go ld no rma l ly adheresbadly because o f the i r pass ivated surfaces . Henc e, these bathscan a lso be used to produce decora t ive gold a l loy coat ings onsuch su bstrates.

The D eposition ProcessFigure 1 is a schem atic diagram of what is thought to happen

during the deposi t ion of a m etal f rom a solut ion of one of i tscomplexes.

The catho de attracts predom inantly positive ions to a regionnear its surface which is known as the Helmholtz dou ble layer.

In addit ion, negat ively charged c om plex ions, such as theAu(CN)-- ions present in the gold(I) cyanide plating baths,which approach this layer become polarised in the electricfield of the cathode.

The distr ibut ion of the l igands arou nd the m etal is therebydistorted and the diffusion of the com plex ion into the Helm-holtz layer is assisted.

Finally, within the Helmholtz layer the complex breaks up.I ts com ponent l igand ions or m olecules are f reed and the m eta lreleased in the form of the positively charged metal cation

which is deposited as the metal atom o n the cathode.

75


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ayer Bulk *

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It will be appreciated from this s imple picture of e vents at

the cathode, that the ease of gold deposition from gold(I)cyanide baths, which contain gold in the form of Au (CN)2 ions,will depend o n the ease w ith which these ions dissociate intoAu+ and CN ions, thus:

Au(CN) u+ + 2C1\1- 1

The m ass action law dictates that for a reversible reaction:

[Au(CN)2]

[Au+ ] [C1N- ]2

where [Au(CN)2], [Aul and [CN- ] a re the concen tra t ions of therespective ions in solution.

This constant (4) provides a m easure o f the s tabi li ty of theAu (CN)2 com plex and is referred to as i ts stabili ty constant j32(the subscript 2 indicates that the reaction is two stage).

The value of1 2 for the Au(CN)2 ion has been determ ined as1038 3. This is exceeding ly large and im plies that the equilibriumin react ion (1) is far to the lef t and tha t the Au(C N); ion is avery s table on e. Moreover, i t indicates that the conce ntrat ion[Au+ ] of Au+ ions is given by equation:

[Au(CN);][Au+ 1 10-383

[CN 2]

This is an exceedingly low concen tra t ion and i t would appearas if reasonable rates of deposition of gold from gold(I)cyanide solut ions are poss ible only becau se of the polar isa tionof Au(CN ); ions which approach the cathod e surfaces and theirdecom posi tion in ac cord with e quat ion. / wi th in the H elmhol tzlayer.

The electrodepo si t ion of gold from i ts other com plexes isthought to occu r in a similar manne r.

Gold(I) Cyanide BathsThese m ay be opera ted unde r a lkal ine , neutra l or ac id con-

ditions. Und er acid cond itions, the value of [Au+ ]is increased a sa result of the equilibrium reac tion:

H+ + CN HCN

in which the formation of undissociated HCN is stronglyfavoured. Acid gold(I) plating baths are therefore characterisedby low free cyan ide ion con centra t ions . However, under a lka-l ine condi t ions and especia l ly i f potass ium cyanide is present ,the value of [CN1 is greatly increased. Following equations1 to

3 th is me ans tha t in ac id baths the value of [Au ' ] i s increased,

wherea s in a lkaline baths i t is decre ased. Despi te the fac t tha tthe s tab i li ty cons tan t o f the Au (CN)2 ion i s un chang ed , th i scom plex ion therefore appea rs to be less s table in ac id than inalkaline media. The range of usefulness of gold(I) cyanidebaths in acid me dia is limited, however, by the tendenc y of theAu(CN )2 complex to decom pose a t pH 's be low about 3 wi thprecipitation of A uCN thus:

Au(CN)z > AuCN + CN-

Both polarographic and potent iome tr ic s tudies have shownthat the gold(III) cyanide co m plex Au(CN )- i s fo rmed in ac idgold(I) cyanide plating baths during operation, presumably byoxidat ion a t the ano de (5 ,6) . I t is im portant to rea l ise tha t anygold oxidised to the t r iva lent s ta te in th is man ner is not a vai l-able for deposi t ion at the catho de because gold(I) and h ydro-

gen are preferentially deposited. The efficiency of the baththerefore fa l ls , not because som e gold is be ing deposi ted f romgold(III) cyanide, but because the effect ive concentrat ion ofgold(I) cyanide in the bath ha s been re duce d. Also gold(III) isjust as susceptible to 'drag o ut ' as the gold(I) and the m etal 'svalency does not affect its price.

Modern acid gold(I) cyanide baths are therefore formulatedwith the addit ion of a mild reducing agen t so as to m inimise theform ation of gold(III) cyanide, but baths form ulated severalyears ago ca n give rise to gold(III) cyanide form ation especiallyunder unusu al plating con dit ions, such as wire plat ing. Sincethe go ld(III) com plex is also very stable, i t is better to preventi ts formation than to reduce it back to gold(I) cyanide.

= a constant 2

76 GoldBult, 1986,19, (3)


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Go ld(III) Cyanide BathsAs h as been indicated above, acid gold(I) cyanide plat ing

baths are norm ally operated at pH values between 3.5 and 5.They cann ot be operated at pH values less than 3.5, since un dersuch ac id condi t ions the gold is prec ip i ta ted as A uC N. More-over, deposi ts f rom acid gold(I) cyanide baths have l it t le du c-t i li ty so that plated ar t ic les can not be su bsequently deformedwithout risk of cracks deve loping in the surface co ating. Theseand othe r l im itations of acid gold(I) cyanide baths have stimu-lated studies in recent years of the deposition of gold fromgold(III) cyanide baths.

Thus, a method for the preparation of HAu(CN)4 and the for-m ula tion of go ld p la t ing ba ths based upon th i s go ld com plex

were de scr ibed in 1971 (7) . The baths were fou nd to have ex-cellent plating charac teristics at pH values from 0.1 to 5.0, andgave opt imu m per formance in the pH range 1 .0 to 3 .0. Theytypical ly contained the go ld com plex, a ci t rate , phospha te orta r t ra te , a weak organic ac id and a m inera l ac id such as phos-phoric acid.

Then in 1979 the plat ing of gold s tr ikes on stainless s teelsubstrates using gold(III) cyanide baths was claimed (8).Potassium nitrate was u sed as the electrolyte in these baths andethylenediamine hydrochloride as a complexant. Nickel,cobal t , copper, t in and indium c ould be incorpora ted as a l loy-ing const i tuents and the baths opera ted a t pH's preferably lessthan 1.5.

The gold( II I) cyanide com plex was genera ted d i rec t ly bywarming a solution containing KAuC 14, KNO3 and KC N, the pHof which was subsequently adjusted by additions of hydro-chloric acid. This gave rise to evolution of chlorine at the an odeand severe corrosion problems, so that significant exploitationof the pa tent d id not fo l low, though the deposi t ion of go ld- tinalloy coatings from baths of this type has been claimed (9).

More rece ntly (10-11), a study of the g alvanic deposition ofgold and gold alloys from baths based upon potassiumgold(III) cyanide has resul ted in the fo rmu lat ion o f chloride-free baths which are finding limited application for special pur-poses. Not only are the d eposits bright, hard an d relatively free

f rom non-meta l l ic com ponents , bu t they a re a l so duc t i le andrelatively pore-free. They have low con tact resistance wh ich isnot affected by heat.

These propert ies can be m odif ied by addit ions of al loyingm eta ls such as C o, Ni , In , Sn , Zn or a l . Other com ponents ofthe baths may include an amine, amino acid or phosphoricacid, with the p11 being ad justed preferably to between 0.6 and2.0 by the addition of phospho ric acid or sulphuric acid or m ix-tures of these, plus citric acid, phosphate and sulphate. Ad-dition of chlorides to the baths was scrupulously avoided.

Other go ld(III) cyan ide baths (12,14) have also been d evel-oped recently.

I t must be s tressed, however, that these baths are not used

commercially to any great extent. The deposits they yield can-not be used in con tact applications in electronics becau se theirwear res is tance is inadequate ag ains t exis t ing g old p la ted con-tac ts . Also the ra tes of deposi t ion f rom them are s low so theycannot be used in m odern h igh speed equipment . Moreover,they are corrosive.

Apart from the ch loride-free baths discussed a bove, otherm odern g old(III) baths are avai lable com m ercial ly which docontain chloride but which contain substances which preventformation of chlorine at the anode.

At present gold(III)/cobalt and gold(III)/nickel baths areused for plat ing stainless s teel or chrom ium with f lash coatings

of gold (abou t 0.1 ,um thick).A gold(III)/indium bath has also been developed whichyields deposits suitable for deco rative applications on su ch sub-strates where an extremely light yellow colour is de sired.

Sulphite BathsDissociat ion of the go ld(I) sulphite com plex occurs by the

reaction:

Au(S03)2- u+ + 2S0 2 -

I t is m uch less s table than the gold(I) cyanide c om plex and i tsstability constant is of the o rder of 1010 or about 1028 t imes lessthan that of the Au (CN)- complex:

[Au (S0 )3 - 010

[A 1_1+ ][80 2,1 2

[Au(S03)3,]and [Au + I -- . 10 -1

[S02 1

However, at neutral and acid pH values the complex isunstable. Signif icant react ions which occu r under these con -ditionsare:

S03,- + F1+ > HSO;

2HSO; + 4e -4 S20 42- + 20H - (at the cathode)

2Au(S03)3 - + 201-1- + HSO; + 4503 + 2Au

SO :- + 2H + -4 SO2 + H2O

Gold B ull .,1986,19, (3) 77


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The form at ion of the d i th ionate ion 8 ,04, which is a power-ful reducing agent, leads to a gradual reduction of the goldcom plex to me tal lic gold (14) . Below pH 4.7 the sulphite ionbreaks down.

Reduc t ion of the gold com plex between pH 4.7 and 8 .0 canbe decrased co nsiderably by strengthening the sulphite com-plex with ethylenediamine (16) or by adding an oxidisingagen t , l ike picr ic acid, that wil l react with the di thionate at afaster rate than the gold sulphite com plex. However, the use ofthese add itives ma kes the bath control difficult and is not com -pletely effective.

BrightenersAlthoug h brightness in i tself is not important in go ld plating

for electronic a pplications, i t is associated with other de sirablequalities suchas wear resistance and lack of porosity.

Brighteners for gold electrod eposits are of three types:

D Organic substances such a s polyethyleneimines and highmolecular weight polyamines. These operate by selectiveabsorption within the Helmholtz double layer, usuallycaus ing an increase in po la r i sa t ion . They a re adsorbedonto the m ore prominent g rowth poin ts so tha t the go ldis deposi ted preferent ial ly on oth er parts of the cathod esurface, producing a levelling and brightening effect. In-variably the organic additive is incorporated into thestructure of the deposit with adverse effects on its physi-cal properties, especially wear resistance. The w ear resist-ance can, however, be improved by co-depositing asecond metal such as cadmium.

0 Co-deposited metals and semi-metals such as arsenic,tha l lium, se lenium , lead e tc . The use of these ca uses de-polarisation and they produce only lustrous depositsfrom cyanide baths bu t fully bright deposits from sulphitebaths . They opera te a t extremely low concen tra t ions of afew ppm an d th is has resul ted in a num ber of theor ies asto how they wo rk, but only one theory withs tands exam -ination. In this theory the semi-metals are regarded asbeing adsorbed evenly on the ca thode surface where theycatalytical ly enhance the nucleation of the gold. In the ab-sence of these br igh teners the go ld nuc lea tes a t g rowthpoints such as crystal dislocations on the cathode , but thecata ly t ic effec t of the sem i-m eta l encourage s more evendeposition by creating more growth points (17). Theeffect can be termed catalytic because the chargedspecies , for exam ple T1; , is avai lable for reads orpt ion.

The me chanism is even m ore efficient for sulphite golds.

0 Co-deposited transitional metals such as cobalt, nickeland iron. For coatings used as contact materials onconnectors and PCB's, these are the most importantbrighteners.

In describing the brightening act ion of t ransi t ional m etals ,cobalt can be taken as the example but the same argumentsapply to som e other m etals . I t has been known for som e t imethat cobalt brightened gold deposits contain carbon (18,) in factif a deposit does no t contain at least 0.1 per cent ca rbon it willnot be bright. C" labelling techniques have shown that thiscarbon originates in the carbon of the cyanide ligand (19).

In som e way the presence of coba l t i s respons ib le for the

creation o f Munier's polyme ric material in the gold de posit (2).Ana lysis has show n that as wel l as carbon these de posi ts a lsoconta in potass ium , n i t rogen and oxygen. I t has been suggested(20) that a com plex such as KC o(Au(C N),), is formed a nd it isthis material that acts as a brightener.

In a plating bath this complex is visualised as being formedwithin the Helm hol tz double layer. Un der the usua l opera t ingcon ditions it is only very slightly soluble, so that as it is forme dit is adsorbed onto the cathode in an analogous manner toorganic brighteners. Any parameter that increases the solubilityof KCo(Au(CN)2)3, such as h igh pH or h igh tem perature , wouldbe expected to red uce i ts br ightening effec t . This is in accordwith the fact that these t ransi t ional metals act as br ightenersonly in ac id gold cyanide baths , whereas the sem i-m eta ls canbrighten deposits from a much wider range of gold electro-lytes. If this understanding is correct we should expect to findK, Co , C and N present in the br ightened deposi ts in the sam eatomic proportions as those in which they are present inKC o(AU (CN),), , i .e. the atom ic ratios of K:Co:C:N in acid golddeposits shou ld be 1:1:6:6.

In practice these ratios can vary enormously, the K:Co vary-ing from 1.0:0.4 to 1.0:5.5 depen ding o n the elec trolyte and theoperating co nditions (21). The Co:C ra tio can vary from 1:3 to1:10 (22). However, the atom ic ratio of C :K is fairly constant at3:1 (22), and the potassium in these depos its increa ses l inearly

with the electrolyte potassium conc entration.These resu l ts do no t ru le ou t KC o(Au(CN)2)3 as the main

brightening ag ent but they strongly indicate that these elem entsare also present in other forms, for instance some workers haveident if ied e lementa l cobal t , CoOOH and c obal t cyanide in ac idgold d eposits (23,24).

Although the format ion of KC o(Au(CN),)3 is s ignif icant inthat i t explains ma ny cha racteristics of the acid gold proce ss, i tmay well be that this substance is only an important inter-m ediate in the deposi t ion process. It may be that other reac-tions contribute to the brightening process, namely the in-clusion of potassium, the reduct ion and d eposi t ion of cobal t ,

and the formation of com plex cyanides and polymers.

78 GoldBull.,1986, 19, (3)


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A typical com posi tion of an ac id gold d eposi t tha t is br ight ,has fair ly low internal s t ress and is ha rd wearing is given inTable I . These figure s agree fairly well with those predicted bythe KC o(Au(CN),)3 theory.

The presence in an ac id gold p la t ing bath of a s t rong l igandfor cobal t can in terfere wi th the format ion of KC o(Au(CN ),) ,and the brightening act ion, by rem oving cobal t ions from thesystem. Thus, i f cobal t is added as the ethylenediam ine tetra-acetate complex, an overall cobalt concentration of about 6giem ay be required to provide suff icient cobal t ions to al low theformation of 1(Co(Au(CN),)3 .

There is, however, an extremely strong ligand in all acidgold cyanide baths , name ly cyanide i tse lf . The cyanoc obal ta tecomplex Co(CN)3 i s one of the mo s t s table com plexes known(4), so m uch so that when co mplexed to C o(III) , cyanide ceasesto be toxic. More significantly, when c obalt is com plexed withcyanide i t i s no longer avai lable to form K Co (Au(CN ),) ,. Theformation of cyanoco baltate is encouraged by high Co (III) con-centration and high free cyanide concentration.

In an acid g old system the free cyanide concentrat ion is verylow, as any f ree cyanide tends to form undissocia ted HCN , butat higher pH free cyanide concentrations are increased.

Therefore i t is im portant , when baths are analysed for co -bal t, that the analysis should different iate between ' inert ' and`active' cobalt. Althou gh the e ffect is not as strong whe n iron ornickel are used as brightening agents, it is nevertheless stilloperative in baths containing these brightene rs.

A s yet there is insufficient evidenc e to say conclu sively whatthe state of the a s-deposited co balt is, but all observers are nowagreed that there is non-metallic material present and thatsom e, i f not al l, the cobal t is present in the form of a cyanidecom plex. However, the ICCo(A u(CN),), theory does provide anexplanat ion of the observed behaviour of cobal t-brightened

acid gold baths.

Golc1Bull.,1986,19 , (3 )

Operating Parameters of Gold(I) Cyanide Baths

pH

pH influences the bath in a number of ways. High pH( 5.0) allows a higher concentration of free cyanide andtherefore cyanoco baltate formation is greater. Also at higher pHthe solubility of 1(Co(Au(CN),)3 increases so that i ts brighteningabil ity is reduce d. As m entioned e arl ier, chelates are used tocontrol the supply of cobal t to the dou ble layer and chelat ings t rength is pH depende nt . Final ly, as the bath o pera tes the pHwill r i se , and to coun ter th i s ac id go ld ba ths need to be wel lbuffered. pH also affects efficiency in that the lower the pH, thehigher the concentrat ion of hydrogen ions;this favours depo-sition of h ydrogen instead of g old.

Temperature

Increasing temperature increases the solubility of1(Co(Au(CN),)3, and this leads to decreasing brightness. Ch e-la t ing s t rengths are a lso temperature depend ent . The effec ts ofhigher tempe rature can usua l ly be correc ted by increas ing thegold and cobalt concentrations.

Barrel Plating

A bar re l can ac t l ike a semi-perm eable mem brane by c re -ating a sepa rate ca tholyte within the ba rrel. Within the ano lytecom par tme n t the Co34 " conce ntra t ion increases , whi ls t wi th inthe ca tholyte the pH a nd f ree cyanide increases . When the bar-

rel is rem oved and a llowed to drain, the two solutions m ix andthere is a rapid formation of cyanocobaltate. These effects canbe coun tered by ma inta in ing a low pH, increas ing the f low ofelectrolyte through the barrel and by using the largest possiblevolume o f electrolyte.

High Speed Plating

High speed acid gold solut ions are designed to produce thesam e type of deposit as conventiona l vat plating. In order to doth is it i s usual to increase the go ld and c obal t content so as toprevent local depletion around the cathode. For the samereason agitation in, high speed plating becomes critical: jetplating is necessary to produce the required turbulence for

very h igh current densi t ies . Higher temperatures are n eeded toreinforce the effect of turbulence.

The m ain charac ter is tics of h igh speed pla t ing are the. highanodic an d ca tho dic cur ren t den s i ti es . Var ious e ffec t s resu l tf rom th is : the pH r ises fas ter ; more C o3+ is produced; conver-sion to cyanocobaltate is faster; and organic c helates tend to beanod ical ly oxidised. As far as br ightness alone is concerne d,semi-metals are better additives at very high current den sities.

Recent ly, a number of ac id gold processes have been devel-oped which m ake use of organic add i tives to increase the cur-rent densi ty range o f baths for deposi t ion of cobal t and n ickelbr igh tened ac id go lds . This enables h igh speed p la ting to be

done a t lower concen trations than was previously possible.

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EfficiencyBy efficiency we mean tha t proportion of the applied cur-

rent, usually expressed as a percentage, that results in the depo-si t ion of gold. The rem aining curren t is responsible for thereduction of other species.

The reaction below is of l i tt le importance in term s of cur-rent consumption but the formation and adsorption ofKCo(Au(CN),), increases the polarisation and thus encouragesthe deposition o f hydrogen:

Co" + e Co"

Any parameters that encourage the formation of

K Co (Au(C N),), , wil l therefore decre ase the efficiency, forexample low pH, high free cobalt concentration, low tempera-tu re_

The use of the correct chelating agents will control the avail-ability of cobalt ions and thus the form ation of KC o(Au(CN)2)3so that the best com promise between brightness and efficiencycan be achieved.

ImpuritiesThe effect of im purities can be exem plified by considering

lead, which can ac t as a sem i-metal brightener, but which in acobalt acid gold bath inhibits the formation o f KC o(Au(CN),),.

In the light of what has been d iscussed abo ve about the catalyticeffects produced in sem i-metal brightening, it will be seen thatlead could be expected to interfere with the formation o f thebr igh tener and cause de polar i sa tion and an increase in ef -ficiency Most sem i-metals interfere in this way, but Group 4contaminants are more acceptable, at least in fairly low concen-tration.

AgitationCom pared wi th most base m etal pla t ing baths , p recious

me tal baths have a low m etal concentration in order to reduceinventory costs.

Nearly al l the current in such baths is carr ied by the con-ducting sal t , which is in relat ively high concentrat ion. TheAu(CN)2 ion is not electrostatically attracted to the cathode andthe supply of gold to the Helm holtz double layer is thus dif-fusion dependent. Agitation is therefore im portant as a mean sof increas ing the supply of bo th go ld and br igh tener to thecathode. This has both advantages and disadvantages in thathigh agitation can increase efficiency, but it also m akes distri-but ion worse by increasing the eff iciency relat ively more athigh current densities than at low current densities.

Modern selective plating techniques have red uced the prob-lems of m etal distribution on the substrate and on e of the otherbenefits of the new c urrent ,density extending additives is that

they also improve d istribution.

Physical Properties of Deposits

The physical properties of gold deposits will now be con-sidered in the light of the above.

HardnessThe ha rdness of a g old deposit is due to its structure or the

impurities in the deposit or both. With acid cyanide de positsthere is an almo st direct relat ionship between hardness an dbase m etal content, and it is possible to vary the hardn ess of adeposit by varying the che late concentration in the e lectrolyte.With organically brightened golds the h ardness increa ses withincreas ing br ightener co ncentra t ion, the ca uses of the in-creased ha rdness being higher im purity level and greater grain

refining. The hardness of sulphite golds can be co ntrolled bythe amount of free brightener, usually arsenic, in the electrolytebut in this case the hardness is not du e to arsenic in the deposibut to gra in re f in ing. It has now been rea l ised tha t the re -lationship between hard ness and we ar resistance is not simpleand straightforward, so that hardness is not regarded a s beingas important as it used to be.

Wear ResistanceWear resistance is not a definite physical property but

depends on very com plex interact ions of numero us factors .Hence, when co mparing wear resistance it is important that the

same experimen tal technique is used . Bright deposits have bet-ter wear resistance tha n dull deposits, acid gold bath depositsare bet ter than organical ly br ightened deposi ts , and these inturn have bet ter wear propert ies than semi-me tal br ightenedcyanide or sulphi te golds. The cobal t brightened acid g oldshave a fine grained structure with a polymer fram ework whichgives a tough deposi t , whi ls t arsenic br ightened sulphi te de-posits are very fine grained and almost amorphous; their finegrained structures being easily deformed. Organically bright-ened go lds show accep tab le wear res i s tance agains t them-selves, but against the tough resilient deposits of an acid goldbath they are unacceptable.

More recent work (20) has indicated that the level of potass-ium ha s an important influence o n the sl id ing wear resistanceof acid gold deposits. However, as the atomic ratios of K :C andC :N in such go ld deposits are fairly constant, it could be that thepotassium level is only an indicat ion of other features in thedeposits which are respon sible for their im proved wear re sist-ance.

PorosityIf the pre-treatm ent of th e substrate is effect ive, the f ine

grained deposi ts f rom a sem i-m etal brightened deposi t showless porosity than ac id gold de posits. Unfortunately the depositst ructure factors that reduce porosi ty tend to preclude otherdesirable properties.

80Gold Bull., 1986, 19, (3)


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The study of the forma tion of pores in acid gold deposits hasma de the form ulation of Iow-porosity acid golds possible, butpretreatment still rem ains the m ost important factor in control-ling porosity.

StressThere are a n um ber of theories that attempt to explain stress

in electrodeposits; none of them are entirely satisfactory andonly two will be co nsidered.

The excess energy theory (23) states that an ion must pass anenergy barrier in order to leave its ligand field and beco me anatom in a solid lattice. This additional energy barrier is respon-

sible for polarisation. Once over this energ y barrier, the atomwil l have excess energy which is converted into heat so thateach fresh ly deposited layer is hotter than the rest of the so lid.On co oling this gives rise to tensile stress. The relatively highstress in acid golds an d the low stress in sulphite golds is ex-plained in this way. On this basis, however organicallybrightened go ld should be high in tensile stress and this is notnecessarily so.

The dislocation theory (24,25) states that impurities cau seextra dislocations in the m etal lattice, which cau se stress in acidgolds and sulphite golds. In terms of this theory certainorganics should give rise to compressive stress because theydisguise som e of the surface vacancies which lead to dis lo-cations.

It is known from em pirical observations that a deposit con-taining m ore than 0.3 per cent w/w C o or Ni will probably betoo highly stressed, and this would fit in with the sec ond theory.

Stress is probably generated in m ore than on e way and it isunlikely, therefo re, that one the ory wil l ever supply all theanswers.

Density

Table II shows the d ensities of acid g old deposits. The figurefor pure gold deposit is included.

The m uch lower de nsi ty of cobalt and nickel brightenedacid gold deposits cannot be explained on the simple assum p-tion that the d eposit contains 0.2 per cent co balt or nickel. Forthe density to have fallen so m uch, the structure of the gold de-posit must h ave been expanded by the inclusion of relativelylight weight material.

It has been indicated above tha t in acid gold deposits up to10 per cent of the atoms arenot gold but are m uch l ighter el -em ents. This explains their lower de nsity.

ConclusionThe early acid gold processes were developed em pirically

and it is only in the past few yea rs that the relationsh ip betweenthe com posi tion and operat ing parameters of the electrolyte,and the co mposi t ion and performance of the deposi t s havebegun to be unders tood. I f s ignif icant advances a re to beachieved, a m ore r igorou s theoret ical basis is required as aguide in exploring avenues of future developm ent. 0

Acknowledgement This paper incorporates contents from an earlier publication (3)by the author. They are reproduced with the perm ission of the Institute of Metal Finishing.

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and 1095 A .Krilidler,Metalloberflache, 1979, 33, 76 Y Okinaka,West. Elect. Eng.,1978, 22, (2), 72-817 E. Freedman and J.N. Shea,U.S. Pat. 3598706, 19718 A. Fletche r and W.L. Moriarty,U.S. Pat. 4168214, 19799 Oxy Metal industries,Gennan Pat. DE-OS 2658003, 1976

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platers' Soc.,' 1985, G3

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1972/1973,1, 3-19;Surfaces (Paris), 1973,12, 67-7517 ST. Rao and R. Weil,Trans. Inst . Met Finish., 1979, 57, 97-10118 D.R. Mason and A. Blair,Trans. Inst. Met. Finish., 1972, 50,138-14019 L Holt and J. Stanyer,Trans Inst, Met. Finish. ,1972, 50, 24-2720 E.T. Eisenmann,J.Electrochem. S oc. ,1977, 124,1957 - 195821 J. Souteret al., 'Conn ectors '85, Leicester', 1985, 4322 P. Wilkinson and C . Martin, '1CT C onf. Brunel U niversity', 198423 H. Leidheiseret al ,J. Electrochem. S oc.,1979, 126 391-39424 R.L, Cohenet at ,J. Electrochem S oc.,1979, 126, 1608-1618

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