High Temperature Gold Deposition From Acid

7/27/2019 High Temperature Gold Deposition From Acid

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H igh T em perature G old D eposit ion from AcidC yanide B athsDEVELOPMENT OF ELECTROLYTES FOR THE LASER-ENHANCED ELECTRODEPOSITIONOF GOLD

Christoph J. Raub, Hamind R. Khan and Manfred BautngrtnerForschungsinst itut f ik Edelm etal le and Metal lchemie, Schwabisch Gm und, Federal Republ ic of G ermany

To simulate some of the conditions which prevail in laser-enhanced electrodeposition (LEE) of gold from three acid cyanidebaths, containing cobalt, nickel and iron respectively, gold platingwas conducted at elevated temperatures and pressures. It wasfound that almost pure gold deposits were formed which were simi-lar to deposits obtained from the baths by LEE. Experiments of thistype produce deposits of relatively larger surface areas, which aremore suitable for analysis, than deposits obtained by LEE. It issuggested that this simulation system may be useful in developingelectrolytes suitable for use in LEE.

In l a se r -enhanced e lec t rodepos i t ion (LEE) of g o ld , m e ta l de -pos i t ion occu r s p referen t i a ll y f rom e l ec t ro ly t e w h i ch i s l oca l l yheated to temperatures of 100C or higher as a result of thel aser- i r radiat ion of the subst rate surface. This s i tuat ion has nopara l l e l in co nvent iona l p l a t ing . In th i s a r t ic l e , we repor t p re -liminary results from studies of gold deposition at similarelevated temperatures but without laser irradiation of theca t hode . The ba t hs s t ud i ed were Au/Co , Au/Ni and Au/Fe ac i dcyan i de ba t hs o f t he t ype used fo r t he con ven t iona l p roduct i onof bright hard deposits. In the temperature range of LEE(% 100C) the code pos i t ion o f o t her m et a l s and o f non m et a l sf rom these b a ths d ropped g rea t ly , t he reby a f fec t ing the ha rd-ness and sur face s tructure of the deposi ts . I t is concluded that i fdepos i t s w i th spec i f ied prop e r t i e s a re t o be ob ta ined in l a se r -enhance d p l a t i ng o f go l d , e l ec t ro ly t es m us t be spec i a ll y devel -oped for t he purpo se . In the sc reen ing of e l ec t ro ly t e s fo r t h i spurpose , depos i t ion f rom t hem a t e l eva ted t em pera t u res by t hem ethod dev e loped f o r th i s s tudy m ay we l l p r ov e r ewar d ing .Heat ing of the elect ro lyte at the in terface between i t and theca thode , l ead ing to loca l i sed e l ec t ro ly t e t empera tures a s h ighas 120C, is a h ighly s igni f icant factor in the LE E of m etals . Anum ber o f pub l i ca t ions (1-4) has appeared i n w h i ch t he e f f ec t so f t h i s loca l i sed hea t ing o f t he e l ec t ro ly t e on t he m echan i sm o fe l ec t rodepos i t ion have be en d i scussed .The sm a l l s ize o f the d epos i t s f r om LEE m ak es i t d i f fi cu l t tostudy their properties. This explains why virtually no infor-ma t i on has been pub l ished so f a r abou t phys i ca l and chem i ca lp roper t i es such as t he chemi ca l compos i t i on , hardness and r e-s i s tance t o w ear o f t he e l ec t rodepos it s fo rmed unde r t hese con-ditions. The chemical composition and structure of the de-posited metal, which determine its other properties, areparticularly important for applications of laser enhancedplating.

In an earlier study (4), however, in which measurementswere m ade on de pos i t s u s ing energy d i sper s ive X-ray ana l ys is( E D A X ) i n th e s c a n n i n g e l e c t r o n m i c r o sc o p y ( S E M ) , i t w a sf ound tha t the depos i t s ob ta ined by LEE f r om a g o ld / coba l t ac idcyanide b a th o f t he no rma l t ype were v i r tua l ly cob a l t - free , i nc o n t r a s t t o t h o se o b t a i n e d u n d e r c o n v e n t io n a l c o n d i t io n s . I nthe case of deposi ts f rom gold/nicke l acid cyanide baths , s imilars tud ie s show ed tha t codepos i t i on of n i cke l w i th the go ld dur -ing LEE was not greatly affected.Si nce t he ext en t t o wh i ch c oba l t and n i cke l a r e codep os i tedwith the gold determines, to a large measure, the functionalproper t i es o f t he r espec t i ve depos i t s (5,6 ), it becam e app aren tthat a more detailed study of the codeposition of other el-eme n t s wi t h go l d f rom t hese and o t her e l ec tro l y tes , under con-dit ions of laser -enhancem ent , was of special impo r tance.O n e p o s s i b l e a p p r o a c h t o t h i s p r o b l e m i s t o s i m u l a t e t e m -perature conditions of laser-enhanced plating. This can bedone by conduct i ng t he p l a t i ng opera t i on in an a u t oc l ave a t in -creased temperature and pressure. Although it did not seempossible to s imulate o ther factors such as , the microconvect ioneffects of the boiling electrolyte under laser-enhanced con-ditions, nevertheless we decided to proceed with a series ofautoclave studies of gold plating. These were carried out atvar ious temp eratures up to 160C (5 bar) .

In t h is ' m anner , coa t i ngs w ere fo rm ed severa l square cen t i -m et res i n a r ea and up t o severa l mi crom et res in t h i ckness , ac-co rd i ng t o t he e l ec t ro l y t e u sed . Copper an d s i l ver shee t s w ereused a s sub s t ra t e s . Tes t s ca r r i ed ou t on the coa t ings inc ludedchemical analysis, determinations of ductility, resistance tow e a r a n d h a r d n e s s , a n d X - r a y , m e t a l lo g r a p h i c a n d s c a n n i n ge lec t r on m ic r oscopic exam ina t ions .This publ i ca t ion i s l imi t ed to an acco unt o f som e of t he re -su l t s o f i n i t ia l expe r imen t s us ing go ld /coba l t , go ld /n i cke l and

7 0 oldBull., 1986, 19, (3)


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8 g/I , Au .0.5 g/I. 40 . g/1014 .100 g/I titri'c acidpH 3.5

40000020 1400Electrolyte temperature, in C

Fig. 1 Varigtion of the-cobalt content of the- gold


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con t en t s i nd i ca t ed t ha t t hese had m i n i mum val ues o f 0 .005 to0.006 per cen t in coat ings f rom gold/coba l t and go ld/ i ron elec-t ro ly t e s be tween about 100 and 120C (F igure 4 ) . In depo s i t sf rom gold/nickel e lectrolytes , the contents were higher .The hardness of the deposits changed roughly in parallelwi th the va r i a t ions in t he i r t o t a l coba l t con ten t s . Coa t ings de -posi ted above 90C had HV10 hardnesses of only 100. Almostcer ta inly, increas ing heal ing of la t t ice defects plays a role in de-t e rmi n i ng t he hardnesses o f coa t i ngs depos i t ed a t h i gher t em-pera t u res . The s t ruc t u res o f the coa t ings as obse rved by SE Ma re in accord w i th the i r op t i ca l appe a rance . Deposi t s fo rm edf rom be l ow 100 t o 120C are s t i l l b r i gh t and ye l l ow, bu t t hosefo rmed a bove 120 a re i ncreas i ng ly m at t b rown i n appearance .At the same time the surface structure altered from smooth(Figure 5) to coa rsely crystal l ine (Figure 6). This lat ter surfaces t ruc tu re co r responds w i th t ha t o f go l d and pa l l ad ium coa t i ngsobtained by laser enhanced plating (Figure 7) (Figure 16 in(4 )) and w i th tha t o f coa t ings f rom g old /n i cke l and go ld / i rone lec t r o ly te s p r oduced a t s im i la r t em per a tur es .The s i mi l a r i ty be t w een t he s t ruc t u res o f coa t i ngs depos i t edwith laser-enhancement and those of coatings formed at el-e v a t e d t e m p e r a t u r e s i n t h e a u t o c l a ve c o n f i r m s t h e d o m i n a n tro l e o f l oca l e l ec t ro l y t e t empera t u re i n l aser -enhanced p l a t ing .It supports the hypothesis that the properties of coatingsfo rmed f rom go l d /cobal t e l ec tro l y tes by l aser -enhanced and by`au toc l ave ' p l a ti ng a t su i t ab l e e l eva t ed t em pera t u res shou l d becom par ab le .

Fig. 5 Scanning electron micrograph of the surface of a gold-cobalt coatingcontaining 0.35% cobalt deposited at 40C.

Fig. 6 Scanning electron micrograph of the surface of a gold-cobalt coatingcontaining 0.02% cobalt deposited at 160 C.

Fig. 7 Scanning electron micrograph of the surface of a gold coating formed bylaser-enhanced electrodeposition.

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Deposits from a Gold/Nickel BathThe com pos i t ion o f t he ba t h and t he var i a t i ons in t he averagevalues of the n ickel conten ts of deposi t s obtained in two ser ieso f exper i men t s a r e show n i n F i gu re 2 . Agreem en t wi t h t he r e-su l t s o f e a r l ie r s t u d i e s ( 8 ) o f d e p o s i t s f o r m e d b e l o w 6 0 C isvery good.

On account of relatively weaker complex formation bynicke l , t he con ten t s o f n i cke l i n t he depos i t s a re g rea t e r by afactor of 4 to 5 than the con tents of cobal t and i ron observed ins imilar exper iments .A fal l in base m etal content , s imilar to that observed with de-posits from gold/cobalt baths, occurs above 80C but takesp l ace m uch m ore g radual l y . Thus a t 140C, a t wh i ch t empe ra-t u re t he go l d /coba l t ba t h p roduce s depos i t s wi t h 0 .02 per cen tcobalt, the gold/nickel bath produces deposits containing0.8 per cent nickel.The r i se in the current ef f iciency/ temperature curve (Figure8) is v ir tual ly ident ical w ith that observed us ing th e gold/cob al tba th . The m in im um in the cur r en t e f f ic iency i s a l so a t about thet empe ra t u re a t wh i ch t he base m et a l con t en t o f the depo s i t i s am axim um (ca. 80C).Although the n icke l con ten t s o f the dep os i t s fa l l m or e g r adu-a l l y above t h is t emp era t u re t han do t he cob a l t con t en t s o f de-pos i t s f rom go l d /cobal t ba t hs , the c u r ren t e f f i c ienc i es i ncreasewith t em per a tur es in an a lm os t iden t ica l m anner .W i th i n c r e a s i n g e l e c t r o l y te t e m p e r a t u r e s , n o t o n l y d o t h en i cke l con t en t s o f t he dep os i t s decrease , bu t a l so t he i r carbonconten t s be low 60 C and abov e 100C, wi th a m axim um in theC-values at 80C (Figure 4).

In accord wi th thei r relat ively h igher concent rat ions of basem e t a l , e l e c tr o d e p o s i t s f r o m g o l d / n ic k e l b a t h s a r e h a r d e r ( s e eF igure 9) t han those ob ta ined f rom go ld /coba l t and go ld / irone l ec t ro ly t es under s i mi l a r cond i t ions . M oreover , t hey decrea sei n hardness mo re s l owl y wi t h increase i n depos i t i on t empera-ture than coat ings f rom these la t ter e lectrolytes .A m aximum of 300 in the HV10 ha rdness o f t he depos i t edgold i s obse r v ed nea r 80 C (F igur e 9) , which i s the t em per a tur ea t wh i ch t he m axi mum am oun t o f n i cke l is p l a t ed ou t wi t h theg o l d ( F i g u r e 2 ) . O n ly a t 1 6 0 C , w h e n t h e i r c o n t e n t s o f b a semetal in the deposits all approach zero, do the hardnessesof the deposits from the three baths fall to similar values(ca. 100 HV).

I n p r e l im i n a r y S E M s t u d i e s , th e su r f a c e s t r u c t u r e s o f d e -p o s i t s f r o m t h e g o l d / n ic k e l b a t h w e r e f o u n d t o b e s i m i l a r t othose f rom gold/cobal t and gold/ i ron baths. At 160C the sames t ruc tu re , name l y t ha t shown i n F i gu re 6 fo r a depos i t f rom t hegold/cobalt baths, was observed in deposits from the gold/nickel bath.Gold Bull, 1986, 19, (3)

Deposits from a Gold/Iron BathThe bath composition is listed in Figure 3. The depositsdi f fe r cons ide rab ly f rom those ob ta ined f rom go ld /coba l t andgo l d /n icke l ba t hs i n t ha t t hey exh i b it no m axi mum i n t he i r con-

tent of iron as the deposition temperature is increased. Inagreement with the results of earlier studies (7), a steep de-crease i s observed i n t he i r con t en t s o f i ron as t he t em pera t u reis increa sed (Figure 3) , and above 100C almo st i ron-free goldi s depos i t ed . At th is t em pe r a tur e a l so , the ca r bon con ten t o f the

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depos i t s has decrea sed t o l ess t han 0 .02 per cen t (F i gu re 4) . Inacco rd wi t h t he s t eady f a l l i n t he i ron -con t en t o f t he de pos i t s ,the cur r en t e f f i c iency of g o ld depo s i tion inc r eases s t ead i ly wi thdeposition temperature to as high as 100 per cent at about160C (Figure 8). The change in the gradient of the currentef f i c i ency /depos i t ion t em pera t u re cu rve a t abou t 80C, wh i choccurs w ith gold/cobal t and gold/nickel baths , is not observed.The hardn ess of depo si t s f rom the gold/ i ron elect ro lyte fal lsr ap id ly wi th depos i t ion t em per a tur e a s i s the case wi th dep os i t sf rom the g old/cobal t bath (Figure 9) .The appearance o f t he go l d /i ron coa t ings , by SEM exam i n-a t io n , w a s s i m i l a r t o t h a t ( F i g u r e s 5 t o 7 ) o f g o l d / c o b a l t a n dgold/nicke l coa t ings .Summary and DiscussionThree wea kly acid br ight gold pla t ing baths have been inves-t igated and the var iat ions in the current ef f iciencies of gold de-pos i t ion and i n t he com pos i t ion , hardness an d s t ruc t u re o f t hedepos i t s have been es t ab l ished over a t em pera t u re r ange o f 40to 160C. The ba ths s tud ied w er e a go ld / coba l t , go ld / n icke l anda g o l d / ir o n b a t h , a n d i t w a s f o u n d t h a t w i t h e a c h o f t h e m t h ecodeposi t ion of base metal , and other impuri t ies , wi th the golddecrease a t tem pera tures above 80C. The C in the Au-Ni sys temforms an exception. There were only minor differences be-tween the ba ths in th i s r e spec t . As a r e su l t , the p r ope r t i e s o f thedeposits formed btween 80 and 160C from all three bathsdiffered greatly from those of deposits formed at lowert em per a tur es .Spec i f i ca l ly , the cu r ren t e f f i c i enc ie s o f go ld de pos i t i on in-creased by a f ac t o r o f app roxim at e l y t h ree unde r cond i t ions o faccelerated d eposi t ion, r is ing f rom about 35 per cen t a t 35C, to70 p e r cen t a t 120C, and to 100 pe r cen t a t 160C. Be tween80C and 120 C, the pe r cen tage n icke l in the d epos i t s f r om thego l d /n i cke l ba t h f e l l f rom abou t 4 .5 per cen t t o 1 p er cen t , t hepercentage cobal t in the deposi t s f rom the gold/cobal t bath fel l

f rom ab ou t 1 per cen t t o under 0 .1 per cen t , and t he percen t agei r o n i n t h e d e p o s i t s f ro m t h e g o l d / ir o n b a t h s f e l l f r o m a b o u t1 per cent to under 0.1 per cent. From all three baths, thedepos i ts a t 160C w ere essent ia l ly pu re gold.As regards hardness , t he changes were equal l y i mpress i ve .At 120C, the dep os i t s f rom the go ld / coba l t and go ld / i ron b a thshad HV10 hardnesses w h i ch were v i r t ua ll y t he same as t ha t o fpu re go l d . In acco rd w i th t he i r h igher base m et a l con t en t s be-tween 80C and 120C, howe ver , t he depos i t s f rom the g o ld /nickel bath had hardnesses which fell more slowly withtemperature, and reached the figure for pure gold onlynear 160C.ConclusionsThese s t ud i es cover t he r ange o f t em pera t u res (100-120C)over which electrodeposition takes place in laser-enhancedp l a t in g o f g o l d . T h e y e m p h a s i s e t h e e x t e n t to w h i c h t h e h i g he lec t ro ly te t em pe r a tur es , which pr ev a i l a t the in te r f ace wi th thesubstrate during this type of plating, can influence theproper t ies and composi t ions of the deposi t s which i t y ields. Asa corollary, it must be concluded that if gold coatings withspecified properties are to be produced by this new platingt echn i que , e l ec t ro l y t es wi l l have t o be devel oped spe c i a l ly fo rthe pur pose .In v iew of t he d i f f i cu l ti e s i nhe ren t i n us ing l a se r enhan ce-men t fo r t he p roduct ion o f depos i t s wh i ch a re l a rge enough fo rde te rm ina t ion of t he i r p rope r t i e s by con vent iona l me thods , i ti s sugges ted tha t the behav iour o f e lec t r o ly te s under ev a lua t ionbe t es t ed a t e l eva ted t em pera t u res by t he m et hods exempl i f ied .Although such testing does not simulate all the conditionsunder which deposition occurs in laser-enhanced plating,exper i ence such a s has bee n recorded ab ove should fac i l it a t ein terpretat ion of the resul t s obtained, and provide an improvedb a s i s f o r t h e e x t e n d e d a p p l ic a t io n o f l a s e r - e n h a n c e d e l e c tr o -deposi t ion of gold. u

Acknowledgements: The authors thank Dr. W.S. Rapson for his ass is tance in prepar ing themanuscript. The work was supported by the A.I.F. (Arbeitsgemeinschaft IndustriellerForschungsvere in igungen , Cologne) f rom funds o f the BMWi (Bundeswir tschaf tsmir i s -teriums far Wirtschaft).

References1 RJ. von Gutfeld, R,L. Melcher, E.E. Tynan and S.E. Blum, Ap p l . Phys. Lett.,1979, 35, 651-6532 RJ. von Gutfeld and J. Cl. Puippe, Oberflache Surface, 1981, 22,294-2983 RJ. von Gutfeld and L.T. Romankiw, Gold Bull., 1982, 15, (4), 120-1234 F. Freidrich and Ch. J. Raub, Metalloberfldche,1984, 38, 237-242

5 R. De Do n ck e r an d J . Van h u mb eeck , Trans. Inst. Met. Finish., 1985, 62,59-636 A. KnOdier, Metalloberflche, 1974, 28, 465-4727 A. K nOdler, Metallobefflache,1984, 38, 539-5468 A. KnOdler , Galvanotechnik,1977, 68, (5), 383-3919 Ch. J. Raub. A. KnOdler and J. Lendray , Plating, 1976, 63, 35-40

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High Temperature Gold Deposition From Acid