Electroplating silver from thiosulphates solutions has received increasing attention recently because of the

emphasis on pollution-free prooesses. This paper reports the results of studies for identifying suitable

compositions for silver plating. Different aspects encompassing stability, mnductivity, cathode current

efficiencv. deposition rate and throwing power of solutions as well as deposit properties like appearance.

microhakness and adhesion were examined. The preplating sequence fro improving deposit adhesion

was also investigated. The studies enabled identification of two methods of wetreatment and production

of sound deposits with good adhesion to copper and.nickel plated copper. specimens

Krishnan, R.M.

Natarajan, S.R.

Sriveeraraghavan, S.

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Bulletin of Electrochemistry 2 (3) May -June, 1986, pp. 257-260 THIOSULPHATE BATHS FOR SILVER PLATING R M KRISHNAN, S SRIVEERARAGHA VAN and S R NATAIUJAN Central Electranemical Research Institute, Karaikudi - 623 006 ABSTRACT Electroplating silver from th~osulphate solutions has received increasing attention recently because of the emphasis on pollution-free prooesses. This paper reports the results of studies for identifying suitable compos~tions for silver plating. Different aspects encompassing stability, mnductivity, cathode current efficiencv. de~ositioniate and throwing ~ower  of solutions as well as de~osit ormerties like amearance. microhakness and adhesion were examined. The preplating sequence fro improving deposk'adhesion was also investigated. The studies enabled identification of two methods of wetreatment and ~roduction of sound deposits with good adhesion to copper and.nickel plated copper. specimens. Key words: P resent by we are in the middle of a new generation of silver plating proceaaes. As a result of the ebcalating cost of gold, silver is under investigation aa a potential substitute for gold, pPrticdars in the semiconductor field. The recognition ofthe need for pollution control is also highly felt nowadays and this haa intensified effortr tc formulate noncyanide compositions as a substitute for cyanide b d  solutions for electroplating. There have been a number of attempta in the recent years in diecovering noncyanide silver complexes for the electrodqmition process. Table I lists the most important complexes and their stability constants [I]. In the choice of a complex of practical value, the most important land marks are in gregard to: (i) iodide electrolyte (ii) silver nitrateammonium pulphate electrolytes and (iii) thiosulphate electrolytes. Table I : Stability constants of Sllver Complexes Complex Ion Logarithm of Stability Constant Ag (CN); (cyanide) Ag (CHJ'JzSh A& (Iodide) Ag (s~o& (thiosulphate) Ag (scN):. (thiocyanate) Ag ( ~ 0 3 ) : -  (sulphite) AgBr: (bromide) Ag (en) 2 + (ethylenediamine) Ag (NH3); (ammonia) A~CI:. (Chloride) Table I1 gives some of the compositions of electrolytes based upon iodide and ammonia complexes as published [2]. Most of the compositions containing silver thiosulphate complex are covered under patents. It waa therefore considered worthwhile by the authors to undertake investigations and this paper presents a report on the work done. Solution characteristics, namely, current efficiency, rate of deposition and throwing power, and deposit properties like appearance, microhardness and adhesion were studied. Procedures were also worked out for preparation of the plating solution and for pretreatment of copper substrates. Table II : Siver platlng solutions based on iodlde and ammonia complexes Bath composition Cathodic current density, temp. etc. 1. Silver sulphate 30 g/l Ammonia (25%) 75 c d l  Potaasium iodide 600 g/l 2 ~ . d m - '  room temperature ~ & u m  pyrophosphate 60 g/l 2. Silver & a t e  30-40 g/l Potaasium iodide 300-400 g/l 0.5 - 3.0 A/dm-' upto 40°C Polyethylene, polyamine '10-1 00 g/l 3. Silver iodide 40-80 g/l Sodium iodide 400-600 g/l 0.5 - 3.0 ~ / d n i '  room temper ature Polyvinyl alcohol 0.5-2.0 g/l Sodium thiosulphate 1-2 g/l 4. Silver nitrate 35 g/l Ammonium sul- phate 150 g/l 0.5 - 1.5 ~ / d m "  pH 10-10.6 upto 30°C Citric acid 4 g/l Ferric sulphate 0.4-3.0 g/l Ammonia 2-50 mV1 5. Silver nitrate 20-30 g/l Sodium pyrophosphate 20-25 g/l Ammonia (1.25%) 60-100 ml Sodium 0.8- 1 .I ~ / d m . '  room temper- ature nitrite 40-70 g/l Ammonium sulphate 40-70 g/l EXPERIMENTAL Repustion of the bath A number of kxperimenta were necessary for identifying suitable compositions for plating. The tendency of a thiosulphate solution containing silver to precipitate black silver sulphide created a need for including a suitable stabilizer, like potansium metabisulphite. The majority of the experimens were performed with solutions I and I1 the compositions of which were selected on the basis of preliminary experiments. Solution I Silver as chloride 50 ,/I Sodium thiosulphate 500 g/l Potaaaium metabisulphite 30 g/l Krishnan et al-Thiosulphate baths for silver plating Silver M chloride 30 g/l Sodium thiosulphate 500 g/l PotPlsium metabi- sulphite 30 g/l Saver chloride was precipitated from silver nitrate and potassium chloride and was dissolved in sodium thiosulphate solution; Potassium metabisulphite was then added and the solution volume made up as required. Effectr of vuiation of concentrationm of the conmtitaentr Different quantities of silver chloride were prepared and the precipitated compound (silver 10-50 g/l) dissolved in an excess of sodium thiosulphate (500 g/l), with the addition of 30 g/l pbtarsium metabisulphite. Silver deposition was carried out from these solutions at different current densities, stirring being resorted to especially when plating at a current density higher than 0.5 A.dm-'. In order to fix the range of sodium thiosulphate for production of accep!able deposits with adequate solution stability, solutions containing 50 g/l silver as silver chloride, 30 g/l potassium metabisulphite and 100-600 g/l sodium thiosulphate were used for silver plating. Po t~s ium metabisulphite being the constituent added for improving solution stability it was considered worthwhile to fix its permissib!e concentration range giving rise to sound deposits. Solutions with 30-50 g/l silver as silver chloride, 500 g/l sodium thiosulphate and 0-50 g/l potassium metabisulphite were studied. Studiem on preplrting treatmentr The conventional preplating treatment with a striking solution was not effective in producing adherent silver deposits from the plating solution. Various preplating treatments were therefore examined and the useful life of the pretreatment solutions was also investigated. The following sequence was employed for obtaining 50 pm thick silver deposits on copper: i) Degrease ii) electroclean iii) preplate treatment like striking in 10 times diluted bath or anodic treatment in the -plating bath cathodic/anodic immersion treatment in ammonium thiocyanate solution and iv) silver plate. Deposits adherence was tested qualitatively by bending the plated specimens. As the conventional preplating treatment was not also effective in producing adherent silver deposits on kckel plated surfaces, silver was deposited on nickel plated copper substrates for qualitatively studying the deposit adhesion. The following pretreatment was investigated for its usefulness: i) degrease ii) electroclean iii) pretreatments such as immersion/anodic treatment in 10% ammonium thiocyanate solution, anodic processing in sodium thiosulphate solution at different current densities, anodically treating in dilute silver thiosulphate bath, or the earlier treatment followed by strike in the same solution iv) strike with silver in the silver strike solution for 1 minute at 0.16 ~ . d m - ~  at room temperature and v) silver plating in the bath proper. Studiem with Solution I Current efficiency experiments weretarried out at different current densities with copper substrates of 12.5 cm2 exposed area and subjected to preplating treatments as explained earlier. Mechanical stirring was employed when plating was carried out at current densities higher than 0.5 A.dm-' The silver deposits were electropolished in a solution of the following solution before testing them for microhardness: Sodium cyanide 10 gA Potassium ferncyanide 100 g/l Applied Voltage 2.5 V Time 1 minute Cathode Graphite The microhardness-values were measured by the Vicker's method of indentation using a Russian made nMT-3 Microscope Hardness Meter at a load of 5 g. Studlem with Solution 11 The current efficiency experiments were performed at different current ddnsities with copper specimens of 12.5 cm2 area after taking them through a suitable pretreatment as described.earlier. Thwwing power This WM determined for each of the two baths employing the Haring-Blum cell and Field's formula The porosity of the silver deposits obtained from both the baths was determined by the electrographic method, with a filter paper of 25 x 25 mm size dipped in a 5 % potassium nitrate solution at 2.5 V for a duration of one minute. Effectr of temperature and pH These were also studied by varying the temperature and pH of solutions 1 and I1 separately and carrying out silver deposition. RESULTS AND DISCUSSION Preparation of the bath The bath composition~finally recommended in this paper had been found to keep well without any decomposition for more than seven months. It was earlier found, however, that solutions prepared making use of soluble silver salts had a noticeable tendency to decompose and hence a proper method of solution preparation involving the use of an insoluble silver compound and a stabilizer was necessary. It was also found that with potassium metabisulphite as an ingredient good solution stability could be achieved. Any large change of pH or temperature of solution decreases its stability markedly. Effect of variation of milver concentration With an increase in silver concentration there is a possibility of the stability of the bath being slightly affected for a given thiosulphate concentration but better deposits are obtained. Acceptable deposits can be produced even at high current densities up to 1.5 A.dm-' with solution agitation. Solutions with low silver contents permit.only low current density operation, rough or spongy deposits being produced at normal current densities (Table HI). Generally solution agitation e.g. by means of a magnetic stirrer improves the quality of silver deposits. Silver chloride equivalent to 10-15 gA silver concentration was used for preparation of the plating bath. Increased quantities of silver chloride dissolve in sodium thiosulphate only slowly and with intensive stirring. Table Ill : Effect of silverconcentration on the stability and performance of bath Composition Observations 1. Silver as chloride 10/20 g/l Stable; the low silver content did Sodium thiosulphate 500 g/l not permit production of sound Potassium metabisulphite 30 g/l deposits even at low current densities 2. Silver as chloride 30/40/50 g/l Stable; current densities up to Sodium thiosulphate 500 g/l 0.77/1.00/1.50~.dm~~ respectively Potassium metabisulphite 30 g/l with stirring could be used. Effect of variation of modirrm thiomdphate concentration An increase in concentration of thiosulphate causes silver chloride ta dissolve more easily. On considerations of solution stability, the d u m  thiosulphate concentration was fixed at 500 g/l (Table IV). Bulletin of Electrochemistry 2 (3) May - June. 1986 258 Krishnan el al- Thiosulphate baths for silver plating Table IV: Effect of sodium thiosulphate on bath stability and performance Composition Remarks I. Silver aa chloride 30g/ I Silver chloride did not dissolve Sodium thiosulphate 100 200 wmpletely 300 g /  l,Potaaaium metabisul- phite 30 g / l  2. Silver aa chloride 30g/ I Sound deposits were obtained, Sodium thiosulphate rU)0/500/ with stability and plating charac- 600g/ 1,PotYsium metabisul- teristics improving succemively phite 30 g7 l with this d t  addition The initial experiments m pmpatim of the both atobILbed the urehrLvg of t h i s o o m p o u n d a a a s t a b i l i r e r . A n ~ i n u m c m ~ d t h i s d ~ ~  the stability of the bath. Aminimumcaacenhtimd20 d l  *urn mehbhu- lphite is eaaential for good bath stability; sdutiauamtainingnot Ian than 2 0 d l  of this compound iue undecompaed even after 6 m c m h  dpqnation(Table V) Table V: Effect of potassium metabiaulphlte on bath stability Compoai tion Rrmsrka 1. Silver aa chloride 30 u)/ l Hinhlv unatable - 2  Sodium thiosulphate "500 g /  l Potassium metabisulphite Og/ I 2. Silver as chloride 30 g/  l Strble for 1 month / mote than Sodium thiosulphate 500 g /  l 3 months respectively Potassium metabiaulphite 10 /mg/  1 3. Silver as chloride 30 g/  I Stable for mom than 7 months Sodium thiosulphate 500 g/ l Potassium metabisulphite 301 401 50 g/  l Experiments on alternative methods of prrtrrcltment rhowed a dip in 10% ammonium thiayannte to be helpful in obtaining adherent rilva deposits. The improved deposit adheaion is  obviously due to formation of a brown w- lo* layer of coppex thiayanate which -mi- the tendency of the s u b  to dissolve, though slightly, and hence parmt immersion deposi- tion of silver. There is also a small loss of substrate copper.which inerrws with duration of treatment in thiayanate solution (Table VI). As far as an undercoat of nickel wan concerned it w u  found that an anod$tratment of the nickel plated specimen in a *dl sodium thisulphate solution with a silver content one tenth of that in the bath, and a uthodic strike in the m e  solution were n e c e s q  to produce an adherent silver deposit T& W : Effect of duration of lmmerdon ol comer wbstrates on metal lo8a and adhesion of s l h r  depodt 4w ef- h 3  With Solution I, an increase in current denaity (cd) up to 1.25 A.dm-' doea not have much iduence on the c-t efficiency which r a n p  Y6-98% (Table W). When however the current dendty is i n u e w d  to 1.5 Adm-' it b m e a  difficult to obtain coherent deposits without stirring and the curremt efficiency is only 67%. In the case of Soltuion 11, an increase of current density does not alter the current efficiency from near 1W)% (Table VIII) in the c.d. range 0.25-0.5 ~ . d m - '  Table VII : Current efflciencles of bath I at dlfferent current densities and correepondlng deposit hardness values Current density A.dm-' Curfent Microhardness efficiency kg. mm-' (%)  (Vickers) Table VHC: Current emciencies of bath II at different current densities and correqumdlng deposit hardness values Currrnt Current Hardness density in efieicncv kg.mm-' kdm-') (%) (Vickers) The m i a n l m h m  of the deposit is also not dected much with c m n t  density. The d-ts have amiavhpcdneg oftherderof 55 and 60 tig.mrn-' respective- ly from baths I and I1 which iue wasiderrbly lower than in the case of deposits from +de sdutioos Thus deposits from thiosulphate baths iue more easilv buffable than umventioaol deposits The tbwingpower(nearly ofSdution I at n o d  cmntdensitieg is also ucepC.My iulequate (Table M). An inwease in either pH or temperature of this bath doa not help in huther improvement of deposits. Table IX: Throwing power of baths I and I1 Current denaity Throwing power (%) (ndm-'1 Bath1 Bath11 p- R d t s  dtbe pausity mummnent d e d  out with Sdution II iue given inTa- Me X Advadepa i to f  not Lea than l5pm thi- is found to be nonpomus. T.Me X: P o r W  ol diver dsposib, from Bath II T h i c k  of silver deposit Number of pore0 per dm' Gem) High1 y porous 16 pores No pore8 Bulletin of Electrochemistry 2 (3) May-June, 1986 259 Krishnan et al- Thiosulphate baths for silver plating If any black silver sulphide precipitate is formed, it may be filtered off and treated with a solution of d u m  hypochlorite, to obtain silver chloride and the latter, rfter thorough washing rediamlved in the plating solution. Excellent silver depoaita can be obtPined from solutions of composition and ~peroting punmetem u below: Solution I Solution 11 Silver u chloride, g/l 50 30 Sodium thioaulphpte, g/l 500 500 Potrssium metabisulphite, g/l 30 30 Temperature Room Room Cathodic currmt denrity ~.dm-'  0.5-1.25 0.25-0.75 pH 4-4.5 4-4.5 1. Rolf Ludwig, IntmjWh Past 11 (1976) p 687 2. FA hwenheim (Ed) M o d m  EhmoplaHv, New Yo& Third Edition, John Wiley and Sons Inc (1974) P' SYMPOSIUM ON ELECTROMETALLURGY 7 - 9 NOVEMBER 1986 *Preparation of feed material for electrolysis *Electrowinning of common metals *Electrowinning of less common metals *Electror&nlng *Electrolytic metal powders *Electrochemistry In mineral processing including elechoflatation *Electrogahranising. electrotinning and electrochemical machining *Physicochemical aspects, modelling. instrumentation and contrd *Corrosion and pollution control in electrometallurgical industries Papers are invited for presentation at the  Symposium Last date for Abstract : 1 May 1986 Last date for full text : 1 September 1986 For further particulars write : Shri R. Srinhrasan Head. Divlsion of Electrohydrornetallurqy. Central Electrochemical Research Institute Karaikudi - 623 006. Shri P.S Desikan Head. Division of Electro~ometallurgy. Central Electrochemical Research lnstitue Karalkudi - 623 006. Further details regarding the  courses may be had from : The Director, Central Electrochemical Research Institute, Karaikudi 623006, India Bulletin of Eledrochemistry 2 (3) May-June, 1986 

Thiosulphate baths for silver plating

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