Palladium has been found to be a viable alternative to gold for contact surfaces in the electronic industry on account of iD

low cost, good resistance to corrosion and tarnishing, good solderability and good electrical conductivity. The present papa

deals with the palladium bath performance, deposit characteristics and bath stabilit

John, S.

Natarajan, S.R.

Sobha , J.

IR@CECRI

0. Electrochem 419) September 1 s .  pp 793-796 PALLADIUM PLATING FOR ELECTRONIC APPLICATIONS SOBHA JA YAKRISHNAN and S R NA TA RA JAN Central Electrochemical Research Institute. Karaikudi-623 006, INDIA [Received: 1988 July; Accepted: 1988 July] Palladium has been found to be a viable alternative to gold for contact surfaces in the electronic industry on account of iD low cost, good resistance to corrosion and tarnishing, good solderability and good electrical conductivity. The present papa deals with the palladium bath performance, deposit characteristics and bath stability. Key words: Palladium plating, electronic industry, electroplating INTRODUCTION E lectrodeposited gold has been the ideal contact material for the electronic industry on account of its desirable characteristics like low and stable electrical contact resistance, excellent resistance to corrosion and tarnish and very good solderability. The replace- ment of gold with other materials is receiving considerable atten- tion in recent years especially because of the rapidly increasing bullion cost. Among the various substitutes, namely rhodium, ruthenium, palladium, silver etc. suggested [ 1,2) for replacing gold in electrical contacts, palladium and its alloys appear to be satisfac- tory alternatives. The economic advantage of substituting gold with palladium is clear from the fact that its cost is approximately one third that of gold and its density is about half that of the latter, Various electrolytes [3,4] have been proposed for electroplating palladium. 'Ike available literature shows however that electrolytes of ammine complexes offer distinct advantages in producing satisfactory deposits. It is reported [5,6] that ductile coatings of palladium can be produced from arnmine chloride and bromide baths. A study of the preparation of these plating baths and some of their plating characteristics has been reported earlier [7]. The present paper deals with the behaviour of a palladium bath based on tetraamminopalladous bromide under varying operating conditions. EXPERIMENTAL The ammine bromide complex was prepared starting from palladiiun chloride through the intermediate preparation of palladium bromide. A concentrated solution of the complex was prepared (for det+ see Ref. 7) and analysed for its exact palladium ;oncentration. This was used to prepare the following plating baths: Palladium as Pd(NH3), Brz @.L-I) 5-25 Ammonium bromide (8. L-1) 40 -- . Ammonium hydroxide As required Hydrobromine acid I !learodeposition studies were carried out with platinum as anode md copper (both of 4 cm .x 1 cm) as cathode. Deposits of com- uuable thickness (10 pm) were produced and nature of deposits xamined visually. Cathode current efficiencies were assessed by ravimetry. The effects of varying metal content, bath pH, cmperature and cathode current efficiency were examined. The deposits obtained were subjected to tests for evaluation 01 their porosity, internal stress and micro hardness. The porosity wa! determined by cadmium sulphide paper test 1181, the stress by H o a  and Arrowsmith method 173 and the micro hardness by the Vickers indentation method. The surface structure of deposits w a  studied by means of Scanning Electron Microscope. RESULTS AND DISCUSSION Effect of varying metal concentrntlon The metal concentration was varied within the range 5-25 g. L-1. The deposits were found to be smooth and adherent with a dull white appearence with 10-20 g. L-1 metal in solution. The in- fluence of metal concentration on cathode efficiency is represented in Fig.1. Curve 1 shows that whereas the current efficiency is low at metal contents below 10 g. L-1, it increases with palladium con- centration to a high level up to 15 g. L-1 and remains nearly cons- tant thereafter. METAL CONCENTRATION,g 2 Fig. 1: Effect of metal concentration (curve 1) and temperature (curve 21 on the cathode current efficiency Sobha Jayakrishnan and Natarajan - Palladium plating for electronic applications Effect of tempemtan Ekctrodeposit charpcterlsstion An increase in temperature renders the deposit darker with no appreciable increase in current efficiency, (curve 2 in Fig. 1). Also operation at higher temperatures is beset with solution evapora- tion and loss of ammonia leading to fall in bath pH. Effect of current density Current density was varied in the range 0.5 - 3.8  A. dm-2 for paIladium solution. The deposits were greyish white below 0.5 A. dm-2, rough and dark grey above 2.5 A. dm-2 and black, powdery and nonadherent at still higher current densities. As shown in Fig. 2 (curve I), there is an appreciable loss of efficiency at  current den- sities higher than .2.0 A. dm-?. Fig. 2: Effect of current density (curve 1 )  and pH lcurve21 on the cathode current efficiency Effect of solution pH The oprlmurn p t I  \ \as fo~~ncl ro be in rhr range 8.5 lo 10.0. T l l a u ~ h  the deposit properties were not affected by operation even at pH 11, the rapid loss of ammonia by evaporation from this highly am- rnoniaca1 soIutian was a problem in operation. At a pH less than 8.5, precipitation at the platinum anode was observed interfering with bath stability, and causing roughness of deposits. All this can be due to the evolution of bromine at the anode which converts the tetra ammino complex to the insoluble diarnmino bromide under the highly acidic condilions prevailing there. The effect of pH on cathode current efficiency is indicated in Fig. 2, (curve 2) rhowing that operation in the pH range 8.5 - 10.0 is possible without much change in efficiency. Based on the above studies, it is concluded that smooth, 3dherent, dull white palladium deposits can be plated from tetra unmino palladium dibromide bath under the folIowing conditions: Metal concentration (g. L-1) 15 - 20 remperature (Ii) 303 - 313 3athode current density (A. dm-?) 0.5 - 1.5 1 H 8.5 - 10.0 The deposits obtained under the optimised conditions were t'uunc to  have the following characteristics: internai .stqss (at 5 pm) 7.0 kg/mrnZ Micro hardness (25 gm load) (25pm) 140 V H N Cathode current efficiency 96% The test on  deposit porosity showed that deposits of more than 5-6 pm thickness were pore free. No cracking was observed in deposits even when applied in 25-30pm thickness. In Fip.3a and 3b are included Scanning Electron Micrographs (SEM) of 25 pn l  palladium deposits produced from the above bath at M3K and 333K Fig.3-a: Scanning electron micrograph of palladium deposit (25 p m )  obtained from the bath at 3U3K Fig.3-b: Scanning electron micrograph of palladium deposit (25 prnl obtained from the bath at 333K 794 R Electrochem 4(9l September 1988 Sobha Jayakrishnan and Netarajan - Palladium plating for electronic applications respecrively. The SEMs (Fig. 4a-4d) which correspond t o  deposiu from the optirnised bath, show a gradual increase in grain size when the plating thickness is inncased horn I Prn to 1Opm and thereafter it is not much affected. Fig. 48: Scanning electron micrograph of deposits with thickness 7 P  where some serious contamination of the bath was suspected, the precipitation of the diarnmine as bromide follovd by filtration, thorough washing and redissolution of the precipitate was found to be effective in renovating the bath. Fig. 4b: Scanning electron micrograph of deposits with thickness 5 j m  Fig. Qc: Scanning electron micrograph of deposits with thickness Fig. 4d: Scanning electron microgmph of deposits with thickness IOurn, l5pni Bath strhilltv The bath w a  found to be quite stable provided its pH was rnain- CONCLUSION tained well above 6 or ' at which palladium complex would get prtcipitattd. No problem was experienced with continuous use of The palladium plating bath with palladium present as tetra the bath. Frequent bath analysis and replenishment was required ammine palladous bromide complex has been found to be capable as the anode used was an insoluble one. The replenishment was of producing smooth, adherent and pore-and-crack-free palladium effected using a solution concentrate of the complex salt. In cases deposits. I euo!aqlddP o!uo~we(e 104 &r!W urn!pqod - un?!arorw pus urul(qn(sr(or wqos 

Palladium plating for electronic applications

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