Ni-P AUTOCATALYTIC PLATING BATH

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1 HALF-REACTION RATES IN A HIGH-TEMPERATURE' Ni-P AUTOCATALYTIC PLATING BATH Douglas T. Mughogho and S. Walter Orchard Center for Applied Chemistry and Chemical Technology University of the Witwatersrand, Johannesburg Private Bag 3, Wits 25, SOUTH AFRICA l?% &Si Abstract Electroless Ni-P was plated from an acidic bath containing nickel sulfite (NiS4), sodium hypophosphite (NaH2P2) and sodium acetate (CH?COONa), succinate or formate, plus minor additives lead acetate ((CH3C)2Pb), potassium thiocyanate (KSCN), thiourea (CH4H2S), or selenic acid (HzSeOd), on to a rotating disk substrate electrode. The effects of reagent concentrations, ph, temperature and rotation rate on the plating rate were measured in this way. The plating mechanism was studied by controlled-potential measurements in the vicinity of the open circuit potential. The anodic and cathodic halfreaction rates thus obtained showed a remarkable correlation with each other. These and other results suggest a direct coupling between the half-reactions, and a highly unusual, non-electrochemical mechanism. Introduction The primary aim of the present investigation was to obtain a better understanding of the fundamental processes occurring in an electroless nickel plating bath. While the process economics and the physico-chemical properties of the plated material may be of greater practical importance, an in-depth mechanistic understanding can he expected to lead to advances in the theory and practice of electroless plating.

2 2 Electroless plating occurs when a solid surface, in a solution, becomes plated with a metal or alloy by a surface-catalyzed oxidation-reduction reaction between solution reagents. In the case of electroless nickel deposition using hypophosphite ion (H2P2') as the reducing agent, the process may he considered to comprise a cathodic half-reaction, where L is a ligand, and an anodic half-reaction, H,PO; + H,O + H,PO; + 2H' + 2e' (2) During plating, the surface typically adopts a potential which is intermediate between the equilibrium potentials of the two half-reactions [l-41. This is referred to as the plating, or mixed, potential. Side reactions may also occur, including in the above case both the codeposition of phosphorus and the evolution of gaseous hydrogen (H2). Wagner and Traud [SI studied a selection of mixed potential processes, including acidic coinxion of zinc, and concluded that the two half-reactions occurred independently of each other; i.e., at the mixed potential, the rate of the one half-reaction did not depend on the simultaneous presence of the second half-reaction. However, more recent studies of both copper and nickel electroless plating systems, have shown that important interactions between the constituent half-reactions do in fact occur [3, We previously studied plating from a low temperature electroless nickel plating solution, at a range of controlled potentials around the mixed potential, using both a rotating disk electrode (RDE) and a quartz crystal microbalance electrode [7, 111. The half-reactions were found to be dependent on cach other to a limited extent, but were not directly coupled. Since high temperature baths are of great practical importance, we felt that a similar investigation of such a bath would be of value: this study is reported here. Experimental The nickel plating bath was based on a formulation published by Henry [ 121. The composition of the "standard" working solution was: NiS4 (.17 M), NaH2P2 (.227 M), CH7COONa (.27 M), (CH3CO)2Pb (3.95 x M), KCl (1. M), ph = 4.6, 81 6

3 3 measured at room temperature and adjusted with H2S4. The KC1 was added to enhance the conductivity and thus reduce the effects of uncompensated resistance in the potential measurements; it did not affect the plating rate. Except where otherwise noted, the plating experiments were carried out at 85 * 2 C. In the first part of this work, the effect of changing the temperature and reagent concentrations, on the open circuit plating rate, was investigated; in addition, salts other than sodium acetate and stabilizers other than lead acetate were studied. In the second part, the dependence of the half-reaction rates on potential was studied, using the standard solution except where otherwise noted. The working electrode was a rotating disk electrode (RDE) of area 4.91 cm2, which has been described previously [ 131. The detachable disks were weighed before and after each run, on a balance capable of reading to.1 g. A planar nickel counter electrode was placed parallel to the RDE and 1-2 cin away. The reference electrode was a saturated calomel electrode (SCE) in a Luggin probe with the tip placed approximately 1 mm from the surface of the working electrode. The electrode assembly was placed in a waterjacketed cell of 1 ml capacity which contained the plating solution. A home-built, MINTEK-designed potentiostat, capable of currents up to 1 ma, was used to control the potential and measure the cument at the working electrode. Plating rates in teims of mass were converted to cathodic partial current densities, jc, using an appropriate conversion factor based of the density and composition of the deposit. From the average net current density ineasurixl during the plating run (j), the anodic partial current density j., was calculated according to the equation We adopt the convention that cathodic current densities are negative, anodic ones are positive. Equation (3) represents a simplification in that side-reactions are ignored in the calculation of ja; however, this simplification reilects our primary concern with the plating rate and not, for example, with hydrogen cvolution. 81 7

4 4 Results and Discussion A. Open circuit plating rate measurements 1. Effect of deposition time Two series of experiments of varying duration in the range 5-3 minutes were performed, using the standard plating solution and a rotation rate of SO rpm for the RDE. In one series, plating was initiated by briefly polarizing the working electrode to a sufficiently cathodic potential; in the second set, plating was allowed to initiate spontaneously. In both cases, the data indicated a constant plating rate up to at least 3 minutes; furthermore, there was no significant difference between the plating rates obtained by the two methods. Since a 3 minute run produced a deposit weighing about 4 mg, far more than was necessary for accurate mass deteimination, the plating times employed in the following experiments were generally shorter, of the order of IO minutes. 2. Effect of rotation rate The rotation rate of the RDE was varied between 25 and 1Sot) rpm, resulting in a mean measured plating rate of 16.7 k 1.5 mg cm'2 hi'. No systematic correlation between the plating rate and the rotation rate was discernible. Subsequent plating experiments were carried out using a constant rotation rate of 5 ipn. 3. Effect of temperature The plating rate was measured at five different temperatures in the range "C. Below this range, plating stopped entirely, and 88 "C was the maximum temperature attainable with our equipment. An Arrhenius plot of the data gave an activation energy of 89 kj mol-'. 4. Effect of bath composition and concentrations Close to the composition of the standard plating solution, the plating rate is independent of the concentrations of nickel sulfate, sodium hypophosphite and sodium acetate (see 81

5 Figures 1-3). However, the rate is sensitive to both ph and the concentration of lead acetate as shown in Figures 4 and 5. In view of the sensitivity of the rate to the hydrogen ion concentration, the bath ph was measured before and after all subsequent plating runs. This was found to decrease dui-ing plating, as would k expected from reaction (2), but the change was usually limited to less than.1 ph unit, which would have an undetectably small effect on the rate. Lead acetate both stabilizes the bath and brightens the deposit. Other additives were studied instead of or in conjunction with lead acetate; these were thiourea (CH4N2S), potassium thiocyanate (KSCN) and selenic acid (H2Se4). The results obtained with these additives are summarized and compared with those with lead acetate, in Figure 6. It is noteworthy that both thiourea and potassium thiocyanate are capable of signiikantly increasing the plating rate when used in appropriate concentrations. Lead acetate produces no such acceleration, but is more effective as a brightener. B. Controlled-potential plating rate measurements Figure 7 shows plots of j and j, measured over a range of potentials on either side of the open circuit plating potential (which is indicated with a vertical dashed line) in the standard bath at 85 C. Also shown are the corresponding j;, values calculated according to equation (3). Figure 7 reveals some sti-iking features which are directly relevant to the underlying mechanism operating in this plating bath. The onset of plating occurs at potentials negative of -.25 V vs SCE. Over a potential range of about.5 V spanning both sides of the plating potential, the current-potential curves display very little dependence on potential. Moreover, since the net current.j is small throughout this range, the ja and ic curves almost mii-ror each other in the -.25 to -.8s V range. At -.25 they both fall sharply to zero or near zero values. Results obtained at 79 C were qualitatively very similar. The behavior shown in Figure 7 is by no means specific to the standard bath composition. In particular, when the lead acetate was omitted from the solution, or when it was substituted by one of the other trace additives, very similar patterns were noted, 81 9

6 6 with due allowance for the modified rates. For example, Figure 8 shows two such sets of data: one with no stabilizer and one with thiourea instead of lead acetate. For comparison purposes, some plating runs were performed with a quartz crystal microbalance working electrode. In Figure 9, the microbalance data of Figure 7 are compared with the RDE data. The results are almost identical in the vicinity of the plating potential, and the only substantial differences (which are seen in the anodic limit) can be explained in terms of the different hydrodynamics. When the standard bath was modified by substituting equimolar NaCl for NaHzP2, no measurable plating was found to occur anywhere near the normal plating potential. Less surprisingly, substitution of Zn2+ for Ni2+ also led to a cessation of plating. In both types of substitution experiment, the measured net current j behaved very similarly to that in the standard bath; i.e. it was small and showed only a slight potential dependence. The general features of the data obtained under controlled-potential conditions are significantly different from those of previously reported studies. Investigations of both copper and nickel baths have demonstrated that substantial, positive interactions between the half-reactions do occur [3, 6-111; however, the remarkable parallels between the j, and ic curves shown in Figures 7-9 appear to be novel. In the present system, it is clear that the anodic and cathodic processes interact; however, the interaction is so extreme that it amounts to a direct coupling between them. Neither half-reaction can occur to any significant extent in the absence of the other. Stated alternatively, the half-reactions (1) and (2) do not occur as electron transfers between electrode and reagents, either in the complete plating bath or in the formulations modified by substituting inert substances for either of the redox reagents. Instead, electron transfer must occur directly between surpace-bound reagents, amounting to a chemical mechanism rather than an electrochemical one. This is in direct conflict with the view that all electroless plating processes can be explained in terms electrochemical mechanisms [4,6, 14, 151. These conclusions receive further support when the data are analyzed in terms of the polarization resistance. Ohno [4] has shown that the measured polarization resistances of 82

7 7 several copper and nickel baths correlate well with the observed plating rates as the baths age and the rates slow. The relationship can be expressed as where R, is the polarization resistance, i, is the plating current and K is a constant, characteristic of the type of plating bath used. For six different copper, nickel and cobalt plating baths, the values of K reported by Ohno lie between.26 and.12 V. From our data such as those of Figures 7-9, the slopes of the j vs E lines can be used to estimate the polarization resistances at the plating potential. Typical K values, calculated according to equation (4), are: - 2 V (Figure 7); 2 8 V (Figure 8, thiourea); 2 5 V (Figure 9, quartz crystal microbalance). While these values are very approximate (not having being determined using optimal methods), they are clearly one to two orders of magnitude greater than the largest values found by Ohno. They are also much greater than the K value of -.8 V estimated from the data in our earlier RDE study of an alkaline electroless nickel bath operating at 35 "C [7]. In the absence of any current due to side reactions, a purely chemical plating mechanism would give infinitely large R, and K values. Our data again support the view that the primary mechanism operating in this bath is fundamentally different to that of most other electroless baths [4, 6, 141 and to the single, general mechanism proposed by van den Meerakker [ 151. We do not exclude the possibility of a small contribution from an electrochemical mechanism, which would establish the mixed potential and reduce the polarization resistance to a finite value. The available data do not provide clear evidence as to the nature of the surface complex involved in the electron transfer process. In the absence of sodium acetate, plating ceases, but at the prevailing concentrations in the bath, the rate is independent of sodium acetate concentration (Figure 3). When hypophosphite is removed from the bath there is no plating even when the electrode is held at the noimal plating potential. These observations suggest that CH3COO and H2P2- both participate in the surface complex and that the H2PO2- ion is not merely a reducing agent, but is actively coordinated to Ni". Hickling and Johnson [ 161 have proposed that the %valent phosphorus tautomer of H2P2-82 1

8 8 (HPO(H)-) is more readily oxidized than the 5-valent form, and it is thought to be an intermediate in the electroless process. We previously suggested that it might also act as a bridging ligand between Ni and the surface, in a low temperature electroless bath [ In the present system a similar structure is probably involved, in which CH3COO- and the bridging H2P2- are simultaneously coordinated to Ni2+. Further work is needed before a more definite assignment can be made. Summary Recent reviewers of electroless plating have argued that the mechanisms of these processes are electrochemical [4, 151. Our data suggest that a chemical mechanism, not an electrochemical one, operates in this particular plating bath. In this connection it is noteworthy that metallic corrosion, while normally an electrochemical phenomenon, can also occur via a chemical mechanism [ 171. A systematic study of a wider range of plating baths could give an indication of how widespread chemical mechanisms are, and under what conditions they are encountered. (Unpublished work from our laboratory has indicated that a particular electroless copper bath is also chemical in nature [ 181.) It may soon be appropriate to reassess current thinking as to the universality of electrochemical mechanisms of electroless plating. Acknowledgments We thank the University of the Witwatersrand and the Foundation for Research Development for their financial support of this work. References [ I] M. Paunovic, Plating 55 ( 1969) 1161 [2] C. Gabrielli and F. Raulin, J. Appl. Electrochem. 1 (1971) 167. [3] P. Bindra and J. Roldan, J. Appl. Electrochein. 17 (1987) [4] I. Ohno, Mciterinls Science cind Engiiweriizg A146 (1991)

9 9 C. Wagner and W. Traud, 2. Elektrochein. 44 (1938) H. Wiese and K. G. Weil, J. Electround. Chein. 228 (1987) 347. S. W. Orchard, Pluting 75 (1988) 56. B. J. Feldman and. R Melroy, J. Electrochein. Soc. 136 (1989) 64. A. Hung and I. Ohno, J. Electrochem. Soc. 137 (199) 918. Z. Jusys, J. Liaukonis and A. VaSkelis, J. Electrocintil. Chem. 325 (1992) 247 A. H. Gafin and S. W. Orchard, J. Appl. Electrochem. 22 (1992) 83. J. Henry, Mrtril Finishing Giiirle Book Directory, 85 (1987) 358. S. W. Orchard, J. Appl. Electrochein. 18 (1988) 666. M. Paunovic and D. Vitkavage, J. Electrochein. Soc. 126 (1979) J. E. A. M. van den Meerakker, J. AppI. Electrochein. 11 (1981) 395. A. Hickling and D. Johnson, J. Electro~inril. Chein. 13 (1967) 1. F. Mansfeld and J. V. Kenkel, Corrosion Science 16 (1976) 653. A. H. Gafin, D. T. Mughogho and S. W. Orchard, manuscript in preparation. 823

10 IO Figure [N is4l/m Dependence of plating rate on concentration of nickel sulfate Figure 2. Dependence of plating rate on concentration of sodium hypophosphite. 824

11 [CHsCOONal/M Figure 3. Dependence of plating rate on concentration of sodium acetate PH Figure 4. Dependence of plating rate on ph. 825

12 928 4 (D - 3 I OE 5 7 I OP 2 W z -b bl oz 3 I N 3-1 I v

13 13 N I E V -=f E \ SO1 I I, I 1 I I I I (E vs SCE)/V Figure 7. Net, cathodic and anodic current densities versus potential y -1 E -2 Q -3 E \ Fig use r I I - + j in bath stabilized by CH4NzS - A 1. in bath stabilized by CH4NzS ' 1 in bath stabilized by CH4N2S. - o 1 in bath without stabilizers - a jc in bath without stobilizers Ja in bath without stabilizers. I I I (E vs SCE)/V 8. Net, cathodic and anodic current densities versus potential, without any stabilizer (open symbols) and with 3.3 x IO-' mol L-' of thiourea in place of lead acetate (filled symbols).

14 14 JUI I ' I I ' : I ' I,,.,.,. ' I 2 1 N 'E -1 Q -2 E v-- "Jc. I 4 jc obtain t ja obtained at a QCME a 1 obtained at a RDE v jc obtained at a RDE la obtained at a RDE I " I. I,I. I. I I I, I I 8 (E vs SCE)/V Figure 9- Comparison of current density versus potential plots obtained at the RDE (open symbols) and the quartz crystal microbalance electrode (filled symbols).

Electrochemistry. The study of the interchange of chemical and electrical energy.

Electrochemistry. The study of the interchange of chemical and electrical energy. Electrochemistry The study of the interchange of chemical and electrical energy. Oxidation-reduction (redox) reaction: involves a transfer of electrons from the reducing agent to the oxidizing agent. oxidation: