The Effect of Thiourea on Alkaline Electroless Deposition

Abstract

C14 or $35 tagged thiourea were used to follow the uptake of C and S into the electroless nickel deposit and to determine adsorption curves for thiourea on nickel and cobalt. Enhancement of the rate of deposition is observed for cobalt but not for nickel. A Raney nickel type of mechanism and an electron transfer mechanism are proposed to explain the results.The effect of thiourea and other sulfur-containing compounds on the rate of the electroless deposition process for acid nickel has previously been described (1, 2). Radiochemical studies of this process utilizing sulfur and carbon tagged thiourea have also been reported (3). None of this previous work has dealt with the effect of thiourea on the electroless deposition of nickel or cobalt from alkaline media. The purpose of the present paper is to broaden these studies into the alkaline region for cobalt and nickel and also to report the results of studies utilizing sulfur and car-_~ bon tagged thiourea in this type of deposition system. EExperimental "e The following systems were studied (i) NiC12.6H20, "~ 30 g/l; NH4C1, 50 g/l; NaH2PO2.H20, 20 g/l; Na cit-~" rate.2H20, 100 g/1. (it) CoC12-6H~O, 30 g/l; NH4C1, 50 g/l; NaH2PO2.HaO, 20 g/l; Na citrate-2H20, 100 g/1. The pH of the system was adjusted to the desired levels with NH4OH.The temperature of the plating systems was maintained at 85 ~ • 2~ by means of an electric heating mantle. The substrates used in this investigation were 1 x 89 in. nickel strips. These substrates were sensitized by previously described techniques (4). Deposits were made at 1-hr periods; agitation of the system was accomplished by mechanical stirring. Measurements of p H were carried out for all experiments using high temperature glass and calomel electrodes.In each experiment the desired amount of C 14 or S 85 thiourea was added to the system. The activity of the solution during deposition and the activity of deposited samples were determined using previously de-36 scribed techniques (3). All nickel substrates were weighed prior to and following deposition to determine the weight of the deposit. All results are re-32 ported as the average of several experimental runs. Results 28The effect of thiourea concentration on the rate of nickel and cobalt deposition from alkaline systems is 24 shown in Fig. 1. Previous work in acid systems has shown a gradual increase in the rate of nickel depo-% -20 sition up to a concentration of about 0.5 rag/liter of thiourea (2). Thiourea concentrations greater than 0.5 rag/liter result in a gradual decrease in the rate L6 until at 1.4 mg/liter, the rate goes to 0. Figure 1 shows that for the electroless deposition of nickel from an alkaline medium, the rate steadily decreases until about 1 mg/liter of thiourea, where there is a very rapid decrease to 0. There is no enhancement of the plating rate due to the addition of thiourea in the 8 alkaline electroless deposition of nickel. The addition of thiourea to an alkaline cobalt electroless system as 4 also seen in Fig. 1 leads to a rate maximum at about 0....

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