STUDY OF THE ELECTRODEPOSITION OF A TIN–BISMUTH ALLOY

O. S. Stepanova, A. N. Ivanova

(Presented by the Scientific-Methodological Seminar of the Department of Technology of Inorganic Substances and Electrochemical Production)

The use of a tin coating alloyed with bismuth (a tin–bismuth alloy) ensures good solderability and, even after prolonged storage, the best appearance, which is of great practical importance.

Deposition of the tin–bismuth alloy is carried out from an acidic gelatin electrolyte \[1–3\]. The technology for applying a tin–bismuth alloy has been presented fairly fully in the literature; however, insufficient data are available on the influence of physicochemical factors on the structure and quality of the coating and on the kinetics of alloy deposition.

In this work we used the experimental method described in \[4\]. For deposition of the tin–bismuth alloy, an electrolyte of the following composition was used (in g/l):

tin sulfate, $\mathrm{SnSO_4}$ (chemically pure) — 50,
sulfuric acid, $\mathrm{H_2SO_4}$ (chemically pure), specific gravity 1.84 g/cm$^3$ — 120,
bismuth nitrate, $\mathrm{Bi(NO_3)_3}$ (pure for analysis) — 1–1.5,
gelatin — 1–2,
wetting agent OP-10 — 2–5.

The influence of the cathodic current density ($i_k$) on the rate of alloy deposition at constant coating thickness ($\delta = 9\,\mu\mathrm{m}$), on the structure and quality, on the cathodic current efficiency (C.E.) of the alloy, and on the bismuth content in the alloy ($C_{\mathrm{Bi}}\,\%$) was studied. The data are given in Table 1 and Fig. 1. From Table 1 it is evident that, with increasing current density, the rate of alloy deposition increases. The range of working current densities in which high-quality coatings are obtained with a current efficiency of about 100% lies from 0.4 A/dm$^2$ to 1 A/dm$^2$. Figure 1 shows

Fig. 1. Macrostructure of tin–bismuth alloy deposits on Kovar.
a — \(i_k = 0.1\ \mathrm{A/dm^2}\); b — \(i_k = 0.5\ \mathrm{A/dm^2}\);
c — \(i_k = 0.8\ \mathrm{A/dm^2}\); magnification \(500\times\).

microstructure of the tin–bismuth alloy deposited on a copper substrate at a magnification of 500×. It is seen that the coarsest crystalline structure of the deposit is observed at \(i_k = 0.1\ \mathrm{A/dm^2}\); with increasing current density the structure becomes finer. However, it is not advisable to use current densities above \(1\ \mathrm{A/dm^2}\) as working values, although the structure of the deposit becomes still more finely crystalline and uniform. The use of elevated current densities is limited by the bismuth content in the alloy.

Table I.

From Table I it is seen that, with increasing current density, the bismuth content in the alloy decreases (\(<1\%\)), and the color of the coating becomes

lighter. The data obtained agree well with the polarization curves (Fig. 2). Figure 2 presents partial polarization curves for tin and bismuth and the total polarization curve for deposition of the alloy at two temperatures. From Fig. 2 it is evident that the considerable polarization of bismuth brings together the deposition potentials of the two components (\(Bi\) and \(Sn\)) of the alloy. Even at low current densities, simultaneous deposition of the two metals on the cathode is observed. As the current density increases, the bismuth content in the alloy decreases, while that of tin increases. Raising the temperature of the electrolyte leads to a decrease in polarization during deposition of the tin–bismuth alloy, a decrease in the bismuth content in the alloy, and a deterioration of the structure and quality of the deposit.

Fig. 2. Polarization curves

1 — alloy, at \(t = 40^\circ\mathrm{C}\);
2 — alloy, at \(t = 22^\circ\mathrm{C}\);
3 — tin, at \(t = 22^\circ\mathrm{C}\);
4 — bismuth, at \(t = 22^\circ\mathrm{C}\).

The throwing power of the electrolyte was evaluated as a function of the cathodic current density. The throwing power \((T, \%)\) was calculated by the method of G. Hering and W. Blum [5]:

\[ T = \frac{K - \frac{g_{\mathrm{b}}}{g_{\mathrm{d}}}}{K}\cdot 100\%, \]

where \(K\) is the primary current distribution, dependent on the geometric factors of the electrolyzer (in our case \(K = 2\));
\(g_{\mathrm{b}}\) is the weight of the deposit on the nearer cathode, g;
\(g_{\mathrm{d}}\) is the weight of the deposit on the farther cathode, g.

The data are given in Table 2.

From Table 2 it is evident that the throwing power of the electrolyte from which tin–bismuth was deposited is fairly high, which makes it possible to coat parts of complex configuration, obtaining thereby

deposits uniform in thickness and fine-crystalline.

Table 2

References

1. V. A. Il’in. Tinning and Lead Plating. Leningrad, “Mashinostroenie,” 1971.

2. V. S. Tsvetaeva. Coating of switching parts for assembly connections with a tin–bismuth alloy. Information Reference Leaflet, No. 020886, Moscow, 1970.

3. V. I. Koikina, L. I. Andrievskaya. Galvanic coating of parts with a tin–bismuth alloy. Exchange of Experience in the Radio Industry, Moscow, 9, 1969.

4. O. S. Stepanova, N. T. Rud’. Study of the electrodeposition of cadmium from a polythiocyanate electrolyte. Izvestiya TPI, 1973, vol. 257, p. 31.

5. Reference Manual on Electroplating. Moscow, “Metallurgiya,” 1969 (translated from the German under the editorship of V. I. Lainer).