Abstract
Conditions, such as of potential difference between Cu/Cu+ and Sn/Sn++ in the baths, over all chemical reaction, effect of additional agent, deposition rate of tin at various temperatures, corrosion properties of the specimen in neutral and acid aqueous solutions, etc., for immersion tin-plating on copper in the following 3 kinds of bath were investigated:
(1) NaCN 185g/l, NaOH 22.5g/l, SnCl2⋅2H2O 18.5g/l
(2) Thiourea 55g/l, Tartaric acid 39g/l, SnCl2⋅2H2O 6g/l
(3) NaCN 85g/l NaOH 54g/l, Na2SnO3 48g/l
In these baths, cyanide and thiourea are working as complicating agents of Cu+ and Cu++ ions, so that the activities of these ions are reduced largely until the potential of Cu/Cu+ in these baths become more negative than that of Sn/Sn++ by 300mV in bath (1) and by 200mV in bath (2) at 17°C.
Total stoichiometric relation can be expressed as follows:
Sn+++2Cu=2Cu++Sn……for bath (1) and (2)
Sn+++++2Cu=2Cu+++Sn……for bath (3)
Addition of 0.1 to 1.0% of anionic surface active agent, “Anionic 08”, reduces markedly the deposition rate of tin in bath (2), but showed very little effect in bath (1). In corrosion test of the specimen in dilute HCl, mostly Sn++ ion came into solution as expected, however, in water containing Cl2 copper was dissolved more than tin.
