Process And Electrochemical Behavior Of Plating Tin And Copper-Tin Alloy (45～55% Tin) From Pyrophosphate Bath

Pyrophosphate system has always been a research focus in non-cyanide plating such as cyanide-free copper and copper-tin electroplating. The feasibility of tin plating with pyrophosphate system was firstly investigated in this dissertation and the effect of different chosen additives was studied. Furthermore, Cu-Sn alloy plating with pyrophosphate solution has been remained in the laboratory research and the coating is not as bright as that from the cyanide bath. In this dissertation, a new additive was introduced based on other research and bright Cu-Sn alloy coating was gained. Finally, the electrochemical behavior of tin and copper-tin alloy plating and the mechanism were discussed thoroughly.Hull tub test and DC electrolytic experiment were used and the optimum bath composition and technological conditions of tin plating with pyrophosphate system were confirmed as:K4P2O7·3H2O 300 g/L, Sn2P2O7 6 g/L, pH 8.5,25℃, the additive W-30.6 mL/L, current density ranges from 0.03 to 1.44 A/dm2. Additive W-3 helps to improve the coating crystallization and gain white matte coating in the DC electrolytic experiment. In addition, the bath has good dispersive ability and the tin coating could form uniform intermetallic compound barrier layer with copper to avoid it’s spread to the coating.The optimum bath composition and technological conditions of Cu-Sn alloy (45～55% Sn) plating with pyrophosphate system using the new additive K-1 (condensation product of amine and epoxy compounds) were confirmed as:K4P2O7·3H2O 300 g/L, Sn2P2O7 8 g/L, Cu2P2O7·3H2O 12 g/L, additive K-1 2.4～4.0 mL/L, reductant 2 g/L, pH 8.5～9.5, current density ranges from 0.7 to 1.2 A/dm2,25 ℃. Additive K-1 functions mainly as the brightness and fine, bright and micro crack-free Cu-Sn alloy (45～55% Sn) coating could be gained while it’s 0.8～4.0 mL/L.Research on the electrochemical behavior indicated that the cathodic reaction of Sn(â…¡) was a first order quasi reversible reaction, the apparent active energy was 13～14 kJ/mol and the electrode reaction took the form of diffusion control. The main existence form of Sn(â…¡) was [Sn(P2O7)2]6- and the the electrode reaction proceed along a quick prepositive chemical trans-formation step. The possible mechanism could be described as follows: [Sn(P2O7)]2-+2eâ†'Sn+P2O74-The cathodic reaction of Cu-Sn alloy co-deposition was an irreversible reaction and the apparent active energy decreased with negative shifting potential. The apparent active energy was 60 kJ/mol at -0.7 V and was electrochemical control, when it decreased to 10～16 kJ/mol, the reaction converted to diffusion control.