Electrochemical Formation of Brazing Alloys On Metal Substrates

Abstract

TürkiyeCusil TM is a eutectic brazing alloy of 72% silver, 28% copper composition; used to join different materials. The large difference between the standard reduction potentials of silver (0.799V) and copper (0.337V) makes their electrochemical codeposition difficult. A complex ion bath containing KAg(CN)2, CuCN and KCN was used to bring the reduction potentials closer together. The effect of electrolyte concentration on composition and microstructure of electrodeposited Ag-Cu films was investigated at 25 o C. A current density of 2 mA/cm 2 was used to codeposit copper and silver based on results of voltammetric studies. It was found that increasing KCN and KAg(CN)2 and decreasing CuCN concentrations increased Ag contents of deposits. Surface morphology was spherical grained in general. Increasing Ag content of the deposits resulted in finer grained structures, but deposits containing higher Ag concentrations were found to develop dendritic structures especially after the initial deposition of the Ag-Cu alloy coating on the surface.

The Role of Electrolyte Composition on Codeposited Ag and Cu

“…It can be seen that silver concentration in the deposit increases with increase in KCN concentration of the electrolyte at 55 o C. The increase was significant at low concentrations of KAg(CN) 2 , but became very small at higher concentrations. This behavior was expected because static potential of Ag becomes higher than that of Cu as KCN concentration increases over 0.18 M at 55 o C as was shown before [16]. The increase in K 4 P 2 O 7 concentration over the 20 to 120 mM range caused a minor decrease in Ag content of the deposit, generally independent of KCN concentrations.…”

“…The electrodes were cleaned and nickel plated prior to Ag-Cu plating. The nickel plating and surface cleaning procedures were described elsewhere in detail [16]. The effect of K 4 P 2 O 7 addition to the electrolyte was examined by employing a statistical, three-level full-factorial design.…”

“…Concentration of CuCN in the electrolyte was kept constant at 0.1 M during the experiments. This concentration was selected [7,16] due to its solubility problem at higher concentrations at 25 o C. Plating temperature was also kept constant at 55 o C to examine the effect of KCN in a wider concentration range; without encountering the solubility problem of CuCN since CuCN does not dissolve completely in solutions containing KCN amounts lower than 0.2 M at 25 o C [7]. However, 0.15 M KCN was taken as the minimum limit to ensure solubility of 0.1 M CuCN at 55 o C. The maximum limit on the other hand was taken as 0.21 M to keep the CN − /Cu ratio close to 3.…”

“…10 mM KAg(CN) 2 concentration was taken as the mean based on the literature data [7]. The maximum KAg(CN) 2 concentration on the other hand was taken as 15 mM, because this concentration was found to give mossy and dark brown deposits at 25 o C unlike the baths with lower KAg(CN) 2 concentrations [16].…”

“…Deposits of 72 wt.% Ag, 28 wt.% Cu composition (Cusil) were obtained successfully by the authors from KCN containing electrolyte [16]. Because, the static Ag potential shifts, in the same direction, more than the Cu potential, when KCN was added.…”