The electrochemistry of a cuprous cyanide strike-plating bath

Abstract

The voltammetric and steady-state polarization response of a copper-disk electrode was used to study the kinetics of copper electrodeposition from a cyanide bath using a solution commonly applied for copper strike-plating (0.1 M Na 2CO 3 + 0.2 M CuCN + 0.6 M NaCN, pH 11.2). Systematic variation of the sodium cyanide concentration from 0.3 to 0.7 M was used to identify cuprous cyanide redox complexes Cu(CN) n ( n − 1)− . The voltammetric response with and without CN − was dramatically different owing to cuprous complexation with CN −, and the voltammetric charge associated with copper oxidation is much higher than that of cuprous cyanide reduction. The major species discharged is proposed to be Cu(CN) 3 2−, although thermodynamic calculations indicate that Cu(CN) 4 3− is the predominant complex. Deposition of the n = 3 complex is suggested to give rise to the relatively uniform macroscopic current distribution inherent with cyanide baths and the fine-grained deposit structure. It is proposed that the cyanide released during deposition shifts the distribution of the complexes at the surface to the completely saturated state and results in a decreased local copper deposition rate since the discharge of Cu(CN) 4 3− is considerably slower. Hence a necessary condition may be posed that any replacement (less toxic) ligand for cyanide should be chosen such that the electroactive copper complex is not saturated with the ligand, and furthermore that the most saturated state be the least electroactive. Cuprous cyanide complexes shift the copper deposition potential to more negative values which avoids displacement deposition on less noble substrates. The predominant copper cyanide species discharged in the copper strike-plating bath is proposed to be Cu(CN) 3 2−; although Cu(CN) 4 3− is at a higher concentration, it is not as electroactive. These kinetics give rise to the inherent good “macroscopic throwing power” observed in the cyanide bath.