Development of a sorbitol alkaline Cu–Sn plating bath and chemical, physical and morphological characterization of Cu–Sn films

A non-cyanide Cu–Sn plating bath containing sorbitol as ligand has been developed, which has the advantage of low toxicity and relative ease of handling, in contrast to cyanide baths. The Cu–Sn deposition voltammetric curve from this bath showed that the reduction process has the same characteristic features as copper deposition, which is controlled by mass transport, whereas the deposition of tin is controlled by activation. The hydrodynamic studies indicate that the Cu–Sn deposition process is controlled by mass transport and that the diffusion coefficient of the Cu(II)/Sn(IV) complex species is 8.1×10−8 cm2 s−1. SEM analysis showed that the films obtained are smooth and cover the substrate totally, showing that sorbitol has brightening and leveling properties. EDS of Cu–Sn films showed that the deposits have a maximum of ∼3% of tin and that sorbitol was not incorporated in the deposit. It was observed that the Cu–Sn electrodeposited films were golden when obtained voltammetrically from −0.6 to −1.6 V or chronoamperometrically at −1.4 V and a charge density of 2.55 C cm−2, from baths containing Cu(30)–Sn(70) plus sorbitol at concentrations 0.15, 0.30 and 0.45 M, as well as Cu(70)–Sn(30)/0.30 M sorbitol and Cu(70)–Sn(30)/0.45 M sorbitol, even when the film Sn content was less than 10%. Bright reddish-colored films were obtained from baths containing Cu(50)–Sn(50) plus sorbitol at concentrations 0.15, 0.30 and 0.45 M. X-ray spectra showed that probably a mixture of Cu and Cu3Sn alloy was deposited at various cathodic potentials.