Effect of surfactant on the bath stability and electrodeposition of Sn–Ag–Cu films

A methanesulphonic acid based bath for the co-deposition of Sn–Ag–Cu films is developed. The bath contains thiourea as chelating agent, while iso-octyl phenoxy polyethoxy ethanol (OPPE) is added as a non-ionic surfactant. Studies show that OPPE plays a major role in improving the bath stability from few hours up to 1 week. However, there is no evidence of any strong chelation between any metal ion and OPPE. There is a slight modification in the reduction potentials of individual ions after addition of OPPE, which is attributed to some weak bonding through the lone-pair electrons of this molecule. Bath investigations confirm that thiourea forms chelates with electropositive Ag+ and Cu2+ ions and alters their deposition potentials. Consequently, the deposition of three elements takes place at a single potential of − 0.541 V. The proposed bath is capable of depositing Sn–Ag–Cu films having near eutectic composition that remains consistent even with increase in current density beyond 5 mA cm− 2. This is attributed to the three metals reaching their limiting current density. The deposited films have compact microstructure with grain size in the range 6–8 μm and thickness in the range 20–100 μm. The studies show that OPPE brings about refinement in film microstructure.