Effect and mechanism of mercaptobenzoheterocyclic derivatives as brighteners in acid sulfate tin electroplating

Three mercaptobenzoheterocyclic derivatives including 2-mercaptobenzimidazole (MBI), 2-mercaptobenzoxazole (MBO), and 2-mercaptobenzothiazole (MBT) were investigated for their potential application as brighteners in tin electrodeposition. Electrochemical measurements demonstrated that all three brighteners effectively inhibited tin electrodeposition and hydrogen evolution, with MBT exhibiting the strongest inhibitory effect, which was evidenced by a negative shift of 14 mV in the potential of tin deposition and a reduction of 8 mA·cm⁻² in peak current density. Optical microscopy (OM) and scanning electron microscopy (SEM) images revealed significant improvements in the uniformity and brightness of the tin coatings. Quantum chemical calculations indicated that the adsorption sites of the brighteners on metallic surfaces were primarily the nitrogen atoms in imine groups and sulfur atoms in thiol groups. It is noteworthy that MBT displayed the highest reactivity due to its lowest energy gap (ΔE = 4.5685 eV). Molecular dynamics simulations suggested that the brighteners were adsorbed in both parallel and tilted configurations on specific tin planes maximizing the inhibition of tin electrodeposition.