Functions of 2-butyne-1,4-diol in the process of tin-silver alloy electrodeposition from the acidic sulfate solution

Abstract

The eutectic (at.%) Sn96,7/Ag3,3 alloy (Tmelt 221 °С) is used for assembling electronic products and soldering jewelry. Control of the Sn-Ag alloy composition during its electrodeposition is a challenging task due to significant difference in electrode potentials of tin and silver and the tendency of Sn(II) to oxidation and hydrolysis. In this work the composition of known acidic sulfate electrolyte for electrochemical deposition of Sn‒Ag alloy was modified by the addition of 2-butyne-1,4-diol (BD). The purpose was to study the effect of BD on simultaneous Sn(II) and Ag(I) electrochemical reduction, the nature of side processes which accompany the deposition of Sn‒Ag coatings, their composition and structure. It is shown that the addition of BD to the electrolyte reduces silver content in Sn−Ag alloy and provides obtaining near-eutectic alloy according to its elemental, phase composition and melting temperature. The presence of SnO and SnO2 oxides and the increased silver content in the near-surface zone of the coatings no more than 3 nm deep are distinctive features of the deposited Sn‒Ag alloy coatings. These features are due to the occurrence of side processes of Ag(I) reduction with tin and the alloy corrosion, which accompany simultaneous Sn(II) and Ag(I) reduction. The BD additive in the electrolyte inhibit Ag(I) electroreduction, slightly accelerates Ag(I) cementation with freshly reduced tin, but retards corrosive dissolution of the alloy and hydrogen evolution.