Electrochemical Action of Fluxes for Soldering

Abstract

The present work describes a study of the action of flux containing Cl-ions and OH- ions on wetting of solder. The corrosion potentials respectively of base metal and solder-copper as base metal and low melting point alloy as solder were used-when dipped in flux solutions of various concentrations, were investigated by an electropotentiometer. To help clarify the meaning of the measured corrosion potential, a reversible potential of anode reaction that would be reasonably considered in those half cells was thermodynamically introduced, after which the resultant corrosion product of the reaction was related in detail with the wetting of solder on the base metal, but the product was not checked here by any other experimental method.
Though the reversible potential was so decided, the corrosion behaviour of the half cell was postulated as an oxygen consumption type on the base metal and a hydrogen evolution type on the solder, since the base metal was noble and the solder was base for hydrogen potential, and it also was assumed that the corrosion potential was nearly equal to the anode equilibrium potential in a local cell of metals, because such a phenomenon was genellary recognized, that is, anode pvervoltage was much samller than cathode over-voltage in a gas evolution type corrosion.
The spreading tests on the base metals cleaned with 10% HNO₃ solution or electropolished were made with the solder (M.P. 47°C) preplaced in baths of the HCI and ZnCl₂ aqueous solutions respectively of different concentrations.
From the results of these investigations, it was noted that the corrosion product had an important action controlling whether the molten solder wets the base metal or not, for example the wetting was seen in the range of flux concentration which was effectively forming CuCl but did not appear in the range of Cu₂O and the wetting limit of the concentration was pH=3.5 in the case of HC1 aqueous solution under the experimental condition studied.