Electrochemical Action of Fluxes for Soldering (Report 2)

Abstract

Authors have reported that both H₊ and Cl_- ions in an aqueous solution of HCl acted well as flux for the spreading of solder on copper base metal. However, the flux action of the individual ion cannot be made clear, because that solution includes simultaneously these ions. So, in the present paper, the aqueous solution of H₂SO₄ was selected as flux not containing Cl_- ions, and particular aqueous solutions including Cl_-ions at various concentrations were prepared by adding NaCl to the aqueous solution of H₂SO₄, and in order to distinguish the electrochemical flux action of each ion, the corrosion potentials of base metal and solder were measured in these solutions arid also spreading tests wre made by the same methods described in the previous report.
In.addition, external cathodic polarization. curves of base metal and external anodic polarization curves of the solder were plotted to investigate whether there was a relation between the spreading of solder and the electrode reaction of a galvanic cell formed instantaneously in contact with the base metal and solder in these aqueous solutions.
The results obtained from these investigations are summarized as follows:
1) When the concentration of H₊, ions in.the particular aqueous solutions was kept constant, the spreading of solder did not increase, appreciably with an increasing concentration of Cl_- ions in these solutions. But a significant increasing phenomenon of the spreading occurred at a certain critical concentration of Cl_- ion in both solutions of pH's about 2.5 and 3.0.
2) A correlation between logarithm of the critical Cl_- ion activity and pH of the aqueous solution is shown by following eq.
log acl-=-4.65+pH (1.0<pH<3.0)
As this experimental eq. approximates to a theoretical eq. (10), the flux action of Cl_- ion for the base metal may be expressed by the meaning of eqs. (10) and (13).
3) When the corrosion potential of the galvanic cell had a higher value than the break potential of the cathodic polarization curve, the spreading of solder did not occur because the formation of H₂O₂ or, H₂O on cathode (that is base metal) decreased H₊ ion concentration. in the aqueous solution. It is concluded that the higher the concentration of H₊ ions in flux solution, the better the spreading of solder becomes.