The solder plating posttreatment we developed involved immersing the plating film in a tin fluoborate solution and substituting tin for lead as follows:
Sn
2+(L)+Pb(s) High temp._??_Low temp.Sn(s)+Pb
2+(L)
where (L) indicates “in solution” and (s) indicates “in solder film”. Increasing the concentration of Sn
2+ and the temperature reversed the redox potentials, M
2++2e_??_M, toward less noble Sn making the forward reaction dominant.
This treatment produced a smooth surface lustrous and enriched in tin content in the outer portions of the film. This resulted in a marked inprovement in antioxidation properties, adhesion to the basis metal, solderability, and heat-humidity stability.
Enriched lead in the bath due to the multitreatment was easily removed as PbSO
4 precipitate by adding sulfric acid.
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