Study on the Soft Soldering (Report 1)

On the Reaction of Copper and Tin

Abstract

This report is researched for the microstructure of joints, which are prepared by the soaking of the copper rods in the bath of molten tin, tin-lead alloy of eutectic composition and in tin powder with flux.
1) The eutectoid temperature of Cu₃₁Sn₈ (δ) is 330°C-380°C It is recognized that δ is not formed at the Cu-Sn interface at the above temperature interval even for 30hr-70hr, but Cu₃Sn is formed and contacts with Cu. But at 400°C δ contacts with Cu.
2) When Cu-Sn is heated for very short time at 50°C-100°C above the melting point of Sn, Cu₃ So and Cu₆Sn₅ are always formed and Cu₃Sn contacts with Cu, Cu₆Sn₅ contacts with Cu₃Sn. Therfore it is thought that these compounds have the important influence on soldering of copper by soft solder and play on important part. Up to now it has been thought that these compounds are not recognized when soldering time is very short.
The microstructure of vertical section alone of solder joins have been studied, but the microstructure of the inclined plane of joins have been not observed, therefore these compounds have been passed over, because there very thin.
3) The microstructure and microhardness of these compounds by the diffusion are same with those of the Cu₃₁Sn₈, Cu₃Sn and Cu₆Sn₅ phase by melting. The Vickers microhardness of Cu₃₁Sn₈, Cu₃Sn and Cu₆Sn₅ are about 590, 530 and 460.
4) Between copper and solid tin are formed these compands Cu₃Sn, Cu₆Sn₅ by diffusion below melting temperature of tin.
5) The free energy of activation calculated from the deposition of tin on copper and brass with flux are 12970cal/g. atom and 12930ca1/g.atom.