Cu-Ni-Sn: A Key System for Lead-Free Soldering

Being the most complex constituent of the quaternary system Ag-Cu-Ni-Sn, the ternary system Cu-Ni-Sn is the key system for the investigation of the interactions of Ag-Cu-Sn solder alloys with Ni as a contact material. Although this system has been thoroughly studied in the literature, there are still many uncertainties left. In the present work, a study of the phase equilibria in four isothermal sections at 220, 400, 500, and 700°C of the Cu-Ni-Sn system was carried out following a comprehensive literature study. The methods employed were x-ray diffraction (XRD), metallography, and scanning electron microscopy including electron probe microanalysis. The ternary solubilities of the Ni₃Sn₂-Cu₆Sn₅ and Ni₃Sn-Cu₃Sn fields were characterized in detail. So far no continuous solubility between the respective phases has been found. At 25 at.% Sn the existence of two ternary compounds formed from the BiF₃-type (Cu,Ni)₃Sn phase and reported in literature could be confirmed. On the other hand, our results differ significantly from the very recent literature related to lead-free soldering.