Effect of Ni on the formation of Cu/sub 6/Sn/sub 5/ and Cu/sub 3/Sn intermetallics

In this paper, the effect of Ni on the formation of Cu₆Sn₅ and Cu₃Sn intermetallics between tin and (Cu,Ni)-substrates has been studied by making use of the thermodynamic assessment of the Sn-Cu-Ni system. The driving forces for the diffusions of the elements in the intermetallic layers were calculated as functions of Ni-content. Assuming constant mobilities of elements, the results showed that all the diffusion fluxes in the (Cu,Ni)₆Sn₅ layer are increased with increasing dissolved Ni, while the dominant fluxes in the (Cu,Ni)₃Sn layer decrease. Therefore it is evident that Ni dissolution retards the growth of (Cu,Ni)₃Sn. When Ni-content of the substrate is high enough, (Cu,Ni)₆Sn₅ should be dominant in the reaction zones and the (Cu,Ni)₃Sn layer would disappear gradually. The shrinkage of (Cu,Ni)₃Sn is associated with larger difference between Sn and Cu fluxes in (Cu,Ni)₃Sn and therefore more extensive "Kirkendall void" formation. In addition, the calculated driving forces also suggest that the growth rate of (Cu,Ni) ₆Sn₅ should further increase after the disappearance of (Cu,Ni)₃Sn resulting in an unusually thick (Cu,Ni)₆Sn₅ layer. The results of thermodynamic calculations supplemented with diffusion kinetic considerations are in good agreements with recent experimental observations