Effect of Ni on the formation of Cu6Sn5 and Cu3Sn 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 diffusion of the elements in the intermetallic layers were calculated as a function of Ni-content. Assuming constant mobilities of component atoms, the results suggest that the diffusion fluxes of all the components in the (Cu,Ni)6Sn5 layer increase with increasing content of dissolved Ni, while the Cu and Sn fluxes in the (Cu,Ni)3Sn layer decrease. Therefore, the dissolution of Ni retards the growth of (Cu,Ni)3Sn. When the Ni-content of the (Cu,Ni) substrate is high enough, the intermetallic compound growth in the reaction zones is dominated by (Cu,Ni)6Sn5 and the (Cu,Ni)3Sn layer disappears gradually. The small thickness of (Cu,Ni)3Sn is associated with large difference between Sn and Cu fluxes in (Cu,Ni)3Sn that encourages also the “Kirkendall void” formation. In addition, the calculated driving forces suggest that the growth rate of (Cu,Ni)6Sn5 should further increase if (Cu,Ni)3Sn disappears, 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.