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)(6)Sn-5 layer increase with increasing content of dissolved Ni, while the Cu and Sn fluxes in the (Cu, Ni)(3)Sn layer decrease. Therefore, the dissolution of Ni retards the growth of (Cu, Ni)(3)Sn. 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)(6)Sn-5, and the (Cu, Ni)(3)Sn layer disappears gradually. The small thickness of (Cu, Ni)(3)Sn is associated with large difference between Sn and Cu fluxes in (Cu,Ni)(3)Sn that encourages also the "Kirkendall void" formation. In addition, the calculated driving forces suggest that the growth rate of (Cu, Ni)(6)Sn-5 should further increase if (Cu, Ni)(3) Sn disappears, resulting in an unusually thick (Cu, Ni)6 Sn-5 layer. The results of thermodynamic calculations supplemented with diffusion kinetic considerations are in good agreements with recent experimental observations.