Solid state diffusion in Cu–Sn and Ni–Sn diffusion couples with flip-chip scale dimensions

The formation and growth rate of intermetallics of frequently used metallisation systems for flip-chip bumping have been studied and are reported for many years. However, no data are available on diffusion couples with flip-chip processing methods and actual flip-chip scale dimensions. In this paper, the interdiffusion coefficients and activation energies of Cu–Sn and Ni–Sn intermetallic formations are measured on flip-chip bumps with 40 μm bond pad diameter. Also the morphology of the metallurgical reactions is described. Furthermore, the ideal case of a binary diffusion system is seldom present in real-life. In practice, the presence of additional alloying elements has an impact on the intermetallic stoichiometry and even on intermetallic growth and morphology. It is shown that small quantities of Cu in a Ni–Sn system can have a beneficial effect on the Ni consumption but larger quantities result in extreme scalloping of the intermetallic interface.