Models of two types of voiding in Cu-Sn reactions

Voiding in Cu-Sn solid-state reactions is a known problem concerning reliability of solder contacts in microelectronics. It is prescribed to non-ideal Kirkendall effect in the growing IMC Cu₃Sn. In this phase, Cu atoms diffuse faster than Sn atoms generating the vacancy flux towards copper side. In this case, the so-called K-sinks of vacancies are not effective enough to absorb all incoming vacancies near the Cu₃Sn /Cu interface. These extra vacancies coalesce into pores (voids) which are the so-called F-sinks. A substantial part of voids remains pinned to the moving interface Cu₃Sn/Cu during phase growth.

Two mechanisms of the Kirkendall voids pinning at the moving interface are discussed. Besides common Zener pinning, voids can be additionally, kinetically pinned to the interface by the additional vacancy concentration gradient, existing due to unideal vacancy sinks/sources at the interface and at the both sides of it. Kinetic pinning may explain the influence of copper substrate defect prehistory on the pinning behavior.

Recently, a new morphology of porous Cu₃Sn with lamellar structure was observed during reaction of Cu with Cu6Sn5. We present a kinetic mechanism for the recently discovered porous Cu₃Sn compound formation with a lamellar structure consisting of alternating crystalline Cu₃Sn phase and void phase. We assume a flux-driven cellular decomposition of Cu₆Sn₅ – 3Sn -> 2Cu₃Sn + Voids, in which the out-flux of Sn from the system by interfacial diffusion along a moving interfacial surface and along the percolating net of the newly formed empty channels.