Pronounced electromigration of Cu in molten Sn-based solders

Abstract

The high local temperature in flip-chip solder joints of microprocessors has raised concerns that the solder, a low melting temperature alloy, might locally liquefy and consequently cause failure of the microprocessors. This article reports a highly interesting electromigration behavior when the solder is in the molten state. A 6.3 × 10³ A/cm² electron current was applied to molten Sn3.5Ag solder at 255 °C through two Cu electrodes. The high current density caused rapid dissolution of the Cu cathode. The dissolved Cu atoms were driven by electrons to the anode side and precipitated out as a thick, and sometimes continuous, layer of Cu₆Sn₅. The applied current caused the dissolution rate of the Cu cathode to increase by one order of magnitude. A major difference between the electromigration in the solid and molten state was identified to be the presence of different countering fluxes in response to electromigration. For electromigration in the molten state, the back-stress flux, which was operative for electromigration in the solid state, was missing, and instead a countering flux due to the chemical potential gradient was present. An equation for the chemical potential gradient, dμ/dx, required to balance the electromigration flux was derived to be dμ/dx = N°z*eρJ, where N° is Avogadro’s number, z* is the effective charge of Cu, e is the charge of an electron, ρ is the resistivity of the solder, and J is the electron current density.