Dissolution and precipitation kinetics of Cu6Sn5 intermetallics in Cu/Sn/Cu micro interconnects under temperature gradient

Abstract

Multiple reflows are often required in 3D packaging. To elucidate the effect of temperature gradient during subsequent reflow on existing intermetallic compounds (IMCs), Cu6Sn5 IMC layers were initially formed in Cu/Sn/Cu micro interconnects. Upon subsequent reflow, synchrotron radiation real-time imaging technology was used to in situ study the dissolution and precipitation behavior of the pre-formed Cu6Sn5 under different temperature gradients. The pre-formed Cu6Sn5 IMC at the cold end continued to grow linearly with increasing aspect ratio, whereas that at the hot end dissolved linearly and then maintained a critical thin layer. The thick pre-formed Cu6Sn5 IMC at the hot end significantly hindered the dissolution of the neighboring Cu substrate until a dynamic equilibrium between chemical potential gradient and temperature gradient was satisfied. The thermomigration of Cu atoms from the hot end towards the cold end was responsible for the asymmetrical evolution of the interfacial Cu6Sn5 between the cold and hot ends. A theoretical model was proposed based on Cu diffusion flux to calculate the IMC thickness at the both ends as a function of reflow time and the equilibrium IMC thickness at the hot end under temperature gradient.