Ultra-thin intermetallic compound formation in microbump technology by the control of a low Zn concentration in solder

Abstract

We report here the extremely slow intermetallic compound (IMC) growth kinetics in the reflow reaction between a Sn-based solder of SnBiIn-2 at.% Zn and Cu. The solder has a melting point about 90 °C, and after reflow for 5 min on Cu at 120 °C, the formed IMC was Cu5Zn8 with a thickness only about 0.36 μm, which is much thinner than the IMC in nowaday packaging technologies. We systematically studied the IMC growth kinetics and built up a model to explain the extremely slow IMC growth rate. The growth kinetics of the reaction is non-parabolic and the activation energy is about 23.8 ± 1.6 kJ/mol. The non-parabolic kinetics is related to the lateral grain growth in IMC during the reactive diffusion along the moving grain boundaries. Our theoretical model shows that the growth rate of Cu5Zn8 compound should be proportional to the square root of Zn initial concentration in solder and a low Zn concentration in the solder will lead to a very slow IMC growth rate. The finding could be applied to control IMC thickness in 3D integrated circuit (3D IC) with micro-bump technology.