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

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.