Study of Al−Cu compounds as soldering bond pad for high-power device packaging

Abstract

Four equilibrium phases, Al2Cu (θ), AlCu (η2), Al3Cu4 (ζ2), and Al4Cu9 (γ2) were produced by annealing the solid-state Al/Cu diffusion couples. With a proper polishing process, the above four Al−Cu compounds were fabricated as soldering substrates. We found that the pure Sn solder ball can wet properly on the Al3Cu4, Al4Cu9 substrates, but, the pure Sn solder ball cannot wet on the Al2Cu and AlCu substrates. Interestingly, Sn can wet on the Al4Cu9 substrate as well as on the Cu substrate. In addition, we found that a dark Al-rich layer formed below the interfacial Cu6Sn5 compound, which is analyzed to be an Al-rich Al4Cu9 phase. We speculate that the Al-rich Al4Cu9 layer is caused by the Sn soldering reaction. During soldering reaction with Sn, the Cu in the Al4Cu9 layer would diffuse out to react with Sn and form the Cu6Sn5 compound. The Al in the Al4Cu9 layer does not react with Sn solder, so, the Al content would stay in the Al4Cu9 substrate. X-ray photoelectron spectroscopy (XPS) analysis has verified the metallic Al phase in the dark Al layer.