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

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.