In this talk, we will report the wetting reaction of Pd-free solders on the Al-Cu compound substrates. The four equilibrium Al-Cu phases, Al2Cu, AlCu, Al3Cu4, and Al4Cu9 were produced by annealing the solid state Al-Cu diffusion couples. With a proper polishing process, 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 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, a dark layer were found below the interfacial Cu6Sn5 compound by the back scattering electron (BSE) microscope image, which is analyzed to be an Al-rich Al4Cu9 phase by x-ray photoelectron spectroscopy (XPS). Compare to the thickness of Cu6Sn5 formed by Sn/Cu and Sn/Al4Cu9, we found the Al-rich Al4Cu9 layer can retard the inter-diffusion of Sn and Cu atoms.