The present paper focuses on the role played by a Zn interlayer in the improvement of the mechanical behavior of Al-Cu Friction Stir Spot Welds. An unusual configuration of lap joint with Cu at the top surface as well as rather thick plates was considered. The effects of the tool plunge depth exceeding or not the top plate thickness and of various Zn layer thicknesses on the microstructure and mechanical properties of the joint were investigated. Zn at once hampers the formation of the deleterious Al4Cu9 compound while favoring the τ′ phase precipitation. Over the range considered in this study, the Zn layer thickness presents no obvious effect on the shear lap tensile properties. At last, various ways of welding improvement are proposed.