Abstract Cu diffusion onto an Ag-plated Cu leadframe surface was detected by Energy Dispersive X-ray (EDX) after de-capsulating the molded package. However, no Cu was detected by EDX on the Ag surfaces of bare leadframe, leadframe after die attachment including die attachment curing, or leadframe after wire bonding. Temperature simulation of molding and post-molding curing show that the temperature and its duration have some impact on the Cu diffusion, which is as expected. Interestingly, this study shows that the Cu diffusion onto the interface of the Ag-plated surface and mold compound is very much dependent on the type of mold compound used, which has never been reported in the literature so far. The Cu diffusion was detected by EDX only for certain types of mold compounds used. It is concluded that the mold compound is a major contributor to the Cu diffusion observed. This is possibly because S and O in the mold compound react with Cu and form copper sulfides and copper oxides, which accelerate the Cu diffusion from the Cu leadframe substrate onto the interface of the Ag surface and mold compound.