Catalytic Pd nucleation is a crucial preparatory step for the activation of electroless (EL) Cu deposition. We have observed that the time evolution of catalytic Pd nucleation can be classified into stages according to the Pd nuclei density and the nucleus size distribution. We have tentatively labeled these stages as growth, secondary nucleation, and ripening. The stage identification is assisted with the observation of distribution range and semi-interquartile range. In this study we found that the smoothest EL Cu film can be achieved with Pd activation corresponding to the beginning of the ripening stage. (C) 2004 The Electrochemical Society.