An electroless nickel bump deposited from nickel sulfate solution was investigated for its structural evolution with respect to deposition parameters. The pad size, pitch size, and lead acetate concentration were found to affect the bump structure. Three types of representative structure, the broccoli edge, anisotropic, and pyramidal were identified. The effect of depositing parameters on the evolution of these structures is explained in terms of the adsorption behavior of additive and the diffusion mechanism of the nickel ions. (C) 2003 The Electrochemical Society.