When plating onto resistive substrates, potential drop in the electrode can cause non-uniformities in the current density distribution. Multiple studies of this so-called "terminal effect" have been conducted, assuming a layer-by-layer growth mechanism, as would apply, for example, to Cu deposition onto a thin Cu seed layer. However, when depositing Cu onto a material such as Ta or Ru, electrodeposition may occur by three-dimensional nucleation followed by growth. In such cases, a reduction in substrate resistance may not be realized prior to coalescence of the deposited film. Simulations show that nucleation can have a very significant impact on film-thickness uniformity. Results show a linear and significant increase in non-uniformity with the coalescence thickness of the depositing film. Simulations are extended to account for spatial variations of coalescence length. Implications of the model to wafer scale plating of Cu for interconnect applications are discussed. (c) 2007 Elsevier Ltd. All rights reserved.