We report on the results of a multi-laboratory study to measure the steady state growth stress of planar copper thin films, electrodeposited from additive-free acidic sulfate electrolyte. Measurements were made using the wafer curvature method, which enabled the stress to be measured in real time as the films were grown. The stress trends toward compressive at low growth rate and becomes increasingly tensile as the growth rate is increased, with a similar dependence observed in each laboratory when the films have comparable grain sizes. The results are explained by a model for stress that focuses on processes that occur at the triple junction between the film surface and the grain boundary as the film grows. (C) 2013 The Electrochemical Society. All rights reserved.