Precursor-type Cu inks were formulated by mixing copper (II) formate and hexylamine. The lowest electrical resistivity obtained in Cu films made from these inks was 5.2 mu Omega.cm. The Cu concentrations in the inks influenced the impurity content after annealing and the microstructure (specifically, the porosity). Inks with a higher volume of hexylamine had a narrower size distribution of Cu particles, so they formed denser films. However, a large volume of hexylamine could not be fully vaporized during a limited annealing time and reacted with formate, creating impurities having boiling temperatures higher than that of hexylamine. Therefore, the Cu concentration governed two factors, porosity and impurity content, which ultimately determined the electrical resistivity of the films. These findings are expected to facilitate the development of copper-based metallic inks for printed electronics. (C) 2011 Elsevier B.V. All rights reserved.