One of the greatest challenges in fabrication of plastic electronics devices by printing metallic nanoprticles is obtaining highly conductive patterns at sufficiently low temperature which will not damage the polymeric substrate. However, to date, sufficient conductivity of silver patterns was achieved only after prolonged heating at elevated temperatures, thus limiting fabrication of plastic devices only to heat resistance polymers.We report on a discovery that assemblies of silver nanoparticles, can undergo a spontaneous two-dimensional aggregation-coalescence process, even at room temperature. The surface coalescence of the metal nanoparticles leads to sintering and eventually to electrical conductivity, much below the melting temperature of the bulk silver. This process is triggered by surface charge neutralization of the nanoparticles, by using a variety of charged flocculants, and takes place in thin layers of various substrates, such as plastic and paper. The resulting high conductivity, 20% of bulk silver, enabled fabrication of various devices, as demonstrated by a flexible plastic electroluminescent ink-jet printed device.