Conductive silver tracks on a polyimide substrate (see figure) are prepared by using microwave radiation to sinter silver nanoparticles printed on the substrate. This method shortens the necessary sintering time dramatically and is independent of the substrate used. Since the polymer substrate is virtually transparent to microwave radiation, a negligible amount of energy is absorbed by the substrate, whereas the conducting silver nanoparticles, with a high dielectric loss factor, strongly absorb the microwaves.