Silver nanoparticle inks printed on temperature-sensitive substrates can be converted into structures with high electrical conductivities within fractions of a second by photonic flash sintering. The key principle is the selective heating of the ink by the absorption of strongly focused pulsed light for which the substrate is transparent. The influence of process parameters like intensity and flashing frequency on the sintering speed is investigated. Furthermore, a setup is demonstrated for monitoring the temperature development in an ink during flash sintering, revealing that the substrate's glass transition point is exceeded only for very short time intervals, which prevents deformation.