In the last decades, fabrication of electrodes and conductive thin films on flexible polymer substrates has gained substantial attention owing to its potential in various applications, including foldable devices, bio-sensors, flexible batteries, and displays. Among the fabrication techniques, pulsed-laser sintering has demonstrated its potential as a novel fabrication technique applicable to heat sensitive polymer substrates. However, despite the importance, the physics of short (femtosecond ~ nanosecond) pulsed-laser sintering, including formation of the microstructures, properties of the sintered structure, and their correlations in different processing regimes, is still vaguely understood. In this regard, this work analyzed the silver (Ag) nanoparticle sintering process on polymer substrates using different laser sources, a KrF excimer laser and a Ti:Sapphire femtosecond laser, by varying the process variables. The characteristics of pulsed-laser sintering were compared between nanosecond and femtosecond processes, specifically in view of the physical mechanisms of sintering and properties of the sintered film, including electrical conductivity, flexibility/robustness, and adhesion strength. When the sintering process was optimized, femtosecond laser sintering exhibited a better performance than nanosecond laser sintering, i.e., better electrical conductivity and flexibility. The nanosecond laser sintering process occurred either by solid-state diffusion (surface necking) or by full melting of the particles, depending on the process condition. On the other hand, the femtosecond laser sintering process took place only by inter-particular necking as the polymer substrate was damaged before the onset of melting of the particle. Finally, the effect of substrate temperature and applied pressure on the sintering characteristics was also examined in this work.