In-situ observation of electromigration behavior of eutectic SnPb solder was performed as a function of line length at $100^{circ}C$, $6{ imes}10^4A/cm$ condition in a scanning electron microscope chamber. The incubation time for edge drift and the edge drift velocity increase as line length increases, which are discussed with the void nucleation stage of solder bump and the electromigration back flux force, respectively. Finally, the existence of electromigration product (jL) and its line length dependency are also discussed.