The critical length of eutectic SnPb solder was investigated using solder stripes. By employing focus ion beam, solder stripes of various lengths, including 5, 10, 15, 20, 30, 100, and 200 μm, can be fabricated. Length-dependent electromigration behavior was observed, which implies that there may be back stress under stressing. The critical length was determined to be between 10 and 15 μm under stressing by 2× 104 A cm2 at 100 °C, and the corresponding critical product was between 20 and 30 Acm. Both values show good agreement with their theoretical values.