Thermomigration in flip-chip solder joints is investigated using alternating currents and infrared microscopy to decouple it from electromigration effect. It is found that the thermal gradient in solder bump can be as high as 2143 °C/cm when 9.2× 104 A/cm2 was applied at 100 °C. Markers fabricated by focus ion beam are employed to measure the thermomigration rate. The thermomigration flux is measured to be 3.3 × 1013 at./cm2. With the known thermal gradient, the molar heat of 26.8 kJ/mole has been obtained for the transport of Pb.