Thermomigration in flip chip solder joints of eutectic SnPb has been studied at an ambient temperature of 100 °C. Redistribution of Sn and Pb occurs with Pb moving to the cold end. A stepwise concentration profile is observed. Significantly, the lamellar microstructure becomes much finer after thermomigration. Since the lamellar interface is disordered and is a fast path of diffusion, it indicates a high rate of entropy production in the thermomigration, in agreement with Onsager's principle of irreversible processes. The effect of entropy production on microstructure change is shown here. The molar heat of transport of Pb has been calculated to be -25.3 kJmole.