Thermomigration in eutectic-SnPb solder with Cu UBM at the room temperature

To investigate the thermomigration issue in flip-chip solder bumps, eutectic SnPb solder joints with typical dimensions were adopted. To observe the temperature distribution in solder joint during current stressing, infrared microscopy was employed. The bumps were powered by a desired current. Then temperature measurement was performed to record the temperature distribution (map) at the steady state. The temperatures in the solder joints were mapped by a QFI thermal infrared microscope. Solder joints were stressed at 1.88 Amp at room temperature for 3, 6 and 10 hours, respectively. An alternating current (AC) is employed to distinguish electromigration and thermomigration effects. The changes in the intermetallic compound (IMC) microstructure were observed with scanning electron microscope (SEM) under thermomigrationt. In this study, IMC formation plays an important role at solder joint during thermomigration phenomenon at room temperature. As stress time increased, IMC grew thicker rapidly. The IMC is thickening at the hot side is a result of temperature and diffusion driving forces.