Thermomigration versus Electromigration in Lead-Free Solder Alloys

Competing mechanisms of electromigration and thermomigration in flip chip SnAgCu (SAC) solder joints was studied experimentally. A chain of solder joints were stressed at 2.0 × 10 Amps/cm, 2.4 × 10 Amps/cm, and 2.8 × 10 Amps/cm current density at room temperature. In the test vehicle, some solder joints were exposed to a combination of electromigration and thermomigration, while some others were exposed to thermomigration alone. The changes in the intermetallic compound (IMC) microstructure were observed with scanning electron microscope (SEM) under thermomigration alone and when both migration processes are present. In all cases, Cu6Sn5 IMC at the hot side disintegrated while at the cold side thickened. The dissolution of the IMC at the hot side and the thickening at the cold side is a result of temperature and diffusion driving force. It is shown that thermomigration driving force, when present is much larger than electromigration.