Thermomigration Versus Electromigration in Microelectronics Solder Joints

Competing mechanisms of electromigration and thermomigration in flip chip SnAgCu (SAC) solder joints were studied experimentally. A daisy chain of solder joints were stressed at 2.0 times 10⁴ Amps/cm², 2.4 times 10⁴ Amps/cm², and 2.8 times 10⁴ Amps/cm² current density levels 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 forces. It is observed that thermomigration driving force, when present, is much larger than electromigration.