Damage mechanics of electromigration and thermomigration in lead-free solder alloys under alternating current: An experimental study

Electromigration and thermomigration reliability of Sn96.5%Ag3.0%Cu0.5 (SAC305 by weight) solder joints under alternating current is studied experimentally with the maximum current density from 2.1 × 10⁵ A/cm² to 1.76 × 10⁶ A/cm², and frequency from 100 kHz to 4 MHz. During the experiment, hot spots are observed at current crowding corners and the skin layer of solder joints. As a result, highly localized current density and joule heating exist. The combined effect of electron wind force and thermal gradient drives mass to diffuse toward the center of solder joints. Resistance change due to temperature change and current crowding was carefully recorded. Ten percent resistance change after thermal steady state was defined as failure. Scanning electron microscopic images show void formation in current crowding corners and in the skin layer of solder joints. Damage of solder joints under alternating current stressing is observed a combined process of both electromigration and thermomigration, and to be proportional to the loading frequency and current density. Owing to the reversed electron wind force, material healing is observed in most of our test vehicles. A statistical mean time to failure equation is proposed for lead-free solder joints under alternating current stressing.