Electromigration Kinetics of SAC/SnBiAg Hybrid Solder

With increasingly complex electronic assemblies, there is greater use of low melting point solders such as SnBiAg. We have studied the kinetics of electromigration of SAC BGAs and SnBiAg hybrid solder. The active energy was found to be about 0.48 eV and the current exponent was about 0.4. The low current dependence can be explained by the polarity dependence of the resistance change. For the electron flow from SAC BGAs into SiBiAg solder, the change in resistance plateaued after an initial increase, due to back flow stress and Bi depletion. When current flows from SnBiAg solder into SAC BGAs, the resistance decreases, since when Bi disperses into SAC solder the resistivity of the solder decreases. Despite the low acceleration factor, the life time at use condition of SAC BGAs/SnBiAg solder can be long or even immortal owing to the unique resistance change behavior. However, mechanical reinforcement of the solder joint, such as with underfill, is crucial.