Electromigration in Pb-free solders

Electromigration (EM)-induced damage in lead-free solders strongly depend on the Sn grain orientation in the Pb-free solder joint. Significant damage can develop at a very early stage when the c-axis of a Sn-grain is oriented close to the current direction. Rapid dissolution of both intermetallic compounds (IMC) and under-bump metallurgy (UBM) that led to significant cavitations at interface is caused by fast diffusion of Cu and Ni through the Sn crystal along the c-axis. On the other hand, when the c-axis of a Sn grain is not aligned with the current direction, cavitations at solder-IMC interface are formed mostly due to Sn-self diffusion which is correlated with failures at a much longer stress time. This is a direct proof of the highly anisotropic diffusion behavior of Cu and Ni in Sn, reported by Turnbull and Huntington many years ago. The stable Ag₃Sn network and cyclic twinning in SnAg solder contributed to the better EM performance of Sn1.8Ag compared to that of Sn0.7Cu solder. The ranking of the three surface finishes, from best to worst, is Ni(P)/Cu, Ni(P)/Au, and Cu, when electrons are entering from the tested surfaces. A Ni barrier layer is needed to retard the electromigration damage. However, the addition of Cu at an optimized level to the Sn-Ag solder drastically improved the electromigration performance.