Improved Understanding of Electromigration Under AC Loads Impacting Microstructure and Reliability of SAC Solder Interconnects

This paper reports the microstructural evolution and failure behavior of Sn-rich solder joints subjected to alternating current (AC) electromigration (EM) loads. SAC (Sn-Ag-Cu) alloy solder joints were tested under both direct current (DC) and AC EM conditions to induce failure. The microstructure of the failed samples was then analyzed to better understand the mechanisms by which AC EM affects solder joint stability. Our study revealed that EM failure is significantly delayed when samples are subjected to low-frequency AC EM (e.g., 0.1 Hz). Microstructural analysis suggests that the primary mechanism responsible for this delay is stress-induced grain refinement, rather than the anticipated mechanisms such as atomic flux reversal, which could reduce or repair damage. The refined grains, formed through recrystallization, were found to shunt fast diffusion paths, thereby slowing damage progression. However, this beneficial effect was less pronounced under high-frequency AC EM (e.g., 10 kHz), likely due to the skin effect, which increases current density along the surface of the solder bump. Accelerated EM along the surface, combined with localized EM damage was found to alter the EM failure mechanism and reduce overall reliability at high frequencies.