Study of Failure and Microstructural Evolution in SAC Solder Interconnects Induced by AC Electromigration Condition

The previously studied low-frequency pulsed direct current (DC) effects on electromigration (EM) results suggest that non-DC effects on EM mechanisms in solder interconnects are much more complex due to joule heat (JH) induced temperature and stress fluctuations and thus cannot be understood by simply extending the mechanisms found in DC EM. In order to correctly identify the EM mechanism without being affected by pulsing temperature/stress effect, a comparative alternating current (AC) EM test with varying frequencies and duty factors (DFs) is conducted. The common assumptions are that the failed samples have the classical intermetallic compound (IMC) microstructure, and the higher frequency AC has more effective healing effects. However, the failure analysis of low and high-frequency AC shows that the samples under high-frequency AC fail faster than those under low-frequency AC. Furthermore, the IMCs of failed samples tested under especially high-frequency AC conditions completely differ from classical EM-induced IMC formation, which is Cu
6
Sn
5
and Cu
3
Sn near the cathode side of interconnects. The IMCs are found to populated more in the middle of the solder interconnects. The faster failure rate of high-frequency AC and the abnormal IMC microstructure suggest that the assumed healing effects are not in operation at least to the degree to significantly affect failure rate and microstructure, and, the IMC formation and dissolution (during EM) are not spontaneous but have different rates.