Electromigration-induced intermetallic growth and voids formation in symmetrical Cu/Sn/Cu and Cu/Intermetallic compounds (IMCs)/Cu joints

Electromigration-induced microstructure evolutions of two types of symmetrical solder joints were investigated. Sn foil sandwiched between Cu plates was reflowed to form two types of joints: a Sn-remained joint with a Cu/Sn/Cu structure and a full-intermetallic compounds (IMCs) joint with a Cu/IMCs/Cu structure. For the Sn-remained joint under current stress (1.0 x 10(4) A/cm(2)) at 150 degrees C, no void propagation was observed until all Sn layer converted to IMC despite the notable polarity effect on IMC growth. However, large voids that are different from Kirkendall one appeared at the cathode side after the formation of a full-IMC structure. In contrast, for the full-IMC joint, the two interfacial Cu3Sn layers kept nearly the same in thickness even under a rather strict condition (1.4 x 10(4) A/cm(2), 200 degrees C). A few Kirkendall voids nucleated in the Cu3Sn layers but did not concentrate dramatically. Surprisingly, small voids were also present in the Cu6Sn5 layer, and they grew and gathered persistently to cause a potential reliability issue.