As the miniaturization trend of electronic packing industry, electromigration (EM) has become a critical issue for fine pitch packaging. The EM effects on microstructure evolution of intermetallic compound layer (IMC) in Sn-3.0 Ag-0.5 Cu + XCo (X = 0, 0.05, 0.2 wt%) solder joint was investigated. Findings of this study indicated that current stressing of Sn-3.0 Ag-0.5 Cu-0.2 Co solder joint with 10(4) A/cm(2) at 50 A degrees C for 16 days, no remarkable EM damages exhibited in solder matrix. Whereas, after current stressing at 150 A degrees C for 1 and 3 days, Sn-3.0 Ag-0.5 Cu specimens showed obvious polarity effect between cathode and anode. Different morphology changes were also observed at both sides. After current stressing for 1 day, two IMC layers, Cu(6)Sn(5) and Cu(3)Sn, with wave type morphology formed at cathode. Sn phases were also observed inside in the IMC layer. However, only Cu(6)Sn(5) formed in anode. Three days later, Sn phases were found in anode. Besides, Co additions, aging treatment, Ag(3)Sn, and other IMCs improved the resistance of EM by the evidence of retarding polarity effect.