The　growth　tendency　of　intermetallic　compound　layer　(IMC)　at　both　anode　and　cathode　of　Sn-3.0Ag-0.5Cu+XCo(X=0,　0.5,　0.2wt.%)　solder　joint　with　Cu　substrate　was　investigated.　In　order　to　eliminating　the　current　crowding　effects,　a　butt　solder　joint　structure　was　used　in　this　study.　The　evolution　of　characteristics　of　microstructure　was　observed　and　studied　at　different　ambient　temperatures.　Results　indicate　that　the　ambient　temperature　is　likely　the　dominant　factor　in　determining　electromigration　life　times.　Current　stressing　of　Sn3.0Ag0.5Cu-0.2Co　solder　joint　with　10(4)/cm(2)　at　50　degrees　C　for　16　d　has　no　remarkable　electromigration　phenomenon.　Whereas,　Sn3.0Ag0.5Cu　and　Sn3.0Ag0.5Cu　-0.05Co　specimen　subjected　to　1　d　and　3　d　current　stressing　with　10(4)/cm(2)　at　150　degrees　C　shows　obviously　polarity　effect　and　difference　morphology　changes　at　both　anode　side　and　cathode　side.　After　current　stress　for　1　d,　two　IMC　layers,　Cu6Sn5　and　Cu3Sn,　formed　in　cathode　side.　Some　of　Sn　phase　still　remained　in　IMC　layer.　However,　just　Cu6Sn5　formed　in　anode　side.　3　d　later,　the　composition　of　IMC　layer　at　cathode　side　and　anode　side　was　contrary　to　the　results　of　1　d.　The　introducing　of　Co　reinforced　the　solder　matrix　under　both　isothermal　and　current　stress　experiment　conditions.