Mechanism　of　Bi　layer　formation　at　the　anode　interface　in　Cu/Sn-58Bi/Cu　solder　joint　was　investigated　under　current　density　of　5×103　to　1.2×104　A/cm2.　During　electromigration　process,　Bi　was　the　main　diffusing　species　which　migrated　from　the　cathode　side　to　the　anode　side　under　electromigration　force.　Bi　was　the　first　species　to　reach　anode　interface　because　the　diffusion　velocity　of　Bi　is　faster　than　that　of　Sn.　Therefore,　compressive　stress　was　formed　at　the　anode　interface　when　much　more　Bi　atoms　accumulated　there　and　compelled　Sn　to　migrate　towards　the　cathode　side,　and　the　continuous　Bi　layer　was　formed　at　the　anode　interface.　In　the　mean　time,　tensile　stress　was　formed　at　the　cathode　interface　due　to　the　departure　of　metal　atoms,　which　led　to　voids　and　cracks　formation.　The　morphology　of　the　Bi　layer　included　planar-type　layer　and　groove-type　layer.　The　diffusing　channels　of　the　Bi　atoms　were　Bi　grain　boundaries,　Sn　grain　boundaries　and　Sn/Bi　interfaces.　With　the　current　density　and　stressing　time　increasing,　the　Bi　layer　thickness　increased.　The　electromigration　force　and　Joule　heating　took　on　the　main　driving　force　for　Bi　diffusion　and　migration.