The　scallop-like　Cu(6)Sn(5)　intermetellic　compound　(IMC)　layer　formed　at　the　interface　of　Sn-3.8Ag-0.7Cu/Cu　joint　as　soldered.　These　IMC　layers　graw　during　aging　at　150　degrees　C.　The　fracture　damage　micro-mechanism　of　the　joints　is　different　when　they　have　a　different　IMC　thickness.　When　the　thickness　of　IMC　layer　is　less　than　5.5　mu　m,　ductile　fracture　occurred　in　the　solder.　When　it　is　more　than　9　mu　m,　brittle　fracture　occurred　between　the　solder　and　Cu.　Scan　Electronic　Microscopy　micro-analyses　show　that　the　fracture　at　the　boundary　could　occur　in　one　of　following　positions.　One　is　the　interface　of　Cu(6)Sn(5)　intermetallic　compound　and　solder.　Another　is　the　cleavage　intermetallic　compound　grains.　The　more　thick　the　I　MC　layer,　the　more　brittle　the　IMC,　the　lower　the　strength　of　the　interface.　The　fracture　position　moved　from　solder　inner　to　the　interfacial　CU(6)Sn(5)　IMC　layer　with　the　IMC　thickness　increased.　in　addition,　when　aging　for　480h,　the　Ag(3)Sn　compound　formed　between　solder　and　Cu(6)Sn(5)　compound.　The　strength　of　solder　is　also　decreased　during　aging.　The　weakening　of　the　solder　matrix　is　caused　by　the　coarsening　of　the　eutectic　solder　structures,　The　weakening　of　the　interfacial　I　MC　layer　is　caused　by　the　size　increase　of　the　I　MC　grains　and　the　decrease　of　connecting　area　of　IMC　layer　and　the　solder.