The　formation　and　evolution　of　the　intermetallic　compound　(IMCs)　between　SnAgCu　lead-free　solder　and　Cu　substrate,　after　isothermal　aging　at　150°C　for　24,　48,　120,　240　and　480　hours,　were　studied.　Scanning　electron　microscope　(SEM)　was　used　to　observe　the　microstructure　evolution　of　solder　joint　during　aging.　The　IMC　phases　were　identified　by　energy　dispersive　X-ray　(EDX).　The　results　showed　that　IMCs　layer　of　Cu6Sn5　was　formed　at　the　interface　of　solder　and　Cu　during　reflowing.　With　the　increase　of　aging　time,　the　grain　size　of　the　interfacial　Cu6Sn5　increased　and　the　morphology　of　the　interfacial　Cu6Sn5　column　was　changed　from　scallop-like　to　needle-like　and　then　to　rod-like　and　finally　to　particles.　At　the　same　time,　the　rod-like　Ag3Sn　phase　formed　at　the　interface　of　solder　and　the　IMCs　layer　of　Cu6Sn5　with　the　aging　time　increased.　In　addition,　large　Cu6Sn5　formed　in　the　solder　with　the　aging　time　increased.　The　tensile　strength　was　measured　for　the　solder　joints.　The　results　showed　that　the　tensile　strength　increases　slightly　at　beginning　and　then　decreases　with　the　aging　time.　SEM　was　used　to　observe　the　fracture　surface　and　it　showed　that　the　fracture　position　moved　from　solder　matrix　to　the　interfacial　Cu6Sn5　IMCs　layer　with　the　aging　time　increased,　The　weakening　of　the　solder　matrix　is　caused　by　the　coarsening　of　the　eutectic　solder　structures.　The　weakening　of　the　interfacial　IMCs　layer　is　caused　by　the　evolution　of　morphology　and　size　of　the　interface　Cu　6Sn5　layer.