In　this　work,　we　have　systematically　investigated　the　evolution　of　microstructure　and　of　intermetallic　compounds　(IMCs),　in　particular,　for　lead-free　SnAgCuEr　solders　during　isothermal　aging　tests.　The　effect　of　trace　amounts　of　the　rare　earth　element　Er　on　this　process　has　also　been　studied.　The　results　indicate　that　diffusion　and　reassembly　occur　in　the　solder　matrix　during　the　aging　process,　and　the　major　influence　of　the　rare　earth　element　Er　is　concentrated　on　the　nucleation　sites.　ErSn3　IMCs　formed　from　the　molten　solder　provide　heterogeneous　nucleation　sites　for　the　IMCs　in　the　soldering　and　aging　process.　Subsequently,　the　Cu-Sn　IMCs　produced　during　soldering　and　Ag-Sn　IMCs　precipitated　during　the　aging　process　have　uniform　size　and　evenly　distribute　in　the　solder　matrix,　and　the　refinement　effect　has　been　achieved　in　Er-containing　solder　joints.　In　addition,　some　cracks　can　be　seen　in　Er-free　solder　joints,　and　the　cracks　may　nucleate　and　propagate　in　the　structure　along　the　compound/solder　boundaries　after　long　aging　times.