A　study　on　microstructure　evolution　and　fracture　behavior　for　the　full　IMC　phase　transited　from　full　Cu3Sn　joints　during　the　600　°C　aging　temperature　was　conducted　systematically.　OM,　SEM,　and　XRD　technology　were　used　to　systematically　study　the　microstructure　evolution　for　the　solder　joints.　Besides,　shear　strength　and　hardness　for　the　Cu-Sn　IMCs　were　investigated　using　uniaxial　tensile　equipment　and　Vickers　micro-hardness　testing.　The　results　showed　that　the　formed　Cu3Sn　phase　gradually　transformed　to　a　metastable　Cu20Sn6　phase,　which　would　continue　to　transit　to　a　eutectoid　microstructure　(Cu20Sn6&α(Cu)),　finally,　the　newly　formed　Cu20Sn6&(Cu)　was　consumed　to　form　Cu13.7Sn　phase　until　the　joint　transformed　to　fully　Cu13.7Sn　phase.　The　average　values　of　the　Cu3Sn,　Cu20Sn6,　Cu20Sn6&α(Cu),　and　Cu13.7Sn　phase　were　as　follows:　374.5,　412.2,　373.4,　and　103.4　HV,　respectively.　When　the　shearing　rate　was　4mm/min,　the　shear　strengths　of　full　Cu3Sn,　Cu20Sn6,　Cu20Sn6&α(Cu),　and　Cu13.7Sn　phases　were　38,　53,　71,　and　41　MPa　correspondingly.　Besides,　the　fracture　mechanism　of　Cu3Sn,　Cu20Sn6,　and　Cu20Sn6&α(Cu)　was　an　intergranular　fracture.　Finally,　the　full　Cu13.7Sn　phase　(solid　solution)　appeared　as　ductile　fracture　following　typical　dimple　morphologies　compared　to　that　was　led　by　brittle　fracture　of　different　full　IMC　joints.　Based　on　the　analysis　of　the　stress-strain　curve　and　fracture　surface,　we　found　that　the　full　Cu13.7Sn　phase　(solid　solution)　was　the　promising　phase　to　replace　the　full　Cu3Sn　joint　with　a　substantial　increase　in　plasticity　and　toughness　while　maintaining　higher　strength.　　©　2022　IEEE.