620　℃　thermal　aging　was　carried　out　on　full　Cu3Sn　solder　joints　for　various　duration,　the　microstructure　evolution　of　solder　joints　were　investigated.　The　mechanical　properties　of　solder　joints　after　aging　were　characterized　by　nano　indentation　test　and　shear　test.　The　results　show　that,　during　the　thermal　aging,　the　Cu20Sn6　is　first　precipitated　in　planar　along　Cu/Cu3Sn　interface,　it　grows　continuously　until　the　complete　consumption　of　Cu3Sn.　Subsequently,　Cu20Sn6　transforms　to　two-phase　layer,　which　consists　of　Cu20Sn6　and　Cu13.7Sn.　As　the　aging　time　increases,　Cu13.7Sn　is　precipitated　in　waves　at　the　Cu/two-phase　layer　interface　by　consuming　the　two-phase　layer　and　grows　continuously　until　it　occupies　the　entire　interface　area.　Meanwhile,　there　is　growth　in　the　number　and　size　of　voids　in　the　middle　of　the　solder　joint,　which　eventually　coalesces　into　microcracks.　The　hardness　of　Cu20Sn6,　Cu3Sn　and　Cu13.7Sn　phases　are　9.62,　7.15,　4.67　GPa,　and　the　elasticity　modulus　are　146.5,　134.0,　133.2　GPa,　respectively.With　the　increase　of　aging　time,　the　shear　strength　of　solder　joints　increases　first　and　then　decreases,　and　remains　more　than　20.1　MPa　within　120　min.　The　fracture　morphology　and　fracture　path　also　change.　©　2022　Beijing　Institute　of　Aeronautical　Materials　(BIAM).　All　rights　reserved.