The　paper　focused　on　the　changes　in　microstructure　and　mechanical　properties　of　the　full　Cu41Sn11　solder　joint　(Cu/Cu41Sn11/Cu)　during　isothermal　aging　at　420　&　DEG;C.　It　was　motivated　by　potential　applications　of　Cu-Sn　intermetallic　compounds　(IMCs)　solder　joint　in　third-generation　wide　bandgap　semiconductor　devices.　Experimental　results　revealed　that　the　Cu41Sn11　phase　was　unstable　under　high-temperature　conditions,　the　full　Cu41Sn11　joint　transformed　into　the　full　&　alpha;(Cu)　joint　(Cu/&　alpha;(Cu)/Cu)　joint　at　150　h　during　thermal　aging.　The　formed　&　alpha;(Cu)　phase　was　a　Cu　solid　solution　with　inhomogeneous　Sn　atomic　concentration,　and　its　crystal　structure　and　orientation　were　consistent　with　the　original　Cu　plate.　The　conversion　of　the　Cu41Sn11　to　&　alpha;(Cu)　was　accompanied　by　the　formation　of　voids　due　to　the　volume　shrinkage　effect,　predominantly　near　the　middle　of　the　solder　joint　interface.　The　&　alpha;(Cu)　solder　joint　presented　a　decrease　in　strength　but　an　increase　in　strain　rate　sensitivity　index　compared　to　the　Cu41Sn11　solder　joint.　Furthermore,　the　strain　rate　sensitivity　index　of　&　alpha;(Cu)　and　Cu41Sn11　is　lower　than　that　of　ordinary　Sn　solders.　After　the　shear　test,　the　fractures　that　occurred　in　Cu41Sn11　grains　were　brittle,　while　the　fractures　in　&　alpha;(Cu)　grains　were　ductile.