The　microstructure　evolution　and　mechanical　properties　of　Sn-3.5Ag　and　its　new　composite　solder　containing　three　different　nano-sized　polyhedral　oligomeric　silsesquioxanes　(POSS)　particulates　with　the　best　mass　fractions　during　isothermal　aging　were　studied.　All　of　the　above　four　solder　joints　were　aged　at　three　different　temperatures　(125C,　150C,　175C)　up　to　1000　h　and　the　intermetallic　compound　growth　was　measured.　The　in-terfacial　layers　between　the　solder　and　the　copper　substrate　were　examined　using　the　optical　and　scanning　electronic　microscopy.　The　growth　kinetics　of　intermetallic　interfacial　layers　formed　between　the　solder　and　the　copper　substrate　was　characterized.　The　growth　rate　of　the　intermetallic　layers　of　the　composite　solders　is　slower　than　that　of　Sn-3.5Ag　solder.　The　activation　energies　of　the　composite　solders　are　80,　97　and　77　kJ/mol,　respectively,　and　are　higher　than　those　of　Sn-3.5Ag　solder.　Besides,　the　shear　strength　of　the　composite　solder　joints　is　higher　and　not　so　sensitive　to　the　different　aging　processes.