The　newly　developed　lead-free　solders　should　have　good　mechanical　properties,　as　well　as　excellent　creep　resistance　in　order　to　meet　the　needs　of　reliability　required　by　electronic　industries.　Nano-sized　polyhedral　oligomeric　silsesquioxane　(POSS)　reinforcement　particles　were　incorporated　into　the　promising　lead-free　solder,　Sn-3.5Ag,　to　improve　the　mechanical　property　of　the　lead-free　solder.　Experimental　results　show　that　the　thickness　of　intermetallic　compound　(IMC)　layer　between　solder　and　substrate　Cu　is　restrained　when　the　mass　fraction　of　POSS　is　less　than　2%.　The　composite　solders　contain　different　POSS　reinforcement　types　and　mass　fractions,　and　their　joints　possess　better　mechanical　properties.　The　creep　rupture　lives　are　enhanced　in　the　composite　solder　joint　at　different　temperatures　and　different　stress　combinations　compared　to　the　Sn-3.5Ag　solder　joint,　especially　at　low　temperatures.