Thermomechanical　fatigue　(TMF)　resulted　from　the　mismatch　in　the　coefficient　of　thermal　expansion　(CTE)　between　solder　and　substrate　would　degrade　the　mechanical　properties　of　solder　joints　during　service.　In　this　research,　nano-structured　polyhedral　oligomeric　silsesquioxane　(POSS)　reinforcing　particles　were　incorporated　into　a　promising　lead-free　solder,　eutectic　Sn-3.5Ag　solder,　by　mechanically　mixing　to　improve　service　reliability　of　the　base　solder　matrix.　Three　different　temperature　profiles,　with　the　same　temperature　extremes　of　-40C　to　125C,　different　dwell　times　at　temperature　extremes　and　ramp　rates,　were　applied　in　the　tests.　Microstructural　characterization　of　surface　damage　and　residual　shear　strength　of　these　solder　joints　were　carried　out　after　0,　100,　250,　500　and　1000　TMF　cycles.　Results　obtained　from　this　study　were　used　to　analyze　the　effect　of　service　parameters　on　the　TMF　behaviors　of　the　nano　composite　solder　joints.　Experimental　results　indicated　that　the　nano　composite　solder　joints　exhibited　better　TMF　performance　than　the　eutectic　Sn-Ag　solder　joint.　It　also　proved　that　the　nano　composite　solder　joints　that　experienced　longer　dwell　time　and　slower　ramp　rate　exhibited　less　surface　damage　accumulation　and　less　decrease　in　shear　strength　of　solder　joints.　©2009　IEEE.