The　research　on　lead-free　solders　has　become　the　focus　in　electronic　industry,　conventional　lead-free　solder　technology　may　not　meet　the　increasingly　severe　service　environment,　so　an　attractive　and　potentially　method　to　enhance　the　performance　of　a　lead-free　solder　is　to　introduce　reinforcements　into　a　conventional　alloy.　This　study　employed　polyhedral　oligomeric　silsesquioxanes　(POSS)　nano-particles　as　reinforcements　for　Sn3.5Ag0.5Cu　(SAC)　solder　matrix.　It　was　aimed　at　investigating　the　effect　of　POSS　on　the　microstructure　evolution　and　recrystallization　behavior　of　ball　grid　array　(BGA)　solder　joints　under　thermal　fatigue.　3wt%　POSS　were　incorporated　into　Sn3.5Ag0.5Cu　(SAC)　solder　matrix　by　mechanical　ball　milling　followed　by　reflow　process,　and　found　to　be　homogeneously　dispersed　into　the　solder　matrix　by　EDX　analysis.　Then,　two　kinds　of　different　components　BGA　assemblies　were　subjected　to　thermal　cycling,　and　the　surface　morphology　change　and　orientation　evolution　of　solder　joints　before　and　after　the　thermal　shock　was　respectively　characterized　by　using　SEM　and　EBSD.　Experimental　results　revealed　that　the　microstructure　of　solder　matrix　could　be　markedly　refined　with　the　addition　of　POSS,　because　of　the　reinforcements　retarded　the　element　diffusion　and　intermetallic　compound　aggregation.　Vickers　hardness　has　obviously　improved,　composite　solder　joint　shows　a　better　mechanical　properties.　After　the　thermal　fatigue　test　of　928　cycles,　at　the　interfaces　extrusion　deformation　produced,　the　recrystallization　accompanied　with　initiation　and　propagation　of　cracks　in　the　BGA　of　SAC305　solder　joint.　But　after　the　addition　of　nanoparticles,　composite　solder　resulted　in　less　grain　boundaries　in　the　same　time,　and　it　can　prevent　the　appearance　of　recrystallization　in　solder.　©　2016　IEEE.