We　reported　on　the　study　of　the　thermal　mechanical　reliability　of　Cu-filled　through　silicon　via　(TSV)　under　450　degrees　C　thermal　loading.　It　was　found　that　the　voids　could　be　generated　at　twin　boundaries　in　TSV-Cu　after　thermal　process.　To　study　the　mechanism　of　void　formation　at　twin　boundaries,　the　voided　regions　were　characterized　by　focused　ion　beamscanning　electron　microscope　(FIB-SEM),　the　crystallographic　orientation　of　Cu　grains　and　the　microstructure　of　twin　boundaries　was　analyzed　by　means　of　electron　backscattered　diffraction　(EBSD)　and　transmission　electron　microscope　(TEM).　Stress　induced　voids　were　considered　to　be　generated　at　twin　boundaries　because　of　the　stress　concentration　induced　by　three　possible　factors:　crystallographic　orientation　differences　of　Cu　grains　near　the　twin　boundary,　interface　decohesion　of　twin　boundary　and　morphology　of　twin　boundary.