Through-silicon　vias　(TSVs)　have　become　an　important　technology　for　three-dimensional　integrated　circuit　(3D　IC)　packaging.　Protrusion　of　electroplated　Cu-filled　vias　is　a　critical　reliability　issue　for　TSV　technology.　In　this　work,　thermal　cycling　tests　were　carried　out　to　identify　how　the　microstructure　affects　protrusion　during　thermal　cycling.　Cu　protrusion　occurs　when　the　loading　temperature　is　higher　than　149A　degrees　C.　During　the　first　five　thermal　cycles,　the　grain　size　of　Cu　plays　a　dominant　role　in　the　protrusion　behavior.　Larger　Cu　grain　size　before　thermal　cycling　results　in　greater　Cu　protrusion.　With　increasing　thermal　cycle　number,　the　effect　of　the　Cu　grain　size　reduces　and　the　microstrain　begins　to　dominate　the　Cu　protrusion　behavior.　Higher　magnitude　of　microstrain　within　Cu　results　in　greater　protrusion　increment　during　subsequent　thermal　cycles.　When　the　thermal　cycle　number　reaches　25,　the　protrusion　rate　of　Cu　slows　down　due　to　strain　hardening.　After　30　thermal　cycles,　the　Cu　protrusion　stabilizes　within　the　range　of　1.92　mu　m　to　2.09　mu　m.