Electromigration　(EM)　of　solder　joints　in　microelectronic　devices　have　become　a　critical　reliability　issue　due　to　the　continuous　miniaturization　and　requirements　for　enhanced　performance　of　the　devices.　At　present,　besides　EM,　thermomigration(TM)　has　been　regarded　as　another　reliability　concern　as　it　has　been　found　to　accompany　EM　in　flip　chip　solder　joints.　Both　EM　and　TM　effect　are　the　main　factors　leading　to　the　failure　of　the　solder　joints.　In　this　study,　TM　and　EM　were　studied　in　Cu/Sn0.3Ag0.7Cu/Cu　solder　joints　by　applying　a　1x10(4)　A/cm(2)　DC　current　and　a　thermal　gradient　of　1x10(3)degrees　C/cm　to　the　joints　respectively.　The　microstructure　and　composition　of　the　solder　joints　were　observed　by　means　of　Scanning　Electron　Microscope.　This　research　include　two　possible　aspects.　Firstly,　the　solder　joint　was　subjected　to　current　stress　and　thermal　gradient.　Secondly,　analyzing　the　microstructure　and　composition　of　the　joints.　According　to　the　microstructure　and　composition,　the　behavior　of　the　joints　under　current　stress　and　thermal　gradient　were　found.　The　results　indicated　that,　with　a　thermal　gradient　of　1x10(3)degrees　C/cm,　the　Cu　atoms　were　found　to　migrate　towards　the　lower　temperature　side　while　the　Sn　atoms　were　found　to　migrate　towards　the　higher　temperature　side　firstly,　with　the　time　prolonging,　the　Sn　atoms　were　found　to　migrate　towards　the　slower　temperature　side.　And　with　a　1x10(4)　A/cm(2)　DC　current　stressing,　The　IMC　in　cathode　decrease　much,　and　some　voids　and　cracks　occur.　In　anode,　the　IMC　increase　a　lot.　And　there　are　some　bump.　The　couple　of　thermal　and　electrical　field　of　solder　joint　will　be　studied　in　the　future.