Currently,　one　of　the　serious　challenges　in　microelectronic　devices　is　the　miniaturization　trend　of　packaging.　As　the　decrease　of　joint　dimension,　electromigration　(EM)　and　thermomechanical　fatigue　become　critical　issues　for　fine　pitch　packaging.　The　independent　mechanisms　of　EM　and　thermomechanical　fatigue　are　widely　investigated　and　understood.　However,　the　coupling　effect　of　both　conditions　needs　further　exploration.　The　current　study　established　the　correlation　between　resistance　and　microstructure　evolution　of　solder　joint　under　the　combination　effect　of　thermal　cycling　and　high　current　density　and　illustrated　the　different　contributions　of　these　two　factors　to　the　reliability　of　the　joint　through　the　comparison　monopoly　tests.　The　results　revealed　that　cracks　had　more　impact　on　resistance　increase　than　phase　segregation.　The　resistance　evolution　could　be　divided　into　three　stages.　First,　the　resistance　mitigated　due　to　the　phase　coarsening.　Second,　Joule　heating　effect　made　the　resistance　increase　slowly.　Third,　EM　led　to　the　resistance　increase　rapidly.　The　high　current　density　can　help　to　improve　the　reliability　of　the　solder　joint　under　the　coupling　effect　of　thermal　cycling　and　EM　at　the　initial　stage,　but　harmful　to　the　consequence　process.