We　reported　a　phenomenon　and　mechanism　of　recrystallization　evolution　in　solder　joint　under　thermomechanical　stress.　Ball　Grid　Array　samples　fabricated　by　Sn3.0Ag0.5Cu　solder　were　placed　into　the　thermal　cycling　test　chamber　with　a　temperature　range　of　-55　degrees　C　to　125　degrees　C.　Characterized　using　electron　backscattered　diffraction　technique,　three　stages　of　recrystallization　occurring　in　the　solder　joint　were　found　to　co-exist　in　one　joint　after　2500　thermal　cycles.　To　better　understand　the　formation　of　recrystallized　grains　during　thermal　cycling,　especially　dislocation　substructures　at　the　early　stage,　thin　foils　corresponding　to　different　stages　of　the　recrystallization　were　extracted　by　focus　ion　beam　from　the　solder　joint　and　analyzed　by　high-resolution　transmission　electron　microscope.　We　summarize　the　recrystallization　process　including　the　annihilation　of　dislocation　dipoles　contributes　to　the　evolution　of　sub-grain　boundaries,　and　then　the　recrystallized　grains　are　generated　by　sub-grain　rotation　mechanism.　In　addition,　the　recrystallized　grains　proceed　to　the　subsequent　same　cycle　until　the　failure　of　solder　joint.　The　study　provides　the　theoretical　basis　and　technical　support　for　further　improving　the　thermo-mechanical　reliability　of　Sn-based　lead-free　solder　joints.