Purpose　-　This　paper　aims　to　analyze　the　morphology　transformation　on　the　Cu3Sn　grains　during　the　formation　of　full　Cu3Sn　solder　joints　in　electronic　packaging.　Design/methodology/approach　-　Because　of　the　infeasibility　of　analyzing　the　morphology　transformation　intuitively,　a　novel　equivalent　method　is　used.　The　morphology　transformation　on　the　Cu3Sn　grains,　during　the　formation　of　full　Cu3Sn　solder　joints,　is　regarded　as　equivalent　to　the　morphology　transformation　on　the　Cu3Sn　grains　derived　from　the　Cu/Sn　structures　with　different　Sn　thickness.　Findings　-　During　soldering,　the　Cu3Sn　grains　first　grew　in　the　fine　equiaxial　shape　in　a　ripening　process　until　the　critical　size.　Under　the　critical　size,　the　Cu3Sn　grains　were　changed　from　the　equiaxial　shape　to　the　columnar　shape.　Moreover,　the　columnar　Cu3Sn　grains　could　be　divided　into　different　clusters　with　different　growth　directions.　With　the　proceeding　of　soldering,　the　columnar　Cu3Sn　grains　continued　to　grow　in　a　feather　of　the　width　growing　at　a　greater　extent　than　the　length.　With　the　growth　of　the　columnar　Cu3Sn　grains,　adjacent　Cu3Sn　grains,　within　each　cluster,　merged　with　each　other.　Next,　the　merged　columnar　Cu3Sn　grains,　within　each　cluster,　continued　to　merge　with　each　other.　Finally,　the　columnar　Cu3Sn　grains,　within　each　cluster,　merged　into　one　coarse　columnar　Cu3Sn　grain　with　the　formation　of　full　Cu3Sn　solder　joints.　The　detailed　mechanism,　for　the　very　interesting　morphology　transformation,　has　been　proposed.　Originality/value　-　Few　researchers　focused　on　the　morphology　transformation　of　interfacial　phases　during　the　formation　of　full　intermetallic　compounds　joints.　To　bridge　the　research　gap,　the　morphology　transformation　on　the　Cu3Sn　grains　during　the　formation　of　full　Cu3Sn　solder　joints　has　been　studied　for　the　first　time.