Purpose　This　study　aims　to　investigate　the　interfacial　microstructures　of　ultrasonic-assisted　solder　joints　at　different　soldering　times.　Design/methodology/approach　Solder　joints　with　different　microstructures　are　obtained　by　ultrasonic-assisted　soldering.　To　analyze　the　effect　of　ultrasounds　on　Cu6Sn5　growth　during　the　solid-liquid　reaction　stage,　the　interconnection　heights　of　solder　joints　are　increased　from　30　to　50　mu　m.　Findings　Scallop-like　Cu6Sn5　nucleate　and　grow　along　the　Cu6Sn5/Cu3Sn　interface　under　the　traditional　soldering　process.　By　comparison,　some　Cu6Sn5　are　formed　at　Cu6Sn5/Cu3Sn　interface　and　some　Cu6Sn5　are　randomly　distributed　in　Sn　when　ultrasonic-assisted　soldering　process　is　used.　The　reason　for　the　formation　of　non-interfacial　Cu6Sn5　has　to　do　with　the　shock　waves　and　micro-jets　produced　by　ultrasonic　treatment,　which　leads　to　separation　of　some　Cu6Sn5　from　the　interfacial　Cu6Sn5　to　form　non-interfacial　Cu6Sn5.　The　local　high　pressure　generated　by　the　ultrasounds　promotes　the　heterogeneous　nucleation　and　growth　of　Cu6Sn5.　Also,　some　branch-like　Cu3Sn　formed　at　Cu6Sn5/Cu3Sn　interface　render　the　interfacial　Cu3Sn　in　ultrasonic-assisted　solder　joints　present　a　different　morphology　from　the　wave-like　or　planar-like　Cu3Sn　in　conventional　soldering　joints.　Meanwhile,　some　non-interfacial　Cu3Sn　are　present　in　non-interfacial　Cu6Sn5　due　to　reaction　of　Cu　atoms　in　liquid　Sn　with　non-interfacial　Cu6Sn5　to　form　non-interfacial　Cu3Sn.　Overall,　full　Cu3Sn　solder　joints　are　obtained　at　ultrasonic　times　of　60　s.　Originality/value　The　obtained　microstructure　evolutions　of　ultrasonic-assisted　solder　joints　in　this　paper　are　different　from　those　reported　in　previous　studies.　Based　on　these　differences,　the　effects　of　ultrasounds　on　the　formation　of　non-interfacial　IMCs　and　growth　of　interfacial　IMCs　are　systematically　analyzed　by　comparing　with　the　traditional　soldering　process.