In　this　study,　the　interfacial　microstructure　evolution　and　mechanism　of　ultrasonic　action　during　ultrasonic-assisted　TLP　soldering　process　(260°C,　600W,　20KHz)　were　investigated.　The　bonding　time　forming　full　Cu3Sn　solder　joints　of　traditional　TLP　and　ultrasonic-assisted　TLP　soldering　was　600min　and　50s　respectively.　Before　forming　full　IMCs　solder　joints,　the　Cu6Sn5　at　Cu/Sn　interface　grew　in　a　non-scallop-like　shape　during　ultrasonic-assisted　TLP　soldering　process,　meanwhile,　the　non-interfacial　Cu6Sn5　distributed　within　the　Sn　layer,　the　Cu3Sn　at　Cu/Cu6Sn5　interface　grew　in　a　non-wave-like　shape　or　non-planar-like　shape,　the　non-interfacial　Cu3Sn　in　the　Cu6Sn5　contacted　with　the　Cu3Sn　layers　at　Cu/Cu6Sn5　interface　with　the　increasing　of　ultrasonic　bonding　time,　which　was　different　from　the　formation　of　scallop-like　Cu6Sn5　layers,　wave-like　and　planar-like　Cu3Sn　layers　by　traditional　TLP　soldering.　The　mechanism　of　ultrasonic　action　was　regarded　as　that　the　solder　joints　experience　generation　of　micro-cracks　in　the　Cu6Sn5,　separation　from　Cu6Sn5　layers　at　Cu/Sn　interface,　being　smashed　to　smaller　size　of　separate　Cu6Sn5　and　moving　into　the　liquid　Sn　of　smaller　Cu6Sn5　in　turn,　while　the　formation　of　non-wave-like　or　non-planar-like　Cu3Sn　layers　was　considered　to　be　the　precipitation　at　Cu3Sn/Cu6Sn5　interface　of　Cu　atoms,　the　formation　of　non-interfacial　CU3Sn　was　attributed　to　the　traversing　Cu3Sn　layers　at　Cu/Cu6Sn5　interface　into　Cu6Sn5　of　Cu　atoms.　In　addition,　the　ultrasonic　wave　accelerated　the　diffusion　of　Cu　atoms　and　Sn　atoms　to　form　IMCs.　©　2019　IEEE.