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The paper investigated the formation and evolution of intermetallic compound at the interface of Ni/Sn with and without USV. Without ultrasonic treatment, the IMC layer are plain and compact at the interface of Ni/Ni₃Sn₄. Towards the solder side, Sn tends to wet and penetrate the grain boundaries to form the scallop-type Ni₃Sn₄. Moreover, intermetallic grains grow with a 1/2 power dependence on time, the growth was controlled by the bulk diffusion. With ultrasonic treatment, acoustic cavitation causes the interfacial Ni₃Sn₄to dissolve and forms many grooves, even forms is reopened and ultrasonic streaming promotes this dissolution. With prolong of ultrasonic time, elongated Ni₃Sn₄grains which was "neck" connection are broken up by the ultrasonic cavitation and fall into the liquid solder. Interfacial IMC was gradually reduction. The Ni₃Sn₄ grains falling into the liquid are further dissolved and broken by ultrasonic cavitation. Finally many fine Ni₃Sn₄ grains are evenly distributed in the joint.