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Eutectic In-48Sn was considered a promising candidate for low-temperature solder due to its low melting point and excellent mechanical properties. Both Cu₂(In,Sn) and Cu(In,Sn)₂ formation were observed at the In-48Sn/Cu interface after 160 °C soldering. However, traditional mechanical polishing produces many defects at the In-48Sn/Cu interface, which may affect the accuracy of interfacial reaction investigations. In this study, cryogenic broad Ar⁺ beam ion milling was used to investigate the interfacial reaction between In-48Sn and Cu during soldering. The phase Cu₆(Sn,In)₅ was confirmed as the only intermetallic compound formed during 150 °C soldering, while Cu(In,Sn)₂ formation was proven to be caused by room-temperature aging after soldering. Both the Cu₆(Sn,In)₅ and Cu(In,Sn)₂ phases were confirmed by EPMA quantitative analysis and TEM selected area electron diffraction. The microstructure evolution and growth mechanism of Cu₆(Sn,In)₅ during soldering were proposed. In addition, the Young’s modulus and hardness of Cu₆(Sn,In)₅ were determined to be 119.04 ± 3.94 GPa and 6.28 ± 0.13 GPa, respectively, suggesting that the doping of In in Cu₆(Sn,In)₅ has almost no effect on Young’s modulus and hardness.