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The phase structure and composition of the Cu/Cu 3 Sn interface were investigated during aging at 150 °C via transmission electron microscopy. The newly formed Cu 3 Sn phase was hexagonal, and the Cu substrate was cubic. However, the fine Cu grains that formed at the Cu/Cu 3 Sn interface were monoclinic. The structure of the Cu 3 Sn/Cu interface exhibited special crystallographic relationships after aging, that is [ 2 1 ‾ 1 ‾ 6 ] C u 3 S n / / [ 1 ‾ 01 ] C u and (0 2 ‾ 20) C u 3 S n / / (404) C u . The mismatch of the crystal plane spacing between the (0 2 ‾ 20) C u 3 S n and (404) Cu crystal planes was 0.959%. In this study, monoclinic Cu precipitated at the Cu/Cu 3 Sn interface in the form of particles but did not produce a layered structure. • During aging, the Cu 3 Sn and Cu grain phases were formed at the interface. However, the fine Cu grains grown at the Cu/Cu 3 Sn interface were monoclinic. • The Cu 3 Sn/Cu interfacial structure exhibited special crystallographic relationships of [ 2 1 ‾ 1 ‾ 6 ] C u 3 S n / / [ 1 ‾ 01 ] C u and (0 2 ‾ 20) C u 3 S n / / (404) C u after aging. • The calculated lattice mismatch of crystal plane spacing between the 0 2 ¯ 20 C u 3 S n and (004) C u crystal planes was α ≈ 0.959%, which facilitated the formation of the interface between the Cu and Cu 3 Sn phases. [ABSTRACT FROM AUTHOR]