Abnormal Cu3Sn growth through grain boundary penetration in space-confined Ni-Sn-Cu diffusion couples.

An abnormal growth of Cu 3 Sn through the grain boundaries of Cu 6 Sn 5 formed in Ni/Sn/Cu ternary diffusion couples is reported in this study. The Ni/Sn/Cu diffusion couples are prepared by thermal compression bonding, with the Sn thickness controlled at 10 μm. The diffusion couples are aged at 200 °C for different periods of time. Due to the thinness of the initial Sn layer, the entire Sn layer ware converted into Cu 6 Sn 5 and Cu 3 Sn only after 52 h or aging. At this stage, both compounds exhibit the layered structure. Afterward, as the reaction continues, Cu 3 Sn grows thicker at the expense of Cu 6 Sn 5. Interestingly Cu 3 Sn now preferably grows along the grain boundaries of Cu 6 Sn 5. The rationalization for this abnormal growth pattern is discussed. • An abnormal growth of Cu 3 Sn along the grain boundaries of Cu 6 Sn 5 is discovered. • Such abnormal growth occurs only after the exhaustion of Sn. • Rapid grain boundary penetration causes a faster conversion of Cu 6 Sn 5 into Cu 3 Sn. • A mechanism for this abnormal growth is proposed from the Cu and Sn diffusivities. [ABSTRACT FROM AUTHOR]