In situ imaging of the soldering reactions in nanoscale Cu/Sn/Cu and Sn/Cu/Sn diffusion couples.

The soldering reactions of three-segmented Sn/Cu/Sn and Cu/Sn/Cu diffusion couples are monitored by in-situ transmission electron microscopy to reveal the metallurgical reaction mechanism and the associated phase transformation pathway. For Sn/Cu/Sn diffusion couples, there is no ε-Cu₃Sn formation due to the relatively insufficient Cu as compared to Sn. Kirkendall voids form initially in the Cu segment and then disappear due to the volume expansion associated with the continued intermetallic compound (IMC) formation as the reaction progresses. The incoming Sn atoms react with Cu to form η-Cu₆Sn₅, and the continuous reaction then transforms the entire nanowire to η-Cu₆Sn₅ grains with remaining Sn. With continued heating slightly above the melting point of Sn, an Sn-rich liquid phase forms between η-Cu6_Sn₅ grains. By contrast, the reaction in the Cu/Sn/Cu diffusion couples results in the intermetallic phases of both Cu₃Sn and Cu₆Sn₅ and the development of Cu₆Sn₅ bulges on Cu₃Sn grains. Kirkendall voids form in the two Cu segments, which grow and eventually break the nanowire into multiple segments. [ABSTRACT FROM AUTHOR]