Interactions between Sn and Cu substrate with various surface finishes

In this paper, the reaction couples were prepared to investigate the interaction between Sn and Cu substrate with various surface finishes. The studied interfacial systems contain Sn/Ni/Ag/Cu, Sn/Ag/Cu and Sn/Cu/Ag/Cu. The corresponding surface finish, Ag or Ni layer, is about 1~2μM. In the Sn/Ni/Ag/Cu reaction, the formed phase was Ni₃Sn₄ at first, indicating that Sn reacted with Ni at the Sn/Ni interface. Sequentially, as the limited Ni layer was consumed completely, Sn kept diffusing across the Ni₃Sn₄ and reacted with Ag layer to form Ag₃Sn. Further, Sn diffused into Cu substrate and produced Cu₆Sn₅ and Cu₃Sn. Meanwhile, Cu also diffused toward the opposite direction and then caused the prior Ni₃Sn₄ layer to gradually transform to (Cu, Ni)₆Sn₅. The (Cu, Ni)₆Sn₅ layer growth thickness depends on the initial Ni layer. Notably, the original continuous Ag₃Sn layer became the discontinuous particulates and intercalated in-between (Cu, Ni)₆Sn₅ and Cu₆Sn₅. For comparison, the result in the Sn/Ag/Cu specimen revealed the Ag₃Sn layer remained continuous and no Cu₆Sn₅ was accumulated at the Sn/Ag₃Sn interface. Moreover, the total thickness of Cu₆Sn₅ and Cu₃Sn layers in Sn/Ni/Ag/Cu is much smaller than that of the Sn/Cu. The initial reaction rate at the interface with the Ni/Ag surface finish is relatively slow. Nevertheless, after the Ag layer is consumed to form Ag₃Sn, Cu would be induced by the Ni₃Sn₄ to rapidly form the thicker (Cu, Ni)₆Sn₅ layer.