Interface microstructure between Fe-42Ni alloy and pure Sn

The interfacial reaction and microstructure of Sn with an Fe-42Ni substrate soldered at 250 °C were studied primarily using transmission electron microscopy. Apparent double reaction layers formed between Fe-42Ni and Sn of the same phase of FeSn₂ containing Ni from 2 to 6 at.% as a substitution impurity for Fe. The first layer, which faced an Fe-42Ni substrate, was thin and grew with a flat interface. The second layer, which adhered to Sn solder, showed needle/square pillar-shaped small crystalline structures. Between these two layers, voids were frequently observed. The morphological difference between these two layers and voids could be attributed to the slow-fast diffusion mechanism during soldering. The fast diffusion occurring along the surfaces or the grain boundary of FeSn₂ in the second layer led to the growth of FeSn₂ products into Sn melt. Simultaneously, the slow diffusion penetrating the Fe-42Ni and FeSn₂ grain contributed to the formation of the first layer and voids. Ni from Fe-42Ni can dissolve into Sn melt at the reaction temperature. The dissolved Ni formed platelets of Ni₃Sn₄ intermetallic compound inside a β-Sn matrix on solidification or on cooling. In contrast, Fe formed FeSn₂ crystallines directly along the interface, because its activity was higher than that of Ni in a Sn liquid at reaction temperature. The joint of Fe-42Ni/Sn/Fe-42Ni, reacted for 2 min, had a joint strength of about 65 MPa, and maintained this high strength regardless of the reaction time.