Interface microstructure between Fe-42Ni alloy and pure Sn

Abstract

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.