Role of Fe impurities in the nucleation of metastable NiSn4

Metastable intermetallics such as NiSn4 occasionally form in solder joints on Ni substrates. Here it is shown that impurity Fe or soldering to Fe–42Ni substrates promotes metastable NiSn4 during the solidification of Sn-rich Sn–Ni alloys. FeSn2 crystals act as heterogeneous nucleation sites for NiSn4, with nucleation occurring on the (001) facet of FeSn2 where the lattice mismatch between (001)FeSn2 and (001)NiSn4 is only ∼2%. The size of active FeSn2 nuclei was 2–10 μm and smaller particles or large, porous FeSn2 particles were ineffective nuclei for primary NiSn4. The use of high purity Sn–Ni alloys, where FeSn2 does not form, is shown to prevent primary NiSn4 formation and promote Sn–Ni3Sn4 eutectic growth. However, metastable Sn–NiSn4 eutectic grew at all Fe levels studied and both Sn–NiSn4 and Sn–Ni3Sn4 eutectics coexisted in high purity samples. Thus, metastable NiSn4 formation is enhanced by nucleation on FeSn2 and also depends on competitive growth between the Sn–NiSn4 and Sn–Ni3Sn4 eutectics.

► Fe impurities typical in solders cause FeSn2 crystals to form during solidification.
► FeSn2 act as heterogeneous nucleation sites for metastable NiSn4.
► When the Fe level was 0.0002 wt%, primary NiSn4 did not form.
► The metastable Sn–NiSn4 eutectic grew even in Sn–Ni alloys containing 0.0002 wt% Fe.