This thesis develops the understanding of microstructure formation in binary Sn-Ni alloys during solidification and during soldering. Where past research has found metastable NixSny phases after solid-state ageing of Ni-Sn couples, it has been shown for the first time in this work that metastable NiSn4 forms in the Sn-Ni system during (i) solidification of Sn-rich Sn-Ni alloys as both a primary and a eutectic phase, (ii) soldering of Sn-Ni alloys to Ni-containing substrates as the βSn-NiSn4 eutectic and (iii) during storage of Sn-Ni electroplated couples as the interfacial intermetallic layer. It has been shown that NiSn4 is an orthorhombic phase with a superstructure closely related to oC20-NiSn4 and that NiSn4 is a metastable compound at all temperatures relevant to soldering. Furthermore, it has been shown that a second metastable phase, Ni3Sn7, forms occasionally during solidification. The research has concluded on the following reasons for NiSn4 formation: (i) NiSn4 has growth advantages over equilibrium Ni3Sn4 due to easier interface attachment kinetics; (ii) the existence of a low planar lattice disregistry (~5%) between NiSn4 and Sn results in highly reproducible low energy NiSn4/Sn orientation relationships; and (iii) Fe impurity levels typical for commercial purity solders result in FeSn2 crystals that act as heterogeneous nucleation sites for NiSn4. Investigation of Sn-Ni electroplated couples showed that the interfacial layer is metastable NiSn4 initially and that Ni3Sn4 nucleates and grows to consume the NiSn4 layer with time at temperature. A similar study on Sn-Ni solder joints showed that the interfacial layer is always Ni3Sn4. The results suggest that, in electroplated couples, metastable NiSn4 nucleates on βSn due to the low planar disregistry between the phases. In contrast, during soldering the interfacial intermetallic nucleates on the Ni substrate when the solder is liquid and the metastable NiSn4 has no nucleation advantages over equilibrium Ni3Sn4.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:631192 |
| Date | January 2013 |
| Creators | Belyakov, Sergey Aleksandrovich |
| Contributors | Gourlay, Christopher ; McComb, David |
| Publisher | Imperial College London |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://hdl.handle.net/10044/1/17998 |
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