Microstructure formation and soldering in Sn-Ni alloys

252 Pages·2013·38.17 MB·English

by  Sergey Belyakov

Imperial College of Science, Technology and Medicine
Department of Materials

Microstructure formation and soldering
in Sn-Ni alloys
Sergey Aleksandrovich Belyakov

Submitted in part fulfilment of the requirements for the degree of Doctor of
Philosophy in Materials Engineering at Imperial College London, May 2013
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Abstract
This thesis develops the understanding of microstructure formation in binary Sn-Ni
alloys during solidification and during soldering.
Where past research has found metastable Ni Sn phases after solid-state ageing of
x y
Ni-Sn couples, it has been shown for the first time in this work that metastable NiSn
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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-NiSn eutectic and (iii) during storage of Sn-Ni electroplated
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couples as the interfacial intermetallic layer. It has been shown that NiSn is an
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orthorhombic phase with a superstructure closely related to oC20-NiSn and that
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NiSn is a metastable compound at all temperatures relevant to soldering.
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Furthermore, it has been shown that a second metastable phase, Ni Sn , forms
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occasionally during solidification.
The research has concluded on the following reasons for NiSn formation: (i) NiSn
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has growth advantages over equilibrium Ni Sn due to easier interface attachment
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kinetics; (ii) the existence of a low planar lattice disregistry (~5%) between NiSn and
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Sn results in highly reproducible low energy NiSn /Sn orientation relationships;
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and (iii) Fe impurity levels typical for commercial purity solders result in FeSn
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crystals that act as heterogeneous nucleation sites for NiSn .
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Investigation of Sn-Ni electroplated couples showed that the interfacial layer is
metastable NiSn initially and that Ni Sn nucleates and grows to consume the NiSn
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layer with time at temperature. A similar study on Sn-Ni solder joints showed that
the interfacial layer is always Ni Sn . The results suggest that, in electroplated
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couples, metastable NiSn nucleates on βSn due to the low planar disregistry
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between the phases. In contrast, during soldering the interfacial intermetallic
nucleates on the Ni substrate when the solder is liquid and the metastable NiSn has
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no nucleation advantages over equilibrium Ni Sn .
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Declaration of Originality
I, Sergey Aleksandrovich Belyakov, hereby declare that this thesis and associated
research are my own work and that all sources I have used or quoted have been
indicated and acknowledged by means of complete referencing.

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Acknowledgements

First and foremost I would like to thank my supervisor Dr. Chirstopher M. Gourlay
for all the help and guidance he gave me throughout this project. Second I would
like express my gratitude to Nihon Superior Co., Ltd. for funding the project, for
producing the Sn-10Ni master alloy and conducting the chemical analysis. I would
also like to thank Dr. Kazihiro Nogita and Prof. David W. McComb for their input
early in this project, Dr. Mahmoud Ardakani for teaching me electron microscopy,
Dr. Ben T. Britton for teaching me EBSD technique and Dr. Vassili Vorontsov for HR
TEM imaging. I would also like to thank my family and friends who supported me
during this PhD project.

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Dedication

to my grandfather
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