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Nucleation and microstructure study of βSn in lead-free solders

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Title: Nucleation and microstructure study of βSn in lead-free solders
Authors: Daszki, Armin
Item Type: Thesis or dissertation
Abstract: Pb-free solders typically exhibit a single nucleation event which results in a limited number of grain orientations, impacting mechanical properties. This thesis explores the influence of common phases on the nucleation of βSn, and βSn microstructure in solder balls and joints. It was found that primary Ni3Sn4 particles and a Ni3Sn4 reaction layer reduce βSn nucleation undercooling. A reproducible orientation relationship between Ni3Sn4 and βSn was found here. Furthermore, it was shown that in the absence of Ni3Sn4, a metastable NiSn4 phase may form, which is able to nucleate βSn. Primary Ag3Sn plates were shown here to frequently nucleate βSn in SAC305 solder balls, with a previously reported orientation relationship measured between these phases. The presence of larger primary Ag3Sn plates was also linked to a more complex βSn microstructure composed of interrelated cyclic twinning. However, it was also shown that thermally cycling SAC305 solder alloy reduced the frequency of primary Ag3Sn plates observed. A nucleation rate approach was used to compare modes of nucleation in similar solder samples. Specifically, it was shown that the nucleation of βSn on primary Ni3Sn4 particles, a Ni3Sn4 reaction layer, and a large Ni3Sn4 single crystal is a result of the same nucleation mechanism at different number densities. Conversely it is shown that nucleation on primary Cu6Sn5 particles and a Cu6Sn5 reaction layer is a result of a different nucleation mechanism. Finally the microstructures of βSn in SAC305 solder alloy were studied in detail and classified into groups based on the presence of primary Ag3Sn, and the complexity and amount of βSn interlacing. Where past research noted a relationship between βSn nucleation undercooling and interlacing amount, this thesis presents an additional relationship to the presence and size of primary Ag3Sn plates. Finally, interlacing microstructures of Double Hollow Cone and Cores are examined here, and show that βSn nucleation can occur near the surface in the absence of primary Ag3Sn.
Content Version: Open Access
Issue Date: Aug-2021
Date Awarded: Dec-2021
URI: http://hdl.handle.net/10044/1/93763
DOI: https://doi.org/10.25560/93763
Copyright Statement: Creative Commons Attribution NonCommercial Licence
Supervisor: Gourlay, Christopher
Britton, Thomas
Sponsor/Funder: Engineering and Physical Sciences Research Council
Funder's Grant Number: EP/N509486/1
Department: Materials
Publisher: Imperial College London
Qualification Level: Doctoral
Qualification Name: Doctor of Philosophy (PhD)
Appears in Collections:Materials PhD theses

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