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• W40014510 abstract "Solder alloys are key materials in the electronics industry and serves as both an electrical and mechanical connection to other components. Due to environmental concerns, the use of lead (Pb) in commercial electronic industries has been prohibited creating a demand for alternatives to traditional Sn-Pb solders. Amongst popular alternative alloys, those based on the Sn-Cu system and in particular Sn-0.7Cu have found acceptance due to a low relative cost and a combination of properties that make them suitable for many current manufacturing techniques. However the manufacturing environment is dynamic and there is a trend towards higher area-density electronic packages and the adaptation of micro and nano-scale solder joints. In these smaller joints it becomes increasingly important to have a fundamental understanding of how the solder material will behave both during manufacture and while in service over a range of operating conditions. Micro-alloying, in which the solidification microstructure is preferably and significantly modified by trace elements, is a key method to improve Pb-free interconnections in electronic devices. In this work, the Sn-Cu system was selected as a viable system for further development using a microalloying approach. Particular focus was given to the effect of alloying additions on microstructure development during the liquid-solid transition and the stability of the two dominant phase, namely the Cu6Sn5 intermetallic and the parent Sn phase. These areas are of interest as solidification is a step common to nearly all soldering operations and both of the dominant phases are known to have the potential for crystallographic transformations and associated volume changes which may impact on reliability. A large range of techniques was used to thoroughly investigate the effect of microalloying additions on these phenomena (these included radiography, X-ray diffraction (XRD), and micro X-ray fluorescence analysis (XRF) with synchrotron radiation, electron microscopy andmicroanalysis etc.). The solidification of the solder alloys was examined primarily through real-time X-ray imaging after the development of appropriate methodology. This allowed visualisation of how minor additions of Ni and Zn influence the development of microstructure in these alloys. In previous studies Ni and Zn had been separately identified as effective micro-alloying elements for Sn-0.7Cu. However, Ni has been associated with the instabilities of interfacial microstructure, and microstructural modification by Zn is not as effective as Ni at similar levels of addition. In this study, by micro-alloying both Ni and Zn concurrently, a significantly refined microstructure was achieved. At the soldered interface, Ni and Zn micro-alloying leads to a more continuous, finer grained and stable interfacial Cu6Sn5 intermetallic and suppressed growth of the Cu3Sn layer, which is commonly found in other solder joints. The Ni and Zn are homogeneously distributed in the interfacial Cu6Sn5 and prevented the polymorphic phase transformation of Cu6Sn5. This stabilising effect minimises the thermal expansion mismatch among interfacial Cu6Sn5, the Cu substrate and Sn based alloys.The two important phases Cu6Sn5 and Sn were examined independently with respect to their phase stabilities due to the implications for reliability. Cu6Sn5 intermetallic shows different types of polymorphs depending on the composition and temperature. The polymorphic phase transformation in slightly Cu-rich Cu6Sn5 occurred in a relatively quick time frame and also at a higher temperature, as compared to the slightly Sn-rich Cu6Sn5. Alloying with Zn, Au and In also had a marked influence on the phase stability and thermal expansion of Cu6Sn5 over the typical service temperature range. It is concluded that either alloying or additional heat treatment processes can prevent the polymorphic phase transformation of Cu6Sn5 and/or mitigate possible damage associated with this transformation. Sn also undergoes an allotropic transformation with a structurally unsustainable volume change. Prior to this work there was little understanding on the likelihood of the β → α transformations of Sn occurring in lead-free solders. To address this, the kinetics of β → α transformations in pure Sn was quantitatively studied along with the effects of α-Sn nucleation and alloying Pb, Cu, Ge and Si. Coefficient of thermal expansions of both β and α phases over the large temperature range was precisely measured. It was established that α-Sn nucleation plays a crucial role in β→α phase transformation. Pb, Cu, Ge and Si particles were found to inhibit the transformation due to their contact with the Sn. The collective results of this thesis demonstrate that the nature of alloy solidification and segregation, interfacial reactions, phase formation, transformation and stability can be significantly influenced by micro-alloying. Concurrent additions of micro-alloying elements were found to be an effective way of modifying microstructure and improving the properties of SnCu solder joints. The results are of scientific and industrial relevance and have implications for new solder alloy composition design." @default.
• W40014510 created "2016-06-24" @default.
• W40014510 creator A5065458315 @default.
• W40014510 date "2015-01-19" @default.
• W40014510 modified "2023-09-23" @default.
• W40014510 title "Phase formation, transformation and stability in micro-alloyed Sn-based lead-free solder alloys and joints" @default.
• W40014510 doi "https://doi.org/10.14264/uql.2015.178" @default.
• W40014510 hasPublicationYear "2015" @default.
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