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7187 |
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Failure mechanisms in SAC305 soldered joints for automotive use |
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Wunmi Olukoya / University of Sheffield, Great Britain Edwin Raj / Johnson Matthey , England Russell Goodall/ University of Sheffield, England |
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Soldered joints are widespread in electrical connections to automotive windscreens. To form the joint, a solder is used between a screen printed silver layer and a connector. Where lead-free solders are used, such as SAC305, a ternary alloys containing Sn, Ag and Cu, failures of the joint can be seen. In this work we are seeking to understand the origin of this problem in more detail. By fabricating samples in the laboratory that replicate the key features of the joints, we explore the phases that are formed and the impact on the failure process. By doing so variations to the materials and processing conditions can be investigated in order to determine the impact on the characteristics that promote failure. We are also using ex-situ investigations of wetting to understand the role of the solder-substrate interactions on the spread of the solder. As well as experimental observation, we are using ThermoCalc (a CALPHAD approach to predict phase formation from thermodynamic data) to assess the phases that will theoretically form, to compare with those observed experimentally. We will discuss the joint microstructure, and the links between this and failure mechanisms that can occur when the joint is mechanically loaded. We will also present the findings from wetting trials using observations of liquid solder at soldering temperatures, in a controlled atmosphere hot stage. |