Physical, electronic and thermodynamic properties of small particles (nano-particles) differ significantly from those of the bulk materials, and the depression of the melting temperature below the melting point of the bulk has been known for a long time. This dependence of the melting temperature on the particle size is not restricted to any particular material and may therefore also be exploited to lower the melting temperatures of lead-free solders. At the same time, the high-temperature solders currently in use (melting temperatures 250-400°C) contain high amounts of lead and are exempt from the European RoHS directive due to reliability reasons. In the search for lead-free substitutes, Sn-Sb and Sn-Sb-M alloys have been identified as possible candidates. With the envisaged reduction of the grain size into the nanometer range and a corresponding lowering of the melting temperature, considerable energy savings would become possible in the first soldering step without sacrificing the stability of the obtained solder junctions against re-melting in the further assembling procedure. Therefore it is proposed to prepare micro- and nanosized particles of lead-free high-temperature solder materials, based on Sn-Sb-M (M=Ag, Cu, Ni) from suitable chemical precursors. These precursors will be prepared by precipitation reactions, followed by thermal decomposition and, if necessary, by adequate reduction reactions, and the reaction conditions will be optimized. The particles will be characterized by chemical analysis, SEM and TEM, and by thermo-analytical methods, and the corresponding properties will be compared with those of bulk materials. At the same time, additional experiments in the bulk systems Sn-Sb-Ag and Sn-Sb-Cu will be performed to complete the available literature information on phase diagram and thermodynamics. It will be attempted to model the phase diagrams for bulk and for nano-sized materials by the well known CALPHAD-method.
Field of science
- /natural sciences/physical sciences/thermodynamics
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