Bi‐Ag alloys with Ag contents of up to 12 wt.% Ag are potential substitutes for Pb‐containing alloys in the high temperature range (T > 260 °C) of soft solders. Bi solders were produced in form of wires and can therefore be used in the same kind of soldering equipment as conventional Pb‐rich solder wires. Bi‐Ag alloys were characterized in terms of melting range and microstructure, mechanical properties (yield strength and elongation to fracture), wetting behavior, thermal fatigue behavior and thermal conductivity. The thermal conductivity is lower than that of the conventional Pb‐containing solder alloys, but all other properties tested are comparable or superior to those of Pb‐rich conventional solder alloys. It thus appears to be possible to use Bi‐Ag alloys as ‘drop‐in‐replacement' for Pb‐rich alloys.

The present study examines the potential of Zn-Al based alloys for replacing Pb-containing solders used in the die-attaching process. Four alloys (Zn6wt.%Al, were cast and characterized in terms of microstructure and mechanical behavior. Based on the criteria of thermodynamics (phase diagram extrapolations) and mechanical behavior (flow curves), it was possible to select the Zn-Al-Ga alloy as best meeting the requirements concerning melting range necessary for die attach and mechanical properties. A wire with 0.76-mm thickness was produced successfully by extrusion. However, the alloy, in the form of wires, is subject to substantial embrittlement after extruding to a degree that further handling becomes impossible. The reason for the embrittlement was investigated using x-ray techniques (x-ray diffraction and x-ray photo spectroscopy (XPS)) and transmission electron microscopy. The temperature dependence of the Ga solubility in the hexagonal close-packed (hcp) Znrich phase in combination with the affinity of Ga to the surface of the Al-rich and Zn-rich phases is held responsible for the embrittlement. A remedy for the embrittlement could be a reduction of the solubility at high temperatures in the Zn-rich phase. This will be attempted by adding small fractions of further alloying elements in future work.

In this work, theoretically dense (N 99%) composites of Ag and WC have been prepared by press-sintering-infiltration for making electrical contacts used as an arc-resistant material in a model switching device. Composites with varying silver content and WC particle size were investigated to get an insight on their electrical contact resistance (R c ) and their ability to withstand enormous thermal stresses during switching. A break-only model switching sequence was used, where the evolution of R c was measured over 50 cycles and the post-switching microstructures were investigated for thermal stress induced crack formation. A well-established 2D computational microstructure based model, object-oriented finite element analysis version 2 (OOF2), was used to determine the composite thermal conductivity (k) for various grades as a function of temperature. R c was observed to be consistently low for the coarser WC containing composite and higher silver content composites. This response was attributed to the ductility of the surface layers formed during switching. Crack formation after switching was found to be a direct consequence of large thermal gradients during 50 cycles, which was minimal for coarser WC grained and higher silver content composites which have a higher thermal shock resistance.

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