Zn-Al based alloys as Pb-free solders for die attach

The present study examines the potential of Zn-Al based alloys for replacing Pb-containing solders used in the die-attaching process. Four alloys (Zn-6wt.%Al, Zn-6wt.%Al-1wt.%Ga, Zn-3wt.%Al-3wt.%Mg, and Zn-4wt.%Al-3wt.%Ga-3wt.%Mg) 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) Zn-rich 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.