Au-Ge Based Alloys for Novel High-T Lead Free Solder Materials – Fundamentals and Applications

Low melting Au based alloys offer an interesting combination of high corrosion and creep resistance as well as good electrical and thermal conductivity, and they support the capability for fluxless soldering. Despite their high price, Au alloys are of special interest for high temperature solder applications where these characteristics are essential, e.g. special MEMS devices, sensors exposed to aggressive media, biomedical applications or in space technology. The alloying system Au-Ge is characterized by a deep eutectic point with a melting temperature of 360°C, but information on the use of eutectic Au-Ge as solder material is very limited.

In this work, the results of a group project within the European COST Action MP0602 “High Temperature Lead Free Solders” on the feasibility of Au-Ge based alloys as high-temperature lead free solder will be summarized. Beside the experimental study and thermodynamic assessment of a number of important alloying systems (Au-Ge-Cu, Au-Ge-Ni, Au-Ge-Sn, Au-Ge-Sb, Au-Ge-Si) wetting and soldering tests using eutectic Au-Ge alloy and Cu, Ni and Si substrates were performed. Sound joints with a high strength could be produced. The successful use of eutectic Au-Ge alloy as solder material for highly loaded components was finally demonstrated on a special part of a mass spectrometer that will be sent to planet Mercury in 2014 within the joint ESA/JAXA space mission Bepi-Colombo.