Electrodeposition of Au-Sn alloys for lead-free solders: Au-rich eutectic and Sn-rich eutectic compositions

The Au-Sn binary phase diagram shows that there are two eutectic alloy compositions. The first one is at ~30 at% Sn (Au-rich) with a melting point of 280°C and the second composition is at ~94 at% Sn (Sn-rich) with an eutectic temperature at 217°C. This paper summarizes work that demonstrates that both eutectic alloys can be electrodeposited from electrolytes that are non-toxic and environmentally friendly. The Au-rich eutectic alloy is electrodeposited as alternating layers of Au₅Sn and AuSn, while the Sn-rich eutectic alloy is electrodeposited as sequential Au₅Sn and Sn layers. In both cases, the entire electrodeposition process is achieved using a single solution, for each eutectic composition, without exposing the sample to air during electrodeposition. Each phase composition is attained by varying the current density only. The microstructure of as-deposited and reflowed solders is characterized and confirmed using X-ray diffraction and scanning electron microscopy.