Difficulties in Determining Eutectic Compositions: The Sn-Ag-Cu Ternary Eutectic
Kil-Won Moon, William J. Boettinger
The determination of a precise eutectic composition is more difficult than the determination of its corresponding eutectic temperature. These difficulties are demonstrated with a Pb-free solder, the Sn-rich Sn-Ag-Cu ternary eutectic. The solidification of this ternary eutectic involves the solid phases (Sn)*, Ag3Sn and Cu6Sn5. The liquid is prone to supercooling, the intermetallics have very steep liquidus surfaces (small phase fractions) and the coupled zone of eutectic microstructure formation is shifted towards Ag- and Cu-rich compositions. These issues were overcome by a combination of methods: preliminary thermodynamic calculation of the ternary phase diagram to anticipate difficulties, increased sensitivity of the thermal analysis, and a cycling heating and cooling method. The experimentally determined composition of the ternary eutectic is Sn-3.58 mass % Ag 0.05 mass % Ag-0.96 mass % Cu 0.04 mass % Cu at 217.2 degrees C 0.2 degrees C.