Nanoindentation measurements on Cu–Sn and Ag–Sn intermetallics formed in Pb-free solder joints

Nanoindentation testing has been used to measure the hardness and elastic modulus of Ag₃Sn, Cu₆Sn₅, and Cu₃Sn intermetallics, as well as Sn–Ag–Cu solder and pure Sn and Cu. The intermetallics were fabricated by solid-state annealing of diffusion couples prepared from a substrate (Cu or Ag) and a solder material (Sn or Sn–Ag–Cu solder), providing geometries and length scales as close as possible to a real solder joint. Nanoindentation results for the intermetallics, representing penetration depths of 20–220 nm and loads from 0.7 to 9.5 mN, reveal elastic/plastic deformation without evidence of fracture. Measured hardness values of Cu₆Sn₅ (6.5 ± 0.3 GPa) and Cu₃Sn (6.2 ± 0.4 GPa) indicate a potential for brittle behavior, while Ag₃Sn (2.9 ± 0.2 GPa) appears much softer and ductile. Using a bulk Cu₆Sn₅ sample, Vickers hardness testing revealed an indentation size effect for this compound, with a hardness of 4.3 GPa measured at a load of 9.8 N. An energy balance model is used to explain the dependence of hardness with load or depth, where the observation of an increasing amount of fracture with applied load is identified as the primary mechanism. This result explains discrepancies between nanoindentation and Vickers results previously reported.