Nanomechanical responses of intermetallic phase at the solder joint interface – Crystal orientation and metallurgical effects

In this study, the relationships between crystal structures, metallurgical effects, and mechanical properties of the most common intermetallic compound formed at the interface of solder joints, Cu6Sn5, were investigated using nanoindentation. Experimental results show that the ( 11 2 ¯ 0 ) oriented hexagonal Cu6Sn5 exhibited anisotropic mechanical behavior compared to those with random growth directions. The closest atomic packing density of the ( 11 2 ¯ 0 ) plane in hexagonal Cu6Sn5 resulted in higher hardness and notably, greater stiffness. Subjected to long time aging at 150 °C, hexagonal Cu6Sn5 was transformed into the equilibrium monoclinic structure, resulting in a reduced modulus and thus inferior ability for plasticity. Alloying of Ni, Mn and rare earth elements (La and Ce) had various contributions to the allotropic transition and thus nanoindentation responses. It was found that the differences in atomic radius between the solute elements and Cu affected the kinetics of the allotropic transformation and also the mechanical performance of Cu6Sn5. There exists a critical value for the modulus/hardness ratio (E/H) of about 17.3–17.5, below which the indent morphology showed a brittle characteristic.