Measurement of deformations in SnAgCu solder interconnects under in situ thermal loading

Optical microscopy was used to discern the different grain orientations and grain boundaries on the polished cross-sections of near-eutectic lead-free board-level SnAgCu (SAC) solder interconnects. Strain distributions with submicron accuracy of the deformations on the cross-sections of the solder interconnects were measured when the package was subjected to thermal loading from room temperature to 100 °C. The results were correlated with the locations of different grains, grain boundaries and larger primary intermetallics. It revealed anisotropic nature of deformations in different grains of the SAC solder, which is similar to the thermomechanical behavior of pure Sn. The strain distribution in a solder interconnect varied significantly in different grain orientations. The primary intermetallics (Ag₃Sn plates) also behaved very differently from the surrounding Sn matrix under the thermal loading. The demonstrated strain localization along the grain boundaries and bigger primary intermetallics provides a clue for the path of fatigue crack growth that leads to a failure because of anisotropic thermomechanical response of SAC solder during thermal cycling.