Mechanical characterizations of single-crystalline (Cu, Ni)6Sn5 through uniaxial micro-compression

Microelectronic industry is experiencing a major paradigm shift from the conventional two-dimensional into hierarchical three-dimensional integrated circuits (3D ICs). In 3D IC technology, through-silicon-vias and micro joints are regarded as the two vital interconnection processes to achieve higher packing density as well as heterogeneous integration. However, the miniaturization of solder joints down to micron level causes the consequence that intermetallic compounds takes up most of the volume of micro joints within a short period of time, in stark contrast to the well-known scenario at the opposite end of spectrum that soft Sn-based solders always occupies a high volume fraction of joints. Thus, intermetallics are expected to shoulder primary responsibilities of mechanical reliability in micro joints. Therefore, a full understanding about the mechanical behaviors of intermetallics at micron scale is essential for validity and reliability evaluation of 3D IC interconnections. The present study focuses on mechanical assessment of single-crystalline (Cu, Ni)6Sn5 by means of uniaxial micro-compression and nanoindentation. The anisotropic mechanical properties of hexagonal Cu6Sn5 are investigated by compression testing on single-crystalline Cu6Sn5 micropillars with different grain orientations. The jammed network of dislocation inside deformed micropillars suggests that intermittent strain bursts in stress-strain curves are relevant to dislocation-avalanche mechanism. In addition, Cu6Sn5 micropillars are found to exhibit brittle cleavage failure with most fractured pieces crushed in the same direction, and one set of preferred cleavage plane are characterized by post-mortem analysis. Moreover, the strengthening and phase-stabilizing effect of Ni element on Cu6Sn5 is also investigated by micropillar compression and X-ray diffraction, respectively.