Hexagonal-Rod Growth Mechanism and Kinetics of the Primary Cu₆Sn₅ Phase in Liquid Sn-Based Solder

A hexagonal-rod growth mechanism is proposed to describe the growth behavior of the primary Cu₆Sn₅ phase in liquid Sn-based solder. After Sn-6.5 at.%Cu solder had been maintained at 250°C for 10 h, a large number of hexagonal-rod-type Cu₆Sn₅ grains were found to have separated within it. Our observations show that these hexagonal rods had side facets in the { 10 1 ¯ 0 } _η family and round ends close to the {0002}_η family. Moreover, the nucleation of the hexagonal rods was studied, and the corresponding growth kinetics found to be governed by a Cu-supply-controlled mechanism rather than an interfacial-reaction-controlled or Cu-diffusion-limited mechanism. More importantly, the anisotropic growth of the Cu₆Sn₅ phase was confirmed to be the dominant reason for production of these primary hexagonal rods with high aspect ratio. This may represent an avenue for synthesis of nanosized Cu₆Sn₅ single crystals for use as anode materials in lithium-ion batteries. Additionally, our Cu₆Sn₅ hexagonal-rod growth mechanism may provide insight into morphological and kinetic studies on interfacial Cu₆Sn₅ grains and similar intermetallics.