Anisotropic behavior of single grain CU₆SN₅ intermetallic

Intermetallic (IMC) layers (Cu₆Sn₅ and Cu₃Sn) are an essential component of a solder joint for good metallurgical bonding. However, the mechanical and physical properties of IMC layers differ significantly from the solder and substrate, and excessive IMC layers can lower the reliability of solder joints due to their brittle nature. Moreover, continuous miniaturization of packages and joints has increased the volume fraction of IMCs to a point where smaller joints could be completely composed of IMCs. Further miniaturization of joints may result in statistical grain size effects. One of the most common types of IMCs in microelectronic joints is Cu₆Sn₅, which is formed in a variety of bonding materials with different compositions of Sn, Cu, and Ag. Due to its large percentage of volume in solder joint; to predict the reliability of micro solder joints, it is necessary to characterize single crystal Cu₆Sn₅ IMC completely. This study reports the information on grain growth orientation and elasticplastic properties such as young's modulus, hardness, yield strength and strain hardening exponent of single grain of Cu₆Sn₅ in Sn-3.5Ag/Cu solder alloy system. IMCs materials were grown using reflow process using an experiment in which the time and temperature of reflow process was varied. Electron backscatter diffraction (EBSD) analysis was conducted after the reflow to measure the grain size and determine the preferred grain orientation. It was found that the growth orientation is in the orientation of the c-axis. Nanoindentation was carried out in 4 individual grains with different crystallographic orientation along normal to the growth axis to determine the elastic properties of Cu₆Sn₅ single crystal. Plastic properties were predicted using the nanoindentation results and Dao reverse analysis model. The results indicate that the hardness for Cu₆Sn₅ grains with different orientation along normal to growth axis is statistically indistinguishable. Lower elastic modulus was observed for a grain with [010] direction parallel to the loading direction. Yield strength of a grain with (001) plane parallel to the loading direction was slightly lower than other grain orientations. Overall, the experimental results obtained were found to be within the range shown in the literature.