Effects of Co/Si Atomic Ratio on Hardness and Electrical Conductivity of Cu-Co-Si Alloys.

Cu-Co-Si alloys are being intensively studied as alternative candidates for Cu-Ni-Si alloys for integrated circuit lead frames. However, the effects of different precipitate types on the comprehensive properties of Cu-Co-Si alloys are not clear. Herein, several Cu-Co-Si alloys with various precipitates (CoSi, Co₂Si, and Co) are designed based on phase diagrams calculation. The designed alloys are melted, cold-rolled, and aged to test the corresponding hardness and electrical conductivity (EC). X-ray diffraction and transmission electron microscopy are utilized to analyze the precipitates generated in those alloys. In the alloys with Co/Si ≤ 1, the excess Si atoms are not easy to precipitate from the copper lattice after CoSi precipitation, which seriously damages the EC. Co-precipitation in alloys with Co/Si>2 is beneficial to improve EC without sacrificing hardness. According to thermodynamic calculation, precipitates in Cu-1.62Co-0.35Si (wt%) alloy with total elements between 1.5 and 2.0 wt% are all Co₂Si and have the highest mass fraction in the equilibrium state. The different precipitates are highly dependent on the Co/Si atomic ratio and essential to Cu-Co-Si alloys' mechanical and electrical performances. [ABSTRACT FROM AUTHOR]