A low-temperature solid-state bonding method based on copper bump coated with nickel microcones and silver buffer

• No high temperature and flat surface because of nano dimension of Ni microcone arrays.
• Ductile Ag can manage CTE mismatch between chips and packages.
• No molten phase involved, joint shape and geometry can be maintained.
• No intermetallic compound formed, reliability issues do not exist.

A low temperature solid state bonding between Cu bump coated with Nickel microcone arrays (Ni MCAs) and Ag layer is put forward. Ag, as an excellent electrical and thermal conductivity material, is used as a bonding medium between Cu substrate and Cu bumps. The bonding process is conducted under low temperature and flat surface because of the presence of nano dimension of Ni MCAs and deformation of Ag. The best bonding quality is obtained under the bonding pressure of 180 MPa, time of 20 min and temperature of 250 °C in ambient air. No molten phase is involved in the bonding process. Scanning Electron Microscopy showed that the Ni MCAs have been effectively embedded into Ag layer without voids and intermetallic compounds (IMC). Thus, reliability is enhanced. Transmission Electron Microscopy results demonstrated a sufficient insertion and atomic scale bonding between Ni MCAs and Ag. This work is expected for extensive practical application.