Cu-Cu joining using citrate coated ultra-small nano-silver pastes

Abstract In this study, ultra-small nano-Ag particles with an average diameter of 4.76 nm and excellent stability have been synthesized on a large scale to meet low-temperature, low-pressure, and lead-free packaging requirements for modern high-power electronic applications. Citrate coated nano-Ag particles’ agglomerations have been used in the form of a nano-Ag paste to solder by a robust Cu to Cu direct bonding. The bonding strength of the formed Cu-Cu joint reaches 28.2 MPa when sintered at 260 °C for 30 min and 1 MPa. The bonding mechanism of such a direct Cu-Cu bonding by using nano-Ag paste has been facilitated by the assistance of an in-situ sintering of nano-Ag particles. The analysis of the bonding has been performed using a transmission electron microscopy (TEM) and a focused ion beam (FIB). The results show that the bonding mechanism can be interpreted by two parts: (1) self-sintering process of nano-Ag paste, and (2) interfacial bonding between sintered nano-Ag paste and Cu substrate.