Highly mechanical and high-temperature properties of Cu–Cu joints using citrate-coated nanosized Ag paste in air

Today, a growing number of third-generation semiconductor-based power devices are used in products that can continuously operate at high temperatures for extended periods of time. Hence, traditional tin-lead and lead-free solders are no longer suitable for modern electronic packaging. A common method is to apply Ag paste for bare Cu–Cu joints under an inert or reductive atmosphere. In this study, the citrate-coated nanosized Ag paste was utilized to generate robust bare Cu–Cu joints under atmospheric conditions. The average size of citrate-coated Ag particles was approximately 4.76 ​nm after being cleaned by deionized water and acetone. The effects of process parameters, such as cleaning, joining temperature, holding time, and joining pressure, on the mechanical properties of the bare Cu–Cu joints were thoroughly investigated. Increasing washing and joining temperatures resulted in a shear strength increase of up to 28.2 ​MPa ​at a joining temperature of 260 ​°C after seven washes. In addition, a holding time of 30 ​min and a joining pressure of 1 ​MPa were selected as optimal process conditions for the application of citrate-coated nanosized Ag paste onto bare Cu–Cu joints. The newly developed Cu–Cu joints showed excellent thermal stability at 150 ​°C using the citrate-coated nanosized Ag paste. After long-term aging, the joints exhibited stability at 250 ​°C for 144 ​h, indicating a good high-temperature reliability for three-dimensional integrated circuits (3D ICs) fabricated under atmospheric conditions.