Mechanism of Ultrasonic-Assisted Sintering of Cu@Ag NPs Paste in air for High-Temperature Power Device Packaging

We successfully synthesized Cu@Ag NPs paste, and they were successfully applied for joining Cu/Cu@Ag NPs paste/Cu firstly in air by the ultrasonic-assisted sintering (UAS) at a temperature of as low as 160 °C. Their sintered microstructures featuring with dense and crystallized phases are completely different with the traditional thermo-compression sintering. The shear strength of the joints reached to 54.27 MPa, exhibiting one order of magnitude higher than thermocompression sintering (TCS) at the same temperature (180 °C). This ultra-low sintering temperature and high performance of the sintered joints were ascribed to ultrasonic effects and systematically investigated. The ultrasonic vibrations had distinct effects on the metallurgical reactions of the joints, resulting in the contact and growth of Cu core and the stripping and connection of Ag shell, which contributes to the high shear strength. For the TCS, the sintering process can be characterized by two stages. Stage I includes the neck formation and growth among Ag layers through surface diffusion. This initial sintering of NPs does not require much thermal activation, so it can be obtained at a low temperature with weak bonding and the sintering rate is very low because of thinner Ag shells on Cu@Ag NPs. With the increase of temperature, part of core-shell nanoparticles would loss the shell and the Cu cores with fresh surfaces would be exposed and contact with others, leading to the occurrence of the Cu NPs sintering, which would step into the Stage II. Because of lower input energy, this Cu NPs sintering took place very limit. However, regarding to the UAS, due to overwhelming effects caused by the application of ultrasonic, the "dewetting" of Ag shells appears since the sharp increase of temperature, resulting in that the sintering process quickly reaches to the stage II. In this stage, the thin Ag shells shrink into tiny nodules on the Cu core surface, leading to fast sintering of the NPs. Because the tiny Ag nodules have extremely large surface to volume ratio, the starting temperature of sintering dramatically decreases based on the thermodynamic theory. Such low temperature sintering in air of Cu@Ag NPs to form die attachments with robust performances can be successfully applied in the fields of high power, auto electronics and harsh environmental device packaging.