Low Temperature Joining of Copper by Ag Nanopaste: Studies on the Strength Behavior

Nanoparticles exhibit a decrease in sintering and melting temperature with decreasing particle size. The utilization of this effect is of great interest for joining at low temperatures. First, the paper identifies possible applications for joining and their significance. Furthermore, a commercially available Ag nanopaste is characterized with respect to its thermal properties and strength behavior. Subsequently, it is qualified for joining at low temperatures. The shape, distribution and size of the nanoparticles are determined using transmission electron microscopy (TEM). The thermal behavior is characterized by differential scanning calorimetry (DSC) and thermogravimetry (TG). Furthermore, first examinations of strength properties were executed for the base material copper in order to characterize the influence of different process parameters on joining strength. The analyses show that the nanopaste has a great potential for joining at low temperatures. It is verified that the organic shell of the nanoparticles oxidatively decomposes at temperatures of about 410 °C followed by a sintering process. The sintered microstructure exhibits the thermal properties of bulk silver. It is also shown, that it is possible to produce joints at even lower temperatures of about 300 °C, whose thermal resistance and joint strength is significantly superior to conventional soldered joints.