Study of Sintered Nano-Silver Die Attachment Materials Doped with Indium

Abstract

Sintered nanosilver paste die attachment is an emerging technology for wide bandgap semiconductors. Apart from the excellent electrical connection and thermal dissipation abilities of the sintered joint, this technology also features its low bonding temperature below 300 °C and high working temperature up to 700°C. However, this technique usually comes up with some problems if we avoid using high bonding pressure, such as high porosity in sintered-silver joint and a weak interface between sintered-silver and bare copper substrates. Besides, the tarnishing problem of silver element under sulfur environment is another serious concern for practical application. Thus, we propose a method by doping indium in the sintered nanosilver joint to aim at solving these problems. In our study, we placed an indium foil between two nanosilver pastes, which had already stencil-printed on two copper substrates. With the peculiarity of low melting point of indium at 156°C, the foil could melt at the beginning of the bonding process. The molten indium then began to diffuse and fill into the pores between silver nanoparticles, and reacted with silver to form silver-indium intermetallic. Eventually, the whole joint became a silver-indium joint, which has the melting temperature above 670 °C, and the porosity inside the joint became lower. In addition, once the indium reached the copper substrate, it presented a great wettability to it. It would form a thin layer of copper-indium intermetallic at the interface, which led to a better connection with copper substrate than pure sintered-silver joint did. In this paper, three different thicknesses of nanosilver pastes and indium foils were used to discuss the propagation of indium diffusing into the pores between silver particles. We found it necessary to transform all the silver into Ag2In by bonding with the thickness ratio of paste and foil smaller than 2.5:1 in our bonding condition. Within this thickness ratio, we are confident of producing a full-Ag2In joint with low porosity and better interface connection. On the other hand, with the thickness ratio bigger than 2.5:1, indium was inadequate to transform the whole joint into Ag2In. Therefore, indium cannot reach to the copper substrates. Instead, it was the remaining pure sintered-silver that connected to the copper substrate, resulting in a poor wettability and bondability at the interface. Furthermore, this joint has also passed the thermal aging tests at 200 °C after 100 h and 500 h. To sum up, this method of improving sintered nanosilver die attachment by doping with indium can assist in lowering high porosity, providing better interfacial connection with copper substrates and a potential of anti-tarnishing property while remaining high melting temperature. It is a promising die attachment material for high temperature, high power applications such as automobile power IC.