Bonding performance of sintered nanosilver joints on bare copper substrates with different grain structures

Nanosilver paste as a state-of-the-art die-attach material for power electronics has been widely used in bare copper bonding. In this paper, bonding performance of sintered nanosilver joints on bare copper substrates with different grain structures was investigated. The bare copper joint with optimum bonding performance was obtained with copper substrate possessing smallest grain size (1.88 mu m), strongest (001) texture and largest surface roughness (750.31 nm). It was thought copper substrate with smaller grain size provided more grain boundaries to facilitate the diffusion rate of Ag atoms on the surface of copper substrate and into the substrate, further leading to the greater contact area and longer inter-diffusion band to achieve stronger interfacial bonding. Meanwhile, faster diffusion at the interface did not interfere with the densification of sintered Ag layer. Therefore, the stronger interfacial bonding and well-densified Ag layer strengthened the sintered joint. It was also found (001) orientation could possibly be the most contributing to bonding strength, since the decrease of relative texture coefficient of (200) coincided with the reduced strength. Besides, rougher surface of copper substrate with smaller grain size after polishing could be another factor to improve bonding strength due to the larger contact area and mechanical interlocking.