Study on Bonding Strength of Sintered Nano-silver Joints on Bare Copper Substrates with Different Grain Sizes

Nano-silver as a potential die-attach material has been widely investigated for bonding with bare copper substrates or copper substrates with metallization layers. Many previous studies have focused on modifying process parameters to achieve better bonding quality. However, the influence of bare copper substrate itself on bonding strength is overlooked. In this study, we prepared bare copper substrates with different grain sizes by annealing at different temperatures (400°C, 550°C, 700°C). And two heating profiles for nano-silver paste were designed for sintering in air and in formic acid atmosphere, respectively. The effect of the copper substrate's grain size on bonding strength can be concluded as bonding strength decreases with the increasing grain size, which was attributed to the fact that the copper substrate with smaller grain size has more grain boundaries to allow the diffusion of silver nanoparticles into substrate to form better bonding joint. This study may provide the basis for a new method to achieve high bonding quality of the sintered joint with nano-silver paste by reducing the grain size of the substrate.