LAUSR

Optimizing oxygen levels in a sintering atmosphere seems to be a more economical method to realize die-attachment on a bare copper substrate with silver paste. In this article, the effects of oxygen content on the mechanical and thermal performance of sintered Ag joint on the bare copper substrate were investigated. The $5\times 10^{3}$ ppm was demonstrated as the most appropriate oxygen content for the optimum bare copper joint, which removed most organics in the paste layer while preventing the copper substrate from oxidation. Besides, the bonding mechanism of joints sintered under different oxygen levels was clarified. For joint sintered at $1\times 10^{3}$ ppm, excess residual organics significantly facilitated the formation of hydrogen bonds, resulting in the adhesive-dominated failure mode and worst overall performance. The increase of oxygen content to $5\times 10^{3}$ ppm reinforced the bonding strength due to the Ag–Cu metallic bond formation at the interface and ameliorative densification of the sintered-Ag layer. As the amount of oxygen further increased, the copper substrate was oxidized, and bonding strength experienced a changing course of decline, rise, and flat, which can be attributed to Ag adhesion on the copper surface and the formation of voids at the interface between the oxide layer and copper substrate.