Effect of Bonding Pressure and Joint Thickness on the Microstructure and Mechanical Reliability of Sintered Nano-Silver Joints

Sintered nano-silver is widely investigated as a die-attach material for next-generation power electronic modules due to its high thermal conductivity, favorable electrical performance, and stability at elevated temperatures. However, how bonding pressure and joint thickness jointly affect densification, interfacial diffusion, and mechanical reliability has not been systematically clarified, especially under the low-pressure conditions required for large-area SiC and GaN devices. In this work, nano-silver lap-shear joints with three bond-line thicknesses (50, 70, and 100 μm) were fabricated under two applied pressures (1.0 and 1.5 MPa) using a controlled sintering fixture. Shear testing and cross-sectional SEM were employed to evaluate the relationships between microstructural evolution and joint integrity. When the bonding pressure was increased from 1.0 to 1.5 MPa, more effective particle rearrangement and reduced pore connectivity were observed, together with improved metallurgical bonding at the Ag–Au interface, leading to a strength increase from 15.3 to 28.2 MPa. Although thicker joints exhibited slightly higher bulk relative density due to greater heat retention and accelerated local sintering, this densification advantage did not lead to improved mechanical performance. Instead, the lower strength of thicker joints is attributed to a narrower Ag–Au interdiffusion region, which limited the formation of continuous load-bearing paths at the interface. Fractographic analyses confirmed that failure occurred predominantly by interfacial delamination rather than cohesive fracture, indicating that the reliability of the joints under low-pressure sintering is governed by the quality of interfacial bonding rather than by overall densification. The experimental results show that, under low-pressure sintering conditions (1.0–1.5 MPa), variations in bonding pressure and bond-line thickness lead to distinct effects on joint performance, with the extent of Ag–Au interfacial interaction playing a key role in determining the mechanical robustness of the joints.