Surface Roughness-Controlled Sinter-Bonding of Binder-Free Ag Preforms Consolidated Directly from Ag Nanoparticles

Ag sinter-bonding is the mainstream die-attach technology for SiC power modules; however, organic residues from pastes and binder-containing sheets can degrade the thermal conductivity and reliability of the bond line. In this study, a binder-free porous Ag preform was fabricated by direct hot pressing of in-house-synthesized, quasi-spherical Ag nanoparticles (70.2 nm) at 200 °C for 1 min under a pressure of 10 MPa to serve as the interlayer. To evaluate its performance, the preform was sandwiched between the Ag-electroplated Cu chips and substrates, and their sinter-bonding behavior was evaluated at 250 °C under a chip-compatible pressure of 10 MPa in air. The surface roughness of the preform, controlled by lapping, was found to be a key factor governing the bonding performance. A precision-lapped preform (Ra = 35.8 nm), smoother than the electroplated Ag layers, allowed the asperities of the counterparts to be plastically flattened under pressure, maximizing the real interfacial contact area. Consequently, the shear strength reached 28.1 MPa after only 1 min and increased to 57.6 MPa after 3 min, with the interfacial connection ratio increasing from 62.9% to 82.5% and the bond-line porosity decreasing to 10.4%. TEM and HAADF-STEM analyses verified that oxide- and void-free metallurgical bonding was formed by the interdiffusion of Ag atoms across the preform/Ag layer interface. Fractographic analyses confirmed cohesive failure with highly stretched metallic ligaments, corroborating the formation of sound bond lines. The developed nanoparticle-derived preform is a promising die-attach solution that combines high-temperature reliability with rapid low-pressure processability.