Novel solid-state bonding for direct cooling of power inverter parts based on partially phase transition and diffusion intermetallic controlling: Experimental, FEM and CFD, material thermodynamic calculation investigation

Replacing conventional non-metallic thermal interface materials (TIM) with direct metal bonding in power inverters can significantly enhance power density by reducing the junction temperature (T_j) and thermal resistance (R_th). This study developed a large-area solid-state bonding technology to prevent re-melting of power semiconductor die bonding, enabling direct cooling of power inverters. Sn-3wt%Ag-0.5wt%Cu (SAC305) sheets were bonded in the solid state, with bonding performance evaluated as a function of temperature (210–220 °C) and sheet thickness (100–200 μm). Thermodynamic calculations guided the control of interfacial intermetallic compound (IMC) formation, achieving a uniform AuSn₄ phase with superior thermal and mechanical properties. Junction temperatures under direct and indirect cooling were analyzed using computational fluid dynamics (CFD) simulation, revealing a 10 °C reduction in operating temperature when we use direct cooling. Results of finite element analysis (FEA) represent that thermal stresses based on operating temperature experienced by the direct cooling layer are also significantly lower than those of TIM. Experimental results demonstrated uniform, large-area bonding under pressure below the melting point of solders, achieving significantly reduced thermal resistance and improved bonding strength. These findings underscore the potential of solid-state bonding for advancing direct cooling technology, providing improved thermal management, enhanced reliability, and increased power density, paving the way for practical adoption in high-performance power electronics.