Heat-resistant reliability of large-area silver microporous connections for direct cooling in power inverter applications

Despite the importance of developing direct cooling technologies for power modules and inverters, especially the method of attaching a completed power module to a heat sink using large-area solder, it has been a controversial topic for decades between power modules and inverters due to their technological implementation difficulty. It is because of the re-melting of the die attachment solder of power semiconductors and distortion due to the multiple soldering peak temperatures. Therefore, in the industry, there is a pressing need for a technology that ensures the quality of power modules by using a direct cooling bonding process. This process must operate at a temperature lower than the melting point of the die attachment to prevent re-melting issues inside the power module and it must achieve bonding at a low temperature without applying pressure. In this study, it is implemented that Ag sintering between electroplated Nil Au finished Al substrates and Cu dies in a direct cooling structure with large areas at a low temperature of 175°C without pressure and the reliability and bonding strength were tested by thermal shock and shear test. Therefore, it succeeded in elucidating the fracture mechanism related to reactivity and diffusivity between Ag-sintered layers, substrates, and dies in large areas. The main key is related to the disconnected interface between sintered Ag layer and lower layer, which means delamination of Ag sintered layer from <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\text{Ni}/\text{Au}$</tex> finished Al substrate due to diffusion during thermal shock tests.