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Low temperature die attach based on sub-micron Ag particles and the high temperature reliability of sintered joints

The rapid development of high temperature SiC power devices has faced a huge challenge to resolve the reliability of devices at high temperature operating environments due to the damage of die attach solder. Ag particles, particularly, Ag nanoparticles have been used to joint these SiC devices because these Ag nanoparticles can be sintered to form a continuous bulk-like network at temperatures far below the melting point of bulk Ag, which greatly reduces manufacturing costs and simplifies the bonding conditions. However, the complex sintering profile, the large volumes of volatilization of organic compounds, the high pressures requirement and their high cost have limited the application of Ag nanomaterials.

In this work, a simple and large-scale two-steps polyol process was used to synthesize submicrometer Ag particles with tailoring sizes, which were applied for sintering joining. The size and size distribution of Ag particles depended on the reaction temperature and determined the shear strength of joints. A shear strength of over 40 MPa were successfully achieved using the Ag paste with the particles ranging from 230 to 800 nm in diameter due to micron-sized connection among these Ag sub-micron particles at low temperature sintering process. The result and joint properties are superior to those obtained with nanoparticle Ag pastes and suggests that the sub-micron Ag particles is a promising alternative for high power die-attach. These joints also have been used to demonstrate the good reliability at high temperature environments. After aging at 250 °C for 500h, shear strength over 30 MPa has been remained. Moreover, the hybrid paste of the Ag sub-micron particles and Ag flake achieved high shear strength of 40 MPa during thermal cycles ranging from -40 and 250 °C for 400 cycles.