Microstructural evolution of sintered silver at elevated temperatures

• High temperature behaviour of sintered silver nanoparticles has been observed.
• Silver microstructure evolves unpredictably at high temperatures.
• This unpredictability worsens long term reliability.
• Exposure to atmosphere blocks surface diffusion up to temperatures of 400 °C.
• Grain growth activation energy of sintered silver has been measured.

Reduction in the sintering temperature of metal powders by lowering particle size into the nanoparticle range has resulted in a new class of porous sintered joining materials. Especially promising are sintered silver based materials which can be used to form bonds between wide-bandgap semiconductor die and circuit boards for use in high temperature applications. This work shows that for these materials the exterior sintered silver surface oxidizes preventing surface morphology changes, while the interior pore surfaces of the porous silver remain largely oxide-free. These pore surfaces facilitate fast atomic movement resulting in grain growth and changes in the internal microstructure. Morphology changes in the temperature range 200–400 °C are presented both as statistical averages of grain size and, uniquely in this type of study, by tracking individual pores and grains. It is shown that the internal structure will undergo changes during high temperature storage in contrast to the stable outer surface. A new technique, utilizing the electromigration effect to check the relative surface mobility of atoms in the interior pores and exterior surfaces was used to support the conclusions deduced from thermal ageing experiments. Finally, we speculate that the stability of the exterior surface could be reproduced in the interior if the chemistry of the paste was altered to allow formation of a passivating layer on the interior pores during the final stages of the sintering process, resulting in formation of a stable die attach material for applications of up to 400 °C, for which there is an urgent need.

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