Microstructural evolution of sintered silver at elevated temperatures

Abstract

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 2 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.

Ultra-Stable Sintered Silver Die Attach for Demanding High-Power/Temperature Applications

“…As can be seen from Figure 4 (c and e) as schematic representations of the likely processes, this effect can be explained by the fact that while silver atoms inside pure silver can migrate and diffuse easily via surface diffusion, the oxidation has the effect of immobilising and pinning the silver atoms and stabilizing the structure up to the decomposition temperature of Ag2O, which has also been observed and confirmed previously through direct exposure to atmosphere [18]. Although calculations of morphology changes driven by surface self-diffusion on pure silver are orders of magnitude faster than those observed experimentally [9], reduction of surface diffusion coefficients in the presence of organic residues from the original paste and partial oxidation can explain the discrepancy.…”

“…The exterior of the air exposed sample is seen to be immediately stabilized but not the interior microstructure [9], which remains stable only after the oxidizing treatment. This is shown in Figure 4 where the microstructural stability seen in sample set 2 is seen to occur throughout the sample cross-section only in the treated case and significant grain growth can be observed for the untreated sample.…”

“…The first two sample sets were sintered on a glass slide and under a coverslip in order to allow continuous optical observations of microstructure evolution without exposure to air during high temperature ageing. It has been shown previously that the cover-slip does not affect the microstructural evolution of sintered silver [9], making this technique ideal for exploring continuous evolution inside sintered silver at high temperatures. Ageing was carried out by placement on a cartridge heater or inside an oven at high temperature to monitor the changes to the microstructure.…”

“…The optical images from these samples were then analysed using ImageJ 1.46 and the Matlab® image processing toolbox to investigate the microstructural changes of sintered silver grains. Refer to references [9,14] for further information on the image processing techniques utilised and algorithms for extracting grain size, which references also include further information regarding the benefits and implications of observations of sintered silver microstructure through a cover-slip.…”

Arresting high-temperature microstructural evolution inside sintered silver

“…Sintered silver goes through massive grain structure changes at temperatures above 200 ºC, introducing uncertainty regarding the suitability of sintered silver for high temperature application due to evolution of material parameters in service and reduced shear strength [6] [7]. One attempt to increase the thermal stability of sintered silver uses a gold mesh interposer [8], and another utilizes addition of SiC [9].…”