Arresting high-temperature microstructural evolution inside sintered silver
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The surface oxidation of internal pore surfaces of nano-scale sintered silver has increased stability for high temperature applications. Operating temperatures of up to 400 °C have resulted in no or minimal changes in microstructure. By contrast, it is known that the microstructure of untreated pressure-less sintered silver continuously evolves at temperatures above 200 °C, grain and pore growth resulting in microstructure coarsening and increased susceptibility to fatigue. Oxidation of the internal pore surfaces has been shown to freeze the microstructure when the contact metallization is also silver or chemically inert. Samples exhibited no change in microstructure either through continuous observation through glass, or after cross sectioning. The tested specimens under high temperature storage resisted grain growth for more than 1000 h at 300 °C. The oxidising treatment can be performed via many different routes. For example, exposure to steam, or even by dipping in water for 10 min followed by immediate high temperature exposure and the effectiveness of these varying treatments is assessed. In this work we explore the mechanism that causes stabilization and explore the hypothesis that oxidation prevents grain boundary movements by arresting the fast migration of atoms along the internal pore surfaces. Analysis of the surface structure of the sintered silver by X-ray photoelectron spectroscopy shows presence of silver oxide (Ag2O) and computer simulation of grain boundary movements confirm the presence of a barrier to atomic movement on the internal silver surfaces. These findings are very promising for potential applications of sintered silver as a die attach material for High Temperature electronics packaging.
The authors would like to thank for Ernest Samuel and Patrick Bergstrom Mann and J. Greenberg for their help during the experiments and the XPS Centre (NEXUS) at Newcastle University.
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