Arresting High-Temperature Microstructural Evolution inside Sintered Silver

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

Arresting high-temperature microstructural evolution inside sintered silver

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

Electromigration Phenomena in Sintered Nanoparticle Ag Systems Under High Current Density

Electromigration (EM) refers to the movement of atoms inside a conductor due to momentum exchange with the conduction electrons. In this work the EM effect in samples of porous Ag fabricated from nanoparticles of Ag in a pressure free sintering process is studied. Current densities of 2.5×104 − 1.7×105 A/cm2 were applied to the samples for periods ranging up to 500 h. In a typical EM setup with a non-porous conductor, void formation occurs at the cathode and hillock formation at the anode. In this study, voids were not directly observed, but cracks were formed after prolonged electromigration, presumably as a result of void accumulation and coalescence. When the samples were placed in 150 °C ambient no hillocks were observed, but at room temperature nanorods were formed with sizes ranging up to 20 μm in length, typically 25 nm in diameter and with aspect ratios ranging from 20 to 1000. It was found that interrupting and restarting the current resulted in growth of new nanorods rather than growth of existing ones, and that growth was limited by welding of individual nanorods when a critical number density was reached. While similar nanorods have been formed from Ag thin films using thermal stress , the location of nanorods was unusual in that while the number density was highest at the anode, significant numbers also appeared at central and cathode locations. Another unusual feature of the observed EM was that the initial porous structure became refined with coarse pores and grains transforming into a fine grained and fine pored structure with elongated and locally orientated pores and grains. Elemental composition studies provide tentative understanding of the nanorod number density, size distribution and growth mechanism. In the geometry utilized for this study, temperature gradients are known to strongly influence the divergence of the EM induced atomic flux and hence resistivity measurements and COMSOL Finite Element modelling was used to determine the temperature in the sample taking into account joule heating, convection and conduction processes.</jats:p