Microstructure evolution during 300°C storage of sintered Ag nanoparticles on Ag and Au substrates

•Shear strength of pressure-free sintered Ag found to increase during ageing at 300°C on Ag substrate.•Rapid collapse of void number density after 24h ageing in the sintered Ag layer.•Higher porosity at edge of joint compared to the middle.•Shear strength of pressure-free sintered Ag decreases durin...

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Bibliographic Details
Published in:	Journal of alloys and compounds Vol. 617; pp. 994 - 1001
Main Authors:	Paknejad, S.A., Dumas, G., West, G., Lewis, G., Mannan, S.H.
Format:	Journal Article
Language:	English
Published:	Kidlington Elsevier B.V 25-12-2014 Elsevier
Subjects:	Condensed matter: structure, mechanical and thermal properties Exact sciences and technology Grain boundaries Microstructure Nanoscale materials: clusters, nanoparticles, nanotubes, and nanocrystals Nanostructured materials Physics SEM Sintering Structure of solids and liquids; crystallography TEM Grain boundaries SEM Microstructure Nanostructured materials TEM Sintering Grain size Gold Tilt boundaries Shear strength Grain boundary migration Inclination Interfaces Nanoparticles Silver Time dependence Twin boundaries Porosity
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Summary:	•Shear strength of pressure-free sintered Ag found to increase during ageing at 300°C on Ag substrate.•Rapid collapse of void number density after 24h ageing in the sintered Ag layer.•Higher porosity at edge of joint compared to the middle.•Shear strength of pressure-free sintered Ag decreases during ageing at 300°C due to high porosity layer growth.•Void free layer and high porosity layer growth explained in terms of atomic diffusion and grain boundary migration. A silver nanoparticle based die attach material was used in a pressure free process to bond 2.5mm square Ag plated Si die to Ag and Au plated substrates. The assemblies were stored at 300°C for up to 500h and the morphology of the sintered Ag and the shear strength were monitored as a function of time. On Ag substrate it was found that die shear strength increased and that the Ag grains grew in size and porosity decreased over time. There was also a clear difference in morphology between sintered Ag at the die edge and centre. On Au substrate, it was observed that the initially high die shear strength decreased with storage time and that voids migrated away from the Ag/Au interface and into the Ag joint. This has led to the formation of a void free layer at the interface followed by a high porosity region, which weakened the joint. The microstructure reveals a high density of grain and twin boundaries which facilitate the Ag and Au atomic diffusion responsible. The grain structure of the plated Au led to diffusion of Au into the Ag via high-angle tilt grain boundaries, and grain boundary migration further dispersed the Au into the Ag layer.
ISSN:	0925-8388 1873-4669
DOI:	10.1016/j.jallcom.2014.08.062