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Comparative study of how additives affect sintered silver microstructure in die-attach applications
Keywords: die-attach, sintered-silver, additives
In the electronics industry, there is a requirement for lead-free interconnection materials which can survive at high temperatures (200 °C or higher) in order to support power electronics and other high temperature electronics applications. Nanoparticle sintering is of interest because the sintering temperature can be brought down to low temperatures, typical of conventional soldering processes, even when the melting temperature of the bulk material is over 1000 K. Sintering of Ag nanoparticle is of particular interest due to its high thermal and electrical conductivity and lack of tenacious oxides on nanoparticle surfaces which promotes ease of sintering. However, it is rare that a material composed of a single element contains the optimal properties required of an engineering material. As well as bond strength and ease of processing, stability at elevated temperature, stability during thermal cycling, resistance to ion migration, thermal conductivity etc. all need to be considered. To help meet these objectives, additives are typically used to engineer better performance. Additional complications that arise when considering additives for sintered materials include the fact that the lack of a liquid phase means that not only the chemical make-up, but also the morphology of the additive needs to be optimised. Features of the added particle’s original morphology will generally be retained in the final sintered structure. Therefore the range of possible additives is unusually large, and although considerable work has been done on exploring additives (notably Cu particles [Woo 2009], hybrid Ag mixtures comprising nanoparticles and micro particles or nanoparticles and nanorods [Chen 2018 , Peng2012]), the vast parameter space has only been sparsely sampled. In this study, a commercially available Ag nanoparticle paste has been used as the base, and a range of additives have been added principally to determine the effect of each additive on the sintered microstructure immediately after sintering, and after long term thermal ageing. The additives include both passivated particles (e.g, Ti) and particles that are expected to react chemically with the silver matrix (Cu, Zn), as well as a variation on Ag morphology - Ag flakes. One common feature of the additives is that they tend to be cheaper than the starting silver nanoparticles and hence can potentially lower material costs even if the improvement in mechanical, thermal and electrical properties is minimal. The additives were added to the existing commercial Ag nanoparticle paste by mechanical stirring and sintering was performed in air. In order to investigate the effects of these additives on the microstructure of the sintered Ag, samples were cross-sectioned and examined by optical and by scanning electron microscopy. The as-sintered structure between Ag nanoparticles and addictive nanoparticles was observed and elemental diffusion and distribution observed using Energy-dispersive X-ray spectroscopy (EDX). Evolution of the microstructure and further elemental diffusion was monitored after high temperature ageing. Chemical reactions between the additives and the paste were monitored using Differential Scanning Calorimetry.
Ming Lo, Student
King's College London
London, London
UK


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