A. James Clark School of Engineering

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    Direct visualization of nanoparticle morphology in thermally sintered nanoparticle ink traces and the relationship among nanoparticle morphology, incomplete polymer removal, and trace conductivity
    (Institute of Physics, 2023-06-19) Chandel, Ghansham Rajendrasingh; Sun, Jiayue; Etha, Sai Ankit; Zhao, Beihan; Sivasankar, Vishal Sankar; Nikfarjam, Shakiba; Wang, Mei; Hines, Daniel R.; Dasgupta, Abhijit; Woehl, Taylor; Das, Siddartha
    A key challenge encountered by printed electronics is that the conductivity of sintered metal nanoparticle (NP) traces is always several times smaller than the bulk metal conductivity. Identifying the relative roles of the voids and the residual polymers on NP surfaces in sintered NP traces, in determining such reduced conductivity, is essential. In this paper, we employ a combination of electron microscopy imaging and detailed simulations to quantify the relative roles of such voids and residual polymers in the conductivity of sintered traces of a commercial (Novacentrix) silver nanoparticle-based ink. High resolution transmission electron microscopy imaging revealed details of the morphology of the inks before and after being sintered at 150 °C. Prior to sintering, NPs were randomly close packed into aggregates with nanometer thick polymer layers in the interstices. The 2D porosity in the aggregates prior to sintering was near 20%. After heating at 150 °C, NPs sintered together into dense aggregates (nanoaggregates or NAgs) with sizes ranging from 100 to 500 nm and the 2D porosity decreased to near 10%. Within the NAgs, the NPs were mostly connected via sintered metal bridges, while the outer surfaces of the NAgs were coated with a nanometer thick layer of polymer. Motivated by these experimental results, we developed a computational model for calculating the effective conductivity of the ink deposit represented by a prototypical NAg consisting of NPs connected by metallic bonds and having a polymer layer on its outer surface placed in a surrounding medium. The calculations reveal that a NAg that is 35%–40% covered by a nanometer thick polymeric layer has a similar conductivity compared to prior experimental measurements. The findings also demonstrate that the conductivity is less influenced by the polymer layer thickness or the absolute value of the NAg dimensions. Most importantly, we are able to infer that the reduced value of the conductivity of the sintered traces is less dependent on the void fraction and is primarily attributed to the incomplete removal of the polymeric material even after sintering.
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    Development of a Shifting Melting Point Ag-In Paste Via Transient Liquid Phase Sintering for High Temperature Environments
    (2008-07-18) Quintero, Pedro; McCluskey, Patrick; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    The increasing demand for electronic devices capable of operating at temperatures above the traditional 125°C limit is driving major efforts in research and development. Devices based on wide band gap semiconductors have been demonstrated to operate at temperatures up to 500°C, but packaging still remains a major hurdle for product development. Recent regulations, such as RoHS and WEEE, increase the complexity of the packaging task as they prohibit the use of certain materials in electronic products such as lead (Pb), which has traditionally been used in high temperature solder attach. The successful development of new attach materials and manufacturing processes will enable the realization of next generation products capable of operating reliably at elevated temperatures. In this investigation a shifting melting point silver (Ag) - indium (In) solder paste that uses a Transient Liquid Phase Sintering (TLPS) process was developed. This novel material and manufacturing process constitutes a major advancement over the conventional soldering process temperature hierarchy, in which the maximum allowable application temperature is limited by the melting point of the attach material. By virtue of a shifting melting temperature, which results from isothermal solidification during the TLPS process, this attach material can be processed at a relatively low temperature while being capable of sustaining much higher temperatures in use, limited only by its new melting point. In order to develop an empirical kinetics model of the Ag-In TLPS process, a design of experiments (DOE) was used to study the effect of multiple factors on the solidification reaction. These factors include particle size, weight fraction of solute, heating rate, holding time, and processing temperature. The physical implications of the empirical model were confirmed by constructing a diffusion based mechanistic model. Pivotal microstructural information was obtained from metallographic analysis where a transition from an In-rich matrix to an Ag-rich solid solution was observed. The metallographic characteristics, mechanical strength, and electrical conductivity of the resulting Ag-In TLPS material were assessed. This study has resulted in the creation of a novel attach material and method that will enable future development of electronic packaging for high temperature environments. The quantitative description of the reaction kinetics during the TLPS process provided a valuable tool for future development and an optimization of this system.