| 1. |
- Svensson, Martin, 1987, et al.
(author)
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Simulation of Droplet Jetting of a Non-Newtonian Mixed Suspension
- 2016
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In: ICMF-2016 – 9th International Conference on Multiphase Flow May 22nd – 27th 2016, Firenze, Italy.
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Conference paper (peer-reviewed)abstract
- The jet printing of a dense mixed non-Newtonian suspension is based on the rapid displacement of fluid through a nozzle, the forming of a droplet and eventually the break-off of the filament. The ability to model this process would facilitate the development of future jetting devices. The purpose of this study is to propose a novel simulation framework and to show that it captures the main effects such as droplet shape, volume and speed. In the framework, the time dependent flow and the fluid-structure interaction between the suspension, the moving piston and the deflection of the jetting head is simulated. The system is modelled as a two phase system with the surrounding air being one phase and the dense suspension the other. Hence, the non-Newtonian suspension is modelled as a mixed single phase with properties determined from material testing. The simulations were performed with two coupled in-house solvers developed at Fraunhofer-Chalmers Centre; IBOFlow, a multiphase flow solver and LaStFEM, a large strain FEM solver. Jetting behaviour was shown to be affected not only by piston motion and fluid rheology, but also by the energy loss in the jetting head. The simulation results were compared to experimental data obtained from an industrial jetting head.
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| 2. |
- Andersson, Martin, et al.
(author)
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Flowing and pressurizing a solid-liquid two phase monodispersed fluid with high solid content in a transparent microfluidic high-pressure chip
- 2017
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In: 28TH MICROMECHANICS AND MICROSYSTEMS EUROPE WORKSHOP. - : IOP PUBLISHING LTD. ; 922:1
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Conference paper (peer-reviewed)abstract
- Handling highly concentrated solid-liquid two-phase fluids in microfluidics is challenging. In this paper, we present the first studies of flowing solder paste with a high solid content in a transparent high-pressure tolerant glass chip, thereby increasing the understanding of how multiphase liquids with high density difference between the phases behave in small channels (840 mu m in diameter). The system, including a custom made high-pressure, low resistance, interface, was continuously operated at pressures up to of 6 MPa and devices where shown to have pressure tolerance up to 17 MPa. During flow through the chip, the packing density of the solder balls displayed inhomogeneity over the channel where chains of solder balls in contact with each other were formed together with voids. These in-homogeneities persisted along the channel during flow. The flow rate of the paste through the chip oscillated between 63 to 350 mu m/s when pumping at constant volume rate of 30 mu l/min. When a pressure of 2 MPa was applied, the volume of the solder paste particle segment decreased 1.6%, and 0.1% was elastically recovered when the pressure was released. It is concluded that this transparent microfluidic high-pressure glass chip with the special developed interface is suitable for flow studies of solder paste with a high solid content.
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| 3. |
- Khorramdel, Behnam, et al.
(author)
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Inkjet printing technology for increasing the I/O density of 3D TSV interposers
- 2017
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In: Microsystems & Nanoengineering. - : Nature Publishing Group. - 2055-7434. ; 3, s. 17002-
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Journal article (peer-reviewed)abstract
- Interposers with through-silicon vias (TSVs) play a key role in the three-dimensional integration and packaging of integrated circuits and microelectromechanical systems. In the current practice of fabricating interposers, solder balls are placed next to the vias; however, this approach requires a large foot print for the input/output (I/O) connections. Therefore, in this study, we investigate the possibility of placing the solder balls directly on top of the vias, thereby enabling a smaller pitch between the solder balls and an increased density of the I/O connections. To reach this goal, inkjet printing (that is, piezo and super inkjet) was used to successfully fill and planarize hollow metal TSVs with a dielectric polymer. The under bump metallization (UBM) pads were also successfully printed with inkjet technology on top of the polymer-filled vias, using either Ag or Au inks. The reliability of the TSV interposers was investigated by a temperature cycling stress test (-40 °C to +125 °C). The stress test showed no impact on DC resistance of the TSVs; however, shrinkage and delamination of the polymer was observed, along with some micro-cracks in the UBM pads. For proof of concept, SnAgCu-based solder balls were jetted on the UBM pads.
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| 4. |
- Vachaparambil, Kurian J., et al.
(author)
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Rheological characterization of non-Brownian suspensions based on structure kinetics
- 2018
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In: Soldering and Surface Mount Technology. - : Emerald Group Publishing Limited. - 1758-6836 .- 0954-0911. ; 30:1, s. 57-64
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Journal article (peer-reviewed)abstract
- Purpose - The purpose of the paper is to develop a methodology to characterize the rheological behaviour of macroscopic non-Brownian suspensions, like solder paste, based on microstructural evolution. Design/methodology/approach - A structure-based kinetics model, whose parameters are derived analytically based on assumptions valid for any macroscopic suspension, is developed to describe the rheological behaviour of a given fluid. The values of the parameters are then determined based on experiments conducted at a constant shear rate. The parameter values, obtained from the model, are then adjusted using an optimization algorithm using the mean deviation from experiments as the cost function to replicate the measured rheology. A commercially available solder paste is used as the test fluid for the proposed method. Findings - The initial parameter values obtained through the analytical model indicates a structural breakdown that is much slower than observations. But optimizing the parameter values, especially the ones associated with the structural breakdown, replicates the thixotropic behaviour of the solder paste reasonably well, but it fails to capture the structure build-up during the three interval thixotropy test. Research limitations/implications - The structural kinetics model tends to under-predict the structure build-up rate. Practical implications - This study details a more realistic prediction of the rheological behaviour of macroscopic suspensions like solder paste, thermal interface materials and other functional materials. The proposed model can be used to characterize different solder pastes and other functional fluids based on the structure build-up and breakdown rates. The model can also be used as the viscosity definitions in numerical simulations instead of simpler models like Carreau-Yasuda and cross-viscosity models. Originality/value - The rheological description of the solder paste is critical in determining its validity for a given application. The methodology described in the paper provides a better description of thixotropy without relying on the existing rheological measurements or the behaviour predicted by a standard power-law model. The proposed model can also provide transient viscosity predictions when shear rates vary in time.
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