Laser curing of inks for plastic electronic applications

Fu, Liwei
Laser curing of inks for plastic electronic applications. PhD thesis, University of Liverpool.

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The development of the plastic electronics industry has drawn great interest and inspired technology innovations in a broad area. This has stimulated the rapid development of flexible circuitry manufacturing technologies, including advances in conductive inks, printing technology and most importantly the novel curing technology - laser based curing (or Laser Direct Write). This has the ability to replace the conventional environmentally damaging and time consuming chemical etching method in current Printed Circuit Board (PCB) manufacturing. The work presented in this thesis is an investigation into a frequency doubled Nd:YAG laser curing process of epoxy-based micro-sized particulate silver inks. This 532nm laser curing process filled the gap as no research reported for solidifying conductive microparticle silver ink using this particular laser wavelength at 532nm. This 532nm laser curing process also extended the curing technology with a fast localized heating process. The composition of the epoxy-based conductive silver ink was studied in this investigation. The laser wavelength of 532nm was selected as the silver microparticles can absorb the laser energy more efficiently without the risk of damaging the material compared to infrared wavelength. Liquid-phase epoxy-based particulate silver inks deposited on flexible substrates were irradiated by laser beam at the wavelength of 532nm. This produced a smooth and cured ink with an effectively reduced electrical resistivity. A new laser curing mechanism theory was proposed based on the presented experimental research. 532nm has shown benefits in protecting the flexible substrate used from thermal damage, owing to the high transmittance of the wavelength through the substrate material. Unlike massive solvent evaporation observed in CO2 laser curing at 10.6µm, laser curing at 532nm, transported the solvent component by expelling solvent liquid from the ink system as a result of a radical change in solvent dynamic viscosity at an increased temperature and the molecular excitation followed by the Marangoni effect. Chemical cross-linking reactions to resin system were evidenced by Fourier Transform Infrared Spectroscopy (FTIR), resulting in a fully cured ink with reduced electrical resistivity. Epoxy-based silver ink's physical properties such as density, thermal conductivity were mathematically defined based on a new temperature evolution for use in a 3-D finite element (FE) modelling. A Time-dependent solver was chosen for modelling the thermal field in a 532nm laser curing process of epoxy-based conductive silver ink within COMSOL Multiphysics 4.3b. The modelling results were compared to the experimental thermal images for FE model validation. The impact to laser curing results by changing the absorption of the epoxy-based conductive silver ink was investigated in this FE model.

Item Type: Thesis (PhD)
Additional Information: Date: 2014-09 (completed)
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Depositing User: Symplectic Admin
Date Deposited: 27 Aug 2015 13:35
Last Modified: 17 Dec 2022 01:27
DOI: 10.17638/02005999