The increasing development of flexible and printed electronics has fueled substantial advancements in selective laser sintering, which has been attracting interest over the past decade. Laser sintering of metal nanoparticle dispersions in particular (from low viscous inks to high viscous pastes) offers significant advantages with respect to more conventional thermal sintering or curing techniques. Apart from the obvious lateral selectivity, the use of short-pulsed and high repetition rate lasers minimizes the heat affected zone and offers unparalleled control over a digital process, enabling the processing of stacked and pre-structured layers on very sensitive polymeric substrates. In this work, the authors have conducted a systematic investigation of the laser sintering of micro-patterns comprising Ag nanoparticle high viscous inks: The effect of laser pulse width within the range of 20–200 nanoseconds (ns), a regime which many commercially available, high repetition rate lasers operate in, has been thoroughly investigated experimentally in order to define the optimal processing parameters for the fabrication of highly conductive Ag patterns on polymeric substrates. The in-depth temperature profiles resulting from the effect of laser pulses of varying pulse widths have been calculated using a numerical model relying on the finite element method, which has been fed with physical parameters extracted from optical and structural characterization. Electrical characterization of the resulting sintered micro-patterns has been benchmarked against the calculated temperature profiles, so that the resistivity can be associated with the maximal temperature value. This quantitative correlation offers the possibility to predict the optimal process window in future laser sintering experiments. The reported computational and experimental findings will foster the wider adoption of laser micro-sintering technology for laboratory and industrial use.

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Recent advances in flexible electronics have highlighted the importance of high throughput, digital additive microfabrication techniques. In this work, we demonstrate the combination of laser printing and laser sintering of a novel copper nanoparticle ink onto flexible substrates in order to produce oxide free conductive copper patterns in ambient atmospheric conditions. The printed patterns exhibit high reproducibility, very low resistivity (about 2x bulk), and negligible oxidation according to Raman spectroscopy. The process has been employed for the fabrication of an on-chip antenna on a flexible substrate for use in combination with a flexible circuit, in applications where a small form factor and simplicity of integration are required alongside ultra-low cost, e.g. consumable tagging.

Current technological trends in the field of microelectronics highlight the requirement to use cost-effective techniques for precise deposition of highly resolved features. Laser-induced forward transfer (LIFT) meets these requirements and is already applied for direct printing of electronic components. However, to improve the process' reproducibility and printing resolution, further research has to be conducted, regarding the rheological characteristics of the printable fluids and their jetting dynamics. Herein, a high-speed imaging setup is used to investigate the liquid jet's propagation during the printing process. Different Ag nanoparticle inks are studied and compared, over a wide range of viscosities and two different values of surface tension. The main focus of this investigation is the influence of the ink's rheological properties, both on the jet propagation and on the spatial and temporal evolution of the printed droplet during the wetting phase on three different receiver substrates (glass, SU-8, and gate dielectric). The results indicate that both the surface tension and the wetting properties of the receiver determine the shape of the printed droplet, whereas the inks' viscosity and laser fluence determine the printed volume.

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