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|Laser Sintering of Aerosol Jet Printed Interconnects on Flexible Substrate|
|Keywords: Flexible Electronics, Laser Sintering , Silver Nano-particles|
|Metal nanoparticle-based inks commonly used in printed electronics  consists of dispersants, surfactants, solvents, and other additives. Even though some of the solvent evaporates during the printing process, these inks require post thermal processing to achieve conductivities closer to bulk. During this post thermal processing, sometimes known as sintering, atomic diffusion takes place resulting in welding (necking) of nanoparticles, densification, recrystallization, and grain growth . Commonly used sintering techniques such as convection oven and hotplates sometimes take multiple hours of sintering time making them incompatible with high throughput Roll-to-Roll fabrication . Moreover, in the case of copper nanoparticles-based inks, the copper nanoparticles oxidize while sintering in ambient condition, thus requiring inert environment . On the other hand, sintering nanoparticle-based inks requires high temperatures and is not compatible with many plastic polymeric substrates; for instance, this is the case of thermoplastic polyurethane (TPU), polyethylene terephthalate (PET), and especially coated papers. Different sintering techniques such as laser, intense pulsed light (IPL) , microwaves , photonic , and electrical sintering  have been used attempting to overcome these limitations. Specifically, laser sintering offers is fast and selective process and can also be optimized to be used for high temperature sensitive plastic substrates. In this work, we used an Aerosol Jet Printer AJP 300/5X for fabricating and laser sintering of silver nanoparticle based conductive interconnects on flexible substrate. The laser tool installed on the AJP 5X is a continuous wave laser with 830 nm wavelength and ~80 µm spot size. The laser sintering process parameters were optimized using statistical design of experiments (DOE) techniques, including screening experiment and general full factorial design. The screening experiment was conducted to define the limits of the laser sintering parameters, which include speed (mm/s) and power (mW), in order to ensure enough power delivery to the printed interconnects without damaging the substrate. Based on the results of this experiment, the laser power range was set to a minimum of 50 mW and a maximum of 950 mW, while the laser speed was defined to be between 1 and 15 mm/s. It is important to notice that higher power could damage the substrate, while higher speed could affect the sintering uniformity along the traces. The two factors, being laser speed and power, were considered having respectively four and seven levels. Each combination of factors levels was replicated three times, generating 84 total samples. The full factorial DOE was generated using MINITAB statistical software. Additional samples were printed to be sintered using a convection oven at 200 ⁰C for 1 hour and 20 minutes; these were considered reference samples. Experiments were also carried out to study the effect of laser sintering parameters on the thickness of the printed traces. The effect of multiple laser sweeps on the printed traces was investigated to define the minimum laser sweep parameters that affect the conductivity of the traces without damaging the substrate. Then, the heat-affected zone on the trace was analyzed to ensure that the heat generated by the laser power had affected the entire trace. Wide printed traces needed special investigation since the laser beam spot size is 80 µm, so multiple passes with some offset were required to sinter the entire trace width. However, since excessive heat generated by the overlap of multiple sweeps could affect the conductivity, a minimal overlap was required. The laser sintered traces were characterized using a Keyence VK-X1050 laser confocal microscope and a Scanning Electron Microscope (SEM). The electrical resistance was measured and correlated to the laser power and the speed, as well as the microstructure of the printed traces. Preliminary results of laser sintering showed that the conductivity of the traces was increased by three times as compared to the conductivity achieved by convection oven sintering. Laser sintering the printed traces while they were still wet caused them to be rough and generated pressure-pockets. This was attributed to the fast evaporation of the ink solvent on the top surface of the printed traces and the trapping of the evaporated solvent in the bulk of the trace. In order to eliminate these air pockets, pre- baking the printed traces was found to be necessary either by convection oven or by laser sintering with low power.|
|Mohammed Alhendi, Graduate Student; PhD Candidate