By: Petri Laakso
INTRODUCTION
Roll-to-roll printing itself is not always sufficient for the production of printed electronics components
and systems. Drying and sintering are the bottleneck processes in metal particle based conductor printing. Lasers show high potential for the curing process, especially in the case of nanoparticulate inks. This is due to fact that the typical sintering temperatures for nanoparticles (100–300°C) are only a fraction of the macroscopic melting point of the corresponding materials. This allows paper or plastic substrates to be used.
MATERIALS AND METHODS
Base material in sintering was polyimide. Laserline fiber coupled diode laser with scanhead was used for sintering.
SINTERING RESULTS
The sintering of printed nanoparticle structures using laser treatment has been investigated at VTT. Laser sintering can be utilised in the manufacturing of printed conductor structures such as antennas, circuits and sensors [1, 2]. A drop-on demand printer was used to print patterns with metal-organic silver nanoparticles on a flexible polyimide substrate. Laser sintering was done with a 940 nm CW fibre-coupled diode laser. The process was optimised using different scanning speeds, laser power levels, line separation and repetition rounds. In sintering tests, three different line thicknesses were printed to gauge the effect of line width. Sintering tests were done with Ink 1 using the hatch technique. The laser speed was 1000 mm/s and the line-to-line distance was 0.2 mm. The beam size was 1 mm in diameter. After a series of pre-tests, the optimal range of average power were estimated to be between 20–50 W. Table 1 shows the effect of average power and conductor width on sheet resistance.
Table 1. Sheet resistance values with different average powers on ink 1 with different conductor widths.
| 50 W | 40 W | 30 W | 20 W | |
| 100 µm | 0.16 | 0.18 | 0.29 | 0.52 |
| 200 µm | 0.17 | 0.27 | 0.31 | 0.59 |
| 350 µm | 0.16 | 0.19 | 0.28 | 0.38 |
SUMMARY
Laser sintering of nanoparticle inks seems to be promising curing technique for R2R sintering. Especially in cases
where only part of the substrate need to be cured, laser has the potential for reaching a high processing speed.
Additionally it can allow low-cost low melting point substrates to be used since heating is well-targeted to inks.
Choosing the right ink for the process and keeping the substrate clean are key factors for successful operation.
Sheet resistance values only slightly exceed the values obtained by heat sintering. This result was obtained in preliminary test and can be further optimised.
BUSINESS POTENTIAL
Laser sintering has a high potential for curing metal particle inks on flexible substrates. It offers a fast processing speed and low temperature processing, and therefore, it often represents an improvement over oven sintering.
REFERENCES
[1] Khan, A.; Rasmussen, N.; Marinov.; Svenson, O.: Laser
Sintering of Nanomaterial on Polymer Substrates.
In: journal of Microelectronics and electronic Packaging
(2008)5, 77-86
[2] Ko S.; Pan H.; Grigoropoulos C.; Luscombe C.; Frechet
J.; Poulikakos D.: All-inkjet-printed fl exible
electronics fabrication on a polymer substrate by
low-temperature high-resolution selective laser sintering
of metal nanoparticles, Nanotechnology 18
(2007) 345202.
ACKNOWLEDGEMENTS
Laser sintering work was done in a TEKES funded PESEP -project. The ink jet printing for laser sintering trials was done in this project by Eerik Halonen from the Tampere University of Technology.
The above brief overview was extracted from its original abstract and paper presented at The International Congress on Applications of Lasers & Electro-Optics (ICALEO) in Orlando, FL. To order a copy of the complete proceedings from this conference click here
