Photostructurable pastes enable 5G applications
The next standard for mobile Internet and mobile telephony is on the horizon: 5G. South Korea, Switzerland and some US cities are already using 5G, while other countries are in the introductory phase. However, the electronics needed to transmit and receive the signals require much finer structures than before. This also applies to the associated antennas, which will initially operate at 3.6 GHz and later at higher frequencies. However, the thick-film technology used to date has reached its limits in terms of miniaturization: with a resolution of around 50 micrometers, the limit has been hit, at least as far as industrial implementation is concerned. For 5G, 20 µm or finer structures are required.
Researchers at the Fraunhofer Institute for Ceramic Technologies and Systems IKTS in Dresden have been able to solve this challenge in cooperation with the British company Mozaik. They developed a technique with which conductor paths can be produced with a structural resolution of 20 µm and smaller – and, as Fraunhofer scientist Dr. Kathrin Reinhardt assures, “suitable for mass production and industry”. It is said that the investment costs are low.
Conventional screen printing technology is used as the basis – so users can continue to use their systems as usual. The principle of screen printing: A screen with the desired printing structure is positioned over a substrate, a thick-film paste is applied through the screen openings and the desired printing structure is transferred. The deposited layer is then dried and sintered at high temperatures to produce the final functional layer properties. However, since the stainless steel wires used for the screens cannot be produced in any desired thickness, screen printing has reached its limit in terms of structure resolution and quality at 50 µm.
The new PI technology adds two steps to this standard process: After the thick film structures have dried on the substrate, a photomask with the desired final structure is positioned above. UV radiation is then applied to the substrate. Where the photomask has structural openings, the UV light penetrates the layer and cures the polymer it contains. At those points where the mask covers the layer, the polymers remain unchanged. This is followed by a wet chemical development process on an aqueous basis. This removes the areas of the layer that were covered by the mask. The thick film adheres to all other areas. This means that the previously 50 micrometer wide structures can be reduced to the desired 20 µm by this process; the final structure is determined by the photomask. Now it goes back to the usual process in which the substrate is sintered. The two additional process steps each take only 15 to 30 seconds and can therefore be easily integrated into the production process.
Tailor-made thick-film pastes are necessary for the PI technology to function properly: These are composed in such a way that they cure reliably under UV lighting but remain unaffected by daylight. A costly “Yellow Room” (as in semiconductor technology) is therefore not necessary. The specific process expertise lies in the precise coordination of the components of the pastes. In the case of metallization pastes, these are powdered metals (silver, gold or alloys), of which the later structures are to consist, the UV-curing polymer and other additives. Developers are currently working on platinum and resistance pastes. The corresponding production facilities are already being developed – the Italian company Aurel is involved here.
IKTS will be presenting their developments at the Productronica trade fair from 12 to 15 November 2019 in Munich (Hall B2, Stand 228).
Further information: https://www.ikts.fraunhofer.de/en.html