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The process of printed electronics and conductive ink applications
Printed Electronics is today a well-defined and understood blanket term for methods of creating devices. The distinguishing element is that in opposition to traditional processes of electronics manufacturing it is purely additive, so ideally no fixtures or preparatory processes are needed because printing conductive ink takes care of creating all the required connections. Typically when thinking about ink – one does not think about electrical conductivity, but this is one of the key characteristics that enable the technology to be functional. Delivery of electrically conductive ink applications requires special high-performance materials of various characteristics, notably electrical resistance, and structure homogeneity which is crucial for producing electrodes.
Exploring the evolution and diversity of conductive inks
In the past conductive inks were based on infusion of carbon and/or graphite into binder. This solution offers acceptable performance for conductive inkjet printing but bears limitations in resistance performance and scale-down of the features. The proliferation of nanotechnology dictates that the materials and processes need to follow into sub-micron scale. Metal nanoparticles (NP) are the engine in transitioning the technology to a smaller scale.
Conductive silver inks are trailblazing the market with excellent performance characteristics: they offer a high percentage of bulk metal conductivity (in the case of XTPL CL85 it is over 40% of bulk silver), excellent homogeneity of post-processed particles, and high adhesion to any substrate. Conductive inkjet inks can be also based on other elements. For example, copper conductive ink will offer the highest compatibility with current PCB technology as well as reduced cost as compared to gold or silver conductive ink, whilst gold conductive inks will offer biocompatibility as well as the lowest resistance.
PEDOT:PSS is an example of a conductive polymer mixture that forms macromolecular salt. The versatility of PEDOT:PSS as a material for a plethora of manufacturing methods lies in the selection of the solvents. For screen printing conductive ink, where a high viscosity and slow drying is expected – high boiling solvents can be used (notably propanediol). To increase the printing speed – freeze-dried pellets can be used, redispersed, with ethanol.
For slot die coating – a water-based PEDOT:PSS is available. All of the mentioned materials require careful design and planning since the same conductive ink type can be used in a very specific technology. For example, high viscous NP paste that enables very specific applications for complex topography printing can use the same NP in conductive inkjet ink, nevertheless, the materials are completely different and not interchangeable. An ink or paste for extra fine printing will also have a strict requirement, which has to be addressed during design, for the anti-agglomeration of NP in order to prevent nozzle clogging.
The field of conductive ink applications is practically endless as conductive inks, printed electronics are expanding the market every day. Especially wearable devices and flexible electronics are matched with printing manufacturing processes which give the ability to create thin layers and extremely small size of the device.
Emerging biosensor applications broaden the market as printed biomedical sensors offer extreme customization and extreme accuracy of employment for a specific case or a patient. The accuracy of some methods enables micro antennas for inlaid RFID or 6g. The areas of use of printed electronics are growing outside of technology intended solely to the connection processes. Printed capacitors are maturing and may soon enter the industry, which will increase possibilities for further miniaturization, customization, and process flexibility. Finally, printed batteries give a promise of further scale-down of wearable electronic devices. Wearables can be enhanced if printed thermally regenerative batteries mature enough to be implemented to consumer purchasable products, giving the user practical freedom from chargers and outlets.
Expanding horizons: diverse applications and future potential of conductive inks
Marketing of such advanced products brings not only endless possibilities but also challenges, especially if one thinks about offering materials for industrial use. Environmental considerations, as well as new sustainability restrictions, put high pressure on R&D departments in order to meet difficult stability, substrate compatibility, and conductivity targets. Especially the last one is a strong barrier for the memory storage sector where line resistance is a limiting factor of the speed of RAM and SSD.
It is an exciting time to be a chemistry R&D engineer in the printed electronics sector as the inks are the foundation of the new industry. This gives an ample opportunity for innovation and to solve complex, sometimes contradicting requirements.
