Direct writing – the printing of electronics of tomorrow

A contemporary designer of electronic applications usually makes use of standard components and pre-assembled modules. Additive Manufacturing (AM) has the potential to change these methods and ways of thinking.

by KU Leuven, UHasselt, Thomas More Mechelen

Characteristics of (passive) components, sensors and antennas are found in datasheets and, during the design, the dimensional and functional constraints are imposed to achieve the desired product. Thanks to a few new printing technologies – grouped under the name of “direct writing” – it is now possible to directly apply interconnects, passive components, sensors and antennas onto flat and curved surfaces. The new opportunities are evident, especially with respect to prototyping, customization, flexible design, repairing, small batch production… Besides, the recent commercialisation of these technologies proves that there is a shift from lab (R&D) to industry.

“Direct writing” in electrical printing typically refers to nozzle based jet technologies, as aerosol jet printing (AJP) and ink jet printing (IJP), along with the use of functionalised inks, as for conductive, or insulating or semi-conductor fluids. The design of the electrical patterns and interconnects is also crucial, especially when dealing with flexible and stretchable foils or free-from substrates.

In this article, some of the aspects of direct writing electronic/electrical applications and potentials are reviewed.

Functional inks and substrates

Functionalized inks with silver nanoparticles are the mostly used owing to their high conductivity. Some examples of manufacturers and sellers of silver inks are Agfa NV, Sun Chemicals Ltd, Sigma Aldricht, DuPontTM, etc. Inks based on Copper nanoparticles and/or Cu-Ag core-shell nanoparticles are also applied for printed electronics applications. Post sintering (e.g. via flash curing, or laser sources, or via oven), at high temperature ( >150°C ) is however necessary to reach sufficient electrical conductivity. This can be problematic when dealing with polymer based substrates. Therefore, a recent research trend is to investigate metal-organic decomposition inks, namely pure solutions consisting out of a solvent mixture with Ag or Cu ions, so that temperatures as low as 60°C are sufficient to reduce the Ag or Cu ions back to their ground state and make the printed structures conductive.

A recent research trend is to investigate metal-organic decomposition inks so that temperatures as low as 60°C are sufficient to reduce the Ag or Cu ions back to their ground state and make the printed structures conductive.

PEDOT:PSS inks are alternative low sintering temperature polymer based conductive inks. They are transparent and commonly used as conductive inks to replace transparent conductive oxides in thin-film organic photovoltaics. Their conductivity however lies several orders of magnitude lower than the Ag and Cu-based inks. Finally, speciality inks consisting out of Ag-nanowires or Carbon-nanotubes, or graphene particles, are applied to combine the high conductivity of the nanowires with the transparency that they can reach when deposited. Those inks are often applied for flexible and stretchable electronics as bending and flexing do not influence the nanowires as much as compared to other inks. In the end, it should be mentioned that dielectric inks, having (semi-)insulating properties are also needed for printed electronics applications to make bridges between two or more conductive patterns and to be able to print a 3D electronic structure with insulating inter-layers. Printing can be on various substrates, as PVC, TPU, kapton, paper and other special flexible foils, but also on rigid (free-form) ceramic, polymeric, metallic or glass substrates, depending on the specific applications and techniques. Pre-treatments, as oxygen plasma activation, or surface functionalization might be relevant to increase adhesion of the printed patterns.

Aerosol Jet Printing (AJP)
pnepneumatic atomisation method
Schematic overview of the pneumatic atomisation method as implemented in an Optomec system (KU Leuven)

Aerosol Jet Printing (AJP) is a comparatively new manufacturing technique to use for printed electronics. It is a trademark of Optomec and it is in the market since 2014. As the name suggests, the jet beam is based on a dense aerosol of the ink material, created in a carrier gas, via a pneumatic or ultrasonic technique, then filtered and focused with the aid of an exhausted and co-axial sheet gas. This process can be considered analogous to spray gun used for the painting with focusing nozzle.

Apart from printing functional inks with range of viscosity from 0.7 to 10 mPa.s, one of the biggest pros of AJP is it can print very small feature size and thin patterns (as down to 100 nm– i.e. roughly 1000X thinner than a human hair) up to several hundreds of microns.

One of the biggest pros of Aerosol Jet Printing is it can print small feature size and thin patterns up to several hundreds of microns.

Printing on 3D surfaces and 5 axes printing are also an option because of the variable stand-off distance between the substrate and nozzle tip in the range of few mm, allowing to develop completely new applications. Printing of sandwich structures with multiple layers of different materials and the capability of wire bonding and interconnects also give substantial freedom in design and manufacturing.

Conductive tracks printed onto a ping pong ball surface. The imageshows the 3D capability of aerosol jet techniques (Source: Adams, Jacob J., et al.)
Ink Jet Printing (IJP)

Inkjet printing is a well-known technology and is used in the scientific community since more than 10 years for printing functional inks. A distinction is made between continuous inkjet printing – where inkjet droplets are deflected by electromagnetic fields to be applied onto the substrate or directed back towards the ink reservoir – and drop-on-demand (DoD) inkjet printing, where a droplet of approximately 20μm is generated and directed towards the substrate only when a droplet is needed to print the desired structure. With this DoD inkjet printing, lines as thin as 100 nm and as wide as 30μm can be applied onto a rigid, flexible or even stretchable, but flat substrate. The way the ink interacts with the substrate is crucial to end up with the predefined thickness and line-width of the printed patterns. Printing on free form substrates is achievable via flexible foil printing, successively applied onto the 3D object via thermos- or vacuum forming technology. This way, the foil is finally formed around the object, still having the conductive patterns as printed onto the foil by DoD inkjet printing. This technology is comparable with in-mould labelling or mould-in design, in which sheets with printed areas are applied to products by placing those foils into injection moulding equipment.

drop on demand piezoelectric system
Schematic view of the working principle of a drop on demand piezoelectric system for ink jet printing (KU Leuven)
Electronic design

Mechatronic designers make typically use of standard components or modules (Commercial Off-The-Shelf (C.O.T.S )) of which the characteristics can be found back in datasheets and that they are designed to use in a 2D way, using a flexible (FLEX) , Semi-Rigid or rigid printed circuit board. Printed electronics instead usually refer to tailored, custom-made and free form applications, based on the needs of the end-user or a creative designer. Design rules then do not only refer to circuit design, interconnections, component but also to a selection of substrates, inks, printing and 3d-forming technology, pre- and post-treatment procedures… opening up new opportunities.

Design rules for printed electronics also refer to a selection of substrates, inks, printing and 3d-forming technology, pre- and post-treatment procedures… opening up new opportunities.

The usage of e.g. strain gauges sensors, for instance, requires specific mounting technique to allow accurate monitoring of the mechanical properties of the underlying structure. Precise positioning, bonding, correct configuration and the way of interconnecting are key factors for quality measurement. Direct writing on 3D surfaces could facilitate all this operations and lead to new 3D shaped mechatronic devices, not possible to produce in the industrial way of today.  Another example refer to in-situ integration of high frequency single or multi-band antennas, which could help product developers to realise new applications on customer defined shape/surfaces. Antenna characteristics however depends strongly on dimensions, 2D/3Dshape, surface roughness, material conductivity and are usually realised on FLEX or PCB substrates with moderate performance.

Some end applications
AJP and IJP tracks in textile elements
A silver track (straight and serpentine) printed on textiles which is tested for stretching and bending in a home-build stretch-test bench (UHasselt IMO-IMOMEC)

World-wide a few nice examples of printed electronics on both flat as 3D substrates are known. The applications can range from industrial electronics over biomedical applications towards automotive and smart packaging. As an example, a well know application includes the 3D printing of antennas onto cell phone cases via Optomec AJP. Moreover, innovative and smart examples combine the printing of transparent interconnects on touch screens for user electronics or the integration of antennas and RFID on bottle caps and blisters for smart packaging applications, or even the integration of sensors, such as stress-strain or humidity, for the development of smart orthopaedic and medical products. Another nice example is the integration of AJP and IJP tracks into textile elements. The authors also provided their technical expertise and manufactured the touch pad/ sensors of the 3D print e-bike.

Flam 3D bike
Touch pad sensor controlling front and rear lights of the Flam 3D bike. The component consists of an electrical pad realized via AJP onto a FDM support. The integrated approach of nozzle based additive manufacturing to realize multimaterial components is pioneer in the context of in-situ manufacturing of embedded devices