Chapter 1
Composite Materials for Application in Printed Electronics

Kamil Janeczek

Tele and Radio Research Institute, Warsaw, Poland

Corresponding author: kamil.janeczek@itr.org.pl

Abstract

Further development of printed electronics requires investigations of new advanced composite materials that can be used to produce different types of devices on flexible or rigid substrates. Among these printed devices, organic light-emitting diodes, organic photovoltaic cells, radio frequency identification tags, sensors, and capacitors can be mentioned. To achieve their high performance, materials used for their fabrication should exhibit excellent electrical as well as thermal and mechanical properties to be not susceptible to environmental factors, in particular to bending cycles. In this study, recently developed different materials used in printed electronics for fabrication of various types of devices are discussed. These materials contain graphene, graphite nanofibers, carbon nanotubes, silver nanopowder, or silver flakes. Properties of layers produced from these materials were discussed, i.e. based on the results obtained using scanning electron microscopy, atomic force microscopy, profilometers, and their durability after thermal and mechanical tests was assessed by measurement of their resistance and analysis of their surface and microstructure.

Keywords: Printed electronics, graphene, carbon nanotubes, silver nanopowder, graphite nanofibers

1.1 Introduction

In recent years, development of modern electronics technologies has been influenced by eco-friendly approach that consists in limitation of hazardous substances and waste created during production of electronics devices. In effect of this approach, the Directive on the restriction of the use of certain hazardous substances in electrical and electronic equipment (in short: Restriction of Hazardous Substances Directive - RoHS) was introduced. Its aim was to restrict the use of the following six substances: lead (Pb), mercury (Hg), cadmium (Cd), hexavalent chromium (Cr6+), polybrominated diphenyl ether (PBDE), and polybrominated biphenyls (PBB) [1].

Another manifestation of the eco-friendly approach in electronics industry was development of printed electronics which is also called organic, plastic, polymer, or organic electronics [2]. This new branch is based on polymer materials that allow to fabricate light-weight, flexible, cheap, and disposable devices. Among them, organic light-emitting diodes (OLEDs) [3-6], organic photovoltaic cells [7-9], radio frequency identification (RFID) tags [10-12], memories [13-15], batteries [16-17], smart textiles [18-19], and sensors [20-23] can be named.

Unlike conventional technologies, e.g. etching used in production of printed circuit boards, printed electronics belongs to additive manufacturing techniques. This means that a produced component is formed by printing different types of materials directly on a substrate, whereas in subtractive techniques some parts of materials are removed to create a designed element. In this way, unwanted waste is produced when subtractive techniques are used. This waste has negative impact on environment and it increases production cost because it is necessary to pay for its utilization [24]. Printed electronics allows to avoid creating as big amount of waste as it is generated in conventional technological processes and with respect to this ability printed electronics can be assessed as eco-friendly. Its additional advantage is less complexity compared for instance to etching, e.g. to produce electronic circuits smaller number of technological steps is required [25]. To see this aspect better, a few chosen printing techniques and etching technique for comparison are described below.

There are many different printing techniques which can be utilized for fabrication of electronics devices. Among them, screen printing, ink-jet, gravure, and flexography can be specified. The first one is a well-known and popular manufacturing technique in which a paste is transferred onto a substrate through a mesh made from polyester, steel, or polyimide. The mesh is stretched on a metal frame. In order to get a required thickness of a printed pattern, it is necessary to choose suitable force and speed of a squeegee and distance between mesh and substrate. After the printing process, the fabricated pattern is cured using elevated temperature or ultraviolet (UV) [26].

Another printing technique is ink-jet, which has been rapidly developed in the recent years. Its two different groups can be named: drop-on-demand (DoD) and continuous ink-jet (CIJ). In the DoD process, a single ink droplet is jetted through a nozzle when pressure within a reservoir grows or due to vibration of a piezo element or a bubble created as a result of rapid evaporation of the heated ink [26-28]. In industrial application, size of generated droplets varies from 15 to 55 µm, drop speed is typically equal to 3-15 m/s, and printing frequencies are up to 100 kHz [29]. In the CIJ process, a stream of fine droplets is ejected out of the nozzle under the pressure inside the reservoir which is undergoing vibration. The generated droplets pass through a charged electrode, and then two perpendicular electric fields can deflect them in two directions. The droplets which are not intended to be printed are collected into a gutter [30]. Typically, CIJ produces droplets with a diameter of 80-100 µm moving at speed of 20 m/s with drop frequencies even above 250 kHz. For both mentioned ink-jet groups, thermal, piezoelectric, and electromagnetic actuators are usually utilized [29].

In comparison to screen printing, ink-jet method makes possible to produce much thinner layers (below 1 µm) [27] and higher resolution (250 lines per cm) [26]. Moreover, it is not necessary to use in-between forms, such as stencils, what is significant advantage of ink-jet printing.

In the classical gravure method, a number of cavities with raster structure are created in a cylindrical metal printing form covered with a thin chrome layer to ensure resistance and hardness to wear. During printing process, the cylinder rotates in an ink reservoir, and the excess amount of ink is wiped away by a doctor blade. Then, the ink remaining in the cavities is transferred to the substrate under pressure created by an impression cylinder. Gravure inks may be solvent- or water-based or UV-curing. Their viscosities can vary in the range of 0.01-0.05 Pa.s [26, 31].

The advantage of the gravure method is highly resistant cylindrical form in comparison to flexography or offset printing. It can be useful when thin layers from low-viscosity inks containing a large amount of aggressive organic solvents are fabricated. However, gravure printing shows some limitations. The first one is relatively high pressure in the contact area and very rigid form surface which limit application of this method to flexible substrates and which cause considerable difficulties when multilayer devices are manufactured. The other limitation is connected with observed deformation of printed pattern which varies depending on the position of printed elements relatively to the axis direction of the cylinder [31, 32].

Another advantage of gravure printing is its capability to be used for the mass production of printed electronics. Large cylinders are capable of producing up to 2000 feet per minute, much more than it is possible to print with ink-jet which is suitable for a small production of printed devices. Furthermore, in gravure method, cavities are constantly refilled with an ink when the cylinder rotates what sustains long print runs and prevents against ink clogging. The latter is a common problem in ink-jet printing [33].

Apart from gravure, flexography can be used for the mass production of printed components. In the recent years, it is often assessed as the most promising roll-to-roll method suitable for printed electronics (initially, it was developed for packaging industry). In flexography method, a printing unit comprises anilox, cylinder holding a flexible printing form and printing cylinder which presses a substrate material against the form. Anilox roller is used to precisely supply a quantity of ink onto the print form [31].

The main drawback of flexography, as a potential technique for production of printed devices, is uneven printing (irregularity on the edges) which is caused by a structure of the thresholds between raster cavities. However, its advantage is capability to produce thin layers and to use a wide variety of substrates, such as corrugated cardboard, paper, board, flexible and rigid polymers, glass, and metals. The ink viscosities can be in the range of 0.01-0.1 Pa.s, and the printed elements can have 10 µm in diameter and 20 µm in width [26, 31].

All above-described techniques belong to additive manufacturing processes. Currently, subtractive techniques are also very popular and used commonly in electronic industry, in particularly wet etching. In this method, liquid chemicals or etchants are utilized to remove unwanted metallic parts in the following steps: deposition of photoresist on the metallic surface, exposure of photoresist to UV light through a photomask protecting a desired pattern on the substrate and finally removing of unwanted metallic areas and photoresist layer [34].

During etching process, only a small part of material is used and the rest is either thrown away or recycled. Thus, a lot of waste materials and chemicals are created what makes etching technique...