Gravure printing of conductive particulate polymer inks on flexible substrates

Abstract

A roto-gravure printing technique has been developed for conductive lines on paper and plastic films. Rotary-screen-printing was used to make comparative prints. The inks contained metal particles in an organic medium and were cured in temperatures of 70–120 °C, limited by the substrate durability. The following conductor line properties were characterised for different substrates: resistance, yield as a function of line width, coil inductance, folding endurance, adhesion, printed antenna properties and maximum current density. A printed resistance down to ∼50 mΩ/□ was obtained, with conductor lines 4–7 μm thick. Minimum line resolution and resistance were affected by smoothness of substrates. Adhesion properties were adequate for the studied components.

Introduction

Particulate polymer inks known also as electrically conductive adhesives (ECA), are used as printed conductors in applications, such as keyboards. Metal particles in inks include silver, gold, copper, nickel [1] and platinum [2]. The benefit of silver is that it has low resistance and a thin oxide layer. By replacing silver with nickel, the electromigration problem is avoided [3]. Carbon is another filler material that is used in electrodes and resistors [4].

Flake-shaped silver particles in inks, compared to those of a spherical shape, are known to cause significant changes in the ink rheology. Flake-shaped particles are better for creating conducting traces, when they are packed in the organic binder material without melting the silver flakes. Conductivity created in such way is often explained by percolation theory, based on contact of the particles [5]. Silver particles in particulate polymer inks are usually coated by a polymer or by stearic acid to prevent particle agglomeration. These coatings, however, have an effect on rheology and obtained conductivity [6], [7]. One way to improve the conductivity is to use metallic particles coated with another metal having a low melting point [8].

Traditionally, particulate inks have been screen-printed, but the method is limited due to line resolution, speed and cost. Particulate (silver) polymer inks have also been gravure and gravure offset printed to produce conductors [9], [10]. A rotary-screen is used for a roll-to-roll (R2R) printing method, but generally, this is used only for fabrics [11]. Examples of emerging novel applications of conductive inks are intelligent packages and product quality monitoring [12]. Generally, intelligent packages refer to systems, such as RFID tags, which include a die-component (e.g. silicon chip). Attractive opportunities for printed conductor inks are RFID- or UHF-antennas. The field is characterised by very large volumes and inexpensive production methods, with or without a die-component. Organic substrates often limit the usage of the highest curing temperatures of the binder. The curing temperature is often a compromise between the curing time [13] and the solvent content. Reported curing temperatures vary for different materials from 80 °C [14] to 200 °C [9].

The R2R printing method needs flexible substrates, such as paper or plastic films. These substrates offer a much less expensive solution than the subtractive method and the method is much more environmentally friendly. Whereas gravure printing is well known, the emerging rotary-screen-printing technique should be tested with conductive silver inks for antennas and RF-components. The particle size (∼10 μm) of conventional inks can be one of the key factors limiting the obtained line resolution.

Thickness analysis of the printed conductor traces is commonly done with a contact scanning profilometer. Such measurements require a smooth substrate surface unlike that of conventional office paper. Although plastic substrates are generally smooth enough for the measurement, solvent absorption from the ink into a plastic surface can cause substrate swelling and twisting. The drying of the ink at high temperatures may also vaporise the plasticisers, causing deformation of plastics. The maximum line height ink line cross-section is not the best measure of a conductor; instead, line cross-section averages give the best indication of ink conductivity [10]. For flexible substrates, ink adhesion to the substrate is essential and is affected by the stress of folding [15] and chemical robustness, as shown in environmental ageing studies [16]. Also, the roughness of a substrate should be considered when characterising printed conductive traces. Yet, more pattern details are required for antennas.

The goal of this work was to study the properties of particulate conductive polymer based ink lines on paper and plastic substrates with desired square resistances < 0.1 Ω/□. R2R was the targeted manufacturing method, enabling non-stop production. The aim was also to study pilot scale printing and find out the major characteristics of a gravure printing process able to print simple electrical structures, such as conductors, coils and antennas on low-cost flexible substrates.