Printing Conductive Inks on Nonwovens: Challenges and Opportunities

Abstract

Flexible printed circuit boards continue to be a high-growth technology in the area of electrical interconnectivity. Over traditional rigid printed circuit boards (PCB's), wire and wire harnesses, flexible circuit boards provide considerable weight, space, and cost savings This study investigates using conductive inks for printing circuits onto flexible nonwoven substrates as a low-cost alternative to traditional PCB manufacturing, because the emerging generation of nonwovens can offer wearability, printability, lightweight, durability and washability.

In this study, instead of incorporating conductive paths by weaving or knitting conductive yarns into the structure of the fabrics, conductive inks are used to print directly onto nonwovens by using polymer thick film technology. There are challenges in printing on fabrics such as achieving printing resolution and durability. Spatial geometry of structure is a critical property for print resolution. The traditional woven fabrics exhibit poor digital printability because the ink disperses mostly through the inter-yarn interstices. The spatial resolution is primarily a function of yarn diameter and thread spacing (e.g., end and picks per inch in woven fabrics). Consequently, in woven fabrics, fine spatial resolution is only possible in high density, light weight, densely structures. Even in the case of such light weight structures, the surface texture can be quite rough for printing purposes. Additionally, light weight fabrics can be limited with respect to properties they offer. Other structural factors such as yarn twist and fiber properties are also factors that may affect the absorbency and the wicking of the ink into the structure and therefore, can limit the utility of the printed circuit lines. In contrast, the new generation of durable micro-fiber nonwoven fabrics offers the opportunity to "engineer" the structure surface properties, the network geometry and the capillary structure to optimize their use for printing. Nonwovens offer the ability to easily manipulate pore size and geometry and create an abundance of small capillaries. Today, it is possible to produce heavy fabrics with fine fibers and different surface textures using nonwovens much more cost effectively than their woven or knitted counterparts. As in other fabrics, the ink substrate interaction determines the printability on nonwoven fabrics. In the case of nonwovens, the main parameters affecting the printability are surface energy of the fibers, fabric structure (fiber orientation distribution), fiber size (controlling surface roughness and pore size) and ink viscosity. Fundamentally, because we are dealing with a porous network as opposed to a non-porous film, the interaction of ink droplet with the structure with respect to its movement in-plane as well as through-the-plane determines the quality of the printing that is achieved. Thus, to control the distribution of the ink on and into the fabric, we have to have control over the structure.

The first chapter outlines and examines existing technologies in the so-called electronic textiles area. Conductive inks, printing methods and polymer thick film technologies will be discussed in detail. The second chapter focuses on printing conductive inks on different nonwoven substrates. The polymer thick film conductive inks and the printed transmission lines will be characterized to demonstrate the properties of the structures used as substrates for flexible electronics. The performance metrics related to the circuits parameters and the manner in which the ink is distributed onto and into the substrate will be examined. The third chapter mainly focuses on the electrical conductivity and wash durability of the printed circuits. A method to control the durability of the printed circuits will be explored. In the last chapter, the interaction of liquids with different substrates using ink-jet printing will be modeled and discussed. The modeling results of droplet substrate interaction will be presented for some topical geometries.