Reliability and interconnections for printed circuits on fabrics

Abstract

E-textiles are specially engineered textiles that perform intelligent functions such as sensing and actuation. They find useful applications in medicine, the military, fashion and entertainment. However the durability of these textiles is limited by mechanical failure resulting in degradation of the electrical performance of the electronic circuits incorporated into them when they are exposed to stresses from washing and bending which leave the host textile undamaged.

This thesis investigates the methods for improving the durability and wearability of printed circuits on fabrics with an initial review of the state of the art interconnection technologies and integration techniques for adding electronic capabilities to fabrics. The durability solutions for e-textiles in the literature are examined to identify their limitations and potential for achieving durable e-textiles. A preliminary assessment of the durability of a printed e-textile fabricated by screen printing is conducted to empirically identify the failure modes that result from washing the e-textile.

Consequently, a theoretical model for characterising the bending behaviour of printed e-textiles is developed to locate the neutral axis (NA) position where printed circuits integrated to fabrics can be positioned to improve their durability. The model integrates Pierce's fabric cantilever test with the classical beam theory to predict the NA positions of four screen printed e-textiles based on different blends of polyester /cotton/lycra fabric. Modelling and empirical results show that piezoresistive strain gauges, screen printed as conductors within ±1% distance from these NA positions, show approximately 0.3 % and 10% change in electrical resistance for a bending radius of 5 mm and five washing cycles respectively. This contrasts to the corresponding changes of the 37 % and 400 % in the resistance of the strain gauges positioned at a 65% distance to the NA.

This thesis also examines the use of photolithography and thin film deposition techniques to integrate functional thin film circuits with resolution down to 30µm on fabrics. The advantages of flexible substrates such as Kapton over polymer coated fabrics for adding these fine circuits to fabrics is discussed. A 0.8mm wide electronic plastic strip containing micron-scale components such as LEDs is fabricated to demonstrate the technology. The strip is 40% of the size of the state of the art offering better concealment in fabrics.

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Identifiers

ORCID for Stephen Beeby: orcid.org/0000-0002-0800-1759

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