FLEXIBLE PCB TECHNOLOGY

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Flexible-Circuit Technology

The advance of electronic systems into our everyday lives is evidence of a major digital technology revolution. The success stories of the personal computer and the mobile phone serve to demonstrate that consumer and business demand for innovative products are significant. Increasingly electrical and electronic systems are entering our lives in many unanticipated ways. They can be found in our homes in the form of cordless phones and digital TVs, in our cars in the form of hands-free communications and telematics, and in business in the form of notebook computers and mobile personal data assistants (PDAs).

Importantly, and also covertly, within the above applications flexible printed circuits have also been entering our lives. Traditionally employed in the role of wire replacement, removing the need for complex wire harnesses, and replacing costly and increasingly complicated wired assemblies, flexible circuits offer a much simpler and often significantly more cost-effective interconnection method.

 However, alongside increasingly innovative applications flexible-circuit technology is branching out significantly from this initial role and it is poised to be a technology that will provide enormous design freedoms for electronic engineers and product designers over the coming years. As the demands of modern electronic systems call for increasing functionality, greater circuit density, higher connectivity, better environmental performance, and all at lower cost, flexible circuitry is poised to deliver on the promise of twenty-first century electronics.

A Definition for Flexible Circuits

Confusion still exists regarding what constitutes a flexible circuit. When asked to envisage a flexible circuit, the image in most people’s mind will be of a bendy printed circuit, typically consisting of a flexible film with a pattern of copper conductors on it.

Whilst the image is not far from the truth, in order to better understand flexible circuits it is important at the outset to establish a working definition. The IPC (formerly the Institute for Interconnecting and Packaging Electronic Circuits), through its role of setting standards and guidelines for the electronics industry, has established such a definition:

Flexible Printed Circuit

A patterned arrangement of printed circuitry and components that utilizes flexible base material with or without flexible cover lay.

The above definition, although strictly accurate, does little justice to the complexity of the technology but does serve to convey some of the potential given the available variations in base materials, conductor materials, and protective finishes.

Flexible-Circuit Constituents

From the above definition, there are a number of basic material elements that constitute a flexible circuit: a dielectric substrate film (base material), electrical conductors (circuit traces), a protective finish (cover lay or cover coat), and, not least, adhesives to bond the various materials together. Together the above materials form a basic flexible-circuit laminate suitable for use as a simple wiring assembly, or capable after further processing of forming a compliant final circuit assembly.

Within a typical flexible-circuit construction the dielectric film forms the base layer, with adhesives used to bond the conductors to the dielectric and, in multilayer flexible circuits, to bond the individual layers together. Adhesives can also be used in a protective capacity to cover the final circuit to prevent the ingress of moisture and dirt, when they are termed ‘cover lays’ (also ‘cover layers’) or ‘cover coats’.

Materials Diversity Overview

Many individual materials exist that time and extensive prototyping have proven suitable for application in flexible circuits. There are numerous substrate materials(termed dielectrics) available as very thin films of 12–120 microns in thickness that have been prototyped as base materials upon which to build flexible circuits. However, the two most common dielectric substrate materials are polyester and polyimide. Both are widely available from a number of global sources and both have unique advantages that make them suitable as base materials.

At costs of pennies per square metre, polyester materials are used to provide millions of exceptionally low-cost flexible circuits that find their way into calculators, cameras, touch panels, keypads and automotive dashboards. Polyesters are also highly flexible and are the material of choice for dynamic flexing applications. One example is the connection between a notebook PC keyboard and its screen, an application where many thousands of flexing operations are required.

Single-Sided Flexible Circuits

Single-sided flexible circuits are the most common types of flexible circuit available. They consist of a single conductor layer on a flexible dielectric film with access to circuit-termination features accessible from one side only. They can be manufactured with or without cover lays and protective coatings, and their relatively simple design makes them highly cost effective. The conductors used can be conventional metal foil, or, for low-cost, polymer thick-film (PTF) ink can be used. This is simply printable conductive ink, loaded with carbon or silver particles, which is directly applied to the flexible substrate in the circuit pattern required by a v Single-sided circuits can offer the lowest cost and relative ease of production. Because of their thin and lightweight construction such circuits are best suited to dynamic- flexing or wiring-replacement applications such as computer printers and disk drives. Nearly all of the world’s calculators consist of PTF flexible circuits on polyester film, a combination that offers an exceptionally low circuit cost.ariety of printing and stencilling techniques.