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Flexible Thinking: Circuit Flexibility (and How to Achieve it)

This column originally appeared in the September 2012 issue of The PCB Magazine.

The multiple definitions of flexible should be kept in mind when one sets about designing a circuit where flexibility is required. Touching on all of the definitions benefits anyone tasked with satisfying the design objectives of the product in their charge.

In the world of electronic interconnections, the word flexible has been used in conjunction with the word circuit for nearly six decades. This is almost as long as the pairing of the words printed and circuit have been co-joined. And just as we have come to discover over the years (and especially in the last five or so) that there are layers of meaning in the term printed,with the advent of, and interest in printed electronics, so also are there layers of meaning to the term flexible.

The most common interpretation of the word flexible, as applied to the flex circuits that the industry currently makes, is something capable of being bent repeatedly without breaking. However, there are other definitions of flexible that are worthy of consideration as well when using the term, for their ability to unlock new thinking patterns relative to what is flexible. For example, there is the definition of flexible where it means something amenable to modification, or simply adaptable, if you will. Additionally, the definition of flexible where the word can be construed to mean something which is easily disposed to yield, that is, something that is pliable. Where the most commonly used definition is viscerally understandable when one envisions a flexible circuit, the second two definitions of flexible are more abstract and could serve as a springboard to new ways of thinking about circuit flexibility, as opposed to the flexibility of the circuit. The logic may seem a little obtuse at this moment, so some examples are offered hereafter to help explain.

To begin, consider the fact that most flexible circuits are not actually used in applications where repeated bending is required; that is, most flexible circuits are designed to be used in static applications. The only bending and flexing required is during installation. This type of application really fits the second definition above in that the circuit is amenable to adaptation to the construct in which it is used. In such applications it is of great benefit to make sure that the design has incorporated some extra length in appropriate areas to facilitate assembly and accommodate any unexpected or unanticipated obstacles that might show up only during the installation process. These extra lengths of material are commonly called service loops. These features provide additional flexibility to that which is already flexible, but it is flexibility of the second type.

The third definition of flexible, something which is disposed to yield or is pliable, is one that comes to mind when considering the emerging subset of flex circuit products: stretchable circuits. Stretchable circuits are an interesting branch of flex circuit technology and one which is getting increased attention from designers of products for a range of different markets, from consumer to medical. Flexible Circuit Technology, 4th Editionhas a chapter on the topic of stretchable circuits and in the interest of brevity and space, those interested in more detail are invited to look there for that information.

The definitions of flexible, as broken down and examined here, apply to circuits which are physically manifest as constructions of copper and flexible polymer. However, there is another type of circuit that one should not lose sight of which can embody all of the objectives and definitions of flexible that have just been discussed, and that is the wireless circuit. No doubt, all readers have found themselves frustrated at some point trying to untangle the wires and cables used to access and control the various external devices used with our computers. To address the problem, increasing numbers of product developers are incorporating devices capable of communicating wirelessly using any or all of the various protocols which have been defined and developed to date. These include all of the various 802.11 protocols along with Bluetooth and Zigby. Clearly, at its core, wireless interconnection is the most flexible of all. It is at once dynamic, static, adaptable and extremely pliable, and if one is seeking flexibility of design, it should not be kept out of consideration.

In summary, the multiple definitions of flexible should be kept in mind when one sets about designing a circuit where flexibility is required. Touching on all of the definitions should be of benefit to anyone tasked with satisfying the design objectives of the product in their charge.

Verdant Electronics founder and President Joseph (Joe) Fjelstad is a four-decade veteran of the electronics industry and an international authority and innovator in the field of electronic interconnection and packaging technologies. Fjelstad has more than 250 U.S. and international patents issued or pending and is the author of Flexible Circuit Technology.