Basics of Flex Design 

Flex circuitry is ideal for many of today’s electronics needs. It is light, compact, and, if properly designed, extremely robust. Because it bends, however, a flex circuit has some very specific requirements that are different from those of traditional rigid circuits. Materials, circuit architecture, placement of features, and the number of layers in the circuit must all be considered in the design process. So must the degree to which the circuit will be bent, how tight the bend will be, how the bend will be formed, and how frequently the circuit will be flexed. By carefully defining the application and design priorities and recognizing the unique demands made upon flex circuits, the designer can work within these requirements to realize the technology’s full potential.

There are plenty of good reasons to use flex circuits. They are light, compact, robust, and resilient, and ideal for today’s smaller and more portable electronics. They are ideal for new product designs, but can be used to replace traditional wire harnesses and circuit boards as well. But to realize their full potential, designers must consider the unique requirements of these circuits and the materials they are made of. It is important to recognize that, while the materials that comprise a flex circuit may be individually flexible, their performance in a completed circuit is greatly impacted by a circuit’s construction.

  Single Layer Flex Bent 90 Degrees

Figure 1: A circuit’s neutral bend axis, ideally located on the central plane of the material stack, experiences none of the tension or compression that affect other layers when the circuit is bent

Every flex circuit has a neutral bend axis. This plane, which is ideally located on the central plane of the material stack, experiences, at least in theory, no compression or tension forces when the circuit is flexed. Toward the outside of the bend, however, outer layers experience increasing tension, which can tear or crack the materials. This can lead to immediate circuit failure or, potentially worse, hairline breaks that will fail after the circuit has been put into service. Toward the inside of the bend, layers are subject to increasing compression. This can cause layers to wrinkle or delaminate, again a potential cause of immediate or eventual failure. Careful design can help prevent these problems.

Figure 2: If a circuit is not properly designed and handled, outer layers can tear or crack and inner layers can wrinkle or delaminate

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