Types of Flexible Circuits

Any circuit that includes a patterned arrangement of wiring printed on a flexible substrate or board with or without flexible cover layers can be classified as a flexible circuit. However, additional design characteristics, such as a specific dielectric material or dimensional configuration, can often be as important as the flexibility rating in determining a flexible circuit’s effectiveness for a given application. Flexible circuits tend to be lighter weight, feature thinner dielectrics, and have lighter boards composed of polyesters or polymides, unlike rigid circuits, which more commonly rely on epoxy-on-glass fabrication materials. The wiring on a flexible circuit board’s surface can be patterned using photolithographic processes after copper foils have been bonded to the surface with an adhesive. Solder joints, terminals, and punched or drilled holes may also play an important part in mounting components on the flexible circuit assembly.

At a basic level, flexible circuitry is a three-dimensional technology for interconnecting components and providing reliable electronic performance. This principle can be illustrated through the “rigid flex” model of flexible circuitry, in which manufacturers can assemble, solder, and test a circuit while it is flat, and then fold it into the necessary shape for its intended system. Flexible circuits serve a distinct and useful role within the field of printed circuitry because they are highly versatile devices with a wide range of applications. Some flexible circuits may function statically, using a flex-to-fit method in their operations, while others may perform dynamically through intermittent operations, such as those for hinged assemblies or continuous work. There are several different types of flexible circuits commonly in use, each with its own set of specialized features and performance parameters.

For more information on dynamic versus static flexible circuitry, see Flexible Circuit Technology.

Single-Sided Flexible Circuits

Single-sided designs are the most common types of flexible circuits, partly because a single-sided configuration is the least expensive and the most effective variant for dynamic flex applications. They are typically manufactured using printed conductive inks or etched metal to produce the intended circuit patterns on a flexible board’s surface. In the case of a metal etched design, the metal can be made accessible from either one side or both sides of the unit. With two side access, laser removal or a similar method is used to strip off the polymer base film on the board. A specialized variant of the single-sided flexible circuit relies on selective etching to produce a circuit pattern with copper of different thickness levels in specific areas of the board. For example, a thinner copper circuit pattern can be applied in sections where flexibility is a priority, while a thicker pattern may be used for areas requiring greater interconnection. Cover layers are normally applied to single-sided circuits to improve their functional lifespan in dynamic flexing applications, as well as to protect the circuits.

Double-Sided Flexible Circuits

Double-sided designs are the second most common type of flexible circuits and are most frequently used in applications requiring high-density interconnection of circuits. Unlike single-sided models, double-sided circuits feature two layers of metal that are usually connected via a plated-through hole. Some variants, however, rely on a non-plated-through hole configuration, with access to both metal layers achieved from the same side. A standard double-sided circuit design features successive layers of adhesive, metal, adhesive, and cover film on both sides of a base film with a plated-through hole extending from one end to the other. The non-plated-through type has a top side opening for reaching the bottom section.

Multilayer Flexible Circuits

As with double-sided circuits, multilayer flexible circuits are used for applications requiring high-density interconnection. They also have some properties similar to those of rigid circuits, but often feature more complex engineering characteristics designed to address specific electronic and electrical connection issues. A multilayer circuit is typically formed from a combination of several single- or double-sided flexible circuits that are laminated with adhesive and bonded onto a single- or double-sided material. The individual circuits are first fabricated, joined, and plated through, and then joined and plated through again to create a compound assembly with multiple conductive and insulating layers between each interconnected device. It is also possible to construct a multilayer flexible circuit with the various layers branching out from a central point.

Rigid-Flex Circuits

A rigid-flex circuit is a type of hybrid configuration that combines certain design characteristics from both flexible and rigid circuitry. The rigid sections of this circuit are normally used to support components and reinforce connectors, while the flexible portions provide interconnectivity for the rigid sections. This method of combining rigid and flexible properties originally found use in military design applications, where it was employed to improve circuit reliability and produce lighter weight interconnection structures. Like multilayer flexible designs, rigid-flex circuits typically involve a higher degree of engineering complexity than other flexible circuitry. A standard rigid-flex model can rely on integral controlled-impedance flexible cables to link the rigid portions of the device and attach them to the body of the circuit assembly.

Types of Flexible Circuits

Single-Sided Flexible Circuits

Double-Sided Flexible Circuits

Multilayer Flexible Circuits

Rigid-Flex Circuits

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