Designing with Ultra-Thin Flexible Printed Circuit Boards

Estimated reading time: 6 minutes

Designing with flexible PCBs is not much different from doing the same with rigid boards, except that the designer must account for the mechanical complexity associated with flex circuits. For instance, a flexible PCB can tear if flexed beyond its capability during installation. Therefore, it is very important to create a mechanical model of the PCB and test it for a proper fit, before taking up the electrical design. This would also involve testing the ergonomics of the installation, any misalignments, and servicing. Additionally, it also makes it necessary to understand the different types of flex circuits available and the way they work. 

Types of Flexible PCBs

Depending on the application, several types of flexible PCBs are available, chief among them being the flex, rigid-flex, and te high density interconnect (HDI) flex types.

Flexible PCBs

These are flexible versions of the commonly available rigid PCBs, with unique capabilities such as flexibility and vibration resistance. The extra features come along with the usual reliability, repeatability, and high-density already offered by the rigid PCBs. The major advantage over the rigid PCBs is flex circuits can assume three-dimensional configurations. One of the most common applications of flexible PCBs is as a replacement for wire harnesses.

Rigid-Flex PCBs

These PCBs are a blend of the rigid and flex, offering the best of both constructions, while adding some unique capabilities that neither possesses alone. For instance, a typical rigid-flex configuration would be a series of rigid PCBs linked by integrated flex circuits. By integrating rigid areas added to the flexible parts, designers can greatly increase the design capability of their circuits.

While the rigid areas are excellent as hard mounting points for chassis, connectors, and components, the flex areas offer dynamic flexing, vibration resistance zones, and flex-to-fit. Such blending offers designers multiple options for arriving at creative solutions for the most demanding applications.

HDI Flexible PCBs

HDI PCBs are useful where the options offered by typical flexible circuits are not adequate. HDI flex circuits offer better design, layout, and construction options by incorporating fine features such as microvias. They offer increased functionality, smaller form factor, and highly dense flex circuitry.

Although it uses thinner materials, HDI technology offers better reliability, improved electrical performance, and access to advanced IC package use.

Advantages of Flex Circuits

As a replacement for a ribbon cable, or discrete wiring, flex circuits provide customized repeatable routing paths throughout the assembly. Flex circuits can reduce service calls as they offer better dependability.

Flex boards cover their conductors with polyimide, a dielectric layer that protects the circuit more than the simple solder mask. Manufacturers use other base and cover materials to suit a broader range of ambient conditions and harsh environments.

Although flex boards can be very thin, they can stand long duty cycles of flexing. Using suitable design material, they can be made robust enough to withstand millions of flexing cycles, while carrying power and signal without interruptions.

The low mass and high ductility of flex circuits is a great advantage when it faces high acceleration and/or vibration. The stress and impact on itself and solder joints on the flex PCB are far lower than that faced by the solder joints and components on a rigid PCB under the same working conditions.

Uses of Flex Circuits

Designers shape flex circuits to fit where it is impossible to use any other type of PCB. One can think of flex circuits as being a hybrid combination of the ordinary PCB and round wires, while exhibiting their individual benefits. With flex circuits, one can retain the precision, density, and repeatability of regular PCBs, while achieving unlimited freedom for packaging geometry.

A very commonplace use of flex circuits is to replace the wiring harness. This allows a single flex circuit replacing several connectors, cables, and hardboards, in one operation. The assembly proceeds much faster, because of elimination of the need to color code wires and wrap them in bundles. Volume production levels go up, while installation costs come down, and there are lower chances of rejects during assembly and in-service failures.

Replacing wire harnesses with flex circuits increases the repeatability of wire routing. It eliminates errors during wire routing, reduces rejections, rework, and test times. The connections are more robust, because flat foil conductors can dissipate heat better and carry more current than do round wires of the same cross-sectional area. As the designer decides the conductor pattern in a flex circuit more uniformly, the impedance, crosstalk, and noise is under better control.

Additionally, use of flex circuitry can reduce the space and/or weight of conventional wiring by up to 75%. Compared to the use of wire harnesses, the recurring costs of flex circuits are lower. As flex circuits are more resistant to vibration and shock, costs for repair and replacement are far lower compared to those incurred for hard boards. Moreover, by placing bonded stiffeners at required areas, surface mount components can be easily mounted on flex boards.

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