Flexible Circuit Board Types: Single-Sided to Rigid-Flex

A flexible circuit is not simply a thin rigid board. It is built on a different material system, assembled with a different set of processes, and available in several constructions that trade flexibility against wiring density and cost. Knowing which construction suits a design is the difference between an elegant solution and an expensive one that fails at the first bend.

What Makes a Flexible Circuit Different

The base material is a polymer film rather than a woven glass laminate, and the most common choice is polyimide because it tolerates solder temperatures and has good dimensional stability. Polyester film is used for lower-cost, lower-temperature applications, and other films appear where a specific property is required. The conductor is usually rolled and annealed copper, which withstands repeated flexing far better than the electrodeposited foil used on rigid boards.

The fabrication sequence differs accordingly. A flexible circuit board is covered with a protective layer rather than with solder mask, and the coverlay is cut and laminated rather than printed. Because the material is thin, the panel is fragile, and every handling step has to be planned. The result is a circuit that can be folded into a product, weighs very little and occupies a fraction of the volume of an equivalent rigid assembly.

Single-Sided Flexible Circuits

The single-sided construction is the simplest and least expensive. One conductive pattern is etched on one side of the insulating film, and the conductor is usually rolled copper. It is the right choice where the electrical requirements are modest, the routing can be done on one layer, and the assembly needs to be flexible only in one direction.

Applications are common in consumer products: interconnect inside a printer cartridge, a link inside a display module, or a folded connection between two sections of a handheld device. The limitation is that the single layer has to carry every net, so crossovers are impossible and the routing must be planned carefully. Where a crossover is unavoidable, a jumper or a second layer is needed, and the design has moved to the next construction.

Adding a second conductor layer doubles the routing capacity but also doubles the process complexity.

Double-Sided Flexible Circuits

In a double-sided circuit, conductive patterns are etched on both sides of the base film and connected by plated holes. That allows nets to cross using the opposite layer, which removes the routing restriction of the single-sided type and makes higher circuit complexity practical.

A coverlay is applied to both sides to protect the conductors and to indicate where components are to be placed. The critical design parameters are the same as on a rigid board – hole size, annular ring, trace width and spacing – but the tolerance for mechanical stress is different, because the plated holes and their annular rings are the places where a flexing circuit is most likely to crack. Holes should be placed away from the bend region, and the copper around them should be as uniform as the design allows.

Multilayer Flexible Circuits

A multilayer flexible circuit laminates three or more conductor layers, which may themselves be single-sided or double-sided constructions, and connects them with plated holes. The advantage is density: the same circuitry that would need a large double-sided area can be folded into a much smaller volume, and the assembly can eliminate separate connectors and cable harnesses.

The trade-off is stiffness. Each additional layer increases the thickness, and a thicker stack bends less easily and requires a larger bend radius. There is no theoretical limit to the number of layers, but the practical limit is set by the assembly: the design has to balance assembly dimensions, layer count and flexibility at the same time. Where the circuit needs both a dense section and a freely bending section, the usual solution is to keep the layer count low in the bending region and to unbundle the construction elsewhere.

Rigid-Flex Constructions

A rigid-flex circuit combines rigid sections and flexible sections in one laminated assembly. The rigid areas carry the components and the dense routing, and the flexible areas connect them, replacing a cable and two connectors. The layers are selectively bonded so that some layers continue through the flexible region and others do not.

The reason to use this construction is reliability and space. Every connector that is eliminated removes a failure point and a mechanical interface, and the assembly becomes a single part that can be mounted in one operation. The counter-argument is cost: the process is more complex, the panel yield is lower and the design rules are tighter. Keeping the layer count as low as the routing allows, and keeping the flexible regions short, is the usual way to control both cost and risk. For a first prototype of this type, the requirements for multilayer prototypes are a useful starting point for what the fabricator will need.

The highest-density versions of rigid-flex combine it with HDI, which changes the fabrication flow again.

HDI Rigid-Flex

Adding high density interconnect to a rigid-flex construction produces the highest-performance flexible product: microvias in the rigid areas, blind and buried vias where routing demands them, and flexible regions where the assembly needs to fold. This combination is used where the product has to be small, fast and multifunctional at the same time, and where the cost is justified by the space and reliability gained.

The fabrication flow is the most demanding of all the constructions, because HDI fabrication and rigid-flex lamination both have to succeed on the same panel. The microvia formation happens before the final lamination, and the flexible materials must survive the process temperatures. Designers should expect a longer quoting and prototyping cycle and should plan the panel and the tooling early.

Choosing Between the Types

The decision follows from four questions. How many nets must cross in different directions? How many times will the circuit bend, and how tight is the bend radius? Is the assembly weight or volume critical? And what is the cost target? A single-sided circuit answers the first question only if the routing is simple. A double-sided circuit handles crossings and remains flexible. A multilayer circuit buys density at the cost of stiffness. Rigid-flex buys reliability and assembly simplification at the cost of process complexity.

Whichever construction is chosen, the mechanical outline has to be defined with the same rigour as a rigid board. Bend regions, stiffener areas and connector locations belong in the drawing, and the board outline and mounting features should be agreed with the fabricator before the artwork is released.

FAQ

How many times can a flexible circuit be bent? A static bend, formed once during assembly, is very different from a dynamic bend that flexes continuously. Dynamic applications need thin copper, a generous bend radius and careful conductor orientation relative to the bend line.

Why is polyimide used rather than a cheaper film? It withstands soldering temperatures and holds its dimensions through processing. Cheaper films are suitable for lower-temperature applications where no soldering is performed on the flexible part.

When is rigid-flex worth the cost? When it removes connectors and cables from a product that also needs to be compact or highly reliable. The saved connectors and assembly steps often pay for a large part of the difference.