FFC vs FPC: How to Choose a Flexible Circuit

The Two Families

Flexible interconnects are grouped under two names that are often used interchangeably and are not the same thing. A flat flexible cable, or FFC, is a set of parallel round or flat conductors laminated between two thin plastic films. A flexible printed circuit, or FPC, is a genuine circuit board built on a flexible substrate, with etched copper traces, pads, vias and sometimes components.

The distinction matters because the two are made by different processes, they have different electrical and mechanical capabilities, and the price difference can be large. Choosing the wrong one either pays for capability the product does not need or produces a part that fails in the field.

How an FFC Is Built

An FFC is made by laying a row of flat conductors between two films, usually polyimide or polyester, and bonding them together. The conductors run parallel and usually at a standard pitch, and the ends are often reinforced with a stiffener so that they can be inserted into a connector.

Its characteristics follow from that construction. It is thin, light and inexpensive at volume, it is available in standard pitches and lengths, and it is intended for a simple point to point connection. Its limitations are equally structural: it generally has one layer of conductors, it has no vias, no pads for components and no shielding, its conductors are round or rectangular rather than impedance controlled traces, and its flex life under repeated bending is limited.

How an FPC Is Built

An FPC starts from a copper clad polyimide film. The traces are imaged and etched, holes are drilled and plated, a coverlay protects the conductors, and stiffeners are added where connectors or components are mounted. The result can be a single sided, double sided or multilayer circuit with controlled impedance, shielding layers, fine features and mounted components.

That construction gives it the design freedom of a rigid board with the mechanical behaviour of a film. It can be routed in three dimensions, it can carry a high speed differential pair with a defined impedance, and it can be given an electromagnetic shield layer or a ground plane. It is also more expensive, needs a custom tooling and a longer lead time, and its design has to respect the bend radius rules of a flexible circuit.

The Differences That Decide the Choice

• Design freedom: an FFC connects A to B with a parallel bus; an FPC can route a complex netlist, include vias and mount components.
• Layer count: an FFC is essentially one conductor layer; an FPC can be single sided, double sided or multilayer.
• Electrical performance: an FFC suits low frequency signals and power; an FPC supports controlled impedance, differential pairs and shielding for high speed or noise sensitive connections.
• Flexibility: both bend, but an FPC designed for it withstands far more dynamic cycles. An FFC is better suited to a bend that is formed once or a small movement.
• Environment: an FFC is adequate in a controlled environment; an FPC handles higher temperature, humidity and chemical exposure better, because the polyimide and the construction are more robust.
• Current and thermal: an FPC can use wider copper, multiple layers and a ground plane to carry more current and spread heat, which an FFC cannot.
• Cost and lead time: an FFC is cheaper and available as a catalogue or semi custom part; an FPC is a custom board with tooling, a longer lead time and a higher piece price.

When an FFC Is the Right Answer

An FFC is the efficient choice when the requirement is a simple connection between two assemblies at a standard pitch, in a product where the cable is installed once and does not move much. Typical examples are a display or a keyboard connected to a main board, a printer head connection, and a sensor or a battery connected in a compact consumer device.

It is also the right answer when the volume is high and the connection is standard, because the part can be sourced at a low cost and the design effort is minimal. What it should not be used for is a signal that needs a controlled impedance, a connection that flexes continuously, or an environment with heat, moisture or chemicals.

When an FPC Is the Right Answer

An FPC is the right choice when the connection has to carry a fast signal, when several subsystems have to be connected in three dimensions, when the circuit has to fold into a shape a rigid board cannot reach, or when the environment is aggressive. It is also the answer when the moving part of the product is the circuit itself, as in a foldable phone hinge or a device that opens and closes in use.

It is usually the correct choice for a product where the interconnect has to be impedance controlled, shielded or capable of mounting components to save space. The extra cost buys capability that the connection genuinely needs, which is the only justification that holds up. Our notes on PCB design and layout cover the flexible layout rules.

Connectors and Assembly

Both cable types usually terminate in a connector on the board. The common types are the zero insertion force connector, which opens its contacts to let the cable in, the low insertion force type, and the non zero insertion force type, which relies on the contact pressure. The choice affects the assembly process, the insertion force the operator has to apply and the number of mating cycles the joint survives.

Three details decide whether the assembly is reliable. The first is the contact orientation, whether the exposed conductors face up or down in the connector, because a reversed cable fits and does not work. The second is the stiffener and the contact finish, because the cable end has to be rigid enough to insert and the exposed conductors have to resist wear and oxidation over the life of the product. The third is the strain relief, because a cable pulled at an angle loads the connector contacts and the solder joints on the board.

Routing matters as much as the part. The cable should meet the connector straight rather than at an angle, the bend radius should be at least several times the cable thickness, and the cable should be held by a feature of the enclosure rather than by the connector alone. On a flexible circuit, the bend should be designed with a single conductor layer and no vias in the moving area. Our notes on flex PCB assembly describe how these assemblies are built.

Reliability and Life

An FFC fails in service in one of two ways: the bond between the conductor and the film delaminates after repeated bending, or the exposed conductor at the connector wears and the contact resistance rises. Both are accelerated by movement, by heat and by contamination.

An FPC fails in service when the copper cracks in a bend area, when the coverlay lifts, or when a plated through hole in a flex region is stressed. All three are design issues: the bend radius, the copper type, the trace orientation and the position of the vias and the coverlay openings relative to the bend. A flexible circuit designed with those rules and built by a fabricator that has run the material before is a reliable part; one designed as a flat board that happens to bend is not. Our notes on PCBA testing describe how flex life and contact resistance are verified, and our notes on PCB manufacturing describe the fabrication process.

What Drives the Cost

An FFC is a low cost part at volume, largely independent of the circuit because there is no circuit, with the cost set by the pitch, the length, the number of conductors, the stiffener and the shielding if any.

An FPC is a custom board, so the cost follows the layer count, the line width, the panel utilisation, the coverlay, the stiffeners and the tooling. The engineering and the tooling are a one off charge that a low volume programme feels heavily and a high volume programme hardly notices. Our notes on quality management cover how these parts are controlled in production.

FAQ

Is an FFC a kind of FPC? No. An FFC is a set of parallel conductors laminated between films, while an FPC is a fabricated flexible circuit with etched traces, vias and pads.

Can an FFC be shielded? Not easily or cheaply, which is one of the reasons it is unsuitable for a noise sensitive or a high speed connection.

Which one lasts longer in a moving application? An FPC designed for dynamic bending, with the right copper and bend radius. An FFC is intended for a static bend or a small movement.

Can components be mounted on a flexible cable? Only on an FPC, and only on a stiffened area, where the joint has the mechanical support it needs.

How do I know which one a quotation refers to? Ask for the layer count and the construction. If it has one conductor layer and no etched circuit, it is an FFC; if it has a defined stack, trace widths and a coverlay, it is an FPC.

Conclusion

FFC and FPC solve the same problem at different levels of capability. An FFC is the cheap, standard bridge between two assemblies in a controlled environment, and an FPC is a real circuit that happens to be flexible, with the routing freedom, the impedance control, the shielding and the flex life that a demanding product needs. Match the part to the requirement rather than to the price, check the connector orientation and the strain relief, and design the bend with the rules the material demands.