Flex PCB | flexible printed circuit board | FPC

FPC stands for "Flexible Printed Circuit" and is - as the name suggests - a flexible printed circuit board. Flexible printed circuit boards, like rigid printed circuit boards (PCB), serve to support the connection of electronic components. FPCs are also often used as cables and are very bendable or even dynamically resilient (many millions of bends). Copper is used as the conductive material in most cases, while the base material of an FPC is usually a polyimide (PI) with or without epoxy adhesive layers.

In contrast to rigid printed circuit boards (PCB), flexible printed circuits (FPC) usually do not use paint as a solderstop, but a yellow coverlay. The openings inside the coverlay for e.g. pads are cut out beforehand. This coverlay is then manually aligned and fixed over the copper image by hand. A yellow coverlay has the advantage that it is a bit more transparent, which simplifies manual alignment. Since this coverlay is then not cured by exposure to light, but is pressed on with heat, the advantages of green solder mask are irrelevant here.

Flexible printed circuits (FPC) are used to connect electronic components via copper tracks. At the same time, a flexible printed circuit can be a carrier of these components. Flexible printed circuit boards are bendable and can, in some circumstances, replace cables more efficiently. Due to very thin copper structures, they can be dynamically resilient, which, with a good design, enables many millions of bending cycles.

In contrast to rigid printed circuit boards (PCB), flexible circuit boards (FPC) are actually in the minority. A main function of any printed circuit boards, in addition to conducting electricity, is usually to “carry” electronic components. In principle, rigid circuit boards are more suitable for this because of their stability. Flexible printed circuit boards are used when a 3-dimensional connection of electronic assemblies is to be created, or when the electronics are to be "movable".

On the one hand, flexible printed circuit boards (FPC) differ in thickness. Very thin materials are only 13µm thick and therefore almost like wrinkled paper. Standard materials of flex PCBs are around 50 µm thick and offer a little more stability in processing and handling. Another difference is the material. Very cheap flex circuit boards are made of PET, but have technical limitations. Flex boards for industrial applications use polyimide (PI) as the base material, either with epoxy (adhesive-based) or without epoxy (adhesive-less/adhesive-free).

Flexible PCBs (FPC) are fully flexible/bendable and will only ever have any vias in these flexible areas (Note: the via areas are bendable but should not be bent as the vias may tear). With a flexible PCB, certain areas can be “stiffened” by gluing on thicker layers of polyimide (PI) or FR4 (this way, e.g. bending in these stiffened areas with vias can be partially prevented). In these stiffened areas, however, there are no vias through the entire board, but only through the flexible, thin part. A rigid-flex PCB (RFPCB), on the other hand, has rigid areas integrated in such a way that vias pass through the entire board, while the flexible parts are routed out of the rigid PCB.

The advantages of flexible printed circuit boards (FPC) lie largely in the possibility of using them like a cable, but also being able to solder components directly onto them. Sometimes sensors that are needed in an area that is further away from the main electronics are soldered, possibly in a movable area. The flexible printed circuit board can then create the connection between the sensors and the electronics under dynamic stress. In other areas, flexible PCBs replace cables and are contacted with one another using suitable ZIF/LIF connectors.

The main component of industrial flexible circuit boards is polyimide (PI). Adhesive-free flex circuits (FPC) are made up of just the polyimide and the copper. More common are adhesive-based flex circuits, where the copper has been bonded to the polyimide using a layer of epoxy. These are called adhesive-based flex circuits. Another material is PET, which is usually only used for low-tech mass applications due to its low thermal stability and other disadvantages.

Flexible printed circuit boards are always manufactured individually according to the data supplied and go through many processes until completion. The costs at Leiton for some prototypes start at around 200 EUR. However, if the flexible printed circuit boards are later mass-produced in larger quantities, they may cost less than 1 EUR or only a few cents. It is therefore difficult to make a general statement. If you are interested, simply send us your data or specifications and we will be happy to calculate the prices for you.

The most well-known application of flexible printed circuit boards (FPC), where everyone has probably held them in their hands, is on the printer cartridges of inkjet printers. Here you can see the flexible circuit board wrapped around one cartridge-corner in the typical brown-yellow color with some golden contacts. The flexible circuit board provides the electrical connection between the printer and the electronics (e.g. chip for identification) within the printer cartridge. In addition, flexible PCBs are used where electrical connections have to be made over flexible paths. Another classic example are today’s very popular LED strips, which are available in all kinds of colors, lengths and designs in every hardware store.