What is an FPC (Flex-Printed Circuit Board)?

FPCB stands for Flex-Printed Circuit Board and is sometimes referred to simply as FPC. Functionally and structurally, FPCs are similar to PCBs (which typically refer to rigid circuit boards). The main difference between FPCs and PCBs lies in their base materials. PCB substrates are similar to glass, making them hard and rigid, while FPC substrates are more like paper—soft and flexible on a flat surface.

FPCs are characterized by being lightweight, bendable, and flexible. This makes them especially suitable for products with limited space where a single rigid PCB cannot meet all design requirements. Some products may require stacking multiple PCBs vertically to make full use of limited internal space, necessitating a flexible FPC for interconnection. Others may use flexible boards to connect distant PCBs for signal transmission in order to save board material, while some designs require PCBs to intersect at vertical or different angles.

Generally speaking, FPCs are not ideal for designing complex circuits or soldering precision electronic components due to the following reasons:

1. Soldering Fragility: The solder joints for electronic components can easily crack on FPCs because of their flexible nature.
2. Lower Heat Resistance: Compared to PCBs, FPCs are less heat-resistant. They are more prone to warping or delamination under high temperatures.
3. Solder Mask Precision Issues: It's more difficult to control the solder mask precision on FPCs. This is because they use a PI (polyimide) film—think of it like a plastic wrap—which typically has its solder pad openings punched with a die before being laminated onto the FPC. This method lacks the precision of the screen printing process used in PCBs, making the openings more prone to misalignment and rough edges.
4. Unsuitability for Multi-layer Circuits: FPCs are not well-suited for multi-layer copper circuitry. As the number of copper layers increases, the FPC becomes stiffer and its bending ability decreases, weakening its main advantage. Moreover, to create precise solder pads, more advanced technologies such as laser-cutting windows on the PI film may be required, which increases costs.

However, you might still see some precision electronic components soldered onto FPCs—like those found in digital cameras. How is this possible? Actually, it’s not too difficult to achieve. A piece of rigid reinforcement (called a stiffener) is attached to the back of the FPC where precision components need to be soldered. This stiffener provides the necessary structural support. However, only one side of the FPC can be used for soldering in the stiffened area, as the stiffener itself has no circuitry—it serves solely as mechanical support. Of course, FPCs with stiffeners are generally more expensive than standard FPCs.