In the watch straps of foldable phones and smartwatches, there is a type of circuit board that can bend like paper - flexible printed circuit board (FPC). It can keep the circuit smooth through repeated folding and bending, not by magic, but by special materials and precise manufacturing processes. From material selection to final molding, every step of FPC needs to balance "flexibility" and "conductivity", which is much more difficult to manufacture than ordinary rigid PCBs.

The "special material" of flexible circuit board: the secret of being soft and conductive

The core of FPC is a "flexible substrate", just like the "elastic skeleton" of a circuit board. The most commonly used substrate is polyimide (P) film, with a thickness typically between 25-125um (equivalent to the thickness of several A4 papers). This material can withstand high temperatures above 260 ℃ and will not break even after repeated bending 100000 times. At the same time, it has excellent insulation performance and can prevent short circuits in the circuit. The FPC of a folding screen mobile phone uses a 50um thick PI substrate. After 150000 folding tests, the substrate has no obvious cracks, far exceeding the national standard requirement of 100000 times.

The surface of the substrate needs to be covered with a layer of copper foil as a conductive line. There are two types of copper foil used for FPC: rolled copper and electrolytic copper. Rolled copper is made by rolling, with excellent ductility and the ability to bend along with the substrate, making it suitable for scenarios that require frequent folding, such as folding screen hinges; Electrolytic copper is formed through electroplating, which has a lower cost but slightly lower ductility, making it suitable for devices with fewer bending cycles (such as smart wristbands and watch straps). Test data shows that rolled copper can withstand 200000 bends at a bending radius of 0.5mm, while electrolytic copper has a lifespan of approximately 100000 bends under the same conditions.