Rigid-Flex PCB - Flex PCB

What is FCB?

Flexible circuit board, abbreviated as FPC, is the abbreviation for Flex PCB, sometimes also referred to as FPCB and FPC. FPC and PCB are similar in function and structure, while PCB (Printed Circuit Board) usually refers to a rigid PCB.

Both FPC and PCB are designed with copper foil circuits on their surface and inner layers for transmitting electronic signals. The biggest difference between the two lies in their substrate materials. The substrate of PCB is similar to glass, making it hard and highly rigid, while the substrate of FPC is similar to paper, soft and bendable on a flat surface.

Characteristics of FPC

FPC has the advantages and uniqueness of being lightweight, flexible, and easy to bend. Therefore, it is particularly suitable for products with limited space that cannot be completed with a single rigid PCB to meet all design requirements.

Some products may overlap multiple PCBs vertically due to the need to fully utilize the limited internal space, therefore requiring bendable FPC for connection.

Some may require the use of FPC boards to connect distant PCB for signal transmission in order to save board material, while others may require certain PCB to intersect with other PCB at a vertical or different angle.

FPC is not suitable for designing complex circuits and soldering precision electronic components because:

1. The solder used to solder electronic components is prone to cracking on easily bent FPC.

2. Compared to PCB, FPC is less resistant to high temperatures. Easy to arch or peel off at high temperatures.

3. The solder mask accuracy of FPC is difficult to control compared to PCB because it uses PI film (you can think of it as a thin film similar to a plastic bag), and the opening of the PI film's solder pads is usually stamped with a die and then attached to FPC. The accuracy is worse than that of PCB screen printing, that is, the opening is prone to offset and unevenness.

4. FPC is not suitable for designing multi-layer copper foil circuits. When the number of copper foil circuit layers increases, FPC becomes harder and its bending ability weakens, which reduces the main advantage of FPC. Meanwhile, in order to produce accurate solder pads, it may be necessary to use advanced laser technology to open windows on the P film mold, which will increase costs.

We can still see people being able to solder some precision electronic components onto FPC, such as many products in digital cameras. How is this achieved?

In fact, it is not difficult to achieve this. Just stick a hard stiffener on the back of the FPC where precision parts need to be soldered. However, in this way, only single-sided soldering of parts can be done at the position of the stiffener, because the stiffener does not have a circuit and only provides pure strength support. However, the cost of this FPC is definitely much higher than that of ordinary FPC.

Flex PCB