When unfolded, a foldable phone is a tablet, and when closed, it is a square brick - this seemingly simple mechanical action places extremely high demands on the internal circuit connections. Traditional rigid PCBs cannot bend, and traditional wiring harnesses will fatigue and break during repeated folding. The only thing capable of carrying this repetitive folding task is a flexible circuit board (FPC).

Inside a typical foldable phone, FPC is ubiquitous: display driver, touch screen, camera module, fingerprint recognition, antenna, wireless charging module - almost every functional module has an FPC behind it to undertake signal transmission tasks. The reason why foldable phones can withstand hundreds of thousands of opening and closing without failure is the core technical support behind FPC's "flexibility" and "fatigue resistance".

2、 What is FPC: Definition and Basic Structure

Flexible Circuit Board (FPC) is a printed circuit board made of polyimide (PI) or polyester (PET) film as the substrate, which has high reliability and excellent flexibility. This technology originated in the 1970s in the field of space rockets in the United States, initially used to solve the engineering problems of limited space and weight reduction in spacecraft.

The basic structure of FPC is composed of four types of materials:

• Base Film Layer- Commonly used PI or PET, with thicknesses of 12.5 μ m, 25 μ m, 50 μ m, etc.
• Copper foil layer (COPPER FOIL)Rolled copper (RA) or electrolytic copper (ED), with thicknesses of 18 μ m, 35 μ m, and 75 μ m.
• COVER LAYER- Insulation, solder mask and protection, with the same material as the substrate.
• Adhesive (ADHESIVE)- Apply adhesive to each layer.

3、 Why PI: Material Logic of FPC Substrate

Polyimide (PI) is the absolute mainstream in the substrate selection of FPC. The reason why PI has become the preferred substrate for FPC is due to its series of irreplaceable comprehensive properties:

contrast:PET film has a lower cost, but it is inferior to PI in terms of heat resistance, dimensional stability, and mechanical properties, making PI the preferred choice for high reliability applications.

4、 From FCCL to FPC: Two Technical Pathways for Substrate

The manufacturing of FPC began with Flexible Copper Clad Laminated (FCCL), which is the core raw material of FPC. According to the different material structures, FCCL can be divided into two categories:

Trend:The glue free 2L-FCCL has taken the lead in the market, and the FPC industry is accelerating its evolution towards thinner, more heat-resistant, and higher density.

5、 Manufacturing process of FPC: from substrate to finished product

The manufacturing of FPC is a precise multi-step process, and the typical process flow includes:

technological process:Cutting → Drilling → Copper Plating (PTH) → Electroplating → Applying Dry Film → Exposure and Development → Pattern Electroplating → Etching → Peeling → Surface Treatment → Applying Cover Film → Pressing and Curing → Nickel Gold Plating → Character Printing → Electrical Testing → Punching → Final Inspection and Packaging

6、 Classification of FPC: from single panel to rigid flex joint board

• Single panel:Only one side has a circuit, with a simple structure.
• Double sided panel:Both sides have lines connected through conductive holes, resulting in higher density.
• Multilayer board:Three or more conductive layers enable complex wiring and are the core of smartphones and wearable devices.
• Rigid joint plate:Rigid PCB and flexible FPC are integrated, with both load-bearing and bending capabilities, used in aerospace, medical, and military industries.

7、 Key Performance of FPC: What Engineers Should Pay Attention to

8、 Application Landscape: From Consumer Electronics to Power Batteries

9、 Conclusion

The essence of flexible circuit board (FPC) is an engineering solution that extends the traditional rigid circuit board's "two-dimensional planar wiring" to "three-dimensional spatial wiring". It uses PI substrate to provide a "skeleton" that is resistant to high and low temperatures and bending, uses rolled copper foil to ensure the reliability of repeated bending, and uses glue free 2L-FCCL to achieve thinner, more heat-resistant, and higher density circuit design - the three work together to teach the circuit board how to "bend".

From the hinge cables of foldable smartphones to the battery management systems of new energy vehicles, from the micro connections of smartwatches to the lightweight wiring of spacecraft - FPC is freeing the "internal space" of electronic devices from rigid constraints. Understanding the material logic of FPC (why PI), structural differences (2L vs 3L), and selection criteria (copper foil type, substrate thickness) is the key to upgrading FPC from a "bendable board" to an "engineering decision".