What are the differences between rigid-flex PCBs and FPCs? Don’t mix them up anymore!

The most confusing aspect of rigid-flex boards and FPC (flexible printed circuit) boards is that they both have "bendable" parts. However, the fundamental difference is clear - FPC is "completely flexible", while rigid-flex boards are "integrated with both flexibility and rigidity".

1) Structural difference: one is fully soft, while the other is a combination of soft and hard

　　FPC(Flexible Printed Circuit)

The entire board is flexible (commonly made of PI substrate and copper foil)

It can bend, twist, and wrap around structures

Usually, reinforcement plates (FR4/steel sheets) are required to strengthen the soldering pad area and the insertion and extraction area

Rigid-Flex PCB

There are both hard areas (rigid PCB) and soft areas (FPC) on the same board

In the rigid area, a large number of components can be directly pasted, and complex multilayers can be made

The soft area is responsible for cross-regional connection, folding, and hinge insertion, etc

Key point: It is not assembled from "a rigid board and a FPC cable", but is laminated into one piece

In a nutshell:

FPC: a piece of "flex circuit"

Combination of soft and hard: a "hard circuit system with soft connections"

2) Difference in usage: FPC is mainly used for "connection", while the combination of soft and hard materials is often used for "system loading and connection"

"The more common role of FPC"

As a flexible cable/interconnection: screen flexible cable, camera flexible cable, button board connection, sensor connection

Pursue thinness, lightness, and flexible wiring

Connect two boards or connect modules

The more common role of combining software and hardware

"Combine multiple boards into one": main board, sub-board, folding connection

Components need to be attached in the hard area, and traces need to be folded and routed in the soft area

Complete machines with higher requirements for reliability, space, and assembly efficiency

3) Process differences: The integration of hardware and software is more complex, with more design constraints

Key points of FPC process

Base material, cover film (Coverlay), reinforcement, window opening, surface treatment

Main concerns: bending life, solder pad reliability, insertion and extraction durability, dimensional stability

Key points of the soft-hard combination process

In addition to the FPC set, it also requires the overlay of rigid PCB for multilayer lamination, interlayer connection, and hard-soft transition structure

Key points and difficulties:

Lamination structure design (soft-hard interface)

Layering/cracking risk control

Impedance and Stack Planning

Bending area copper foil routing and reinforcement design

Therefore, the combination of software and hardware demands higher capabilities from suppliers, and early DFM communication is also more crucial.

4) Cost difference: Generally, a combination of hardware and software is more expensive, but it may result in "overall cost savings"

Based on board price alone: combination of soft and hard components is usually > FPC

However, looking at the complete machine BOM and assembly, the integration of software and hardware may be reduced

Number of connectors

Number of cables

Assembly process and rework

Failure rate due to poor contact/loosening, etc

In mass production, many products are actually "more cost-effective overall".

5) Reliability difference: The combination of software and hardware reduces connector failure points, but the transition area needs to be well designed

Common faults of FPC solutions: poor contact of connectors, loose locks, wear from insertion and extraction, and broken flat cables (caused by use/assembly)

Common risks of the soft-hard combination solution: insufficient lifespan due to layered soft-hard transitions, copper cracking in bending areas, and unreasonable lamination structure

The conclusion is:

The integration of hardware and software significantly reduces "connector failure points", but it is more demanding on design and craftsmanship;

FPC is more flexible and cheaper, but if there are many connectors, high vibration, and tight space, the risk will be higher.

6) How to choose? Here's a straightforward method for you to make a decision

Situations where FPC is preferred:

You just need "flex cable connection", as the components on the rigid board are all at both ends

Space allows for the use of connectors/plug-in structures

Cost-sensitive, small batch sizes, or frequent structural changes

Priority is given to situations where a combination of software and hardware is employed:

You want to integrate multiple boards to reduce connectors and cables

Ultimate compression of product space (folding, rotating shaft, wearing, miniaturization)

High reliability requirements (vibration, shock, long-term use)

For high-speed signal cross-board interconnection, it is desirable to have continuous impedance and a cleaner link

7) The most easily confused "third type": rigid board FPC (external flat cable)

Many projects actually utilize a rigid board (PCB), a FPC cable, and a connector.

This is not called a combination of hardness and softness. The combination of hardness and softness refers to "integrated lamination", where instead of inserting flat cables during assembly, the entire board is directly folded/fixed.