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Rigid-Flex PCB: A Genius Mix of Rigid PCB and Flexible PCB

PCBONLINE Team

Thur, Jun 02, 2022

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rigid-flex PCBs

Once, a guy left a comment that said the above circuit boards were merely rigid FR4 PCBs connecting with soft adhesive.

Well, that's wrong. These circuit boards are rigid-flex PCBs, also known as semi-flex PCBs.

On the whole rigid-flex board, you can design any areas to be bendable and any other places to be rigid. Circuits are interconnected in the whole rigid-flex PCB, and you don't need any independent connectors or jacks to take up the compact space anymore!

Part 1: Rigid-Flex PCB is a Genius Mix of An FPC and rigid PCB Layers

From the top to the bottom, a rigid-flex PCB is a PCB that has a flexible section and one and above rigid sections that are interconnected.

rigid-flex PCB structure

The flex section refers to the flexible PCB layers in the middle. Its size is the same as the rigid-flex PCB size.

The rigid sections refer to the rigid PCB layers on the top and bottom of the flex section.

As you can see, a rigid-flex PCB basically is a flexible PCB having rigid layers.

So how did engineers come up with such an ingenious design?

rigid-flex PCB board

Rigid-flex PCBs were developed from aerospace research for reliable wiring in a compact space. High-density wiring, flexibility, limited space, lightweight, and mechanical strength were all the challenges that were met. Rigid-flex PCBs offer a perfect solution to these challenges.

Now, rigid-flex PCBs are not only used in aerospace, they also find applications in medical, consumer electronics, industrial, communication, automotive, defense, and research.

Part 2: Rigid-Flex PCB Manufacturing

PCBONLINE is a rigid-flex PCB manufacturer. In rigid-flex PCB fabrication, we first fabricate the flexible section and then laminate the rigid sections on the flexible section.

According to different designs, rigid-flex PCBs need different fabrication processes.

Here we reveal the normal manufacturing process for rigid-flex PCBs below.

Step 1. Prepare the overlay, prepreg, flexible laminates, and rigid PCB laminates

flexible PCB lamintes

First, we need to prepare the overlay for the flex section, prepreg (PP), flexible laminates, and rigid PCB laminates and cut them to be in the required size.

Note
For the rigid sections, the areas that will cover the to-be-exposed flex section have only PP. So in these extra rigid areas, we need to remove the copper foil from the laminates.

Step 2. Drill on coverlay, PP, rigid laminates, and flex laminates

flexible PCB fabrication

The PCB layers are interconnected by different vias, and we need to drill to make the holes.

On the coverlay, PP, rigid laminates, and flexible laminates, we locate mechanical drill positioning holes.

If the rigid-flex PCB has HDI requirements, we also need to laser drill microvias.

Step 3. Pre-process overlay, PP, and rigid laminates

For the rigid section, the areas that cover the to-be-exposed flex section need to be removed at the end of rigid-flex PCB fabrication.

For this reason, we need to pre-process the overlay, PP, and rigid laminates so that these extra rigid areas can be removed easily later.

Step 4. Circuit generation on the flex section

If the flex section has only 1 layer, we transfer the circuit image on the laminates circuit, etch the rolled copper, and keep only the circuit copper traces.

If the flex section has 2, 3, 4, 5, or 6 circuit layers, we need to first generate circuits on the core layers, laminate the alternating layers of PI and rolled copper on the core, and then generate circuits on the rolled copper layers.

Step 5. Laminate coverlay on flex PCB layers

After the circuits are generated in the flex section, we need to implement the automatic optical inspection (AOI). Then we add coverlay to the flex section and laminate them.

Step 6. Laminate the rigid laminates on the flex section

At this step, we need to laminate the alternating layers of copper and PP on the flex section.

Step 7. Drill plating-though holes and generate circuits on rigid sections

If the rigid-flex PCB has only one or two rigid PCB layers, we drill PTH holes on the whole rigid-flex PCB and then generate the circuits on the rigid laminates.

If the rigid-flex PCB has 4 or above rigid PCB layers, we need to generate circuits on the internal rigid PCB layers, laminate the alternating layers of PP and copper on the core, drill PTH holes on the rigid-flex PCB, and then generate circuits on the two external rigid PCB layers.

Step 8. AOI, solder mask, surface finish, and silkscreen

After all circuits are generated on the rigid-flex PCB, we implement the AOI to ensure the rigid-flex PCB doesn't have catastrophic defects.

Then we apply solder mask, surface finish, and silkscreen on the rigid-flex PCB.

Step 9. Remove the extra rigid areas on the to-be-exposed flex section

We use the laser to remove the extra rigid areas. In this way, the coverlay of the flex section is exposed. And the rigid-flex PCB can bend now.

Next, we need to route out the PCB shapes on the production panels.

Then, we implement the power-on self-test to the rigid-flex PCB to ensure the PCB can well function.

At PCBONLINE, if the rigid-flex PCB is used for defense, medical, automotive, and aerospace, we also need to implement the four-terminal sensing test to prevent the minimum defects or resistance tolerances.


The above process is only for the most common rigid-flex PCBs. Form the rigid-flex PCB manufacturer PCBONLINE, apart from common rigid-flex PCBs, you can also order high-density rigid-flex PCBs, FR4 PCBs bonded with a clear flexible PCB layer, rigid-flex PCBs whose exposed flex section is mounted with components, etc.

Part 3: One-Stop Rigid-Flex PCB Manufacturer

PCBONLINE provides one-stop rigid-flex PCB manufacturing, including PCB design, PCB fabrication, component sourcing, PCB assembly, and box-built.

rigid-flex PCB supplier PCBONLINE

We are not only a rigid-flex PCB fab. If you let our engineering team join your project in the research and development stage, we can help solve technical problems and reduce manufacturing costs. Our PCB engineers will work with engineers on your side and provide a reasonable PCB design.

PCBONLINE has no quantity limit for rigid-flex PCB orders. Prototypes and bulky production are both available.

Please refer to our rigid-flex PCB manufacturing capabilities below:

PCBONLINE's rigid-flex PCB capabilities
Flex section layer
1 to 6
Rigid-flex PCB layer
2 to 30
Minimum rigid-flex PCB size
4mm × 4mm
Maximum rigid-flex PCB size
457mm × 610mm
Rigid-flex PCB thickness
0.5mm to 6mm
Minimum trace width/space
1.6mil/1.6mil
Through-hole size tolerance
PTH: ±2mil, NPTH: ±1mil
Flex section copper thickness
1/3oz to 3oz
Rigid section copper thickness
1oz to 6oz
Minimum mechanical via aperture
0.15mm
Minimum laser blind via aperture
0.1mm
Minimum PTH via pad aperture
0.35mm
Minimum blind via capture pad aperture
0.25mm
Minimum PP layer thickness of the rigid section
38µm

If you have any rigid-flex PCB needs, please feel free to contact PCBONLINE and send your Gerber/IPC-2581/requirements by sending an email to info@pcbonline.com.

Part 4: Rigid-Flex PCB Applications in Flexible Electronics

Flexible PCBs empower flexible electronics, and rigid-flexible PCBs add more possibilities to flexible electronics.

For some flex electronics applications, mechanical support to the circuits is also needed. Though flexible PCBs can add stiffeners, they can't provide circuit connection or mount components as rigid-flex PCBs.

In flexible electronics, rigid-flex PCBs can be used for:

1. Rigid-flex PCBs for smart wearable devices

rigid-flex PCB for wearable devices

Above is a set of PCBs manufactured from PCBONLINE for wearable VR products, which is a rigid-flex PCB and two rigid PCBs.

Rigid-flex PCBs can be used for wearable electronic products, such as VR headsets, wireless headsets, smartwatches, and smart shoes/rings/glasses/clothes.

Rigid-flex PCBs for automotive electronics

rigid-flex PCB for automotive

Cars and vehicles are getting smarter and more electronic-driven. Rigid-flex PCBs can work in the compact space at high temperatures in vibration.

In a car, rigid-flex PCBs can be used for control unit modules, sensors, switches, etc.

3. Rigid-flex PCBs for communication electronics

Rigid-flex PCBs are used for cameras in mobile phones, the bending areas of the display screen of notebooks, screen signal connection, and any mobile devices.

4. Rigid-flex PCBs for medical, defense, aerospace, and industrial electronics

Rigid-flex PCBs can be used for any bending areas in medical, defense, aerospace, and industrial electronic instruments and devices, such as wearable health detection devices, control systems, rotation axis in satellites, industrial control panels, etc.

Conclusion

Rigid-flex PCBs combine flexible and rigid PCB technologies. Via locating and laser drilling are the reason for the difficulty in rigid-flex PCB fabrication. The one-stop PCB manufacturer PCBONLINE can give you free design help and suggestions to make sure successful fabrication, provide rigid-flex PCB fabrication, PCB assembly, component sourcing, and value-added services such as box-built. If you have any rigid-flex PCB needs, welcome to contact PCBONLINE by email or online.