Semi-Flex PCB: All You Need To Know


Oakley Mae

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Engineers must use a flexible or semi-flex PCB when designing compact electronics and devices with tighter spaces.

From their names, we can tell that both flexible circuit boards and semi-flex boards can bend and twist thanks to their flexible materials. 

However, when fabricating semi-flex circuit boards, manufacturers combine traditional board fabrication processes like drilling, etching, and photolithography with technologies for handling flex substrates.

As a result, they generate a product that can bend limited times than a flexible circuit board. But still, this circuit board presents some benefits worth your attention. 

Today’s article looks into semi-flex PCB boards, highlighting their advantages, applications, fabrication problems, and solutions. 

Table of Contents

What’s A Semi-Flex PCB Board

Now that we all understand flexible circuit boards, we can borrow from them to define a semi-flex board.

Generally, a semi-flex PCB is simply a flexible circuit board that follows traditional board fabrication processes like drilling, etching, and photolithography.

Therefore, unlike flexible PCBs that can bend limitless times, a semi-flex product can flex to a certain degree while maintaining its structural integrity. 

Semi-flex circuit boards are common in tight electronic spaces where manufacturers try to avoid connectors.

These boards have the following specifications: 

Base material: FR-4 is the base material manufacturers use in these circuit boards.

Flexible area: We achieve flexibility in these circuit boards by sequential stack-up or z-milling the flexible area in FR-4 material.

As a result, the FR-4 will provide a certain degree of static flexibility, which could be up to ten bending cycles. 

Lines and spaces: We restrict the board’s spacing width and minimum trace to guarantee trace reliability and strength. 

Layer count: There is no limit to the number of layers you can have in your FR-4 semi-flex circuit board. However, the flexible part can only hold up to two layers. 

Surface finish: You can have all surface finish types on the FR-4 semi-flexible circuit board. 

Solder mask: Finally, you can use either a flexible solder mask or PI coverlay as the solder mask in the circuit board’s flexible area.

Moreover, other board sections can do well with a standard liquid solder mask. 

Advantages of Semi-Flex PCB Boards

PCB fabrication process

(PCB fabrication process)

This section outlines the advantages of using semi-flex circuit boards. 

Low cost: We manufacture semi-flexible circuit boards using epoxy resin FR-4.

Afterward, we employ depth milling to reduce the traditional material thickness selectively.

The aim here is to create a good flexibility level.

Generally, the semi-flex PCB fabrication process reduces manufacturing costs.

Moreover, these boards are the perfect solution for cost-challenging uses. 

High reliability: Semi-flex circuit boards are gaining increased use in electronics thanks to the limited connector entanglements.

And after you’re through with the fabrication process, it’s easy to bend the PCB into its final shape without having to deal with the flexible substrate.

As a result, you eliminate the reliability challenges resulting from flexible and rigid substrate contractions. 

Time-saving: A semi-flex circuit board saves time regarding design and assembly. Moreover, it’s easy to integrate these circuit boards into boxes and housings. 

Flexibility: Although semi-flex circuits don’t have as much flexibility as their flexible counterparts, they still offer enough flexibility to fit into smaller, tighter areas.

Therefore, you’re not limited to designing an electronic device based on the circuit board specifications.

Moreover, when you use a semi-flex PCB, you’ll achieve flexibility in your connections. 

Special installation performance: With the semi-flex circuit boards, you can achieve local bending depending on your product requirement. 

Applications of Semi-Flexible PCB Boards

Fragment of a flexible electronic board

(Fragment of a flexible electronic board)

Semi-flex circuit boards have a high quality and flexibility that allow application in the following areas: 

Automotive: Semi-flex circuit boards are critical in the automotive industry, especially in applications without bending area dynamic stressing.

Some vehicle parts on these circuit boards include airbag systems and antilock brakes. Moreover, they are common in engine control systems. 

Industrial electronics: Since semi-flex circuit boards have high reliability and enhanced flexibility, manufacturers employ them to create industrial electronics. 

Consumer electronics: Some consumer electronics like hearing aids devices and smartphones use semi-flex circuit boards. 

Medical devices: You’ll find semi-flex circuit boards in medical devices like hearing aids and heart

monitoring devices: Most telecommunication equipment like GPS trackers and satellites use semi-flex circuit boards. 

Differences Between Semi-Flex and Flex PCBs

An old rigid circuit board

(An old rigid circuit board)

Although semi-flex and flexible circuit boards can bend without damaging their structural characteristics, they still experience some unique differences. 

For example, a flexible circuit board is fabricated on a flexible polymer film base.

Then, you add conductive material to the film to facilitate signal and power transmission.

Moreover, a coating layer is meant to protect the board from damage.

The flexible film allows you to bend the board severally without damage. 

On the other hand, semi-flex PCBs combine flexible technologies and traditional approaches like drilling, etching, and photolithography to create a functional product.

These boards can only be bent in specific areas you intend to flex.

Therefore, a semi-flex PCB is similar to a rigid board but has predetermined flexible sections.

However, it would be best if you treated these sections to prevent cracking cases. 

Generally, you can bend flexible circuit boards limitless times, while a semi-flex board can only twist a few times. 

FR-4 Semi-Flex PCB Fabrications Problems and Solutions

A PCB fabrication machine

(A PCB fabrication machine)

One common semi-flex circuit board problem is controlling the PCB’s tolerance and precision of depth control milling.

Therefore, you must at all times check oil peeling and resin fractures.

Generally, it would be best to focus on the following during depth control milling. 

  • Resin material
  • Milling tolerance
  • Board thickness

Depth controlling milling test A: Use the mapping approach to carry out depth milling for 0.25mm, 0.275mm, and 0.3mm thicknesses. 

Depth controlling milling test B: For this option, the copper thickness between the solder mask and L2 should be between 0.213mm and 0.188mm.

Also, pay attention to the warpage that could damage the overall thickness. 

Depth-controlling milling test C: After prototyping the panel, you should conduct depth-controlled milling to ensure the size is fixed at 6.3”X10.5”. 


What are the semi-flex PCB operating conditions

The following operating conditions define semi-flex PCBs:

  • A maximum bending angle of 180 degrees
  • Bending radius of at least 5 mm for semi-flex1-deep-miled and 3 mm for semi-flex2-rigid-flex technology. 

What’s the semi-flex PCB manufacturing method?

Generally, semi-flex circuit boards undergo a straightforward manufacturing process that borrows a lot from the fabrication of rigid FR-4 PCBs.

After fabrication, you’ll apply a flex coverlay on the bending area.

Remember, a semi-flex PCB doesn’t use a solder mask ink. 

Afterward, you deep-mill the section below the flexible overlay.

In short, you reduce the thickness of the bending area until you can fully bend the circuit board. 

Final Remarks

In conclusion, semi-flex circuits are flexible circuit boards that follow traditional fabrication processes like drilling, etching, and photolithography.

In short, its fabrication process borrows so much from the rigid FR-4 fabrication process.

However, apply a flexible coverlay instead of the solder mask ink on the bending areas. 

Generally, semi-flex PCBs have a lower cost, high reliability, and excellent flexibility.

Moreover, they are time-saving and provide special installation performance.

Some application areas for these boards are medical devices, industrial electronics, consumer electronics, and automobiles.