Semi-Flex vs Flex PCB: All You Need To Know


Oakley Mae

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The electronics industry is full of debates and comparisons, with the latest being the semi-flex vs. flex PCB.

Although these two boards have similarities, major differences arise, especially concerning design and applications. 

For instance, a semi-flex PCB, as the name suggests, combines some characteristics of flexible PCBs and rigid options.

In short, their manufacturing process involves traditional approaches like etching, photolithography, and drilling.

On the other hand, flex circuits are designed purely from flexible boards that can twist and bend without breakage or damage. 

But that is not all! We still have other major differences that every engineer, designer, or manufacturer should understand.

And that’s why we’re here.

Today’s piece examines the semi-flex vs. flex PCB debate while highlighting their differences and similarities. 

Table of Contents

Understanding Semi-flex PCBs

We can otherwise refer to these circuit boards as semi-rigid PCBs since they borrow from the qualities of both rigid and flexible circuit boards.

In short, these circuit boards can bend without damage but retain some rigidity.

And while we create these circuit boards using flexible circuit board technologies, there are still some elements of traditional approaches, such as etching, photolithography, and drilling. 

You design semi-flex PCBs by combining flexible layers with rigid materials like FR-4.

Next, you etch the layers to create the circuitry and drill the vias to connect the layers electrically.

And after creating the circuitry, you cut and fold the semi-flex PCB to create your desired flexibility and shape. 

Finally, you add the circuit board components using through-hole or surface mount technologies.

Generally, the semi-flex PCB manufacturing process is complex and requires specialized equipment because of combining rigid and flexible materials.

Therefore, expect the manufacturing process to be more costly than the traditional manufacturing process. 

Understanding Flexible PCBs

A plastic strip flexible circuit electronic

(A plastic strip flexible circuit electronic)

As the name suggests, flexible PCBs are electronic boards that can be bent into different configurations and shapes without any cases of damage.

These boards have thin-flexible insulating materials, including polyester film and polyimide. 

Additionally, a flexible circuit board has conductive layers either on both or a single side using technologies like inkjet printing, screen printing, and etching.

Generally, these circuit presents many benefits over traditional circuit boards, including design flexibility, the ability to fit into tighter spaces, and lightweight. 

Furthermore, flexible circuit boards are designed with features that provide better thermal management and resistance against vibrations.

And in most cases, we use them in different electronics, including consumer electronics, medical devices, wearables, aerospace technology, and automotive systems. 

We have the following types of flexible circuit boards: 

Single-layer: These are products with a single layer of substrate and another layer of conductive layer. Therefore, they’re the simplest and cheapest option. 

Double-layer: From the name, we can tell that these PCBs have two conductive layers. Also, we drill vias through the circuit board to connect the different layers. 

Multilayers: This is a high-density circuit board with at least three layers. They’re very complex to create and are more costly. 

Rigid-Flex PCB: Finally, we have rigid-flex PCBs that combine features of the traditional rigid PCBs and flex circuit boards.

Therefore, they are firm enough but can still fold into various shapes. 

Semi-Flex vs. Flex PCB: Differences

A technician holding an electronic flex PCB

(A technician holding an electronic flex PCB)

Major differences between flexible and semi-flexible circuit boards arise in their application and flexibility.

Generally, flexible circuit boards are the most flexible options, with the ability to bend and twist into various configurations and shapes.

Moreover, these circuit boards have a polyimide or polyester substrate and a conductive layer on one or multiple circuit board sides. 

Flex PCBs’ main features include great flexibility, the ability to fit into tighter spaces, and reduced weight/volume.

You can use these boards where rigid circuits aren’t feasible, including wearables, medical equipment, and aerospace technology. 

On the other hand, a semi-flex circuit board is a hybrid product combining flexible and traditional rigid circuit boards.

In most cases, the core is rigid with flexible materials around it.

Therefore, the circuit board uses traditional manufacturing techniques like photolithography, etching, and drilling. 

Semi-flex PCBs are less costly to manufacture compared to flexible circuit boards.

You will likely find them in industrial equipment, mobile electronics, and automobiles. 

Semi-Flex vs. Flex PCB: Similarities

Despite the above differences, these two have major similarities that make them almost interchangeable in their application. 

  • First, both the flex and semi-flex circuit boards are flexible, making them useful in applications with smaller spaces. Therefore, they have many benefits over rigid circuit boards. 
  • Secondly, they use the same material and almost similar structures. In most cases, they use polyester film and polyimide substrates. Also, they commonly use copper as the conductive layer. And besides the substrate and conductive layer, they have a surface finish and through-holes. 
  • Thirdly, they share similar features like improved thermal management, vibration resistance, better flexibility, and reduced weight/volume. 
  • Finally, you can employ the two circuit boards in automotive, consumer electronics, and medical equipment applications. 

Advantages of Semi-Flex PCBs

LEDs on a flex PCB

(LEDs on a flex PCB)

Semi-flex circuit boards have gained much popularity thanks to the following benefits they offer in electronics. 

Less costly: Semi-flex circuit boards combine rigid and flexible materials.

Therefore, you’ll manufacture them more cost-effectively than flexible boards. 

Design flexibility: Although not as flexible as flex PCBs, these boards offer better design flexibility than rigid boards.

Therefore, it’s easy to fit them in tight spaces, thus supporting compact electronic design. 

Improved thermal management: This is one area we think semi-flex circuit boards perform better than flexible circuit boards.

These boards are designed for better reliability and increased heat dissipation. 

Time-saving: If you have ever dealt with semi-flex boards, you understand how easy the design and assembly steps are.

Also, you’ll find it easy to integrate them into housing. 

High component density: These circuit boards support a higher component density than the traditional rigid options. 

Semi-Flex PCB Design Rules

A female engineer designing PCB

(A female engineer designing PCB)

There is no chance of designing semi-flex PCBs or any other circuit board.

Therefore, you must follow these rules for a successful and efficient design and assembly: 

Choose the right material: Almost everything depends on the quality and type of material you select.

For the semi-flex PCB, choose the right substrate and conductive materials.

And where possible, seek assistance from the experts.

Remember, semi-flex PCBs mix both rigid and flexible materials. 

Limit layer count: If a few layers can serve you well, using more than four layers is unnecessary.

Doing so will cause the board to be less flexible and too stiff.

Also, it’ll automatically raise the manufacturing costs. 

Never use sharp corners: We know that sharp corners are a recipe for stress points that translates into cracks.

Therefore, we recommend round corners. 

Use short traces: Generally, using long traces might limit PCB flexibility.

Employ filleting: There is no better way to smoothen the transition between rigid and flexible parts than filleting.

As a result, stress points become minimal or non-existent. 


What materials do we use for semi-flex PCBs?

As mentioned, semi-flex PCBs combine flexible and rigid materials.

Generally, the circuit board core is a rigid material like metal or FR-4.

On the other hand, the flexible material could be polyester film or polyimide.

The rigid part provides stability and mechanical support, while the flexible one offers flexibility. 

And nothing stops you from using thermosetting and thermoplastic materials.

Material selection should depend on mechanical strength, temperature, and chemical resistance. 

What are the main characteristics of semi-flex PCBs?

Semi-flex PCBs have the following characteristics:

  • Reduces manufacturing costs
  • A fine combination of flexibility and rigidity
  • Improved thermal management
  • High reliability than flexible PCBs
  • High component density

Final Remarks

We hope that this article has settled the semi-flex vs. flex debate.

But in summary, semi-flex PCBs combine the flexibility and rigidity of flexible and rigid PCBs, while flexible PCBs are designed fully with materials that bend without damage.

Therefore, semi-flex PCBs use traditional manufacturing approaches like drilling, photolithography, and etching. 

These circuit boards share some features and applications, meaning you can use them interchangeably.