Flat Flex PCB: The Ultimate Guide

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Oakley Mae

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So what is a flat flex PCB and how can you design it?

Generally, the circuit board fabrication process gives rise to three PCB types: flexible, rigid, and rigid-flex.

Then we have flat flex PCB, a special class of flexible PCBs. 

Flat flex PCBs are small outlines, low profile, and lightweight circuit boards that find great application in high-density applications.

Their unique characteristic includes the ability to bend into multiple shapes and continuous dynamic flexing, thus distributing stress throughout the board. 

Moreover, these boards can hold many components finding applications in consumer products, communication, and medical applications.

Today’s piece will help you understand more about flat flex PCB. Let’s get rolling. 

Table of Contents

What are flat flex PCBs?

You’ll hear some people refer to them as flexible flat cables representing electrical circuits or wires with a flat and flexible surface.

Some consumer products like computers and smartphones have these PCBs.

Also, what’s surprising is that these boards are kink-resistant, filler-free, and halogen-free. 

Moreover, almost all flat flex PCBs have the following features:

Flexibility: These circuit boards have ultra-flexible copper and silicone materials, making them easy to bend and ideal in tight areas. 

Longer Lasting: You can use these boards in flexing uses for over 10 million cycles. 

Operation in harsh environments: These boards support applications in a vacuum, making them ideal for designing satellite devices.

And regardless of whether you expose them to chemicals, steam, or salts, you can comfortably employ them between -65°C to 260°C. 

Fire-resistant: Besides functioning well under a wide temperature range, these boards are fire-resistant.

They’re designed to meet all fire standards, including the common UL-94 vertical flame standard. 

Designing A Flat Flex PCB

A flat ribbon cable isolated

(A flat ribbon cable isolated)

Although challenging, designing a flat flexible circuit board could become easy if you follow the steps below.

  1. The first step in any design process involves understanding the electrical parameters relating to the circuit board. Therefore, you must consider important factors like voltage, capacitance limitations, current maximums, and impedance characteristics. 
  2. After you finish with the electrical parameters, it’s time to handle the schematics. Start by defining your board’s shape and determining whether you intend it to be dynamic or static. And your decision here depends on the function your board will serve. Simply put, a dynamic option allows regular flexing than a static board. Furthermore, choose from a list of available quality software, including Altium and KiCad
  3. Align components, ensuring maximum compatibility and minimum noise or interference. Therefore, designing component holes and applying air wire connectors is important. But later, remember to replace the wires with real ones. You can then incorporate markings and reference designators to represent components. This way, any engineer can identify electrical components easily. 
  4. Lastly, you can generate layout files from the software containing all the circuit board formatting information. This final stage means the circuit board is ready for assembly. 

Flat Flex PCB Material Composition

A photo of a flat cable

(A photo of a flat cable)

Material composition is an important aspect of flat flex circuit boards as it determines their quality. Below are the common materials. 

Insulator and cover layer: The most common insulators are dielectrics and polyimide insulators.

The purpose of these materials is to insulate the copper layers. 

Conductor: Copper foil is preferable in flat-flex PCBs due to its high conductivity.

Although we have alternatives like silver and gold, they’re expensive and not cost-effective for such products.

Fortunately, you have to use a small amount of copper, as using a massive amount might make it heavy. 

Finish type: Lastly, we have a surface finish that eliminates copper oxidation and creates a solderable surface.

The most common surface finish is Nickel Immersion Gold (ENIG) which is available in almost 80% of the circuit boards.

Generally, ENIG creates a gold surface layer over the copper with nickel in between.

We also have a worthwhile alternative in Electroless Nickel Electroless Palladium Immersion Gold finish. 

Flat Flex PCB Cost Determiners

An electronic flex printed circuit reflection

(An electronic flex printed circuit reflection)

As an engineer or designer, your goal should be creating a high-performance yet cost-effective board.

However, you might not have a choice as certain factors independently decide the flat flex PCB cost.

Therefore, the following factors increase PCB costs: 

Circuit size and shape: Designing many large boards on a single panel is not feasible.

Therefore, you’re left with the option of using many smaller flat flexible circuits.

However, since these circuits consume more components, they’re often expensive. 

Layer count: Anytime you design a multilayer circuit, you’ll add more materials and use more time per layer.

Also, you might find it challenging to stack up layers; the more layers you use, the higher the cost. 

Circuit class: Funny enough, the PCB’s IPC class determines its price.

In most cases, a class 2 board requires fewer validation coupons and serialization than a class 3 board. 

Dimensional tolerances: The more dimensional tolerance a circuit board is, the more expensive it gets. 

Final finishes: As highlighted, most circuit boards employ a nickel immersion gold finish.

And in most cases, the finish materials cost increases with quantity.

Therefore, the more material you use, the higher the final product cost. 

Blind and buried via A vertical interconnect access routes vertical traces to connect boards and layers.

Generally, you’ll find vias more costly than through holes

Flat Flex PCB Advantages

Image sensor optical laser computer that uses flex PCB

(Image sensor optical laser computer that uses flex PCB)

Flat flex circuits are ideal for designing products requiring weight minimization and limited space.

But generally, they offer the following benefits: 

Constantly reliable: A flat flex circuit board makes it easier to customize a routed path, thereby making the board reliable. 

High-temperature tolerance: Engineers mainly prefer flat flexible PCBs because of their endurance for high-temperature conditions.

They usually use it to design medical, aerospace, and military systems. 

High vibration: Flat flex PCB is highly foldable and allows you to adjust its shape without breakage.

Therefore, they can withstand high mechanical vibrations. 

Better signal reception: Because of their high-speed transmission, you’ll find flat-flex PCBs in high-speed digital systems. 

Extended duty cycles: Flat flex PCBs transmit enormous power and signals without interference, last longer, and withstand large cycles. 

FAQs

Where are flat flex PCBs applied?

These boards are low profile, lightweight, and have small outlines making them effective in high-density applications.

Also, since the materials have a high-temperature tolerance, you can use them in high-temperature environments. 

Moreover, they’re perfect for high-temperature, high-vibration, and high-speed transmission systems.

For this reason, you can apply them to design medical, aerospace, and military systems. 

Is Flex PCB reliable?

What do you expect from a circuit board with high accuracy, quality materials, and top-level standards?

First, these boards let you customize routed paths and support high-density applications while at the same time maintaining the high-performance.

As a result, all designers and engineers seeking to create high-performance electronics can count on flat flex circuit boards. 

Final Remarks

Flat flex PCB represents electrical circuits or cables with a flat surface.

These boards are known for high flexibility, long life, operation in harsh environments, and fire resistance.

Also, the boards are designed with insulators, conductors, and finish materials. 

Focus on the dimensional tolerance, layer count, and circuit class to control the flat flex circuit board cost.

However, as much as a lower cost is important, balance it and quality.