Flexible Circuit Technology: The Complete 101

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Oakley Mae

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Often, many consider flexible circuit technology as making PCBs (Printed Circuit Boards) that can bend.

But the reality is that there are several other distinct differences between flex circuits and PCBs in terms of fabrication, design, and functionality.

This 101 on FCBs explains what they are, how they differ from standard printed circuits, and more.

Table of Contents

What are Flexible Printed Circuits

A flexible FPC circuit comprises conductive traces on a thin, flexible substrate.

So that means they can twist or bend. In addition, these boards may feature surface-mount aspects such as LEDs and resistors and usually have a termination or connector point for integrating with other components.

(Caption: Image of circuit board connectors)

Although several functions FPCs perform are similar to those of standard printed circuit board counterparts, they differ greatly in structural and mechanical design.  

For example, it’s impossible to twist or bend a standard printed circuit board, which is impossible with flexible printed circuits. 

Because of this unique feature, the two require several distinct manufacturing processes, designs, and materials.

Flexible Circuits Technology Advantages

The following are the advantages of flexible circuits technology:

Lightweight

In today’s manufacturing industry, some of the essential elements of a circuit include lightweight and compact size features.

With flexible circuit technology, technicians can save as much as 60% weight by employing flex substrate instead of soldered joints or bulk wires.

Thermal Resistance

These circuit boards can withstand higher temperature changes without interfering with data transmission or other key elements.

Also, such features help to cut down on repair and maintenance costs.

Easy to Assemble

Industrial flex circuit technology allows for easy assembly while promoting effective data handling.

Many companies today utilize this kind of board because of such features.

Flexible Circuits Manufacturing Process

FCB manufacturing

(Caption: FCB manufacturing)

Concept Design and Prototyping

Concept Design and Prototyping

When it comes to the PCB, it must follow a design capable of supporting contraction and expansion for extended periods without falling apart.

For that reason, it undergoes optimization (in terms of production) that adheres to various design regulations.

Some of these design standards include being wider to increase the routing complexity, which is not the case with standard PCBs.

Preparing the Materials

The preparation involves chemically cleaning the production panels before applying a photoresist film to ensure it remains firmly in place. Also, a protective layer of ultra-thin material cores.

Circuit Pattern Exposure

This is the overlaying of photoresist-coated panels with circuit artwork patterns.

During the process, images get transferred to the production panels via collimated ultraviolet light that helps expose both sides simultaneously if necessary. 

Etching

Etching is vital for the creation of surface mount boards and through-hole designs. It involves using sacrificial material to protect components from damage throughout the chemical process.

Further, etching helps to remove unwanted material.

The etching process occurs in a plating tank containing a sulfuric acid-based etchant that can eliminate unwanted copper traces.

Also, the etchant helps with exposing the pads for drilling holes.

Drilling

After the etching process comes drilling to make through holes in the board.

And once it’s done, metal pads on the other side of the board get exposed, allowing for the insertion and soldering of components into the board via through holes.

Additionally, drilling helps to create desired hole patterns for different production panels. 

Copper Plating

Plating chemicals help remove unwanted materials by filling traces and pads on circuit boards.

Then, gravity takes over and keeps the hole plates on their surfaces.

As a result, there’s a flat area on which to distribute the chemical evenly on the board.

Applying Coverlay

This step involves the application of polyimide coverlay panels during production before laminating them together.

That way, components remain safe from damage during the soldering and drilling processes.

Once done, applying permanent glue is best to ensure the components are properly enclosed.

Applying Stiffener

Applying a stiffener allows for strong interconnections between the various PCB layers.

Usually, to make stiffeners, manufacturers use fiberglass-based materials that they can apply locally to stiffen materials before the final lamination takes place.

Treating the Surface

Treating the surface helps boost the product’s mechanical features, as some tend to be harder than your conventional PCBs.

Engineers prepare the surface with plasma-enhanced vapor deposition and PVD or vacuum lamination.

Doing so extends the product’s lifespan and helps increase dielectric strength whenever the circuit gets used on any FR4 substrate.

Spacing the Edge

This is crucial in the flex circuit manufacturing process because it’s where all of the effort put in during the design and construction phase eventually pays off.

Correct spacing between the different laminate layers ensures that the product fits seamlessly into its enclosure or packaging. 

Trimming

For the best results, manufacturers do the trimming accurately to eliminate the excess laminate material once the lamination of the circuit patterns is over.

And it takes place before the product enters the last phase of assembly.

Flexible Printed Circuit Finishing and Assembly

An FPC comprises a metallic layer of traces. Often, these traces are copper bonded to a polyimide dielectric, for example.

Interestingly, the thickness of the metal layer can be pretty slender at <.0001″ or fairly thick, averaging > .010″, although the dielectric thickness can be anywhere between .0005″ to .010″. 

Usually, engineers use an adhesive when it’s time to bond the metal to a substrate. However, they may apply other bonding, like vapor deposition, to attach the metal.

Since copper is usually readily available to oxidize, it’s common to cover all exposed surfaces with some protective layer, gold or solder.

That’s because of their conductivity abilities and environmental durability. Regarding the non-contact regions, a dielectric material is often used to protect the circuitry from factors like electrical shorting or oxidation.

Factors to Consider When Choosing Flexible Circuits

Manufacturer Selection

it’s important to ensure that the flexible PCB producer you settle on applies state-of-the-art technologies and materials that can handle the thermal coefficient and movement needed for a strong circuit board.

Bend Radius

The bend radius is the minimum radius a flex can bend without damage.

Generally, it’s advisable to determine the bend radius early in the production process to ensure that the circuit can tolerate flexing. 

Adaptable Routing

The wire routing phase is an integral part of designing PCBs.

To achieve the levels of safety needed, the installed components have to pass through low-stress areas, and this is where trace routing can help to prevent electrostatic discharges or short circuits on the PCB.

Adhesive Thickness

Usually, it’s not a  good idea to base the adhesive width for a flexible printed circuit board on a drawing during the design phase.

Instead, the maker should understand that the description needs to state the width of the circuit board and include some important dielectrics that could affect impedance.

Cover Material

The type of cover material chosen during the fabrication phase of the circuit board can greatly impact its lifespan and general performance.

Therefore, selecting appropriate cover material based on the circuit board’s application is essential.

Delamination Risk

Sometimes, people refer to delamination risk as layer detachment. Undoubtedly, it’s a fairly common issue in producing PCBs and their usage.

That’s because the layers tend to demold, leading to abrupt damage to the circuitry and occasionally causing the PCB to collapse. As a result, engineers use a solder paste to prevent delamination.

FAQs

What are Flexible Circuits Used For?

Most flexible circuits are passive wiring components for interconnecting electrical devices like integrated circuits, capacitors, and resistors.

That said, some FCBs are commonly employed in interconnections between electronic assemblies using connectors or directly.

What Is the Flexibility of the Circuit?

It’s an arrangement of conductors mounted atop a flexible circuit substrate or base and supported through a dielectric film.

What Is an Example of the Flexibility of the Circuit?

You’ll often find flexible circuits in established medical and manufacturing industries where there’s a need for several interconnects in a compacted package.

Another good example is cellular telephones with their extensive flexible circuits.

What Are Examples of Flexible Electronics?

The following are some examples of flexible electronics:

  • Flexible batteries
  • Automotive circuits
  • Input Devices
  • Displays
  • Solar cells
  • Skin-like circuits
  • Printers
  • Cellular telephones

Conclusion

Now you understand more about flexible circuit technology, the manufacturing process, and factors to consider when choosing flexible circuits.

The designer’s imagination can limit the potential of flexible printed circuits.

It’s no wonder more flex design engineers today are investing heavily in trying to learn more about flexible circuit technology and its amazing possibilities.