Flexible PCB Manufacturing Process

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Oakley Mae

The Flexible PCB manufacturing process is complex and requires professional experts to achieve perfect results.

It starts with developing a concept, designing, and prototyping and ends with the final fabrication and testing.

Moreover, the process requires some of the best materials and equipment in the industry.

And what makes this process even more complex is that the process is always making improvements in packages and materials to make everything attractive.

Also, there are specific rules and standards every designer and manufacturer must follow.

But worry not if you find any of the above new to you. 

Today’s piece puts together critical information about the flexible PCB manufacturing process. Let’s get rolling. 

Table of Contents

How Are Flexible PCBs Different From Standard PCBs?

The main difference between flexible and standard circuit boards is their material.

We have two main materials that we use to design flexible PCBs:

First, we have ABS, an off-the-shelf plastic we use for general-purpose manufacturing, which is easy to use.

Secondly, there are thermoplastics which are polymers that change structure upon pressure and heat application.

Therefore, it makes materials like PBT and PET relatively stiff, strong, and malleable.

As a result, they find applications in designing lightweight parts, especially for aircraft and vehicles. 

Another major difference involves the fabrication process.

Generally, flexible PCBs require unique, special, and complex manufacturing procedures.

Some procedures include surface treatment, laminating, bonding, cutting, and molding. 

FPC Manufacturing Dynamics

Technological process of soldering and assembly chip components

(Technological process of soldering and assembly chip components)

Flexible circuit boards are new in the industry compared to rigid boards.

Normally, it’s a thin and electrically insulated polymer that makes printing on conductive circuit board patterns possible.

And in most cases, you’ll have to wrap a thin coating to prevent cases of damage. 

Additionally, flexible circuit boards have gained popularity in the electrical and electronics industry thanks to their high flexibility.

Generally, manufacturers prefer them since you can bend and fold them without damage or causing electrical shorts. 

The common devices using these PCBs include smartphones, refrigerators, smartwatches, and televisions.

Furthermore, the boards have proven to be less costly than regular boards over time. 

Flexible PCB Manufacturing Process

Female technicians working on a PCB

(Female technicians working on a PCB)

Generally, a flexible PCB has a unique manufacturing process that is different from that of standard PCBs.

This section outlines the processes you follow to manufacture a flexible PCB. 

Concept, Design, and Prototyping

Nothing that succeeds starts without a concrete plan.

Therefore, you must start your flex PCB manufacturing process with a concept and design outlining the foundation for manufacturing the board. 

First, a good design should support contraction and expansion for extended periods without breaking.

Also, the design should abide by certain rules and undergo product optimization.

Therefore, make the tracks wide to increase the PCB routing complexity.

Remember, flexible PCB design rules are larger and wider than those of related circuit boards to achieve better performance. 

Then there is an issue of a prototype to guarantee that components have enough space and fit well together.

However, ensure the prototype uses the same materials and procedure as the final flexible PCB. 

Material Preparation

We mostly use FR-4, a polyimide-based material, to manufacture circuit boards.

And in most cases, the selection depends on solder mask requirements, production speed, and component placement. 

When you deposit several copper layers on a substrate using a stencil, you must use a buildup process.

Afterward, you etch and pattern the copper to expose internal components and develop traces.

Also, before applying the photoresist film, you must chemically clean the production panels for the film to stick. 

Circuit Patten Exposure

Next, you overlay photoresist-coated panels with circuit artwork patterns.

During this stage, collimated ultraviolet light facilitates the transfer of circuit images to the production panels.

Therefore, this step exposes both sides at the same time if there is a need.

Generally, exposing the copper traces that link internal components on the PCB is very important. 

Etching

The etching process helps in creating through-holes and surface mount designs.

When applying chemicals, this process involves using sacrificial materials to shield PCB components from destruction.

Also, the etching process makes it easier to remove unwanted materials.

Additionally, the process happens in a plating tank using a sulphuric-based etchant that exposes drill pads. 

Drilling

This step happens after etching and involves creating holes in the flexible PCB.

You’ll note an exposure of metal pads at the PCB backside to allow component insertion during the soldering and insertion processes.

And we have laser-based systems which can provide very accurate ultra-small holes. 

Copper Plating

This electrochemical process involves depositing a copper layer on the PCB surface.

The process offers surface wear resistance and corrosion protection.

Therefore, you can make copper plating easier by appropriately aligning pads and leveling conductors. 

Coverlay Application

We apply coverlay to protect the PCB components from destruction during the soldering and drilling phase.

And in most cases, we apply polyimide coverlay panels.

Additionally, after you drill holes through the coverlay, you must apply permanent glue to cover the components. 

Stiffener Application

Next, you apply stiffeners to create strong interconnections between layers.

Stiffeners are mainly fiberglass-based, and you apply them before the final lamination.

Their main purpose is to prevent warpage during high-speed inserts, bending, and soldering. 

Moreover, you can place the stiffener on either one or all PCB sides.

Be guaranteed that the flexible PCB improves in rigidity upon applying stiffeners. 

Final Fabrication

An expert engineer in the lab doing visual PCB inspection

(An expert engineer in the lab doing visual PCB inspection)

The final fabrication process involves visually inspecting the PCB to identify irregularities like missing components or solder splashes.

This is also the time to test whether the PCB meets the essential customer requirements. 

Electrical Tests

Electrical tests help you identify shorts or faults that might cause operation or design failures.

Therefore, this process should happen after you finish the other processes and after a visual inspection.

This is the only chance you have to ensure the reliability and quality of the circuit board. 

Moreover, you must verify the functionality of connections, circuits, components, and internal parts.

And if you identify a faulty component, replace them immediately to avoid stressful repairs and more costs later.

That is why we always recommend that only professional and competent engineers and technicians be allowed to do electrical tests. 

A flexible circuit board in human fingers

(A flexible circuit board in human fingers)

Besides the above-mentioned processes, we also have important PCB manufacturing-related processes. 

Surface Treatment

Surface treatments help to improve the flexible PCB’s mechanical properties.

Therefore, to prepare the surface, use plasma chemical vapor deposition, which employs PVD or plasma-assisted vacuum lamination.

As a result, the PCB’s dielectric strength and durability improve. 

Edge Spacing

The proper laminate layer spacing guarantees that your flex PCB fits well into its enclosure or package.

Although this step is often ignored, it’s crucial and will always pay off during manufacturing. 

Trimming

This process happens before the PCB enters the final assembly steps and involves cutting excess laminate materials.

However, ensure you cut the laminate accurately without affecting other areas. 

FAQs

What are The Different Types of Flexible PCBs?

We categorize circuit boards based on configurations and the number of layers.

As far as configuration is concerned, we have HDI PCBs and rigid-flex PCBs.

First, HDI PCBs are high-density interconnections with many features on the board.

On the other hand, rigid-flex PCB combines the features of flexible PCBs and rigid boards. 

Also, we categorize PCBs based on layer count. The first category involves single-layer PCBs with one substrate and a copper layer.

Moreover, we have double-layer PCBs with two copper layers and a single substrate.

Therefore, the layers are on either side of the substrate. 

Finally, we have multilayer PCBs, which, as the name suggests, have multiple copper and substrate layers.

In short, they combine many double and single-layer circuits. 

Flexible PCB cost and how to reduce it?

The most competent and successful designers and manufacturers estimate their manufacturing costs before initiating the process.

And we have several factors that contribute to PCB costs.

First on the list is the layer count, with more layers representing a higher cost.

Therefore, if a single-layer PCB works well for your project, there is no need for a multilayer PCB. 

Also, the material you select will determine the PCB cost. If possible, avoid polyimide and related materials since they’re more costly. 

Furthermore, drilling small via will cost more. However, small vias enable the board to provide more durability and reliability. 

Final Remarks

Flex circuit boards are the future of electronic devices, and the earlier you understand them, the better for you and your project.

The boards are highly flexible and reliable, thus perfect for developing compact devices with tight spaces.

Moreover, the boards provide better adhesion and bonding between components, enhancing product performance. 

In conclusion, flexible PCB is the best option for functionality, reliability, performance, and durability.

Therefore, understanding the above manufacturing steps puts you ahead in the industry.