Flexible Circuit Board Design, 7 Things You Need To Know(The Ultimate Design Guide)

Circuit boards are a major component of a wide variety of electronics products.

In today’s electronic world any product must have a circuit board.

However, the use of flexible circuit boards is now growing rapidly due to their versatility.

As the days go by new technologies are coming and almost all the big companies are trying to make their devices more flexible.

In order to make any device fully flexible, the PCB inside it must be flexible and the technology that has brought the biggest revolution in this field is the flexible PCB technology.

With the introduction of flex and rigid-flex circuits, engineers have had the opportunity to become more creative in designing new products. Flex and rigid-flex boards are designed to ensure mechanical wear and vibration resistance.

1)What is a Flexible Printed Circuit Board?

A flexible circuit board is a type of circuit board similar to a rigid circuit board where the required components are placed on a flexible substrate instead of on a rigid substrate.

This board is made in such a way that it flexes to any desired shape when applied.

Flexible circuit boards are of different types according to the layer and configuration.

Rigid-Flex PCBs:

The rigid-flex PCB is a hybrid PCB of rigid PCB and flex PCB. Many rigid PCBs are used in a rigid-flex PCB.

The rigid part of the rigid-flex PCB is usually used for fitting connectors and for installing chassis.

The rigid-flex design helps to increase the capacity of any PCB. The biggest advantage of rigid-flex type PCB design is that it does not require any compromise in the design of the device so that any type of design can be done keeping in mind the convenience of the user.

 Rigid-Flex PCB

HDI Flexible PCBs:

HDI Flexible PCBs: The full form of HDI is High Definition Interconnect.

This type of PCB is used in devices or applications that require more performance than the general Flex PCB.

This type of design is especially helpful in creating a perfect layout.

It uses a much thinner layer than the general Flexible PCB which helps in total size reduction and uses power efficiently.

This type of PCB is chosen to create high-performance PCBs.

Classification of Flexible Circuit Board Based on Layers:

Single-Sided Flexible Circuit Boards:

It is a PCB board consisting of a single-layer polyimide and a thin copper layer. The electrically conductive copper layer can be accessed from only one side of the board.

Single-Sided Flexible Circuit Boards with Dual Access:

It is a PCB board consisting of a single-layer polyimide and a thin copper layer. However, in this case, the electrically conductive copper layer can be accessed from both sides of the board.

Double-Sided Flexible Circuit Boards:

This type of PCB board has layers of electrically conductive copper on both sides of the base polyimide layer and these two layers are connected to each other through a hole.

Multi-Layered Flexible Circuits:

This type of circuit board consists of a combination of many double-sided and single-sided flexible circuits. These flexible circuits are connected to each other through different holes of the plate.

2) How to Design a Flexible PCB:

When designing a flex circuit, it is important to analyze what the board is being made for.

Will it be used in a static or dynamic environment? If the board is used in a static environment i.e. does not require any movement of the board, then the board should be flexible enough so that it can be easily installed in the product.

Again, if the board is designed for use in a dynamic environment, i.e. the board needs constant movement, then the amount of flexibility of the board should be kept so that it does not break during the movement.

Now we need to see if the device or application needs a simple flex board or a rigid-flex board. If the use of one-sided mounting technology seems most appropriate for the product, an all-flex board is the best option.

If two-sided mounting technology is suitable for DVC, a rigid-flex board is the best option.

No matter how experienced a PCB designer is, everyone’s a little scared to design a flex circuit for the first time.

In fact, they think that designing flex circuits and rigid circuits is completely different.

So they do not want to design flex circuits easily. But the fact is that there is no such difference between them.

Just like you design a rigid board, you need to do the necessary design in a PCB design software and the output files will be the same.

However, there are some differences. The differences are to be aware of the cover layer and the Stiffener layer and to follow some basic rules. Since flex circuits are flexible in nature, there are a few things to keep in mind, such as vias, terminating traces, and sharp angles and keeping these features away from the bending region. Since flex circuits are usually made of polyamide material, the process of PCB assembling and soldering is very difficult, so keep your vias, traces, annular rings, pads, and spacing as much big as possible.

Flexible PCB

One question that often comes to us is how small the trace or via size can be.

Our answer is that you can make it as small as you want.

But the thing is that it can create problems in manufacturing and it is also doubtful how reliable the PCB designed in this way will be.

The biggest advantage of rigid circuit boards is that traces of the required size can be used, and more features can be used than flex PCB.

Solder masks are usually used on top of the PCB to protect the conductive layer in rigid circuits.

Similarly, a cover layer is used to protect the conductive layer in Flex PCB.

For this, the PCB designer should set this cover layer in advance in the design file.

Another thing to keep in mind is stiffener. An important part of the flex circuit is the stiffener.

A stiffener is used to support different parts of the circuit. You can use as many stiffeners as you want.

It can be used on both sides of the PCB. However, during the design of PCB, if the stiffeners are kept in the same file in the design file, then the direction of the PCB ( or which side of the PCB it should be applied) should be shown in the fabrication drawing.

Again a separate file can be used for the upper stiffeners and a separate file for the lower stiffeners.

A stiffener is a very good option when a special part of a flex circuit board needs extra support.

This protects the circuit from excessive movement and helps keep the components stable.

Moreover, a stiffener is very important to keep the soldering joints safe.

Many types of stiffeners are available such as polyimide, FR4, stainless steel, aluminum, etc.

The thickness of the stiffener has to be determined as required.

The thickness of the stiffener should be kept low while making the board for use in such a small space.

However, the thicker the stiffener, the greater its strength.

Now it is clear how the flex circuit is designed. Then the question may be raised about how the rigid-flex circuit is designed.

In this case, it is better to say that the method of rigid-flex circuit design is the same as before.

But many new designers get confused and think that the flex part of the rigid-flex circuit may be stuck with some glue.

But that’s not true. As before, PCB is designed by stacking layers.

We have to do the same here. However, for rigid-flex circuit design, some parts of the rigid layer have to be left blank in the design file so that the manufacturer could understand that the blank parts are made for flexible regions and can prepare the board accordingly.

A flex circuit has many more variables than a rigid circuit. Thus a data aggregated fabrication drawing is required.

So that any information during manufacturing does not escape the attention of the manufacturer.

Detailed fabrication drawing is very important because incomplete drawing can cause the manufacturer to have to guess a lot of things so it is very normal to have errors in the PCB.

3) What is Flexible PCB Board Material?

The main function of the film substrate is to supply the conductor carrier and act as an insulator between the circuits.

Since we want to create flexible circuits, the substrate material must be flexible.

PI(polyimide) film and PET (polyester)film are commonly used to make this type of substrate.

Apart from these, various types of polymer films are also available. Such as PEN (polyethylene phthalate), PTFE and Aramid, etc.

All of these materials can be used considering performance and cost.

Usually, a resin called FLCC or flexible copper clad laminate covers the PI.

This resin is a type of thermosetting resin that does not melt or soften at room temperature.

Although it can maintain flexibility after thermal polymerization which other thermosetting resins usually cannot.

PI has a high temperature-resistant capacity. The humidity absorption capacity of the upgraded PI is 0.6% which is much lower than normal conditions (1.6%). Also, it has high dimensional stability. Flexible CCL and rigid CCL are highly environment-friendly materials. Because they are halogen-free which is very harmful to the environment.

PCBs with halogen and six other substances have been banned in several other countries, including the EU.

The biggest disadvantage of its bad heat resistance.

Moreover, the performance of PEN is much higher than the of PET so the uses and applications of PEN are increasing.

The adhesive is one of the vital substances which plays a major role in joining substrate material.

Its classifications such as cover PI resin, PET resin, modified epoxy resin, and acrylic resin are used.

However, modified epoxy resins and acrylic resins are more used due to their high adhesive nature.

 Flexible PCB

Flexible CCL consists of three layers: 1. adhesive, 2. polyimide, 3. copper foil.

Since adhesive reduces the electrical and mechanical performance of a flexible PCB, a two-layer flexible CCL or 2L-FCCL without adhesive has been developed.

It is also good for the environment because it does not contain any halogen. It is also suitable for lead-free soldering.

Film coating, Electroplating, and Lamination are the three methods of manufacturing 2L-FCCL.

Using electroplating metal layers on polyimide film is not difficult to create a more slender substrate material and copper foil can be gotten with minimal effort.

Film coating is relevant for mass-volume creation with ease.

Lamination is perfect for double-sided board fabrication.

LCP (Liquid crystal polymer) is a newly invented material. It is developed due to reducing the disadvantage of PI substrate material.

We all know that copper foil is used to cover the LCP film so that it can deal with heat, pressure.

We can develop both single-sided and double-sided LCP as necessary and which has a water absorption rate of 0.04% and a dielectric constant of 2.85, which can be used for high-frequency digital circuits.

TCLP is another high-quality material for flexible PCB.

The halogen-free substrate material is now trending for both rigid PCB and flex -PCB.

Flexible substrate material and reinforcement board must be fire-resistive and Halogen-free.

The best conducting material for flexible PCB is copper foil. An alloy such as aluminum, gold, and silver is also a good option.

The most vital condition for a conductive material is that it should be flexibility resistant.

Now copper foil is also classified into two categories as per crystal shapes. One of them is ED(Electro Deposit) copper foil and another of them is RA(Rolled and annealed) copper foil. For dynamically flexible PCB RA is the best option on the other hand ED copper foil is responsible for high-density flexible PCB.

Conductive Silver Paste:

Generally, during the process of PCB fabrication, we use conductive ink which is mainly silver paste to print on insulating film with a shielding layer or wire. The conductive layer on a PCB must be low resistance and highly flexible. Newly developed conductive silver paste is highly flexible and low resistive and it also can form conductive images on thermosetting or thermoplastic polymer film. This type of conductive layer is also useful to create different graphics that are used in different products such as RFID.

Photosensitive Coverlay: PIC or photo imaginable Coverlay has been developed due to so many reasons. One of them is that traditional PI Coverlay fails to attain the requirement of high dimensional stability, high density, and environmental protection. Newly developed PIC has high flexibility resistance which is almost like solder mask oil. Depending on modified epoxy resin or acrylic resin two types of PIC (Liquid PIC and Film type PIC) can be applied in a wide range of applications due to their flexibility, binding force, and high resolution.

Flexible PCB

4) Flexible PCB Stack-up:

Now many of you may be wondering what actually a flexible PCB stack up. And why is stack-up documentation important? Now we will talk about this. And we have chosen 4 special technologies to achieve this.

1 layer flex circuits: most simple structure for the construction of a flexible printed circuit board is one layer flex construction the following things are the stack-up for adhesive and adhesiveness of 1 layer flex

circuit.

1layer Flex PCB stackup

2 layer flex circuits:

In 2 layer flex circuit stack up there are two-layer of copper traces. For standard layer stack up for 2 layer flex 8 types for adhesive and 5 types for adhesiveness.

2layer Flex PCB stack-up

4 layer flex circuits stack up: Multilayer board is necessary when the circuit is too complex to rout or we may have to use copper foil to cover the ground layer. As an example, we provide a 4 layer flex circuit stack-up:

4layer Flex PCB stack-up

Flexible PCB stackup documentation:

The flex PCB stack-up documentation is a vital element of the datasheet of a flexible PCB design. It mainly contains a brief description of the flex PCB such as the requirement of the specific material and construction of the design.

Stack-up documentation is required by the manufacturer so that they can perfectly manufacture your PCB without any undesired error.

Material Stack up:

Material stack-up documentations for flex PCB are basically the side view of the flexible printed circuit board which is mainly a part of mechanical drawing in Gerber file, pdf file, or DXF file format.

So it is now clear that stack-up documentation contains a significant amount of data related to the construction and design of a flexible printed circuit board.

It contains some vital information such as the types of specific material; used in PCB, their location within the PCB structure, material layer thickness, part number, etc.

One of the biggest reasons why they stack up documentation is vital for flex PCB and rigid-flex PCB because in flex PCB and rigid-flex PCB different areas need different requirements.

Such as some parts of the PCB may have to meet certain conditions like specific bend, specific impedance, or different connector specifications.

Although we have to be very careful at the time of designing as well as at the time of manufacturing. The designer and manufacturer should follow the IPC 2223C design guideline.

Specific Elements include:

1. Flex core should be adhesive less.
2. Selective coverlays should be applied only on the exposed flex areas, not on the rigid areas.
3. No-flow prepreg for rigid layer lamination and high TG FR4 rigid cores.

 Material Details

Some additional details that should also be included:

Flex Cores:

Type (Adhesive or Adhesive less)

Thickness, Copper weights

Copper Type (ED or RA)

Lamination Adhesive Thickness

Coverlays / Solder Mask:

Thickness

Adhesive Thickness

Adhesive Type (Epoxy, Acrylic, etc.)

Color

Stiffeners:

Material Type (FR4, Polyimide, Aluminum, Stainless, etc.)

Thickness

Attachment Adhesive (PSA or thermally bonded)

Additional Materials:

PSA (Pressure Sensitive Adhesive)

EMI Shielding Films

Epoxy Strain Reliefs

Depending upon the layer count Flexible PCB has a thickness tolerance of +/- 30um to 50um.

Rigid-flex and Complex-flex PCB Design:

Complex flex PCB designing and manufacturing is much more challenging than other flex PCB designing and manufacturing. When it comes to creating stack-up documentation for a complex PCB then it is hard to represent as we did previously. If we want to do it as before then there is a high chance of overlapping different elements. So we will follow a different way to execute stack-up documentation. Here the material stack-up has to be divided into different regions to get a clearly defined description of the design.

Example of Advanced Multiple Area Material Stack up

Flexible PCB Material specification:

We suggest that the adaptable and inflexible materials (whenever required) be characterized by IPC guidelines as a feature of the drawing notes. This considers the utilization of comparable material brands that meet similar IPC details.

It’s not down to earth for a provider to stock every one of the accessible material brands and arrangements.

What’s more, not all material brands are promptly accessible or financially savvy in various geological regions.

The exemption for utilizing these particular guidelines is for plans that require uncommon materials for which there are either no current IPC principles or have explicit execution necessities. This most usually applies to superior flex materials, PSAs, protecting movies, and epoxy strain reliefs.

Whenever realized reciprocals are free, the drawing notes should state “or same”.

Instances of claim to fame materials:

PSA(s): 3M9077, 3M467MP, etc.

EMI Shielding Films: Tatsuta SF-PC6000

Flex materials: DuPont TK, DuPont HT

Epoxy Strain Relief: Eccobond 45/15 Flexible Formula

CONCLUSION:

Flex PCB or All flex plans require a total and exact PCB stack up as a component of their included documentation to guarantee the provided parts meet all the plan prerequisites for the client. Guaranteeing that this data is right takes out pointless specialized inquiries from springing up and dodging delays in the conveyance season of your parts.

So it is now clear that stack-up documentation is vital for flex-PCB designing and manufacturing. So focus more on it so that you are able to provide a perfect and error-free flexible PCB to your customer.

5) Assembly process of Flexible PCB:

The assembly process of flexible PCB is almost the same as rigid PCB. But there are some differences according to some technical requirements.

Because of its thinness, the flexible PCB has a tendency to bend and deformed with a little bit of excessive pressure and temperature.

So it is not possible to assemble flexible PCB on a manufacturing line like rigid PCB. So to assemble a  flex PCB we have to follow some rules such as it has to be fixed on a plate or carrier so that at the time of assembling it can behave like a rigid PCB to avoid error and deformation of PCB.

Few things have to remember to maintain the product quality such as accuracy of positioning and consistency of board carrier, etc.

As flexible PCB is more expensive than rigid PCB so the uses of flexible PCB are not applied everywhere. Sometimes it is only used for some connection between modules. Generally, fewer components are required for flexible PCB.

Flexible PCB is mainly used to minimize the size of devices such as cell phones, digital cameras, etc so that it can be used more efficiently. Flexible PCB takes less space than rigid PCB because of its thinness. Now we can use a large number of panels and a small number of a component on each flex PCB so that total circuit can run smoothly and efficiently

Environment-friendly assembling technology:  when it comes to PCB assembly we know this is a major issue to maintain environment-friendly conditions. To maintain this we have to care about a few factors such as repeating bending and control accuracy, the material of the components assembled on flexible PCB, etc. Moreover, we have to maintain anti-static precautions and soldering reliability.

6) Applications of Flexible PCB:

As flexible PCB is now one of the most in-demand technologies due to its easy-to-use features and super flexibility it has now a wide range of applications. Both flex and rigid-flex PCB is now trending basically after 2017 companies become more interested in flex-PCB technology.

Applications of flexible PCB in Computer:

Today’s computers largely rely on flexible PCB.  We know that a hard drive is a vital part of a computer. In a hard drive flexible PCB is used due to keeping up with high-speed data processing. If the hard drive is running for a long time it can cause a high temperature within the drive so the flex-PCB within the hard drive must have high-temperature absorption power. Flexible PCB plays a major role in the longevity of a hard drive.

  Flexible PCB in Computer

Automotive Industries:

Modern cars are full of computer technology and they contain many electronic components and units to manage the whole car functions. Flexible PCBs are used in these electronic units such as in EMU (Engine Management Units), anti-lock braking systems, airbag controllers, instrument panels, and dash systems, etc.

As flexible PCB has the flexibility it is easy to fit even in a much smaller space which gives more freedom to engineers to design as they desired. Moreover, flexible PCBs are made for extreme conditions which are useful for electronics units of cars.

Flexible PCB in Smartphones:

Smartphones are portable and small so it is not easy to use only rigid circuits to build one. So now almost every smartphone manufacturer is using flexible PCB to build it. As flexible PCBs have high-temperature absorption capacities so it is easy to deal with heating issues while charging.  Another great thing is that now smartphone manufacturers are trying to build fully flexible smartphones so there is a high possibility of using flexible PCB more in number to build such a kind of smartphone.

Flexible PCB in Wears

Flexible PCB in the Health and Medical Sector:

Flexible PCB has numerous applications in the health and medical sector such as Pilcam. We all know that Pilcam has to deal with extreme human body conditions and take different pics from inside the human body. It is made up of superfine flexible PCB which is not toxic in nature. Researchers are now trying to develop different kinds of flexible PCBs to use inside the human body.

Flexible PCB in Robotics:

Flexible PCB is used in robotics to build super flexible robotics arms. As most of the robots run on automation and are built for long-term use so flexible PCBs are the best option for that.

There is much more application but here we discussed a few of them to give a concept.

7) How to Choose the Best Flexible Circuit Board Manufacturer?

There are a few important factors to manufacture PCB:

High-end equipment and expertise:

Manufacturing flexible PCB is not as easy as rigid PCB manufacturing. It needs some of the most latest and High-end equipment and powerful software that analyzes and predicts material movement. So before choosing a manufacture you have to verify whether the manufacturer can meet all these factors or not. Depending on that you can choose your manufacturer. Another thing is that you have to confirm that that manufacturer has special expertise for manufacturing flexible PCB.

UL 94 V-0 flame ratings:

One thing you have to be careful about when you choose a flexible PCB manufacturer that is UL 94 V-0 rating. This rating is a must-have rating for medical and industrial equipment. As it is difficult to get separately so request your manufacturer to provide UL-certified ratings.

Modeling your requirements:

Always try to choose a manufacturer who can help you, in the beginning, to model the requirements of your board and help you model your requirements with their high-end software. It is also mandatory to choose those manufacturers who can test your board once it is completed and can crosscheck all the impedance values you want.

Maintaining Quality to Cost ratio:

Always try to find a manufacturer who is able to provide you higher quality to cost ratio.

Higher customer rating:

Of course, customer rating is a big sign of a good manufacturer. After checking all the above factors you have to check the customer rating of a manufacturer and choose accordingly.