Flexible Circuit Design: All You Need To Know

About Flexible Circuit Design, Designers are now reducing the size of electronic devices but increasing functionality, partly due to flexible circuit design.

If you have ever used a flex PCB, you understand how reliable and effective it can be, especially when working with tight spaces.

The circuit boards can fold to fit the smallest devices, like wearables.

However, a challenge arises regarding designing and fabricating flexible circuit boards.

Unlike the traditional rigid boards, these boards follow a unique and complex manufacturing process.

For example, you’ll have to consider material selection and board outlines, which only expert designers can do.

Luckily, we have prepared this article to help you understand everything related to flexible circuit design. Let’s get rolling.

Flexible PCB Design: What You Need To Know Before Starting

Even before you begin the design process, there are critical things you should understand:

First, you must understand the flex PCB bendability. The main concern is how often and to what extent your circuit board will flex. This determines whether the flex circuit board will be dynamic or static.
Second, you must understand the different materials, their characteristics, and their specifications.
Third, you must consider the circuit board’s routing and layout.
Fourth, it would be best if you focused on Via design considerations. This is the time to evaluate the risks of using Vias.
Fifth, you should know how to select the perfect flexible PCB stack-up.
Moreover, you should understand and follow the different manufacturing, testing, and design standards.
Furthermore, it’s important to the factors that affect PCB cost and turn-time.
Finally, you should know that only a reliable manufacturer can help you fabricate a quality flexible PCB. Therefore, knowing and considering these manufacturers is in your best interest.

How To Design Flex PCB

(A light-emitting diode flex PCB)

We’ll take you through a step-by-step guide on how to design flex PCBs:

Understand The Electrical Parameters

The first step is to understand the electrical parameters of the system and circuit board you seek to design, including:

Voltages
Impedance characteristics
Current maximums
Signal types
Shielding considerations
Detailed net wire schematic and listing
Capacitance limitations
Location and type of PCB connectors and components

Develop The Schematic

A schematic is a general representation, shape, and design of your flex PCB using symbols.

This is the time to consider your PCB bendability and decide whether you want it to be dynamic or static.

While a dynamic circuit board might bend for a lifetime, a static one will only bend a few times.

This step might require a lot of design experience.

Alternatively, you can contact any professional engineer to assist you where possible.

The step involves creating a layout using software like Allegro and Altium to show the location and appearance of different components on the board.

Design The PCB Stackup

The stack-up plays a significant role in designing and fitting your PCB in a system or device. Therefore, the trick is to have the flex layers in the middle of the stack-up to offer protection.

Define Design Requirements and Rules

This step mainly depends on IPC requirements, standards, and rules. The standards tell you the steps and rules for the design, fabrication, and testing phases.

Therefore, select a circuit board layout provider who understands the IPC standards to avoid future errors and revisions.

Place The Components

In most cases, you’ll have to discuss the layout and design guidelines with your circuit board layout provider regarding component placement.

For example, we have standards prohibiting the installation of certain components next to each other to eliminate electrical noise.

Insert Drill Holes

The connection and component types will drive this process. For example, a double-sided PCB connects the drilled hole bottom to the top layer.

Funny enough, these circuit boards make up almost half of the flex PCBs on the market.

Route The Traces

After the drill holes and component placement, you can now route traces. This involves connecting the path segments.

Add Identifiers and Labels

This is the time to add markings, reference designators, identifiers, and labels to your layout. As a result, it’ll be easy to determine where you’ll place the different components.

Generate Layout/Design Files

Finally, use your software program to generate files with design information. You’re now free to proceed with the assembly and fabrication process.

Basic Circuit Board Materials for FPC Design

(A plastic flex PCB LCD panel)

The most common materials for FPC design include:

Conductor

Copper is the most common conductor in FPC design thanks to its high conductivity.

Also, you have to use a small copper layer for effective electrical conductivity.

Moreover, copper is available in low-temperature and fully annealed forms, thus offering excellent flexing properties.

However, some manufacturers use gold and silver instead of copper. Unfortunately, these two options are costly.

Coverlay and Insulator

We use these materials to insulate the copper layers. The most common overlay or insulator material in use is a polyimide film.

Finish Type

A PCB finish is used to provide a solderable surface or eliminate cases of copper oxidation.

Therefore, you can use ENIG (Electroless Nickel Immersion Gold), which is common in about 80% of circuit boards.

Flexible Laminate

Here, we use polyimide and copper foil as the core component of the circuit board.

We often employ rolled annealed copper to improve copper flexibility and offer a tighter bend radius.

Adhesives

Also, adhesives like epoxy and acrylic strengthen the bonds between layers.

Since acrylic adhesives remain malleable even after curing, they’re the perfect choice for dynamic applications.

Stiffeners

Finally, we have rigid materials that increase rigidity, thickness, and strength and provide mechanical and physical support for mounting your components.

For the best result, use stainless steel, FR-4, or Kapton.

Considerations When Designing The Vias

(Close view of a colorful transparent flex electronic)

Every circuit board designer should evaluate the risks of employing Vias. It’s common for vias to break or crack in the design.

Therefore, to mitigate such risks, do the following:

Use teardrop-shaped vias to link plated through holes or traces in a flex PCB. As a result, they’ll minimize potential stress concentration points.
Ensure the large annular rings establish a reliable connection between copper traces and vias. Therefore, ensure the annular rings have a minimum size of 8 mils.
Finally, you can apply vias over the stiffener but not on the edge.

Choose The Best Flex PCB Stack Up

You should optimize the stack-up by considering the bend radius, board thickness, application type, and the number of layers. Therefore, keep the below guidelines in mind:

First, choose the material depending on the operating environment
Second, mention each layer’s impedance requirement and thickness
Third, for a balanced stack-up, use an even number of layers
Fourth, always select industry standard MCAD and ECAD tools providing a toolset to layout, customize, and design stack-up.
Finally, if you develop rigid-flex PCBs, flex the layer in the middle to eliminate slippage cases.

IPC Standards for Designing Flex PCBs

(Factory supervisors designing PCBs)

Verifying and testing the quality and integrity of raw materials and the complete circuit board is common.

However, this can only happen if you understand the IPC standards and regulations that govern PCB design and fabrication.

Therefore, in this section, we consider the different standards that govern flex PCB design:

IPC-2221: This standard provides the general PCB design requirements. Also, it offers different interconnecting guidelines and component mounting standards.

IPC-2223: You use these standards alongside the IPC-2221 standards. Moreover, it provides directions in different areas, including impedance control, capacitance, and plating.

IPC 600: This one defines the non-admissible, negotiable, and acceptable PCB condition as per visual interpretations. Therefore, the latest version available for designers is the IPC-600 K.

IPC-6013: It provides the performance specifications relating to impedance and thermal testing.

IPC-FC-234: This standard outlines the available adhesives and their proper usage. Additionally, it defines the correct process of fabricating PCB and the relevant limitations, weaknesses, and strengths.

FAQs

How many layers are in a flexible PCB?

There is no limit to the number of layers in a flex circuit board.

Therefore, you can design single, double, and multi-layer circuit boards.

However, understand that all these circuit boards function well in unique use cases.

What are the benefits of flex PCBs?

Flex circuit boards boast several advantages, including high vibration, long-duty cycles, and repeatable reliability.

Moreover, they can withstand tough conditions and work well under high temperatures.

Furthermore, they have improved signal integrity and saved space because of high foldability.

Finally, most designers who use these PCB reports limited errors and low manufacturing cost and time.

Conclusion

If you’re keen enough, you’ll note that flexible circuit boards are slowly facing out rigid boards.

Moreover, flex circuit boards offer improved signal integrity and repeatable reliability and work better in harsh conditions.

Therefore, designers and electronics engineers must adopt new and better design procedures to design quality PCBs.

And what better start than using the best design software, following IPC design standards, and choosing a reliable layout provider?

All in all, this guide is all you need to kick-start flex PCB design.

Table of Contents