Working with Flex PCB Design Standards

30 October 2022
New

Most electronics devices don't contain a single PCB (printed circuit board). They are made up of many flex and rigid-flex PCBs. Due to this, PCB manufacturers might create unusual enclosures for their boards, incorporate motion into new devices, and create distinctive form factors that are not achievable with rigid boards. After all, devices are shrinking daily due to technological advancements, and so are the PCBs inside them.

What Is A Flex Circuit?

As the name implies, a flex circuit is a flexible printed circuit. In essence, flexible circuits are constructed from a thin layer of insulating polymer over a substrate attached with conductive lines in a pattern. Flex circuits have been in use since World War 2, and their range of applications continues to grow rapidly.

Types of Flex Circuit:

The four main types of flex circuits are mentioned below:

Rigid-Flex Circuit

Rigid-flex circuits are composed of several circuit layers joined by an epoxy pre-preg bonding film. Rigid circuit boards and flex circuits are almost interchangeable.

Multi-Layer Flex Circuit

Dielectric layers separate multiple copper layers in multi-layer flex circuits. Typically, through-holes join the metal layers in this circuit type.

Double-Sided Flex Circuit

Multiple layers are present on both sides of a single dielectric layer in a double-sided flexible circuit. Plated through-holes are used In order to join the two layers together.

Single-Sided Flex Circuit

Single-sided flex circuits have a single layer of metal traces on one side of the dielectric layer. Polymer or Polyimide is typically used as the dielectric. For insulation and defense against deterioration from the environment, a polyimide layer is applied.

Flex PCB Design Standards

Compared to rigid PCBs, flexible PCB design requires a somewhat different approach. Flex PCBs can significantly reduce manufacturing costs and weight and space requirements compared to other PCB types. However, their design must be optimized according to their use cases and materials. On that note, below are a few flex PCB design standards and guidelines you need to follow.

Understand the Bendability of Your Flex PCB

Flex PCBs provide great bendability for applications involving constant movement or motion. However, as a PCB designer, you must remember the frequency, size, and number of bends matters a lot. Depending on this information, one must choose between a static and dynamic flex board.

The bend radius is critical when determining your flex PCB's flexibility or bendability. The bend radius denotes the flex area's minimal amount of bend. To enable the design to accept any number of bends without damaging the copper, you must always determine the bend radius early in the design process.

Things to Consider For Laying the Bend Radius

Stagger the conductors in multi-flex circuits to achieve better circuit effectiveness.
Don't place plated through-holes in the area where the PCB bends.
Keep your bend under 90 degrees to avoid unnecessary stress on your PCB.

Insert Cutouts and Slots

You can always reduce the region or area of your flexible PCB if it has no traces. However, the bend radius must be reduced by adding cutouts or slots. The amount of materials needed for the bend is reduced via cuts. You can also reduce the bend radius by deleting some flex parts without electronics. Although you would also require routing after that, consider going with lengthwise routing.

Know Your Flex PCB Materials

Polyimide is typically used as the primary material for a flexible PCB's cover-lay and core layer. When compared to rigid printed circuit boards, flexible materials offer superior properties.

Flex material's uniform thickness and improved dielectric constant of 3.2 to 3.4 are two more significant advantages. Moreover, DK variations are eliminated when woven glass reinforcing is absent.

Due to its distinct production method (cast), Polyimide also has a consistent thickness; a typical polyimide layer will range from 0.5 mils to four mils.

Flex materials are available in both adhesive-based and adhesive-less varieties. Acrylic glue is used to attach the copper to the Polyimide in adhesive-based products. On the other hand, the adhesive-free kind has the copper cast directly into the Polyimide.

Adhesives are often used to attach the core and copper layer (Polyimide). However, fractures in the copper plating, particularly inside the vias, might occur when using adhesive. It happens due to the acrylic adhesive's propensity to soften when heated. As a result, you must incorporate anchors and tear-drops into your PCB design to avoid issues.

Keep an Eye on the Flex Trace Routing

Your printed circuit board's circuitry layout has the power to make or break it. For instance, a larger bend radius is preferred for a flex circuit containing sharp angles, which would otherwise reduce the board's lifespan.

You can also reduce the circuit-thinning stress effect by avoiding I-beaming. Keep in mind that curved traces result in less stress than angled traces. So, keep your traces perpendicular to the bend's main arc to avoid putting unnecessary stress on your board. Additionally, consider top or bottom staggering while placing the traces on a flexible PCB with two or more layers.

Reduce Via Breakage through Teardropping

Vias usually peel off of or split in a flex PCB. However, you can always avoid this by including tabs and anchors while tear-dropping the vias. Moreover, expanding the annular rings can also reduce the vias peeling and breakage.

Place the Flex Layer in the Middle Of the Stack-Up

This mostly applies to Rigid-flex PCBs, where the flexible layer must be positioned between two rigid layers. However, to reveal the flex section, grinding the item is required. Such a configuration shields the flexible component from contact with the outer layer plating. Such an approach will improve control and impedance in the flex region and streamlines the PCB production process.

IPC Standards for Flex PCBs

In order to ensure your flex PCB's integrity, testing is a crucial step. The Association Connecting Electronics Industries (IPC) has a set of standards that will help you determine the quality of both the materials used and the final products. Some of these standards are listed below:

IPC-2223 and IPC-2221

IPC-2223 and IPC-2221 deal with the interconnecting structures and the mounting of components on your flex circuit board.

IPC-FC-234

IPC-FC-234 deals with the use of pressure-sensitive adhesives (PSAs) for your flex PCB's assembly. Furthermore, this document also highlights each adhesive type's benefits, flaws, or limits, along with suggestions on how to utilize them properly.

IPC-6013

IPC-6013 deals with performance and qualification for flexible circuits. Additionally, it describes several tests for flex PCBs, including temperature, bend, and impedance testing.

Wrapping Up

This wraps up our overview of flex PCB design standards and how to ensure that your PCB design will be effective. However, depending on your PCB use case, these design standards are subject to change. After all, one type of design consideration might not work for a certain application.

But, all in all, flexible circuit boards undergo a ton of stress and bending during real-world application. So, religiously follow these guidelines and standards to expand your flexible PCB's lifespan!

Furthermore, if you want to learn more about flex PCBs and different PCB design tricks and tips, visit HemeixinPCB today!