All You Need to Know About Flex PCB Design Guidelines

Flex PCB are also known as FCBs or Flex Circuit Boards. They are employed in a variety of common applications, including tablets, smartphones, printers, cameras, and laptops. They can even be found inside our bodies, where they are used to build cochlear implants, pacemakers, and defibrillators.

Designing high-quality flex PCBs requires you to understand different properties and applications. This guide can help you learn all you need to know.

How Is a Flex PCB Different from Rigid-Flex PCB?

Although most printed circuit boards serve fundamentally the same functions, there are three major varieties — rigid, rigid-flex, and flex — that vary in design and manufacturing. Rigid boards do not bend, as their names suggest, but flex PCBs can be bent and tailored to fit into certain devices and systems. A rigid-flex PCB is a hybrid of the two types.

What To Know When Designing Your Flex PCB

Flex PCB design differs from rigid PCB design in that it necessitates a slightly modified technique. Whilst flex PCBs can save a lot of money in terms of manufacturing costs, area expenditure, and lightness when compared to rigid PCBs, their construction must be optimized according to the resources available and their application.

Benefits of a Flex PCB

A flex PCB is made using tough, durable materials that can withstand variable gravitational forces and extreme conditions. This, coupled with the fact that you can customize a routing path with a flex circuit, is reliable and dependable.

It can carry power without disruption thanks to its tolerance of multiple flexing cycles, high vibrations, and notable strain without compromising on space efficiency and signal integrity. Moreover, they are able to tolerate high temperatures, making them useful for countless industries, such as the military, aerospace, or even the medical industry.

Materials Used

Polyimide is the most common material for both the coverlay layers and flex core. The thickness is consistent, with a Dk value that ranges from 3.2 to 3.4. Variations in Dk are eliminated considering the absence of woven glass reinforcing. Polyimide's thickness is also exceptionally consistent thanks to its "cast" production technique. Layer thicknesses typically range between 0.5 to 4 mils.

Since copper foil is incredibly conductive, it is the most widely utilized substance. F or the flex PCB to be functional, you don't need to have a lot of copper. You can also find completely annealed or low-temperature annealed copper, which improves bending properties.

Lastly, the finish is important to prevent oxidation and offer a solderable surface. Electroless Nickel Immersion Gold (ENIG) is widely used as a finish due to its ability to give a thin, gold layer that protects the copper.

Copper is protected by electroless nickel, which prevents it from coming into touch with other metals. Palladium acts as an anti-oxidation and wire-bonding surface, protecting the nickel from corrosion, while the final gold layer increases castability and acts as an oxidation shield.

What Affects the Cost?

Naturally, the features, layer counts, and materials you use depend on your project, but keeping cost-effectiveness in mind is important when you are designing your flex circuit.

When more materials and time are used, a flex circuit with numerous layers usually has a greater cost. A few of these multilayer designs could also be technically complex. Certain features such as buried vias are more expensive.

The number of flex circuits that can be fitted on a manufacturing circuit is determined by their size and form, which has an impact on pricing. Similarly, when it comes to the IPC class, Class 3 will have greater serialization specifications and, therefore, more validation coupons than Class 2, which could increase the cost. 

Flex PCB Trace Routing

The layout of your circuit determines the effectiveness of your design. Sharp angles limit the longevity of a board; hence, a big radius is recommended. Furthermore, I-beaming should be avoided to reduce the stress that could erode copper circuits.

Curved traces put less strain on the body than angled ones. Traces also should be positioned vertical to the overall bend and staggered on the two ends if set on a flex PCB with multiple layers.

Flex PCB Vias

The vertical interconnect access is a plated through hole that routes a trace between the layers in the board. Vias are an important component of the multi-layered PCB design.

Vias can cause peel flex PCB designs to split or crack. It is better to make sure that vias are tear-dropped shaped, tabs or anchors are included, and the annular rings are as big as feasible to keep them from doing so.

Impedance and Signal Integrity

How quickly a certain amount of electricity can travel down a trace is known as impedance, and matching impedance allows your circuit board to correctly function and transmits signals. The factors that affect impedance include the width of the traces, the relative permittivity of the insulating materials employed, the distance of the signal traces from the layers, the thickness of the traces, and more.

Assembling Your Circuit

When assembling your circuit, you should make sure you follow all the right steps. For professional advice, you can consult us. Before exposing the circuit to high temperatures, check to ensure all moisture has been completely removed.

The use of flex layers around the interior of the stack-up protects the exterior plating from damage. In the flex area, this location also streamlines production and increases impedance and stability.

Lastly, after baking, you should process the flex circuit right away. If this isn't feasible, you should keep your circuits in a nitrogen chamber or a desiccant-filled, airtight, dry container.

Although there are a variety of ways to ensure the reliability of your raw materials and completed products, standards from the Association Connecting Electronics Industries (IPC), including IPC-6013, IPC-2223, and IP-FC-234 and their counterparts, are among the most widely adopted.

Last Words

Flex PCB design standards require experience and expertise. If you choose to hire a manufacturer, they should also provide individualized customer service as well as a variety of design and layup alternatives. Feel free to contact us for a quote!