Flex PCB: Flexible Circuit Boards for Compact Devices

As the name suggests, a flex PCB is a special type of circuit board that can be bent into the desired shape for the application. Compared to conventional rigid circuit boards, this type places conductive pathways and electrical components on a flexible substrate.

The flexible design has several benefits that come in handy when building modern compact electronic devices such as phones, cameras, hearing aids, and more. So, if you're a techie working on this type of project, here's an in-depth guide to flexible PCBs to help you understand it better.

Although both rigid and flexible PCBs aim to connect electronic components in a compact circuit design, the two are different. The main difference is the substrate material. Flex PCBs have a flexible substrate usually composed of a polyimide film, while rigid types have a fiberglass substrate. Manufacturers also use flexible rolled annealed copper instead of rigid electrodeposited copper for the conductive layer because of the flex.

Flex PCBs protect exposed circuitry by covering the process while the part is being manufactured, while rigid PCBs use a solder mask. The former is also more expensive, but the overall product cost may be cheaper because you can shrink the size of the device you plan to make.

So, which is better?

It depends on the application. Flexible PCBs are ideal for compact electronics and appliances such as phones, hearing aids and wearables. However, rigid boards are better suited for devices with more space, such as computers and televisions.

You can combine the two to form a rigid-flex board with half of the design on the rigid board and the rest in the flex circuit.

A basic rigid PCB requires only the Gerber file layer during the design phase. However, rigid-flex boards require 3D design software to create the board in 3D space prior to manufacturing.

They have more dimensions and require a precise definition of the transition point from flexible to rigid, so the 3D design is essential.

The base film on the flexible side is polyimide with copper foil and a cover retaining film layer. However, rigid PCBs use FR4 as the base material.

The first step is to apply an accessible glue to the copper layer during assembly. After that, you can use the copper plating method or the next stage of the capping process.

Once complete, tiny holes are drilled in the flexible substrate using a precise laser penetration method.

Next, copper is filled into these holes using a through-hole plating procedure (synthetic copper plating).

The next steps involve applying a photosensitive engraving pair overlay on the flexible surface and then engraving the copper film. After these two procedures, the drawn object is expelled from the board.

Next, a cover layer (usually polyimide) is applied to the top and base layers of the flexible substrate.

After this, the board goes through a blanking process, which cuts the substrate to the required size. The most commonly used blanking procedures are hydraulic punches or kick bucket groups.

Finally, the cut flexible blank is covered between rigid layers. The final product can be tested afterward to make sure it works correctly.

Save space and weight.

Improve stability and reliability

Minimizes wiring errors by eliminating the need for connectors and wire harnesses

They can be stacked in various configurations

High tensile strength and high resistance to radiation, chemicals, and extreme temperatures

Durable (suitable for harsh environments)

Easy to install

Provides strong signal quality

Improved impedance control

The simplest type, single-sided flex circuits have a conductive layer on one side of a dielectric layer (polyimide or polymer film).

Double-sided flex PCBs have a conductive copper layer on each side of the dielectric material. A plated via connects the two copper planes.

This type consists of multiple copper layers separated and encapsulated by dielectric layers. Through holes, each metal layer is connected.

As mentioned earlier, rigid-flex PCBs are multilayer circuits that combine rigid and flex boards, creating a hybrid design with high circuit density.

High-Density Interconnect (HDI) circuits are efficient and reliable circuit boards with multiple electronic components. They offer better electrical performance in a smaller package size due to their thin non-conductive substrate material.

Sculpted flex circuits have circuit traces that vary in thickness throughout the circuit design. Therefore, they may have different copper thicknesses in some areas.

Polymer thick film is an easy and affordable method to make reliable FPCBs by screen printing using a stencil. You can find such PCBs in low-voltage applications as they are not strong enough to handle high voltages.

This is a special single-sided PCB with conductive tracks accessible from both sides of the board.

Two things make this possible. First, a pre-perforated dielectric film is applied over the metal layer. Second, the conductor layer etches runs under the access points (openings) on the flexible film (dielectric).

The four main layers in a flex board can have the following materials.

The metal used in this layer is usually copper because of its excellent electrical properties and low cost. Heat dissipation is also a key consideration, and copper can do that pretty quickly. Other suitable conductor materials include stainless steel, beryllium copper, and cupronickel.

Between the copper and the insulating film (polyimide), there is an adhesive layer containing acrylic or epoxy. Acrylic adhesive materials have good heat resistance, but poor electrical properties and low bond strength. Epoxy, on the other hand, doesn't offer the best heat resistance, but it has better electrical properties and stickiness.

Insulators, also known as substrates or dielectrics, are flexible materials that give circuit boards their flex properties. Polyimide is the most common material, but some sheets are available in polyester, polyetherimide, various copolymers, or fluoropolymers.

Exposed copper circuits may be electrocuted or shorted if electrical conductors touch the pathway. A cover film, usually composed of polyimide film, seals and protects the copper.

In summary, flex PCBs are ideal for building compact packages and devices due to their flexible nature. If you want to buy a flexible PCB board, welcome to contact us.

Singo is a professional custom PCB board manufacturer. Some of our products are often required to operate in harsh environments where quality and reliability are critical. With many years of experience, we have won a good reputation from customers in the field of electronic production with reasonable prices, abundant resources, and punctual delivery.