As the name suggests, a flexible PCB is a particular type of circuit board that can bend to the desired application shape. Compared to the regular rigid circuit boards, this type places the conductive pathways and electrical components on a flexible base material.
The flexible design has several benefits that come in handy when building modern compact electrical devices, such as phones, cameras, hearing aids, etc. Therefore, if you are a technician working on such a project, here is an in-depth guide to flexible PCBs and where to buy them.
Rigid vs. Flexible PCBs
While the purpose of both rigid and flexible PCBs is to connect electronic components in a compact circuit design, the two are different.
The main difference is in the substrate material. Flexible PCBs have a flexible substrate usually consisting of polyimide films, while the rigid type features a fiberglass base.
Because of the bending, manufacturers also use flexible rolled annealed copper instead of the rigid electro-deposited copper to make the conductive layer.
A flexible circuit board with a white connector
When manufacturing the pieces, flexible PCBs go through an overlay process to protect the exposed circuitry, while the rigid type uses a solder mask.
The former is also more expensive, but the overall product cost might be cheaper because you can shrink the size of the device you plan to make.
A rigid circuit board for a computer
So, which one is better?
It depends on the application. Flex PCBs are ideal for compact electronic devices and appliances, such as phones, hearing aids, and wearables. However, a rigid board is better for devices with lots of space, like computers and TVs.
You can combine the two to form a rigid-flex board with half of the design in a rigid board and the rest in a flexible circuit.
A rigid-flex board
Means to Make a Rigid-Flex PCB
Basic rigid PCBs only need Gerber file layers for the design stage. However, rigid-flex boards need 3D design software to create the board in a 3D space before manufacturing.
They have more dimensions and need precise definitions of the flex to rigid transition points, so the 3D design is essential.
The base film of the flexible side is polyimide on which a copper foil and overlay holding film layer above it. However, rigid PCBs have FR4 as the base material.
The first step is to apply accessible glue on the copper layer during assembly. After that, you can use the copper plating method or the overlay process for the next stage.
Once complete, use the precise laser penetration method to bore tiny holes on the flex substrate.
Next, fill copper into these holes using the through open plating procedure (synthetic copper plating).
A PCB copper layer electroplating machine
Source: Wikimedia Commons
The following step involves applying photosensitive engraving opposes covering on the flex surface, then carving of the copper film. After these two procedures, the draw oppose gets expelled from the circuit board.
Next, apply the overlay (usually polyimide) to the top and base layers of the flexible substrate.
After this, the board goes through the blanking process, which cuts the substrate into the required sizes. The most commonly used procedures for blanking are hydraulic punch or kick the bucket set.
Lastly, the cut flex blanks get overlaid between the rigid layers. The final piece can go through testing afterward to ensure it works correctly.
Advantages of Flexible PCBs
- Save space and weight.
A flex PCB. Note how it takes up less space when folded or rolled.
- Increased stability and reliability
- Eliminates the use of connectors and wire harnesses and minimizes wiring errors
- Possible to stack them in various configurations
- High tensile strength & high resistance to radiation, chemicals, and extreme temperatures
- Durable (ideal for harsh environments)
- Easy to install
- Delivers a strong signal quality
- Improves impedance control
Disadvantages of Flexible PCBs
- It has a higher cost compared to rigid PCBs in specific applications
- It uses a complex assembly process
- Difficult to repair or rework
- Needs proper storage conditions
- Easily damaged through improper handling
Types of Flexible PCBs
As the simplest type, the single-sided flex circuit has a single conductive layer on one side of the dielectric layer (polyimide or polymer film).
Double-sided flex PCBs have a conductive copper layer on each side of the dielectric material. Metalized through holes connect the two copper sides.
This type contains multiple copper layers separated and encapsulated by dielectric layers. Through holes, connect each of the metal layers.
As stated earlier, rigid-flex PCBs are multi-layer circuits that combine rigid and flexible boards, creating a hybrid design with a high circuit density.
HDI Flexible PCB
High-Density Interconnect (HDI) circuits are efficient and reliable boards with multiple electronic components. They deliver better electrical performance at a reduced package size due to their thin non-conductive substrate material.
Sculptured Flex Circuit
Sculptured flex circuits feature circuit traces that vary in thickness throughout the circuit design. As such, they can have varying copper thickness in some areas.
Polymer Thick Film Flexible PCB
Polymer Thick Film is a simple and affordable way to make reliable FPCBs via screen printing using stencils. You can find such PCBs in low voltage applications because they are not powerful enough to handle high voltage.
Double Access/Back Barred Flexible PCB
This is a special single-sided PCB with conductive tracks accessible from either side of the board.
Two things make this possible. First, the metal layer gets a pre-punched dielectric film applied over it. Second, the conductor layer etching runs under the access points (openings) on the flexible film (dielectric).
Flexible PCB Design: Typical Schematic Diagram and Stackup
A flex PCB consists of several layers, but the number varies depending on the type. These structure diagrams and stack-ups can help you during the design process when building one.
Single Layer Flex PCB
Single-layer FPCBs have a simple stack-up that comprises a flexible base layer (usually polyimide), then an adhesive layer followed by copper. A protective layer (cover lay) consisting of an adhesive and polyimide covers the metal. However, the conductive part cannot be left exposed.
A single-sided flex PCB
Double Access/Back Barred FPCB
Like the single-layer electrical circuit, this one has five layers but with one difference. The polyamide outer layer on the base has a laser opening to access the copper circuitry.
A double access flex PCB
Double Layer FPCB
This PCB features a flexible laminate material as the base, separating the conductive layers. Each metal layer has its cover lay to protect the circuitry and copper-plated through holes to connect the two sides.
A double layer flex PCB
Multi-Layer Flex PCB
Multi-layer flexible circuits combine single and double-sided flexible circuitry designs. Inside, the middle section has dual-sided flex circuits, while the surface has a single-side conductive design covered in a protective film.
A multilayer flex PCB
Materials Used in Flex Circuits
The four main layers in flexible boards can have the following materials.
The metal used in this layer is usually copper due to its superb electric properties and low cost. Heat dissipation is also a key factor to consider, and copper does this quickly.
A flex PCB with a copper conductive layer
Other suitable conductor materials include stainless steel, Beryllium copper, and Cupronickel.
A glue layer containing acrylic or epoxy is between the copper and insulating film (polyimide). Acrylic bonding adhesive materials offer better heat resistance but have poor electrical properties and low binding strength.
On the other hand, epoxy does not give the best heat resistance, but it has better electrical properties and stickiness.
Source: Wikimedia Commons
The insulator is also known as the base material or dielectric and is the flexible material that gives this circuit board its bending properties. Polyimide is the most commonly used material, but some boards have polyester, polyetherimide, various copolymers, or fluoropolymers.
An exposed copper circuitry can shock you or short circuit if an electrical conductor touches the pathways. A cover film usually consisting of a polyimide film keeps the copper sealed and protected.
Flexible PCB with an orange/golden polyimide film finish
- High-speed digital/RF/microwave connectors
- Industrial sensors
- Instrumentation consumables
- Medical devices - wearables
- Consumer electronics
IPC Standards for Flexible Circuit Board Manufacturing
OurPCB: A Leading Flexible PCB Manufacturer
OurPCB is the top PCB manufacturer, and we have the expertise required to build any PCB type, including the flexible one. We specialize in multi-layer flex and rigid-flex manufacturing and provide SMT component assembly for these boards.
We assure you of quality, reliability, and durability, meaning you can use our products in high-temperature, high-density applications worry-free.
Furthermore, our prices are reasonable, and we have a fast lead time of 5 - 6 days and 3 - 14 days for flexible and rigid-flex PCBs, respectively. The build time varies depending on the design.
In conclusion, flexible PCBs are ideal for building compact packages and devices due to their bending properties. If you have a project requiring this type of circuit, contact us for more details about flex circuit boards and get free quotations for your designs.