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What is Flexible PCB: An Introduction to Flexible Circuit Boards


Fri, Feb 11, 2022

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Introduction of Flexible PCB

A flexible PCB is a special printed circuit board that uses the thin and bendable polyimide (PI) substrate to replace the rigid FR4/metal/PTFE/ceramic substrate and PI coverlay to replace the solder mask. It uses non-conductive PI materials to insulate and protect the conductive copper traces that connect different flexible PCB layers by plating through holes or buried vias.

flexible circuit boards

Compared with normal rigid PCB boards, flexible Flexible PCBs are extensively used in electronics devices and systems in telecommunications, automotive, industrial, medical, defense, aerospace, and consumer electronics. Why are flex PCBs so favored? Let's see the details of flexible PCBs and find the answer!

What is Flexible PCB

A flexible PCB is also known as a flexible circuit board, flex PCB, or FPC. Copper circuit traces are etched on the non-conductive flexible polyimide materials, and different circuit layers are interconnected by plated holes (PTH) or buried vias. On the two sides of a flexible PCB, electronic components can be surface-mounted on the PCB pads. For better mechanical support, sometimes, PCB stiffeners can add to the ends of flexible PCBs for connection with the system/device/other parts.

flexible PCB stiffeners

Like traditional PCBs, flexible PCBs are custom designed and manufactured, and they can be single-sided, double-sided, single-layer, dual-layer, or multi-layer. One circuit layer counts one PCB layer. But flexible PCBs are so different from traditional rigid PCBs, and everyone can identify them immediately - flexible PCBs are thin, light, and usually, yellow.

And the thickness range of flexible PCBs falls from 0.05mm to 0.6mm. This is also the finished flex PCB thickness range from PCBONLINE.

Flexible PCBs are usually double-sided. To better understand flexible PCBs, you need to know the basic structure of flexible PCBs.

Structure of a Flexible PCB

flexible PCB structure

The above image shows the structure of a 4-layer flexible PCB with a PTH hole. A flexible PCB consists of:

  • PI substrate - a PI substrate is the core of an FPC. The PI material is usually yellow, but it can also be black, white, and transparent.
  • Rolled copper foils - the etched copper foils are the circuit layers of the flexible PCB. Rolled copper foils dramatically increase the ductility and repeatable bending capability of the FPC. The maximum FPC copper thickness is 3oz.
  • PI layers - PI layers insulate the copper circuit layers, and the materials are the same as the substrate but only the thickness is smaller. The two external PI layers of the flexible PCB are coverlay, which is equal to the solder mask in normal PCBs.
  • Adhesive layers - adhesive layers are optional for flexible PCBs. They are between the copper layers and PI layers to bond them. Flex PCBs with adhesive look darker than the PI materials, and those without adhesive look bright and have a close color to the PI materials.
  • Flexible PCB vias - PCB vias are laser drilled on FPC layers and interconnect different circuit layers. The vias that penetrate all FPC layers are plated-through holes (PTH); vias that penetrate from the external layer to an internal layer are blind vias; vias that penetrate the internal layers are buried vias; vias with an aperture of 0.25mm and below that penetrate only one layer are microvias.
  • Flexible PCB stiffeners - stiffeners are the rigid pieces laminated at the end of flexible PCBs for connection with the system/device interfaces. Stiffeners are optional for flexible PCBs. There are FR4 stiffeners, PI stiffeners, and stainless steel stiffeners.

A PI substrate is laminated with two copper layers on its two sides. A two-layer flexible PCB consists of a PI substrate, two rolled copper layers, and two PI layers (coverlay). Multilayer flexible PCBs are manufactured by laminating alternating layers of copper and PI on the flexible PCB core.

Flexible PCB Types

According to circuit layers differences, there are single-layer FPC, two-layer FPC, and multilayer FPC.

According to component mounting side differences, there are single-sided FPC and double-sided FPC.

According to FPC laminate differences, there are yellow polyimide FPC, clear flexible PCBs, and flexible aluminum PCBs.

According to FPC vias differences, there are HDI flexible PCBs and normal flexible PCBs.

Here I give a brief introduction to some special flexible PCB types.

Clear flexible PCB

clear flexible PCB

Clear flexible PCBs use transparent polyimide laminate. From PCBONLINE, a leading FPC manufacturer, the normal clear flexible PCBs have a thickness range of 0.06mm to 0.1mm, and a maximum PCB layer of 8. The surface finish of clear flexible PCBs is usually immersion silver or immersion tin.

Clear flexible PCBs are about 5 to 10 times more expensive than normal yellow polyimide PCBs. The detailed price should be quoted according to your Gerber files. You can get free design/engineering help or a clear flexible PCB quotation by sending your Gerber to our email info@pcbonline.com.

HDI flexible PCB

HDI flexible PCBs are high-density-interconnect flexible PCBs that have buried/blind microvias. According to IPC standards, HDI flexible PCBs have the FPC via aperture of 0.25mm and below and a trace width/space of 0.1mm and below.

HDI flexible PCBs are multilayer, with a PCB layer of 4, 6, 8, or 10. You can have HDI flexible PCBs manufactured from PCBONLINE.

One-Stop Flexible PCB Manufacturer - PCBONLINE

PCBONLINE is a one-stop flexible PCB manufacturer providing flexible PCB manufacturing, assembly, design, component sourcing, and end-product assembly. PCBONLINE, founded in 1999, has two large advanced PCB manufacturing bases in Jiangsu and Jiangxi, and one flexible PCB assembly factory in Shenzhen.

flexible PCB manufacturer PCBONLINE

From PCBONLINE, you can order 1 to 10-layers of yellow polyimide flexible PCBs, 1 to 8 layers clear flexible PCBs, single/double-sided flexible PCBs, HDI flexible PCBs, flexible aluminum PCBs, and aluminum LED PCBs.

Here are the reasons to let PCBONLINE manufacture your flexible PCBs:

    • PCBONLINE provides turnkey flexible PCB manufacturing from FPC design, manufacturing, PCB assembly, and testing, to box-build assembly.
    • High-quality flexible PCB manufacturing that passes ISO 9001, IATF 16949, UL, RoHS, and REACH certification.
    • Free and professional one-on-one engineering/design assistance for flexible PCB customers.
    • Free complete samples and PCBA functional testing for batch flexible PCB/PCBA orders.
    • Traceable flexible PCB/PCBA manufacturing and quality money-back/free-rework guarantee.

Please feel free to contact PCBONLINE by email at info@pcbonline.com to get a flexible PCB quotation or free engineering consultancy.

Flexible PCB Applications and Advantages

Flexible PCBs can be used in almost all electrical and electronics industries. However, flexible PCBs are more expensive than normal rigid PCBs, and they have been extensively used in these industries:

1. Flexible PCBs in Automotive Electronics

Flexible circuit boards are used in automotive systems/parts including anti-lock braking systems, airbag controllers, dash systems, instrumental panels, etc. Flex PCBs fit into the small parts and provide flexibility for engineers. Automotive components need to withstand heat and cold and experience vibration during use. There is no need to worry about flexible PCBs in such circumstances.

2. Flexible PCBs in Aerospace Components

Flexible PCBs are used in different parts of the aviation industry, such as in holographic waveguide helmet-mounted displays providing high-resolution 3D images. In satellites, flexible PCBs are used to replace connectors that may cause interconnect errors, and they can fold and bend thousands of times, carry components, and directly connect different parts.

3. Flexible PCBs in computer electronics

Computer operations rely on flexible PCBs as computers have a small space and flex PCBs pass signals. Desktops usually apply 4 and 6 layers FPC boards, and laptops with a smaller space require 8-layers FPC boards.

4. Flexible PCBs in smartphones

Smartphones are tiny, light-weighted, portable, and multifunctional. Inside the small smartphones, there are plenty of flexible PCBs used to carry and connect various components. Flex PCBs fit tiny spaces and keep our smartphones safe, and they are resistant to heat and the environment.

5. Flexible PCBs in medical devices

Flexible PCBs are extensively used in medical appliances such as swallowable pill cameras, gastroscopy, glass hearing devices, etc. Flexible PCBs are small, thin, foldable, and durable, and these metrics make them necessary for medical devices that have a small space and require precision. Below is a double-sided flexible PCB for gastroscopy.

medical flexible PCB

Besides, flexible PCBs are also used in wearable devices, digital cameras, etc. Flexible PCBs are extensively used in these middle and high-end devices because of the below advantages.

Advantages of Flexible PCBs

  • Direct board-to-board/device connection, without the need for bulky cable connectors and wires.
  • Lightweighted, thin, and smaller than conventional PCBs.
  • Flexible, able to bend thousands of times without harm to the board.
  • Able to withstand up to 400°C high temperatures.


This article gives a comprehensive introduction to flexible circuit boards. I hope you can have a deep insight into flexible PCBs by reading this article. If you want to know more specifications of flexible PCB manufacturing and assembly, or you want to design/manufacture/assemble flexible PCBs, please feel free to contact the reliable one-stop FPC manufacturer PCBONLINE.