Electronics Manufacturing, PCB Fabrication, PCB Manufacturing

Flex Circuit Boards: Introduction and Applications

Flex Circuit Boards

Flexible circuit boards, often known as flex circuits, possess unique possibilities that go beyond typical rigid PCB technologies. Their flexible structure allows them to bend, fold, and flex dynamically while in operation, making them indispensable in modern electrical devices and products due to increased miniaturization and innovation in electronics.

Flex circuit PCBs are becoming more common in a variety of industries, particularly aerospace, automotive, and medical, as well as high-definition and slimline display applications.

In this post, we will explore what flex circuit PCB is, how it is made, what its advantages are, and where it may be utilized.

What is Flex Circuit Board?

Flex circuit PCB is a type of PCB which can be bent. The circuit components are arranged on a flexible substrate unlike FR-4 for regular PCB, it is called an FPC stack up. PI material is used as core which is a high performance plastic type and adhesive material in general. Flex circuit boards are generally used for high performance, low current compact lightweight systems. Flex circuit boards can generally handle more signal lines as the trace-to-trace thickness can be as small as 0.06mm. The bending radius of the Flex circuit PCB is very minimal and it can be also bent flat on top of each other.

Different Types of Flex Circuit Boards

  • Single layer flex PCB

The flex circuit PCB thickness can be as small as 0.08 mm thick with a single layer and up to 0.15mm FPC. The layer stack-up stands with top coverlay, adhesive, top copper layer and PI to support from the bottom using the aid of an adhesive. 

  • Double Layer Flex PCB

Double-layer Flex circuit boards are also used in various applications with standard thickness varying from 0.1mm to 0.26 mm where the layer stack up stands with Top coverlay, top copper layer, adhesive, PI, adhesive, bottom copper layer and bottom coverlay orderly. Customers can also customize the stack-up based on their requirements and manufacturing capability. 

  • Multilayer Flex PCB

Multilayer Flex circuit PCBs are a little expensive because of the construction and complexity of the process. The thickness can vary from 0.2mm to 0.38mm from 4 to 6 layers, as the stack up stands similar to a double-layer PCB with additional Adhesive materials sandwiched in between copper layers.

  • Rigid Flex PCB

There is one more type of flex circuit PCB rigid flex PCB, it is nothing but a combination of rigid standard PCB and flex PCB. It is recommended to use the same stack-up for both the rigid part and flex part of the PCB. Though rigid flex PCBs are of many types these stack-up recommendations have to be followed to achieve integrity. The difference between the PCBs will be the outer layer where the right part has FR4 and a copper layer with a Top solder mask similar to standard PCB, whereas the Flex part consists of similar PI and adhesive materials to maintain flexibility.

The Composite of Flex Circuit Board

Rigid Flex PCB

  • Base PI material

The base material for Flex circuit PCB is Polyimide used as an insulating material for both bendable circuit board and Flexible cables. The properties of PI material are withstanding high temperatures, flexible and it is also capable of shrinking during the manufacturing process yet providing insulation similar to FR4, PET and ET. PI is also used as a s cover film in many areas to isolate gold pins in minute connectors. The thickness of PI is very low and we get more thickness by laminating multiple PI layers in a Multilayer flex circuit board.

  • Copper Layer

Copper layers are very similar to standard FR4 PCB. The conductive copper layer is itched as usual and the thickness of the copper varies from 12um to 30um or sometimes above.

  • Coverlay

The top layer is called as top coverlay used for insulation. In multi-later PCB the stack up can be changed and the thickness of the bendable circuit board can be made up to 0.3 mm.  In single layer Flex PCB, we have a Top coverlay and in two-layer or multilayer flex circuit PCB we have both TOP and Bottom coverlay adjacent to the top and bottom copper layers. The solder mask coverlay is usually yellow, white and sometimes black. The silkscreen is also printed on top of coverlay with white or black colour as requested by the customer.

  • Stiffener

In a few cases, stiffeners are connected to bendable circuit boards or Flex cables which are used to primarily give mechanical support when it needs to be mated with a connector. Stiffeners on the PCB can also be requested on both sides of it. The manufacturer will request if there needs to be a specific thickness for the stiffener and the metal required for it. The material for stiffeners is PI or FR4 or Stainless steel. We can also get flex circuit PCB without stiffener.

  • Surface finish

Surface finish generally comes with very few materials unlike standard PCBs.  The materials are generally Immersion gold, OSP, Immersion silver, and Immersion tin with thicknesses of mental 1U’ 2U’ and 3U’. The finished copper thickness is also very vital as it is generally 0.5Oz, 1Oz, 1.5Oz or 2Oz.


The Advantages of Flex Circuit Board


Flexible Printed Circuit

In the printed circuit board industry, flexible circuit board haven’t been around for very long. However, it has already revolutionized the circuit board business. Flex circuit boards have been the preferred choice of most PCB manufacturers and people working with PCBs in many industries due to their numerous potential benefits.

  • Lightweight and thin

The bendable circuit board’s polyimide material is lighter than FR4. Their low mass makes them well-suited for mobile devices. A typical 2-layer flex can range in thickness between 4.4 and 10 mils.

  • Flexibility

Flexible circuit boards are commonly utilized in portable devices such as cameras, RFID tags, and cellular phones. Unlike rigid boards, which have restrictions on how they may fit into smaller devices, this material is highly convenient to utilize since it can easily twisted and folded. Flexible printed circuit can adapt to the shape and geometry of diverse surfaces.

  • High Temperature

Compared to FR4, polyimide flex materials can withstand extremely high temperatures. They are also stable and durable, gasses, and oils.

  • Excellent reliability

The flexible circuit board features fewer interconnections between the substrate and other components. As a result, contact crimps and solder connections are reduced. Because flexible printed circuits have less room for failure, you could thus experience exceptional reliability with them.

The Applications for Flex Circuit Board

Flex PCB Fabrication

Flexible circuit boards have a wide range of applications in numerous industries due to their unique characteristics and benefits. Some of the identified applications are discussed below.

  • Flex Cables in all Industries

In the market, the major purpose of bendable circuit board is with Flex cable design where the smaller FPC connectors can be used instead of bulky connectors with ribbon cable design and replacing the wiring harness. In automotive applications even various temperature monitoring sensors inside battery packs are mounted on flexible circuit board design to make the commuter industry safer and more reliable. These techniques are adapted to use very minimal space and achieve more safety.

  • Flex PCB on Display

Display connections with the Display driver are designed using flexible circuit board for HDMI connections used in extra thin LED or OLED Television, Head’s up display and Helmet displays in the aerospace and automotive industry.

  • Flex PCB on Medical Applications

A variety of sensory digital signals are communicated to DSP in numerous operation machines. Almost all hearing devices use these flexible circuit boards to carry actuating digital signals from the inner part of the lobe to the signal processing units on the outer side of the ear.

  • Flex PCB on the Consumer Electronics

Consumer Electronics flexible printed circuit is used in a variety of electronic applications. Calculators, computers, smartphones, cameras, and other gadgets are among the items that make use of this bendable circuit board.

  • Flex PCB on Automotive

Flex PCBs have gained significant popularity in the automotive industry due to their ability to conform to unique shapes and contours of automotive components. Flexible LED lighting systems, flexible display panels, sensors, infotainment systems, and electronic control modules are a few examples of its uses.

  • Flex PCB on Automation and Robotics

Flex PCB is essential for managing and running the electrical parts of robotic systems. It manages the main parts of a robot, including the batteries, body, actions, feedback, and sensors. Robots, drones, and automated land, sea, and air vehicles all depend on PCBs that fit their particular requirements. PCBs are the building blocks of components such as power distribution boards and robot control system sensors.

  • Flex PCB on Industrial equipment

Flex PCBs enable flexible machine design and are capable of operating under demanding circumstances. The industrial sector uses bendable circuit boards for power circuits, industrial automation, radio frequency communication, and other applications. There are more industrial spaces available when a flexible circuit board is used.

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