Everything You Want to Know About Flexible PCBs

What Is a Flex PCB ?

Flex PCB is a unique type of printed circuit board that provides extra capabilities as compared to traditional rigid PCBs. These types of boards can be used dynamically allowing the circuit to bend ,fold and flex. In today’s modern innovative and miniature world of electronics flex PCBs have become ubiquitous in modern devices.

Flexible PCBs Classification Based on Configugration

• Single-Sided Flex PCBs: Featuring one conductive layer on a flexible substrate, these are used in simple, low-cost applications.
• Double-Sided Flex PCBs: With conductive layers on both sides, these PCBs are suitable for more complex circuits.
• Multilayer Flex PCBs: Consisting of multiple conductive layers separated by insulating layers, they are used in highly complex and dense electronic assemblies.
• Rigid-Flex PCBs: Combining flexible and rigid board technologies, these PCBs offer the benefits of both worlds, making them ideal for applications requiring both flexibility and structural integrity.

Materials used in Flex PCBs :

Polyamide substrate is used in flexible PCBs. Due to the flexible base these boards have the best toleration for vibrations ,heat dissipation and can be folded into many shapes. They consist of conductive circuits having very thin insulating polymer film fixed at the conductive circuit and most of the time it gives protection to conductive circuit with a thin polymer coating. The quality of PCB would be determined by PCB raw material and flex PCB stack up. Keeping in view the market  considerations flex PCBs are categorized in three types of polyester ,adhesive polyamide, and less adhesive polyamide. Here is a chart for choosing the raw material according to the requirement.

Layer stack up of Flex PCBs: 

Flex PCBs have alternative patterns of copper conductors and flexible dielectrics which are bonded in a multilayer circuit.  The key components of flex PCB layers are

Base material:

A flexible substrate formed from thin polyamide or other flexible polymer film with typical thickness range from 12um to 75um.

Conductor:

It consists of copper foil with a pattern of traces pads and interconnects having thickness ranging from 18um to 35um.

Dielectric adhesive: 

A 25um thick adhesive made up of polyamide or acrylic is used to bond layers together.

Cover layers:

To protect conductors an outer layer of flexible material or adhesive coated polyamide is used .

Stiffeners: 

For mechanical support metal plates or additional polyamide material is used.

Finished thickness :

The total thickness of multilayer flex circuit usually ranges from 75um to 150um. In the industry, a flex circuit thickness of roughly 0.062 inches, or 1.57 mm, is preferred. However, the majority of designers will ultimately come up with a range of measurements, ranging from 0.7 mm to 6.40 mm and beyond. A 4-layer flex PCB has a thickness of 0.42 mm, compared to a typical 2-layer PCB's 0.32 mm. The total thickness of the flex PCB increases with the number of layers.

There is no theoretical limit for layer count of flex PCB but if practically considered flex PCBs can have maximum 12 or 16 layers. Higher counts are only feasible for specialized applications due to increased complexity cost and manufacturing challenges.

                        Layer Stack up in Altium Designer

Calculation of bend radius is very important for layer count of flex PCB. It is the minimum radius up to which circuit can be bent without any damage. It should be identified properly at the start of the design. The bend radius depend upon the number of layers in layer stack up.

 The width of the flexible PCB depends upon various factors including size and weight and copper thickness of PCB.

Key characteristics of flexible PCBs :

The flex circuit based on their material and construction possess following properties.

Thickness :

The overall thickness of flex PCBs ranges from 12um to 180um for single layer to multilayer construction respectively. This gives extremely light weight thin electrical circuits.

Bend radius :

Flex PCBs can be bent to three times their thickness in tight radii. For certain architectures, dynamic flexing down to 10X thickness is achievable.

Heat resistance :

The base polyamide material gives the thermal strength to flex PCBs to withstand temperatures up to 400^oC. Other types of material do not provide temperature resistance for such high temperatures.

Chemical resistance :

The most common fluxes being used during soldering and assembly may damage the PCB but the substrate used in flex boards provides good chemical resistance.

Weight :

The weight of the polyamide or polyester substrate is extremely small which makes flex PCB weigh less than 80-90% as compared to the conventional rigid PCBs.

High frequency :

Thin dielectric material and small signal paths provide high frequency performance and some substrates like LCP give superior RF properties to such boards.

Current ratings :

The current carrying capacity gets limited by copper thickness. For common flex circuits typical current ratings range from 0.5 to 5A.

Application of flex circuit Boards :

Flex PCBs in LED display :

In led displays transparent flexible PCBs are used , Led is surface mounted over the flex PCB and gets illuminated if the power is given to it. In addition to being transparent, PI flexible PCBs are utilized as touch displays on tablets and LED screens in smartphones. Particularly, flexible, transparent PCB is used in folding screens.

Flexible PCBs for Electronic Automotive Systems :

Automobile systems and components such as airbag controllers, instrument panels, dash systems, and anti-lock brake systems all use flexible circuit boards.
Flexible Circuit Boards in Aerospace Parts :

Flexible printed circuit boards (PCBs) find application in various aviation-related fields, including holographic waveguide helmet-mounted displays that offer sharp, three-dimensional (3D) pictures.  Flexible PCBs, which can fold and bend thousands of times, carry components, and connect various elements directly, are used in satellites to replace connectors that may result in connectivity issues.

Flex PCBs in computer :

Since computers are compact devices, flexible printed circuit boards (PCBs) are essential to computer operations. Typically, desktop computers use 4 or 6 layer FPC boards, whereas smaller laptops use 8 layer FPC boards.

Smartphones with flexible PCBs :

Smartphones are compact, lightweight, multipurpose, and portable. Numerous flexible printed circuit boards (PCBs) are employed to carry and link different components inside the tiny cellphones. Our smartphones are safe and compact because to Flex PCBs' resistance to heat and environmental factors.

Medical devices using flexible PCBs: 

Medical products like glass hearing aids, gastroscopy equipment, and swallowable pill cameras all make heavy use of flexible PCBs.

Conclusion: 

An extensive introduction to flexible circuit boards is provided in this article. I hope that after reading this post, you will have a thorough understanding of flexible PCBs. We have explored the world of Flex PCBs and their benefits in this post. Adhering to specific criteria in order to account for mechanical bending loads and steer clear of typical failure scenarios is essential for a good flex PCB design. A strong implementation is achieved by utilizing hard parts joined by flexible interconnects. You may take advantage of the various advantages of flex circuits by using good flex design principles.