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Flexible Printed Circuit Board, Introduction and importance.

In this post today, I’ll walk you through the Flexible PCB in detail.

A PCB (printed circuit board) is an electrical circuit that provides mechanical support to electrical components connected through conductive copper wires. It is the basic building block of many electronic projects, opening the door for small, compact, and less erratic designs.

The PCBs you see today have evolved over years into a more sophisticated circuit board – compare the circuit boards installed in TVs of the 1990s with the ones available in modern LED designs and smartphones.

The PCB boards are categorized into different types including single layer, double layer, multiple layers, and rigid, flexible, rigid-flex, and aluminum circuit boards. Here in this article, we’ll only cover flexible circuit boards.

Let’s jump right in.

What is Flexible PCB?

Flexible PCB, also known as flex prints or flex circuits, is a special type of circuit board that you can bend into the desired shape. They are widely used for high-density and high-temperature applications. The flex designs are composed of polyimide or a transparent polyester film as a substrate material that comes with high heat resistance which makes it a right fit for solder mounting components.

Image of a flexible PCB

A flexible circuit comes with a conductive layer of traces made up of copper that is combined with a polyimide dielectric layer. The thickness of the copper conductive layer can vary from 0.0001’’ to 0.010’’ and the thickness of the dielectric material can range from 0.0005’’ to 0.010’’. An adhesive is required to bond the conductive copper layer to the substrate; while sometimes vapour deposition is equally handy to attach the copper with the substrate.

The material selection for making flexible circuit boards is a bit tricky and depends on many factors including chemical and mechanical resistance, current, temperature, capacitance, and types of flexing.

The flexible designs are more reliable as they come with fewer interconnections that ensure fewer solder joints and contact crimps. And these circuits require less space due to their flexible bend capability and they cover only 10% of the area compared to rigid circuit boards.

Flexible PCB Manufacturing Process:

Polyimide is used as a base substrate material to manufacture flexible PCBs. This material is more expensive than FR-4, thus needs to be used carefully. To appropriately use the polyimide material, the nesting technique is used to keep circuit boards close to each other.

Flexible PCB shown being flexed

Steps to manufacture flexible boards include:

Looping

The service loop ensures the servicing length and circuit assembly which is the amount of extra material you add beyond the designer’s limit.

Sizing Conductor

Thin copper is used as a conductive material on the board. The thin copper is responsible for the circuit’s flexibility and makes it suitable for dynamic applications.

Etching

The etching process is used to compensate for any isotropic losses during the manufacturing process. In this process, copper foil thickness is half the line width loss. Factors influencing the line width include conductor, etch mask and the different types of conductor used.

Routing

Routing is simple. Routing ensures reduced stress and improves bending and folding.

Ground Planes

Creating ground planes will reduce the weight of the board and leads to better circuit flexibility.

Next, you need to create holes before applying filleting where you increase the pad area to divide stress. Then add adhesive-backed films for dynamic flex circuit applications. Next, you need to apply screen-printable liquid overcoats before adding photo imaginable liquid and film polymers that act as a solder mask and prevent the circuits from both internal and external damages.

Types of Flexible PCBs:

Flexible printed circuit boards are divided into four different types.

Single Layer Flexible PCB

The single-layer flexible PCB, as the name suggests, contains only one conductive layer that sits on the flexible dielectric film. The electric components rest on only one side of the PCB.

Double Layer Flexible PCB

The double-layer flexible PCB offers a conductive layer on both sides of the PCB, allowing you to connect electrical components on either side of the conductive layer.

Image showing double layer flexible PCB

Multiple Layer Flexible PCB

The multi-layer flexible PCB comes with three or more conductive layers separated by a dielectric material. The irregular lamination ensures high board flexibility and lamination thickness is normally less in the bonding area.

Rigid-Flex PCB

The rigid-flex PCB is a combination of both rigid and flexible circuit boards that offer higher component density compared to other circuit boards.

HDI Flexible PCB

The HDI stands for high-density interconnect. These circuit designs are efficient, reliable and come with better design, construction, and layout compared to other boards. HDI flexible boards offer better electrical performance and reduced package size due to the thinner substrate material used for the manufacturing of these boards.

Importance in Modern World:

The flexible boards can be bent into any shape which is the reason they are widely used for dynamic flexing and static applications. Dynamic circuit boards are made for regular flexing while the circuit boards used for static applications offer minimum flexing.

Flexible circuit designs are made to withstand high temperatures – between -200° C and 400° C – the reason they are a right fit for borehole measurements in the oil and gas industry.

Flexible boards come in handy in many applications but you cannot use them as a replacement for rigid circuit boards. Because rigid boards are cheap and can be installed in high-volume automated fabricating applications.

The flexible circuit boards are a right match for applications that require better performance, high accuracy, precision, and regular flexing. Applications of flexible circuit boards include:

  • Cameras
  • Antilock brakes
  • Fuel pumps
  • Ultrasound probes
  • Semiconductor test
  • Medical devices
  • Motion systems
  • Satellites
  • Avionics
  • Battery packs
  • Manufacturing devices
  • Airbag systems
  • Bar code equipment

That’s all for today. Hope you’ve found this article helpful. If you’re unsure or have any questions, you can approach me in the section below. I’d love to help you the best way I can. Thank you for

HI, I am a student of Electrical Engineering. I am currently working for an electronic store. I work at the electrical components there. I am learning many useful practical concepts there. On the other hand, I give online tuitions to some students of high school I love electrical and electronics devices and have a plan to have a Master's Degree in Electronics.

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