A Complete Guide To Flexible Printed Circuit

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Oakley Mae

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About A Complete Guide To Flexible Printed Circuit, The ongoing growth of technology necessitates manufacturers to develop better circuit boards with reliable, flexible printed circuits.

The PCBs must work well in specific applications while tolerating vibrations and extreme temperatures.

But do you know what a flexible printed circuit is? If so, what are its benefits over traditional PCBs?

In today’s article, we highlight vital information about flex PCBs. Let’s begin. 

Table of Contents

What’s A Flexible Printed Circuit Board

A hand holding a flex PCB

(A hand holding a flex PCB)

A flexible PCB is a circuit board with metallic traces attached to a dielectric layer.

Generally, most manufacturers use copper traces and polyimide dielectric layers for many flexible PCBs.

Additionally, they are light, thin, and with good bending properties.

Types of Flexible Circuit Boards

Here are the types of flexible PCBs:

Single-Sided Flex PCBs

They are the most common flex PCBs that you can find. They have one conductor layer with a flexible dielectric film. They are also printed on one side, making them cheaper to produce. 

Double-Sided Flex PCBs

They have prints on both sides of the dielectric material. Consequently, they pack more components and perform better power-handling than other boards.

On the downside, they are costlier to manufacture. 

Multi-Layer Flex PCBs

They have more than two layers of conductors, making them suitable for military and aerospace applications.

Moreover, they pack dense circuitry and are costlier to manufacture.

Rigid-Flex PCBs

From the name, they are a blend of rigid and flex circuit boards. More specifically, they are rigid circuit boards with flexible connection substrates.

Due to their construction, you’ll find them mostly in smart devices and military applications. 

Materials Used In Flexible PCBs

Curling a flexible PCB into a circle

(Curling a flexible PCB into a circle)

Materials we use to manufacture flexible PCBs include:

Conductors: Allow for the easier flow of electric current through the circuit. Moreover, copper and aluminum are popular conductors for flexible PCBs.

Adhesives: Allow for better binding of surface mount components. Examples include acrylic and epoxy adhesives. 

Insulators: Separate conducting layers without carrying current through the circuit.

Examples include solder masks, polyester, and polyethylene terephthalate. 

Applications of Flexible PCB Boards

Flexible PCBs have many applications in all spheres of life. Here, we’re looking at their most common applications:

Static Application

It is also known as the flex-to-fit application of flexible PCBs. PCB manufacturers design the PCBs to fit in spaces that are tough to work with rigid PCBs.

Consequently, they install flexible PCBs in a one-time bending. For instance, when you want a PCB that fits into a curved part of a device or appliance.

Dynamic Application

If you want a PCB for a flexible device, you’re better off going with a flex PCB. We use it in applications whereby the PCB bends according to the shape of the final product. 

More specifically, here are applications for flex PCBs:

Wearing a hearing aid

(Wearing a hearing aid)

Electronics: Flexible PCBs are lighter and gaining popularity for use in handheld technology. These include smart devices, scanners, and cameras, among others. 

Aerospace: Flexible PCBs handle stress and vibrations better than rigid circuit boards. Accordingly, they find greater use in aerospace applications to substitute circuits prone to failure.

Automotive: Flex PCBs enable better space allocations and thermal performance in automobiles. Additionally, manufacturers are incorporating more sensors and electronics to improve performance. As such, there’s a need for lighter boards that fit in small spaces.

Medical devices: We use flexible PCBs in many medical devices like hearing aids

Benefits of Flex Circuit Boards

An open smartphone without the back cover

(An open smartphone without the back cover)

Flexible circuit boards are beneficial in devices that bend or fold. For instance, you’ll prefer a flex PCB in a wearable device to a rigid PCB.

Moreover, the rigid PCB will limit your device’s functionality, and you risk breaking it forcefully if you try bending it. 

Here are additional benefits of the flex circuit board:

  • Flex PCBs increase the applications of many devices across sectors.
  • It decreases the likelihood of wire connection failure hence improving overall dependability.
  • They are smaller and lighter than rigid boards.
  • Flex PCBs are appropriate for tougher settings because of their wide temperature range.
  • Flex circuit boards can accommodate a high circuit density.

Manufacturing A Flexible PCB

Manufacturing flexible PCBs differs from the more traditional one of producing rigid PCBs.

Let’s look at three vital steps that go into manufacturing:

Flex PCB Build Up

The substrate material for manufacturing flex PCBs is costlier than the one you can use for traditional PCBs.

The material is polyimide, and you’ll need to use it properly for the best results. 

And the best way to do so is to design the circuits so that they are as close to each other as possible.

Moreover, the best way to make a minimalist design is by employing the nesting technique. Let’s examine the process you must use to manufacture your flex PCB.

Looping: It is the technique of including additional material over the design limit. The reason to do so is that you need the extra material that serves as a service loop.

In other words, the extra material will make it easier to service the board and assemble the circuits.

Sizing conductor: You must use the thinnest copper conductor to ensure it doesn’t reduce your board’s flexibility.

It is more the case in scenarios whereby you would like to use the board for dynamic tasks.

Etching: This procedure makes up for any isotropic losses during manufacture. The line width loss during this operation is almost twice the thickness of the copper foil.

Additionally, some variables affecting the line width include the conductor and different copper kinds and etch masks.

Routing: During manufacture, ensure that you route your conductors well. You can do so by bending and folding them while in a perpendicular position.

Accordingly, you’ll improve the board’s folding by lowering stress levels. 

Ground planes: Crosshatch the ground sections if there is enough electrical capacity. Doing so makes the PCB more flexible as it makes it lighter.

Fabricating your Flexible PCB

Inspecting a circuit board under a microscope

(Inspecting a circuit board under a microscope)

The second step involves all the processes necessary to fabricate your flexible PCB. They include using polymer films with industry-standard widths.

Furthermore, silver-based films will pass a rated amount of current through it for a particular application.

Hole size: We use flexible PCBs in small devices. As a result, you’ll need to use special technology to ensure the holes are tiny.

For example, some equipment can drill holes as small as several micrometers. 

Filleting: You need to fillet all land termination points and pads to increase the area of the PCB. Doing so will reduce the stress that applies to the board. 

Button plating: Printed circuit board manufacturers employ copper to create holes and vias. 

Add Cover Layers

In this step, printed circuit board manufacturers tackle issues with cover layers. Let’s look at some techniques you can use while concentrating on physical constraints. 

Screen-printable liquid overcoats: You can use screen-printable liquid overcoats to coat your flexible PCB. They are cheap and work well with thick polymer layers. 

Adhesive-backed films are mainly from raw materials that make them suitable for dynamic flex PCB applications. Moreover, manufacturers employ them for coating custom circuit boards. 

Photo imaginable film and liquid polymers: If you need the newest over-coating technique, use photo-imaginable liquid polymers.

They have some excellent functionalities that improve the quality of your circuit board manufacturing process. 

First, they guard against external electrification of circuits. Second, they shield the printed circuitry from internal and external harm.

And thirdly, it functions as a solder mask to stop traces from being circuited by the solder.

FAQs

Are flexible PCBs reliable?

Generally, most flexible printed circuit boards are reliable and durable. It is because manufacturers make these PCBs using flexible substrate materials like polyimide.

Afterward, they use them in devices that need to be flexible. 

For example, we use flexible PCBs in wearable devices like watches.

On the downside, the reliability of your flexible PCB will depend on the quality of its construction. Poorly made ones won’t be as reliable as high-quality PCBs.

Why is Flexible PCB circuits expensive?

Some flexible PCBs are costlier than rigid PCBs for several reasons. First, they are more complex to design and manufacture.

Consequently, manufacturers allocate more resources to manufacture a flexible PCB than they would with traditional circuit boards.

Second, flexible PCB substrates are more expensive than the materials used to make rigid PCBs. 

For instance, polyimide substrate costs more than rigid materials manufacturers use in traditional PCBs.

Third, the flexible nature of the PCBs makes the manufacturing process intensive and time-consuming.

Overall, it reduces the number of boards a manufacturer can make in a given time.

As a result, manufacturing costs increase, translating to higher selling prices. 

The Takeaway

Flexible PCBs have many applications in our daily lives due to their flexibility and versatility.

For example, most wearable devices have a flex PCB. Additionally, you’ll find them in military applications for their high tolerance to heat and vibrations. 

However, they are costly to manufacture and difficult to repair. Moreover, their manufacturing process is complex and limits their size.

If you’re to make a flexible PCB, we’ve given you an excellent overview of what you need to start.