Flex PCB Material: A Comprehensive Guide

Introduction

The flex PCBs
The flex PCBs

A flexible printed circuit board is also known as a flex PCB, it is made up of a layer of metallic traces, typically copper attached to a dielectric layer that is made up of polyimide. Usually, the metal is attached to the dielectric by use of an adhesive. Nowadays, adhesive-less laminates are becoming popular due to their excellent performance and thermal benefits. In this article, we will discuss everything about the flex PCB material in deep detail.

What is a Flex PCB?

A flexible PCB
A flexible PCB

 

Flexible PCBs are also known as flex circuits or flexible printed circuit boards. They belong to the electronic and interconnection family. These circuits are made up of a thin insulating polymer film with conductive circuit patterns attached to it. They are usually coated with a thin polymer layer to protect the conductor circuits.

Flex PCB Materials and construction

A flexible PCB of a dismantled keyboard
A flexible PCB of a dismantled keyboard

 

Materials for flexible PCBs should meet several design and operational quality criteria. The flex PCB materials might seem unconventional in their usage. The production of flex and rigid-flex PCBs at a large scale is dependent on a relatively limited range of materials. The fundamental characteristics of flex PCB materials are given below:

The upper dielectric layer

The upper dielectric layer serves as the base for the construction of the conductive. The selection of a suitable material is very important. Common choices are given below:

Polyimide (Kapton)

Polyimide sheet
Polyimide sheet

It is widely preferred with a high-temperature rating and exceptional chemical resistance.

Polyester (PET)

Polyester
Polyester

It is a more cost-effective option with a very high temperature rating.

Polyamide

Polyamide sheet
Polyamide sheet

It maintains flexibility even at very low temperatures.

Fluoropolymers (PTFE)

PTFE sheets
PTFE sheets

They offer powerful resistance but come at a relatively higher cost.

LCP

LCP sheet
LCP sheet

It is ideal for high-frequency applications with minimal signal loss.

Conductors

The conductors are the necessary conductive paths or traces that are created on the copper foil through lithographic procedures. Flex PCB conductor fabrication usually utilizes a subtractive method for its production.

Copper Foil

It is a very thin and rolled copper foil that is attached to the underlying substrate. If the foil is thinner then it would have more current-carrying requirements to enhance flexibility.

Stiffeners

The extra dielectric stiffening layers can be incorporated to reduce the wrinkling in flex PCBs due to thermal stresses.

Finish and Coatings

A solder mask is applied to insulate the conductor pattern and prevent oxidation. Surface finishes like hot air solder leveling (HASL) or alternative treatments can also be used for the best results.

Coverlay

For insulation and safeguarding, a slim and flexible dielectric coverlay can be attached to the conductor layer. The coverlay is 25 to 50μm thick and is made up of Kapton or polyester films.

Adhesives for bonding

Adhesive films are made up of acrylic or epoxy and they are used to join the base substrate with the copper foil. It connects the coverlay with the conductors. These adhesives ensure strong bonding while preserving flexibility.

 

Which Materials Should You Use as Flexible PCB Substrates?

The materials selected for a flexible printed circuit board are different from those used in standard units. Let’s explore three material types that are frequently used by clients for their applications:

Polyimide (PI):

  • Polyimide presents a cost-effective option with satisfactory reliability and performance.
  • It exhibits notable resistance to chemicals and heat, making it well-suited for such requirements.
  • It possesses average electrical characteristics, making it a popular choice for a variety of industries.
  • Polyimide-based flexible PCBs can be found in smartphones, tablets, entertainment products, calculators, cameras, and other devices.

Polytetrafluoroethylene (PTFE):

  • PTFE offers an impressive temperature range, high stability, and low dissipation for the flex PCB.
  • It is suitable for various applications. It is used in the aerospace and automotive industries.
  • PTFE is often utilized for applications like guidance telemetry, driving assistance control and antennas.

Polyether Ether Ketone (PEEK):

  • PEEK has exceptional chemical resistance and resilience to radiation.
  • It has an impressive heat tolerance, even at extremely high temperatures.
  • The medical industry widely utilizes PEEK-based materials in flexible printed circuit boards. X-ray machines in local medical facilities often utilize this type of PCB.

Cladding Components

The cladding layer is the protective cover of a flexible PCB. It is Important to select the suitable materials for it. The primary role of a flex PCB is to provide insulation for board protection. This component enhances the reliability and quality of the product.

Utilization of a copper-clad laminate

 

A copper-clad laminate (CCL) is a logical choice for flexible boards. Professionals often use a polyimide resin due to its exceptional thermal control. This means that even when exposed to exceptionally high temperatures, the CCL remains stable, minimizing the risk of softening and reducing the likelihood of board failure. The capacity to maintain flexibility and elasticity is a major factor that increases the preference for this material.

Exceptional qualities of a polyimide

Beyond its thermal control and resistance properties, it is necessary to highlight the electrical characteristics of polyimide. Selecting a high-quality film is vital to reduce the absorption of moisture and increase in strength against tear. It involves a higher initial investment but the results are the best. The value is evident in the form of a dependable and long-lasting solution.

 

Why CCLs should be Adhesive-Free?

Previously the manufacturers followed an approach of a polyimide film adhesive as a connector and a copper foil for Copper-Clad Laminates (CCLs). Adhesives have various advantages but now experts believe that the board’s performance can be enhanced without them. Manufacturers and customers were dissatisfied with the electrical performance and dimensional stability limitations related to adhesion, therefore, forcing the industry to explore alternatives. Ultimately, the most effective solution resulted in the form of a two-layer CCL that ended the need for adhesive, resulting in highly satisfactory results.

The process of assembling the flexible copper-clad laminates

Nevertheless, the end of adhesive usage in modern industry has introduced changes in the assembly process of a flex PCB. This modern technique has resulted in increased production costs for the board. Here are the processes manufacturers utilize to assemble these flexible copper-clad laminates:

  • Lamination: It is a suitable choice for double-sided flex PCBs.
  • Film coating: It is highly recommended for large-scale production of flex PCBs.
  • Electroplating: It is the most cost-effective option among the other provided methods for the production of flex PCBs.

The process of creating a flex PCB

Designing the Flex PCB: The first step to creating a flex PCB is to design the layout of the PCB using PCB design software. The design is according to the expected mechanical and electrical requirements of the device, the placement of components, and the routing of traces.

Preparing the Copper Layers: After designing, output files are generated from the PCB design software. These files are used for creating the copper layers. The copper layers are designed with proper tolerances. The correct impedance ensures the circuit functions optimally.

Printing the Layers: After preparing the copper layers, the next step is to print the copper layers on the substrate. A photochemical etching process is used to remove the unwanted copper from the substrate.

Drilling the Holes: Once the copper layers are printed, small holes are drilled and then plated to connect the layers of the board.

Plating the Layers: After drilling the holes, the layers of the flex PCB are plated with copper.  A solid and stable connection is created between the layers.

Applying the Solder Mask: Apply a solder mask to the board to protect the copper traces from corrosion and oxidation. The solder mask helps to prevent short circuits or accidental electrical connections between different sections of the flex PCB.

Adding Silk Screening: Silkscreen the PCB to indicate the location of components, test points, and other key information.

Testing the Flex PCB: In this step, the flex PCB is tested for electrical connectivity, impedance, mechanical stability, and durability.

 

Conclusion

A Flex PCB or FPCB is a structured printed circuit. Its components include flexible materials as a base. These are produced using the parts employed for hard PCBs but have the flexibility of converting to a desired shape (flex) during their usage. Their pliable design enables them to bend, fold, and flex seamlessly during operation. As electronic devices continue to shrink in size and evolve, flex PCBs have become common for utilization in contemporary gadgets and items.

FAQS

What is a Flex PCB material?

Flex PCB material is a flexible circuit board material. It is made up of a thin, flexible laminate material, typically composed of a polyimide film that is coated with a copper layer.

What are the advantages of using Flex PCB material?

Flex PCB material is advantageous because it is lightweight, flexible, and durable. It offers the ability to create complex shapes and designs in small spaces, making it ideal for many applications.

What is the most common material used for Flex PCB?

The most common material used for Flex PCB is polyimide. This material is known for its heat resistance, flexibility, and chemical resistance.

How is Flex PCB material made?

Flex PCB material is made by laminating several layers of thin, flexible materials such as polyimide film, copper, and adhesive. These materials are then etched with a circuit pattern and other features using a specialized process.

Can Flex PCB material be soldered?

Yes, Flex PCB material can be soldered. It can be used with traditional through-hole components, or with surface mount technology.

What are the challenges of using Flex PCB material?

The main challenge of using Flex PCB material is the intricate manufacturing process required to create it. Additionally, it can be more expensive than traditional rigid PCBs.

What industries typically use Flex PCB material?

Flex PCB material is used across many industries, including aerospace, automotive, telecommunications, medical, and consumer electronics.

What are the benefits of using Flex PCB material in medical applications?

Flex PCB material is ideal for medical applications because of its flexibility and durability. It can be used to create intricate designs that allow for small devices to be implanted into the body, while also being able to withstand the harsh conditions of the body.

What is the lifespan of Flex PCB material?

The lifespan of Flex PCB material varies depending on the application and use, but they are typically designed to last for several years.

What is the maximum size of a Flex PCB?

The maximum size of a Flex PCB is dependent on the manufacturer and design requirements. Some manufacturers are capable of producing Flex PCBs that are several feet long, while others specialize in smaller sizes for precision applications.

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