Flex PCB Material: Know Your Options

word-image-5696-1
_flex PCB materials by layers (1)
This image show the different flex PCB materials by layers
Resource: https://www.edn.com

The choice of flex PCB material depends on many factors: the end use application, thermal and electrical performance requirements, PCB design complexity, flexing cycles, and the board’s ability to withstand extreme temperatures or chemical exposure. In this post, we take a look at the different types of material for flex PCB, layer by layer.

What are PCBs Made Of?

PCB is an acronym for Printed Circuit Board, a board that’s used to connect electrical components in electronic devices. The primary components of a PCB are the substrate, copper tracks, and thin solder mask or other cover layer type.

The substrate forms the main layer and can be thick and rigid, or thin and flexible. In a rigid board, the base is usually glass fiber material that’s been impregnated with epoxy resin (also called FR-4), or it can even be metal.

The flexible PCB substrate, on the other hand, is commonly polyimide or PET. These types of flex PCB material allow it to fit tighter spaces as well as bend without breaking. This property fits applications where space is limited, or when the board needs to be bendable.

Highly bendable flex PCB
Highly bendable flex PCB material for actuator application
Resource: https://hackaday.com

Flex PCB Materials

The type of flex PCB material defines its physical and electrical properties. As such, it must be chosen carefully, usually according to the board’s intended use. In view of that, common flexible PCB materials include those that make the substrate and coverlay or, if needed, the adhesives and stiffeners.

1. Flex PCB Substrate

A PCB substrate is the layer that serves as the foundation for the entire board. In a flex PCB, this layer must maintain its integrity when bent and flexed once or repeatedly. Manufacturers use a range of materials for the flex PCB substrate, with the most common being polyimide (PI) and polyester (PET).

Polyimide is the most popular flex PCB material for the substrate layer, providing excellent flexibility for both dynamic and non-dynamic applications. A polyimide flex PCB material is also heat resistant.

In terms of electrical performance, polyimide flex circuits offer good dielectric strength, among other benefits. Learn more about the polyimide PCB material properties here.

Polyester is another popular flexible PCB substrate. Compared to polyimide, it makes an economical choice for simple flex circuits. Polyester is also less expensive than polyimide and an attractive choice for cost-sensitive projects.

2. Flex PCB Trace

Bonded to the substrate is the trace. It typically consists of thin copper foil that’s laminated onto the substrate, a conductive metal layer that forms the electrical pathways between components.

The flex PCB copper thickness and width depend on the application and can vary from 0.3 oz/ft² to 0.45 oz/ft² for the copper layers, with minimum and maximum values for the trace width depending on the applications.

The most common types of this flex PCB material are rolled annealed copper and electrically deposited copper. Rolled Annealed Copper (RAC) is suitable for dynamic applications as it offers a high level of flexibility and can withstand large-radius bends.

3. Flex PCB Coverlay

The flex PCB coverlay, or top layer of a flex PCB, is the protective material that is applied to the surface of the board. It helps protect against moisture and chemical exposure, while providing additional strength. It also serves an aesthetic purpose, allowing your board to convey a professional image.

Polyimide is popular choice for flex PCB coverlay, mostly due to its excellent properties. It’s a strong, durable material that can withstand extreme temperatures and stress. Additionally, polyimide boasts excellent dielectric properties for high frequency applications.

Instead of coverlay, you can also choose to use a cover coat. This is a thin coating that is applied to the surface of the board, providing a layer of protection against moisture and contaminants. A thin and flexible solder-mask may also be used.

4. Flex PCB Adhesives

This is the flex PCB bonding material used to join the different laminations or layers together during the fabrication process. The type of flex PCB adhesive will depend on the application and temperature requirements, as well as its ability to withstand shock and vibration.

Acrylic and epoxy are the two most commonly used adhesives for flexible PCBs. Acrylic does not harden with curing, and is often preferred for dynamic applications PCBs. Epoxy hardens when exposed to heat and is often used in static flex PCBs.

Another option for this flex PCB material is the pressure-sensitive adhesive (PSA), which provides excellent adhesion and improved coverage over larger areas. Note that adhesive-less PCBs are a popular option today. In these boards, the layers are plated onto the substrate.

When choosing a flex PCBs adhesive, consider the strength of the bond you need and its compatibility with the application that the PCB will be used in.

5. Flex PCB Stiffener

Stiffeners are additional layers that increase flex board rigidity in certain areas. Various requirements can lead to the need for stiffeners: the need for a stronger surface for mounting components, the need for thicker board sections, and solder joint reinforcement.

Other reasons for the addition of flex PCB stiffener include weight balancing, improved thermal performance or heat dissipation, and strain relief or abrasion protection.

A flex PCB stiffener can be different materials, such as Kapton (a form of polyimide) and FR4. These are usually added to the board in the form of a reinforcing layer and attached using adhesives, usually acrylic based or PAS types.

Kapton is strong and thin, while FR-4 is more rigid and thicker. Other options for flex PCB stiffener, although not as common, include aluminum foil and stainless steel. It’s important to understand that polyamide is also used for the different flex PCB laminates, from the substrate to the coverlay and stiffener.

Conclusion

PCB manufacturers use a range of materials to make flex PCBs, each of them with its own advantages and disadvantages. The type flex PCB material chosen should match the application’s requirements in terms of electrical performance and thermal properties, as well as shock, vibration and bending resistance. This should, in turn, ensure the board’s optimal performance and longevity.

Share:

Table of Contents