Flex PCBs Technical Specification

Technology has advanced dramatically from traditional printed boards to High-Density Interconnector printed boards. We have miniaturized devices to the palm of our hand or smaller, and the trend continues. Now we have flexible PCBs. These are a particular type of printed circuit board that can bend without breaking.

Flexible circuit boards are in high demand in the defense and medical industries due to their unimaginable uses. Suppose you want to design a flex PCB for your project. In that case, you need to consult professional PCB manufacturers and provide them with complete details and specifications required for its design and assembly.

This article will discuss the technical specifications of a flex PCB to help you understand how this technology is possible and helpful.

Flex pcb Technical Specifications

Structure
There are a total of 6 structural types available for flexible PCBs.
1. Single-sided flex: It has a single-sided FPC core bonded by a coverlay, with or without stiffeners—simple structure and easy to fabricate.
2. Double-sided flex: Coverlay attached on both sides of the double-sided core with through-hole vias. Complex structure with control difficulties.
3. Multilayer flex: It has 3 or more conductive layers plated through holes and is bonded by a coverlay.
4. Single-sided rigid-flex
5. Double-sided rigid-flex
6. Multilayer rigid-flex

The three rigid-flex structures are difficult and costly to assemble and thus not commonly used. They have extra layers, and quality control is challenging to maintain.
Material

There are two types of flexible PCB materials:

• Adhesive-less material
• Adhesive based material

In adhesive-less material, the copper is cast directly on the polyimide. The removal of adhesive bond layers enables manufacturers to make thinner circuit boards. Moreover, it also improves the operating temperature rating and provides higher copper peel strength. The flex PCB is less likely to break because it doesn’t absorb moisture from the environment.

The copper is bonded on the polyimide using acrylic adhesive in adhesive-based material. The adhesives absorb moisture from the environment and become soft when heated; thus, cracks may appear in rigid areas. Also, the thickness of the adhesive can change/reduce after fabrication, which leads to dimensional errors.

A flex PCB consists of 4 primary materials; a copper layer, a cover layer, adhesive, and insulating material. In most flexible PCBs, FR4, polyimide, and polyester are used. Many new materials have also been introduced with different capabilities.

• FR4: It has maximum tensile strength but offers minimum elongation. FR4 is also unstable under constant bending and can break.
• Polyester: Good electrical and physical performance, although low cost and can break during the soldering process.
• Polyimide: Most commonly used due to excellent electrical and physical performance. It is also noninflammable.

Stiffeners
Stiffeners are additional mechanical pieces used to provide rigidity to some areas during the assembly process. It also provides support for mounting components. And if components are close to a flexible area, stiffeners are used, but this can vary depending on the surface area.

There are two commonly used stiffeners for flexible circuit boards; FR4 and polyimide. Stiffener thickness is also important. For example, FR4 is around 0.010” – 0.059” thick, and polyimide is 0.002” – 0.008” thick.

If a stiffener needs to be added manually during the manufacturing process, time and cost can increase significantly.

Flex pcb Stackup Type

When it comes to stackup types for flexible PCBs, there are multiple varieties available, such as:
1. 1 layer flex/ adhesiveless
2. 1 layer flex/ adhesive
3. 2 layers flex/ adhesiveless
4. 2 layers flex/ adhesive
5. 3 layers flex/ adhesiveless
6. 4 layers flex/ adhesiveless
7. 2 layers flex with multiple stiffeners

Besides these general types, you can also customize and optimize your build-up depending on the number of layers and requirements. For example, in a 3-layer rigid-flex stackup, the solder mask is on the top, and the cover layer is on the bottom, making the bottom flexible. You can create numerous combinations in this way for flex PCBs.

Thus, it’s best to consult the Flex PCB manufacturer before designing the layout of the flexible circuit.

Impedance Control
If you want to create a flexible PCB with controlled impedance, you need to add 2 sheets of polyimide instead of 1. Impedance control is achievable with consistent width and spacing between each component to have correct resistance measurement.

An extra polyimide core is added between an impedance layer and plane layer. This increases the manufacturing cost but is necessary for controlled impedance.

Surface Finishes
There are many options available for surface finishes for flex PCBs.
• Electrolytic tin plating: It is used for anti-corrosion protection. They are easy to flex and bend but poor-quality for extended use. It is also not used in dynamic flex applications.
• ENIG: It is the commonly used surface finish. It is easy to solder, has a smooth surface, and easily places small components.
• Immersion tin: This surface finish is not useful for high-degree flexing and bending. It also forms an intermetallic layer over copper, and the layer continues to grow over time.
• Immersion silver: It has a limited shelf life and should be soldered quickly after fabrication. It is very conductive.
• Electroplated soft gold over nickel: It is used for direct chip attachment. It is an expensive option as it is not solderable. It is more flexible and thinner than a full hard gold plate.

Each surface finish is suitable for different purposes. It is recommended to avoid multiple surface finishes on the same part.

Aside from these, you also need to provide information about plating, shielding, hole and pad size, and panel and conductor dimensions.

It is important to provide this information upfront to the PCB manufacturer to design an accurate layout and have your budget settled beforehand. You can consult Hemeixin to ensure strict guidelines are being followed. They offer various services, such as PCB assembly, rigid-flex PCB design and manufacturing, adhesive-based and adhesive-less construction, and more.