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What does flex PCB thickness depend on?

Flex PCBs are like super flexible wires which allow the circuit to twist, bend, and fold just like a sheet of paper and even sustain the twists and turns with ease. The design of this circuit board is like a build blocks, with various materials bonded together in many layers to build a lightweight and flexible circuit system.

flex pcb for connect the device

Introduction: understanding flex PCB thickness

Flexible printed circuit boards are gaining popularity across various applications such as consumer electronics to medical devices as they have characteristics such as flexibility and reliability in tight volume which can never be achieved from rigid PCB. The another crucial factor that determines the capabilities of flexibility of these boards is their thickness. Were you aware that the thickness of the flex PCBs mounted in the smartphones and the tablet computers is not haphazardly decided?

Flexible PCB thickness is not just a specification, it also affects the durability, flexibility and electrical performance of the FPC. If the thickness of the circuit board is not appropriate, it may limit the durability of the final product.


The main factors that determine the thickness of the flex PCB


1. The thickness of the base material

Let's talk about the base material first. This is the main part of the circuit board, usually made of PI or polyester. The thickness of the base material is generally between 12μm and 125μm. So, if you want to make a super flexible circuit board, the base material must be thinner; if you want it to be stronger, it must be thicker.


2. Thickness of copper foil

Copper foil is the metal layer used for conducting electricity on circuit boards. The thicker the thickness, the better the conductivity, but it also increases the overall thickness and stiffness of the board. The thickness of the copper foil largely determines the flexibility of the board.

Common copper thicknesses used in FPCB manufacturing include:

  • 12μm (0.5 oz): A very thin layer suitable for low-current applications or highly flexible designs.

  • 18μm (0.75 oz): A medium thickness that balances flexibility and performance, commonly used in mobile devices and wearable technology.

  • 35μm (1 oz): A thicker copper layer, usually used in high-current applications or where additional mechanical strength is required.

In some specific applications, copper foil is sometimes thicker, even up to 70μm


3. Thickness of Coverlay or Protective Layer

Coverlayer are used to protect circuits from the outside environment, such as moisture and physical damage.

  • Typical coverlay can range in thickness between 12.5μm and 50μm

Thicker coverlays provide greater protection and are more durable, but increase overall thickness and may reduce flexibility.

In many designs, there is a balance between protection and flexibility. Highly flexible applications such as wearables may choose a thinner coverlay material to maintain the required flexibility, while industrial applications may prioritize thicker coverlays for enhanced durability.


4. Thickness of adhesive layer

The adhesive layer is used to bond the base material to the copper foil, and the cover film to the circuit layer. Common adhesive thicknesses range between 20μm and 50μm. Some special flex PCBs may also use adhesive-free technology, which can reduce thickness.

The thickness of the adhesive layer can affect both the mechanical flexibility of the FPCB and its heat dissipation characteristics. Thin adhesive layers preserve flexibility, while thicker layers may offer greater bonding strength but reduce flexibility.


5. Solder mask

If there is a solder mask on the flex PCB, although its thickness is very thin (usually 10μm to 15μm), it will still have a certain impact on the overall thickness.


6. Number of layers

In addition, the number of layers of FPCB will also affect the thickness. Multi-layer design can make the circuit density higher and the signal isolation better, but the thickness will also increase. So when designing, you have to choose the number of layers according to specific needs.


What is the conventional thickness of flexible circuit boards?

In flexible printed circuit boards, the conventional thickness of single-sided boards, double-sided boards, and multi-layer boards depends on different application requirements and material selection. The following are common thickness ranges:


1. Single-layer Flex PCB

single layer flex pcb structure
  • Structure: single-layer copper foil (9μm, 12μm, 18μm or 35μm) + base material + coverlay or solder mask

  • Thickness range: usually between 0.05mm (50μm) and 0.2mm (200μm).

This is the simplest type of FPCB, consisting of a single copper layer laminated onto a flexible substrate. It offers the thinnest possible design, with minimal materials contributing to the overall flexible circuit board thickness.

Single-layer flex PCBs are ideal for applications where space is tight, and high flexibility is required, such as in cameras and hearing aids.


2. Double-layer Flex PCB

double layer flex pcb structure
  • Structure: copper foil on both sides (usually between 12μm and 35μm) + intermediate base material + coverlay.

  • Thickness range: generally between 0.1mm (100μm) and 0.3mm (300μm).

The extra copper layer increases the current-carrying capacity and allows for more complex circuit designs but also increases the total flex circuit boards thickness.

These boards are commonly used in automotive systems, medical devices, and telecommunications.


3. Multi-layer flexible board

  • Structure: formed by alternating multiple conductive layers and insulating layers, with substrate and adhesive material between each conductive layer.

  • Thickness range: the thickness of multi-layer flexible board will increase as the number of layers increases, usually between 0.3mm and 0.5mm or more.

These designs are used when highly complex circuitry is required, but the trade-off is a substantial increase in FPCB thickness.

 

Multi-layer FPCBs are found in high-performance electronics such as aerospace technology and advanced consumer devices.

 

Other factors affecting flexible PCB thickness

 

While the base material, copper foil, adhesives, and layer count play the most significant roles in determining circuit board thickness, other factors also come into play:

1. Processing Techniques

2. Tolerance Requirements

3. Environmental Considerations


Industry standards for flex PCB thickness

 

In designing of the flexible pcbs, there is a need to ensure compliance with the established norms, Standard’s documents to ensure integration, performance and quality. IPC-2223 is one of the most frequently referenced standards for FPCB design and includes guidelines regarding the thickness of FPCBs.

IPC-2223 includes data on the number of layers and materials from which the thickness for the design is to be constructed as it aims to achieve certain mechanical and electrical characteristics.

The norm also defines the procedure for determination of minimum and maximum thickness in terms of bending radius, flex and electrical characteristics.

We as a flex PCB manufacturer follow the industry standards for product thickness if the customer failed to indicate the thickness of the product during order placement.

 

How to choose the right flex PCB thickness for your project


Choosing the right FPC board thickness depends on a variety of factors, such as application, environment, and performance requirements. Here is a step-by-step guide to help you choose the optimal thickness:

1. Determine the application:

Start by determining the primary function of the FPCB. Does it need to be flexed frequently, or does it remain stationary most of the time.

2. Consider electrical requirements:

Analyze the current-carrying capacity required for the application. Higher currents will require thicker copper layers.

3. Evaluate environmental conditions:

Evaluate the environmental stresses your flexible circuit board will face. High temperatures or corrosive environments may require additional protective layers, resulting in increased thickness.

4. Balance flexibility and durability:

If flexibility is a key requirement, choose thinner substrates and coverlays. However, make sure the board is still durable enough to withstand physical handling and stress.

5. Consult with the manufacturer:

- Work closely with the flexbile PCB manufacturer to understand the available material options and manufacturing processes. Flex Plus can provide insights on how to optimize thickness without sacrificing performance.

 

By considering these factors, you can select the appropriate flex PCB thickness that meets both your performance needs and the requirements of your specific application.

 

FAQs about flex PCB thickness

 

1. What is the standard thickness for a flex PCB?

The standard thickness for a FPCB typically ranges from 0.1mm to 0.3mm for single- or double-sided boards. However, this can vary depending on the number of layers and materials used.

 

2. How does the number of layers affect the thickness of a flex PCB?

Each additional layer, including copper, substrate, and adhesive layers, increases the total thickness. Multi-layer boards can be significantly thicker than single-layer designs, impacting flexibility.

 

3. Can I customize the thickness of an FPCB for my specific needs?

Yes, FPCB thickness can be customized based on the requirements of the project, including the materials and layer count.

 

4. How does copper foil thickness influence the performance of the PCB circuit board?

Thicker copper foil allows the FPC circuit board to handle higher currents and improves conductivity but can make the board less flexible.

 

5. What are the typical material options for flex circuit board base material?

The most common materials are PI and PET, with PI offering better flexibility and temperature resistance.

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