Flex PCB Stackup

30 May 2022
New

Flex PCB stackup defines the arrangement of the copper layers and insulating layers that make up PCB board. Knowing more about the stackup allows designers to fit more components on a single board and minimize exposure to external noise, radiation, and reduce impedance.

The more layers you have, the better your flex PCB’s ability to support high-speed signal. Good layering can reduce cross-talk and has a major impact on the performance of the product. On the other hand, a bad stackup design can significantly result in radiation and noise.

The Purpose of Flex PCB Stackups

Most PCBs consist of at least three layers, with the bottom layer being synthesized with an insulation board. PCB stackups offer various purposes including:

Maximizing functionality: Multilayer flex PCBs can multiple the speed and functionality of a device, making it a more functional board.

Reduce vulnerability: PCB stackups can provide protection to the internal layers from external noise, making them less vulnerable to exterior forces.

Minimize Radiation: A well-designed stackup can protect the device from radiation, especially in high speed applications. It is worth noting that poor PCB design with impedance mismatches may result in higher EMI radiation than a normal circuit board.

Lower Costs: Good Flex PCB stackup can lower the cost of manufacturing by placing multiple circuits on a single board.

Factors to Consider When Designing Flex PCB Stackup

Designers have to consider the following factors before designing the flex PCB stackup:

Total layers
Sequencing of levels
Spacing between levels

Other minor factors include:

Operating frequency
Total signals that are to be routed
Whether the flex PCB will require a shielded container or not
Whether the PCB has to adhere to EMC regulations

Below are a few tips for flex PCB stackup design that can be followed.

Determine the Number of Layers

The first consideration for any PCB stackup is determining how many layers are needed. This includes the speed of the signal (whether it is high speed or low speed), whether it needs more power, and ground layers or planes. It

Determine Vias and Routing

The designer needs to consider routing of the traces, including figuring via placement, and the type of vias to utilize. They will have to work with the board’s manufacturer to make informed decisions because some boards avoid certain via types, such as via-in-pads.

Determine Layer Thickness

Another important consideration for any PCB flex stackup is the thickness of each layer. The designer should know these before planning out the assembly of the board. Different board material types have different properties, including thicknesses. You should select materials based on their electrical, thermal, and mechanical properties.

The Flex PCB Stackup Structure

The PCB stackup structure is the foundation on which all components are placed. This is why an improperly designed stackup can adversely impact the performance of the board, including signal transmission, manufacturability, reliability, and power delivery.

The PCB stackup is constructed from multiple layers of core, copper foil materials, and prepreg that are heat pressed together. The core material is a thin electrical insulator (usually a fiberglass weave material with epoxy resin) and copper foils bonded on the sides. The dielectric will act as an insulation layer. The internal copper will form the power, signal, and ground planes in the board.

During the PCB flex manufacturing process, the prepreg layers and core layer arrangement are laminated together with a top and bottom copper layer. The PCB is formed with the help of heat and pressure to bind all the different layers together.

There are a few rules needed for managing a good flex PCB stackup. We’ll discuss a few of these below:

Ground planes are more convenient since they improve signal routing in a stripline configuration. They also reduce ground impedance and ground noise.
High speed signals should be routed on intermediate layers that are located between various levels. This will allow the ground planes to act as a shield and stop the radiation at high speed.
Signal layers should be placed in close proximity to each other and must be adjacent to a plane.
It is beneficial to have multiple ground planes because they lower a board’s overall impedance and shield against radiation.
The mass and power planes must be coupled together.

Common types of PCB Layer Stackups

Here are some of the most common PCB layer stackups:

2 layer:

Signal layer
Laminate Core
Bottom layer

4 layer:

Top layer
Prepreg
First inner layer
Core
Second inner layer
Prepreg
Bottom layer

This design is affective for most applications.

8-layer:

This PCB stackup offers more options than 6-layer board by adding two or more layers for performance or routine. Here’s what it looks like:

First signal layer
Prepreg
Ground plane
Core
Second signal layer
Prepreg
Power Plane
Core
Ground plane
Prepreg
Third signal layer
Core
Power plane
Prepreg
Fourth signal layer

Multilayer PCB stackups are great options if you need increased functionality on your board. While it is true that manufacturing costs can increase with each additional layer, it is worth having because of the improved functionality and EMC improvements.

Wrapping Up

Flex PCB stackups are important for various applications. They provide engineers with the ability to come up with high quality electronic products and take a lot of things into account. Without a high quality electronic product, the performance of the product will be greatly affected. This is why designers should spend a lot of time strategizing how to put together different components and materials in PCB stackup design.

To achieve all their objectives, designers should work with a minimum of 8 layers with symmetric configurations. For example, in the case of an eight layer PCB, if level 2 is plane, then level 7 should also be plane.

A high quality stackup design goes a long way in improving the quality and output of a flex PCB board. By applying the above tips, you can easily overcome the challenges associated with multilayer flex PCB design.

Get in touch with Hemeixin PCB for the best flex PCB stackups.