Standard 8 Layer PCB Stackup Techniques and Challenges

October 28, 2019 Cadence PCB Solutions

An 8 layer PCB stackup requires more than a 6 layer PCB stackup

 

Obviously the more layers you add to something, the more difficult it becomes. Take 3D chess, for example. Not only are you responsible for memorizing strategies and plays, but you also have to keep track of multiple boards at once. I thought a rubix cube was hard enough, but I’ve been introduced to the pentamix iteration of a rubik’s cube which seem like an entirely new level of challenge. 

Fortunately, most multilayer PCBs are not going to rely on the same amount of memorization and require attention to thousands of moveable pieces. Instead, you’ll get instances of 8 layer PCB stackups where you need to carefully balance designs around EMI, frequency, analog and digital signals, and power planes. Thankfully, I’ve had much better luck in handling standard 8 layer PCB stackup in electronics designs. 

Why Should You Use A Standard 8 Layer PCB Stackup

In typical PCB designs, a single or double-layer PCB is used. Often, a double layer PCB is sufficient to handle the requirements of the project. Also, it is best to keep the PCB design simple to reduce the risks of unnecessary problems. But some circumstances call for a multilayer PCB design. Having an 8-layer PCB stackup is less common, but it’s a practice in some applications. 

A multilayer PCB generally refers to a printed circuit board with 3 or more conductive layers. These conductive layers are sandwiched between prepregs and insulative core in a PCB stackup. Prepregs are raw insulated material, usually FR4 while the core has copper layers attached to it.

As electronics become smaller in size, and signals speed is getting faster, a multilayer PCB becomes a viable option. In a standard PCB layer stackup, there are at least 4 layers of copper for signal traces. This increases space on the outer layers for components. 

A board with plenty of vias for heat management and power plane distribution

Better EMI control is a motivation for a standard 8 layer stackup

 

PCB designers also rely on a multilayer PCB to increase the signal integrity of the design. Signal layers on a standard 8 layer PCB stackup are separated by power and ground planes. These inner planes help to reduce crosstalk between signal layers and thus, increase signal integrity.

How A Standard 8 Layer PCB Stackup Looks Like

Designing a double-layer PCB, even with hundreds of components, is considerably easy for seasoned PCB designers. However, introducing inner layers on a PCB design brings forth challenges and considerations of its own. 

 

Here’s a typical conductive layer arrangement of a standard 8 layer PCB stackup.

  • Top layer

  • Prepreg

  • Ground plane

  • Core

  • Inner Layer 1

  • Prepreg

  • Power plane

  • Core

  • Ground plane

  • Prepreg

  • Inner layer 2

  • Core

  • Power plane

  • Prepreg

  • Bottom layer

 

Note that signal layers are separated by power or ground plane to reduce EMI susceptibility and emission. 

PCB designers will need to decide on the prepregs and core material that are used to construct the multilayer PCB. The thickness and the choice of materials may affect impedance control which is important for transmission line design. You’ll need to ensure that the PCB manufacturer has the capability to produce the 8-layer stackup that you’ve specified.

Useful Techniques For Standard 8 Layer PCB Stackup Design

It’s common to fall under the impression that having an 8 layer stackup will solve all your EMI woes. No, it won’t. There are still some best practices that you’ll want to adhere to get the best of the multilayer stackup. 

 

Traces and components placed in a circuit

Design techniques are equally important to ensure a functional 8-layer PCB. 

 

1. Routing Direction

In some cases, an 8 layer PCB stackup may consist of 6 signal layers. Signal traces on adjacent layers must be routed perpendicularly to minimize cross talks. To be safe, make it a habit to route signal layers differently on subsequent layers even if they are separated by power or ground planes.

2. Return Path

It is important to visualize the return path of high-speed signals, even if they are on the inner layers. Ensure that the signal has a short return path and doesn’t cause interference with other components.

3. Ground Plane

The rule of thumb is never to have a split ground plane as it may create impedance discontinuity. Also, ensure that components on the outer layer have low impedance connected through vias to the inner ground planes.

4. Buried or Blind Vias

To further increase space for routing, you may opt for buried or blind vias. However, it is important to check with your manufacturer if it is within their capability to produce that in the PCB.

Your task in designing an 8-layer PCB will be easier with the right software tool. The Cross Section Editor in OrCAD PCB Designer gives you control on the layer stackup. Whether its with strong DRC tools, easy documentation management and production processing, or reliable analysis and simulation tools, you can trust Cadence to have what you need for your designs. 

If you’re looking to learn more about how Cadence has the solution for you, talk to us and our team of experts

About the Author

Cadence PCB solutions is a complete front to back design tool to enable fast and efficient product creation. Cadence enables users accurately shorten design cycles to hand off to manufacturing through modern, IPC-2581 industry standard.

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