Designing a 6 Layer Stackup for Enhanced EMC

April 22, 2019 Zach Peterson

 A 6 layer PCB stackup is just like a great sandwich

Designing a multilayer board is like putting together a great sandwich. The right arrangement of components can be satisfying in both cases. You won’t have to worry about signal integrity in your BLT, but you will have to design your PCB layer stack to keep your signals clean.

6 layer PCBs are an economical and popular stackup for a variety of applications. With the right stackup, you can suppress EMI, accommodate fine-pitch components, and even work with RF devices. You won’t need to choose one of these advantages; the right stackup, routing choices, and component placement allows you gain all these advantages in a single board.

 

How Many Power, Ground, and Signal Planes Do I Need?

The answer to this question is extremely important and really depends on the application for your board. If you are routing a dense board with limited space, you’ll want to opt for one power, one ground, and four signal planes. If you need to significantly reduce susceptibility to EMI, you should opt for more ground planes with two signal layers. The arrangement of ground planes will have a significant shielding effect in your PCB without the need for shielding cans.

If you will be mixing digital and analog signals, mixing high and low frequency signals, or a combination of all of these, you can still make some creative use of an 6 layer PCB stackup. At some point, you will need to opt for a larger board or more layers in your stack (or both!), but you can still make use of a six layer board for a number of applications. Note that there are plenty of combinations for 6 layer PCB stackups, so only a few useful ones will be shown below.

With this in mind, let’s jump dive into a few 6 layer PCB stackups:

Signal Layer/Power/2 Signal Layers/Ground/Signal Layer

This stackup is a 6 layer PCB stackup that it is excellent for providing shielding for sensitive traces as well as tight coupling to solid planes. You can route signals with different switching speeds or different frequencies through an interior layer. Typically, the higher speed digital or higher frequency analog signals will be routed through these interior layers in order to shield them from the lower speed/frequency components on the outer layer.

Signal Layer/Ground/Power/Ground/Signal Layer/Ground

This is a good stackup for boards with limited space and that need to mix analog/wireless capabilities with digital signalling. The interior signal layer will be shielded from the surface signal layer as it is encased between two ground planes. It is also useful for suppressing EMI from interfering with the interior signal layer as the solid conductors provide effective shielding.

The alternating ground/power/ground planes also provide effective decoupling for RF devices. It is a good idea to place grounded vias between the alternating ground/power/ground plane in order to suppress radiated EMI from the board edge. If you are placing, say, a printed dipole antenna on the surface layer, you can extend the nearest ground plane directly beneath it and place a via fence at the back edge between the remainder of the stack. This nicely ensures that radiated emissions will not interfere with other components on the board.

The right 6 layer PCB stackup lets you ditch the shielding can

If you are converting to digital data directly on the board, you will need to include a slot in the ground/power/ground stack to create some separation between the analog and digital sections. Make sure you do not route any traces across this slot, either in the interior or exterior signal layers as these traces will act like strong radiators and have large loop area.

Ground/Signal Layer/Power/Ground/Signal Layer/Ground

If your board will be deployed in an electrically noisy environment, or if it will be placed near a board that emits strong radiation, this stackup provides excellent EMI suppression. The downside is that there are only two signal layers, so board space for routing signals will be limited. That being said, placing the signal layers between stacked conductors is the best choice from an EMC standpoint.

You can also place mounting pads on the surface layers for components, and the nearby ground planes on the surface provide a convenient path to ground. If designed to the right tolerances, you’ll be able to easily route signals and power to your components with vias.

This layer stack provides another not-so-obvious benefit: better thermal management. If you will be working with high currents, the conductors on each side of a signal layer can absorb heat and transport it to the board edge, where it can be dissipated with passive or active cooling.

 

A Note on Routing Between Multiple Layers

We often talk about routing vias through multiple layers, but doing this can create a discontinuity in the return path that increases the loop area for the circuit. In this case, the parasitic capacitance for the board will provide some discharge that induces a return current near the signal via. Unfortunately, the capacitance is usually too small to provide a low reliable impedance return path. One option is to place a decoupling capacitor in parallel with the signal via to provide the return path.

Integrated circuits should contain a nearby bypass capacitor that connects directly to the same reference plane as the signal vias/traces. This helps reduce fluctuations in the supply voltage, provides a charge reservoir that combats ground bounce, and provides a path for the induced return current for signals that transition between layers. If the ground connection is made through a via, this also provides a return path for signals traversing to the top layer through a via.

Ferrite choke and electrolytic capacitors

Your PCB design package should include the tools you need to design your stackup entirely from scratch. With Altium , you’ll have full control over your layer arrangement, material constants, and dimensions. You’ll even be able to use the layer stack manager to create rigid-flex and multi-board systems with ease. All these design tools integrate directly with your schematic design, layout, and deliverable generation tools in a single program.

Download a free trial of Altium to see how the powerful tools give you full control over your board. You’ll also have access to the best design features the industry demands in a single program. Talk to an Altium expert today to learn more.

 

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