Guide to Six-Layer PCB Stackup Design

After exhausting the space on a 4-layer PCB, it's time to upgrade to a 6-layer circuit board. The additional layers offer space for more signals, extra planes, or a mix of conductors. The key isn't just having these extra layers but how to arrange them in the PCB stackup and how to route on a 6-layer PCB. If you've never used a 6-layer board before or encountered challenging stackup EMI issues, continue reading for some 6-layer PCB design guidelines and best practices.Why use 6 layers?

Before starting to build the board, I think it's important to consider the reasons why people might want to use a 6-layer PCB. There are several reasons beyond simply adding more paths for the signals.The most basic version of a 6-layer stack would use the same methodology as the SIG/PWR/GND/SIG stack in a 4-layer board, except that the signals would be placed on the other two in the center of the stack. In fact, the SIG/PWR/SIG/SIG/GND/SIG is the worst 6-layer PCB stack from an EMC standpoint, and it's probably only suitable for boards running at DC.

Some of the reasons I chose a 6-layer board over a 4-layer board include:

You are using a 4-layer SIG+PWR/GND/GND/SIG+PWR stack, and you need more space for components on the surface. Placing the PWR and SIG in the inner layers allows for more decoupling through the PWR/GND plane pair.

For mixed-signal boards, you can dedicate the entire surface layer to the analog interface and will have an additional internal layer for slower digital wiring.

You are using a high-speed board with a high I/O count and you want a good way to separate the signals into different layers of the board. You can implement the same strategy in #1.

In all of these configurations, only one additional signal layer is added. The other layer is dedicated to the GND plane, the power rails, or the full power plane. Your stacking layer will be the main determinant of EMC and signal integrity in the board as well as layout and routing strategy.

• Read this article for more tips on 6-layer PCB stacks and their EMC characteristics

How to wire signals？

Before we start routing, let's look at the typical PCB stack you will use in a 6-layer PCB:

6-Layer PCB Example

In this stack, the top and bottom layers sit on a thin dielectric, so these layers should be used for impedance control signals. 10 mils is probably the thickest dielectric you should use, as this will require the use of microstrip routing with a width of 15-20 mils, depending on the dielectric constant. If you are wiring a digital interface with differential pairs, the spacing will also allow you to reduce the width of the traces, which will allow you to wire into finer-pitch components. As an example, we use a version of the above stack for many of our smaller networking products that support multiple multi-gigabit Ethernet channels.

If you need to use smaller trace widths in the outer layer, simply reduce the outer dielectric thickness (perhaps as low as 4-5 mils) and then add some thickness to the L3-L4 dielectric to meet your board thickness goals. The next point to consider is how to route the power supply.

How to Wire a Power Supply？

In the above example of a 6 layer PCB stack, there is an entire layer dedicated to the PWR. in a 6 layer PCB, this is usually a good practice as it frees up the surface area for components and makes it easier to power these components through the via holes.

Just as an example, take a look at the BGA shown below. this particular BGA is typical of high speed interface controllers that need to supply a lot of current at multiple voltages, so many of the balls will be connected to power and ground. With something like an FPGA, you may find multiple pins for power and ground throughout its package. By dedicating a single layer to the power supply, you can break up the plane into tracks so that multiple voltages can be used at high currents if necessary. This eliminates the need to overlap these power rails at different voltages, preventing additional EMI issues.

In this FPGA BGA package, you can see multiple pins in the center area dedicated to GND and multiple VCC rails.The GND pins can be connected directly to the planes on Layer 2 and the VCC pins can be connected to different power rails on Layer 3.

Note that just because you place the power supply on an internal layer does not mean that you cannot place the power supply in other locations. You can still wire the power supply on other signal layers using copper-layered power rails or thicker alignments.

If you need to operate at high currents in a 6-layer board, possibly at multiple voltages, I would recommend using an additional power layer rather than an additional signal layer. In other words, you will have two power layers interleaved with ground on internal layers within the stack. You could even go a step further and place a power plane on the back layer for more current handling capability. This will give you enough room to wire the power supply over a large area, possibly with heavier copper, thus ensuring low DC resistance and low power loss.

In addition to these points, other important wiring strategies used to ensure EMC on 4- or 8-layer boards also apply to 6-layer boards. If you use elements similar to the example 6-layer stack above, you will have an easier time routing and ensuring signal and power integrity. the same DFM considerations for 4- or 8-layer boards apply to 6-layer boards; get your stack approved by the manufacturer before you begin to create the layout, resize the alignment, and route the wiring.

Be sure to follow these 6-layer PCB design guidelines before you create your stacks and begin routing. When you're ready to build your 6-layer board, use the best PCB design toolset available in Altium Designer®. You'll have a complete set of tools for laying out, routing, and preparing your board for production. Once the PCB package is created and ready to be shared with collaborators, your team can work together through the Altium 365™ platform. You can find everything you need to design and produce advanced electronics in one package.