Once you run out of space on your 4-layer PCB, it's time to upgrade to a 6-layer board. Additional layers can provide room for more signals, additional pairs of planes, or a mix of conductors. It doesn't matter how you use these extra layers, what matters is how you arrange them in the PCB stackup and how you route them on a 6-layer PCB. If you have never worked with a 6-layer circuit board before, or have encountered such stack-up EMI issues that are difficult to resolve, read on to learn about some 6-layer PCB design guidelines and best practices.
Why use 6 layers?
Before starting to build the board, I think it's worth considering the reasons why one might want to use a 6-layer PCB. There are several reasons besides simply adding more paths to the signal. The most basic version of a 6-layer stackup will take the same approach as the SIG/PWR/GND/SIG stackup in a 4-layer board, just putting the signals on the other two in the center of the stack. In fact, from an EMC perspective, SIG/PWR/SIG/SIG/GND/SIG is the worst 6-layer PCB stackup, and it may only work 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 to leave more space for the components on the surface. Placing PWR and SIG on internal layers allows for more decoupling through the PWR/GND plane pair.
With a mixed-signal board, you could dedicate an entire surface layer to the analog interface, and have an extra internal layer for slower digital routing.
You're working with a high-speed board with a high I/O count, and you want a good way to separate signals into different layers of the board. You can implement the same strategy in #1.
All these configurations add only one additional signal layer. Another layer is dedicated to the GND plane, power rails, or full power plane. Your stackup will be a major determinant of EMC and signal integrity as well as placement and routing strategies in your board.
-
Read this article for more tips on 6-layer PCB stackup and its EMC characteristics
How to route signals
Before we start routing, let's take a look at the typical PCB stackup you will use in a 6-layer PCB:
6 layer PCB example
In this stack, the top and bottom layers are on a thin dielectric, so these layers should be used for impedance control signals. 10 mil is probably the thickest dielectric you should use, as this will require using 15-20 mil width microstrip routing, depending on the dielectric constant. If you are routing a digital interface that comes with differential pairs, the spacing will also allow for reduced trace widths, which will allow you to route into finer pitch components. As an example, we use a version of the above stackup for many small networking products that support multiple multi-gigabit Ethernet channels.
If you need smaller trace widths on the outer layers, just reduce the outer dielectric thickness (maybe as low as 4-5 mils) and then add some thickness on 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 power
In the 6-layer PCB stackup example above, there is an entire layer dedicated to PWR. This is generally a good practice in a 6-layer PCB as it frees up surface area for components and makes it easier to power those components through the vias.
Just as an example, take a look at the BGA shown below. This particular BGA is a typical high-speed interface controller that needs to deliver a lot of current at multiple voltages, so many balls will be connected to power and ground. Like an FPGA, you may find multiple pins for power and ground throughout its package. Dedicating a single layer to the power supply allows the plane to be broken up into tracks to allow the use of multiple voltages 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 central area dedicated to GND and multiple VCC rails. The GND pin can be connected directly to a plane on layer 2, and the VCC pin can be connected to a different power rail on layer 3.
Note that just because you put the power supply on the inner layer, it doesn't mean you can't put the power supply somewhere else. You can still route power on other signal layers using copper-threaded power rails or thicker traces.
If you need high current operation in a 6 layer board, possibly at multiple voltages, I would recommend using an extra power layer instead of an extra signal layer. In other words, you will have two power planes interleaved with ground on the inner layer within the stackup. You can even go a step further and put a power plane on the back layer for even more current handling capability. This will give you enough room to route power over a large area, possibly using heavier copper, thus ensuring low DC resistance and low power loss.
In addition to these points, other important wiring strategies used to ensure EMC for 4-layer or 8-layer circuit boards also apply to 6-layer circuit boards. If you use similar elements to the example 6-layer stackup above, you'll have an easier time routing and ensuring signal and power integrity. The same DFM considerations in a 4- or 8-layer board also apply to a 6-layer board; have your stackup approved by the fabricator before you start creating the layout, sizing traces, and routing them.
Sourcing: Zachariah Peterson