It’s time to upgrade from a four-layer PCB to a six-layer board. The extra layers may be used to accommodate more signals, additional plane pairs, or different conductors. What matters is the way these extra layers are arranged in a stack on a six-layer PCB. Read on for some 6-layer design guidelines if you have never used 6-layer boards before or had EMI problems with these stacks.
Why six layers?
Consider the reasons why you might want to build a 6-layer board before starting. Other than adding more signals, there are other reasons to use a 6-layer board. The most basic version would use the same technique as the SIG/PWR/GND/SIG stacked on a four-layer board, except that the signal would be placed in the center. The SIG/PWR/SIG/SIG/GND/SIG is the worst 6-layer PCB stack from an EMC standpoint, and it is probably only suitable for boards running at DC.
Choose a 6 layer board instead of a 4 layer board for the following reasons:
- If you have a 4-layer stack consisting of SIG+PWR/GND/SIG+PWR and need to add more components, then the outer layers are not enough. Placing PWR in the inner layer allows more decoupling by using the PWR/GND pair.
- If you are using a mixed signal board, the surface layer can be dedicated to the analog interface. An additional layer is available for the digital wiring.
- You want to separate the signals for different layers of a high-speed, high-I/O board.
- All these configurations add only one additional layer of signal. The GND plane is the other layer. Your stacking layers will determine the EMC, signal integrity, and board layout.
How to route signals?
We will look at a typical 6-layer PCB before we begin wiring.
In this stack, the top and bottom layers sit on a thin dielectric, so these layers should be used for impedance control signals. Ten mils is the thickest dielectric you should use, as this will require microstrip cabling 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 for a reduced alignment width, allowing you to wire into finer-pitch components. For example, we use a version of the above stack for many small networking products that support multiple multi-gigabit Ethernet channels.
If you need to use a smaller alignment width on the outer layer, reduce the outer dielectric thickness (perhaps as low as 4-5 mils) and 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 route the power supplies?
In the 6-layer stack shown above, the PWR is given its own layer. This is usually a good idea for a 6-layer PCB, as it gives the components more surface area and makes it easier to power them through the vias.
As an example, look at the BGA below. This BGA, which is common in high-speed controllers for interfaces that require a lot of current at different voltages, will have many balls connected to both power and ground. If you have an FPGA in your package, there may be multiple pins that are used for ground and power. If you dedicate a single power layer, it allows you to divide the plane into multiple tracks for high-current voltages. This will eliminate the need to use multiple voltages and high currents on these power rails.
You can still place the power rail in another position if you use a thicker alignment or copper laying. By using thicker alignment or copper laying, you can still connect the power supply to other signal layers.
On a 6-layer circuit board, if you require high currents or multiple voltages to be operated, we recommend adding an extra power layer rather than an additional layer of signal. On the internal layers, you’ll have two power layer interleaved together with the ground. You can even add a power plane to the back layer if you want to increase current handling. This will allow you to distribute the power supply across a large surface, and possibly use heavier copper.
Conclusion
These points and other important routing techniques used to guarantee EMC on boards with 4 or 8 layers also apply to boards with 6 layers. If you use similar elements to the example 6-layer board, you will have an easier route and ensure power and signal integrity. DFM is the same on 4- or 8-layer boards as it is on 6-layer ones. Before you start creating layouts and routing wires, get your stack reviewed by the manufacturer.
Before you begin routing, please remember to adhere to these 6-layer PCB design guidelines.
