1- EMI problems caused by improper stackup selection.
2- How planes affect EMI performance and tips on their placement.
3- Advantages of using higher layer count boards.
With modern electronics and ever-increasing signal speeds, PCB design has evolved into something very different compared to the past. The complexity and challenges of designing boards increased dramatically. There are issues to deal with in high-speed designs like signal integrity, crosstalk, and EMI. While a proper stackup helps greatly with these issues, a badly designed one, no matter how hard you try or pay attention to other aspects, may make it impossible to overcome them.
Considering commonly used high speed interfaces today like PCIe, USB, HDMI, etc., using multilayer PCBs have become mandatory. To route these high-speed signals effectively, a reference ground plane is required. It is also possible to use a reference power plane, but it is very EMI risky. Power planes will most likely have splits on them, and routing critical signals over these splits is a big no. Also, in most of these stackups, power and ground planes are widely separated, which results in low interplane capacitance and causing EMI problems.
Four-layer stackups are the starting point for high-speed designs and they offer a huge improvement over two-layer boards. However, traditional SIG-GND--PWR-SIG stackup may cause the EMI issues mentioned above due to large distance between PWR and GND planes. The better performing (SIG/PWR)-GND-GND-(SIG/PWR) stackup lacks the dedicated power plane, which will make the layout process much harder in complex boards.
Six-layer boards solve some of these issues, but not all of them. As explained above, it is not recommended to use stackups like SIG-GND-SIG--SIG-PWR-SIG due to widely separated planes. A better one is SIG-GND-SIG--PWR-GND-SIG, since all signal layers and PWR plane has an adjacent GND plane as reference. The issue here is symmetry though, which is a problem for fabricators since upper half has two signal layers and one plane while lower half has two planes and one signal layer. This may be partially avoided by pouring copper fill in layer 3, which will make it have a similar copper density compared to a plane.
Figure: 6-Layer PCB Structure Example
Eight-layer boards perform well in handling all these problems, and our preferred stackup in ForeCR products. Considering the stackup SIG-GND-SIG--PWR-GND--SIG-GND-SIG, all the aforementioned issues are addressed. All signal layers have an adjacent GND plane, PWR and GND planes are closely coupled, a dedicated PWR plane is present and symmetry criteria is satisfied.
It may sound a little cliché but the answer to the question in title, what is the best stackup, is it depends. It depends on the complexity of your design, your budget, standards you should comply with, speed of the signals, etc. As a final advice, always ask your manufacturer before production to optimize your buildup. Some very small and easy to make changes can help you save considerable time and money.
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