Ask the Experts: 12-Layer Stackups With Multiple Powers and Grounds

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Q: We’re having lots of problems designing good stackups for 6-, 8-, 10- and 12-layer boards with multiple powers and grounds. Any suggestions?

Stephen Chavez: When it comes to stackups, getting it right makes all the difference from the very beginning. The stackup is the foundation of a PCB design. It sets up the structure for any power and impedance requirements. Using multiple powers (PWR) is the norm and usually required in today’s complex PCBs, especially when implementing a complex FPGA. As for multiple GND (reference) planes in a design, you need to truly understand how to design a quality stackup to set yourself up for overall success. It is suggested to only have “one” GND reference utilized on multiple planes throughout your stackup. It’s also recommended to have this GND reference plane adjacent to a PWR plane for better coupling when to comes to controlling the energy within your stackup design. The general goal with this PWR and GND pair setup is to increase your capacitance and reduce your inductance. This will help in mitigating SI and EMC issues that may present themselves. Keep in mind that good signal trace-to-reference plane setup is also required to mitigate similar issues related to SI and EMC.

Most SI and EMC issues can be traced to design errors such as bad PCB stackup, poor PWR and GND utilization, and poor signal routing. Placing PWR and GND planes adjacent to one another within a PCB stackup, and routing signals adjacent to their appropriate reference plane, without routing the signal over any interruptions (gaps) in that reference plane, is the key. It amazes me how often designers simply make this type of mistake. So, the key to success is to get the PCB stackup correct early in the design phase. This would include working with your PCB supplier in creating and verifying the stackup. Then, implement industry best practices for both GND and PWR utilization along with quality signal routing. Take the time to do the due diligence to get it right the first time. It will be worth it.      

Rick Hartley: Placement is everything. I’ve said this for at least 40 years. I have put as many as 20 different voltages on a single power plane of a high-speed board, and made the circuit function perfectly. Is this easy? No. It requires perfect part placement. On a very dense board with 4,000 to 6,000 components, placement this good can take weeks. If you have taken the time to achieve placement that good and you still cannot get the job done with one or two power planes, plus some small areas of localized power (IC cores) on signal layers, then you will likely need to increase layer count. I know you have already figured that out.

One other comment: If you have more than one ground in the design, it’s likely that someone does not understand the basic concepts of energy movement and the reasons why you almost never need more than one ground plane.

To read this entire feature, which appeared in the July 2021 issue of Design007 Magazine, click here.


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