“PCB creatures, stay well grounded!”
We’re living in a world where we get bombarded with so many things fighting for our attention. We’re facing tough challenges, emotionally charged situations, and stimulation that can become too much to handle. Every human being should be able to relate and know that we sometimes need to tell ourselves to stay grounded and calm down.
The daily overflow of stimulation in our world is something PCB creatures can relate to all too well. They are living in cramped settlements with noisy neighbors. Everybody is “crosstalking” each other and the noise from the nearby factory radiates into their living room giving PCB creatures sleepless nights. However, don’t lose hope my lovely PCB creatures, you can count on us! We electrical engineers promise to keep you grounded.
While for humans staying grounded can mean to spend more time with family, doing sports or even trying meditation, PCB creatures need a different treatment.
One of the golden rules in PCB design is to use thick ground (GND) traces and even better to pour a whole layer with copper and connect it to GND. However, GND planes are not the only situation where it helps to use planes. The main advantage of copper pours is that they provide a very low impedance path.
There are several situations where we absolutely need the lowest possible impedance path.
- Signal and power return paths
- Power distribution network
- Improving electromagnetic immunity
Signal and power return paths
In our schematics we use wires to connect different signal pins to each other. GND and supply pins are usually connected by using global power ports. However, as soon as we enter the layout phase we have to switch to the physical representation of a circuit. Every single signal and supply path needs its own return path. Additionally, the return path should be as close as possible to the signal path to reduce the loop generated by the two paths. This in turn, will decrease susceptibility to electromagnetic disturbance and radiation. To cut a long story short, we have to use GND planes to get as close to the optimum return paths as possible.
Plane cut-outs — a dangerous but powerful weapon
Unfortunately, there are situations where a GND plane also makes a circuit design worse. Remember we said a plane’s main task is to guide the return path and have a low as possible impedance. However, we assumed the return current would always closely follow the signal traces. There is a big difference between high frequency and low frequency signals in this regard.
In the case of high frequency signals, the return current stays as close as possible to the signal trace. However, lower frequency signals’ return current, behaves more like a point-to-point connection, where the return current forms a straight line between point A and B of the signal current. Therefore, our assumption that return currents are always right next to the signal currents depends on the frequency.
Such behavior can cause problems for other circuit components which intersect the return current. In conclusion, we must determine where we expect the return current to flow and what its behavior will be.
If we’d like to prevent current from flowing through an area which contains highly sensitive components or we’d like to separate a noisy component form the rest of the circuit, we need plane cut-outs or multiple planes.
However, make sure to use plane cut-outs wisely and only if there is no other way. Cut-outs and traces on the GND layer can also increase the impedance of return paths and/or create electromagnetic compatibility and signal integrity issues.
Situations where cut-outs are used include:
- Separation of digital and analog grounds
- Partly separating high frequency oscillators from the rest of the circuit (the purpose is to avoid a patch antenna structure)
Power Distribution Network (PDN)
Ensuring as low impedance as possible for the power distribution network is, right after the GND plane, the second most important concern in designing copper pours for signal integrity and electromagnetic compatibility. Almost all the same guidelines as for GND planes apply. However, one major difference is that power planes are often used to separate different function blocks in the layout. Separate power planes can be applied in the following situations:
- Analog area
- Digital area
- FPGA / microcontroller area
- PLL, Quartz, Oscillator area
Complex designs and chips such as FPGAs usually require five or even more different power supply rails, each of which will have its own power plane.
After having talked about GND planes and the power distribution network, we should also briefly mention decoupling capacitors. Even the best copper pours will never achieve an optimal impedance path. Therefore, decoupling capacitors are used as buffers and located as close to chips as possible. You can read all about decoupling capacitors here.
Improving electromagnetic immunity
Most designs will be built with the intention to pass some sort of electromagnetic certification process (e.g CE, FCC certification). Part of the tests is to apply high voltages and high power spikes on all external interfaces. In order to protect the sensitive internal components from these spikes, protection circuitry is used. Most protection circuits rely on the basic principle to employ a second much lower impedance return path, than the interface going deeper into the device provides. Thereby effectively protecting the sensitive electronics by diverting the spike to GND or mechanical earth before it can do any damage. To ensure a very low impedance path, copper pours and stitching vias are the ideal tools.
Summary for copper pours
We’ve covered several different scenarios where it helps to use planes. Here are the most important points we’ve covered.
- The main purpose for copper pours is to ensure a low impedance path.
- Do not interrupt GND planes with cut-outs or traces unless you know what you are doing
- If you can’t predict where return currents will flow, you risk running into signal integrity and electromagnetic compliance issues.
- Avoid GND currents between different functional blocks
- Having a proper GND plane is first priority, second priority is to have a low impedance power distribution network.
Copper pours are a versatile tool and help to keep our PCBs grounded and more robust to disturbance. There are many guides available to learn more about copper pour design and signal integrity in general. However, there are also different experts contradicting each other. Signal integrity and especially electromagnetic issues are very hard to predict, expensive to measure and often impossible to calculate. As the signal integrity guru Dr. Eric Bogatin puts it:
With very few exceptions, every equation used in signal-integrity analysis is either a definition or an approximation! — Dr. Eric Bogatin, Signal and Power Integrity — Simplified
Therefore, most of the knowledge about when to use which plane and how to use cut-outs effectively is based on hard earned experience.
In this article we only looked at the usage of copper pours from a signal integrity and electromagnetic compatibility perspective. Another very useful purpose of copper pours is in thermal management.