Although some of the first boards I designed were not intended to be manufactured, they still needed the right grounding techniques in order to ensure power and signal integrity. I soon learned that unless I wanted to spend hours trying to diagnose mysterious signal integrity problems, I needed to take my grounding strategy into account when designing my PCBs. Grounding is, in fact, a major determinant of board performance and encompasses more than just routing return signals to a ground point or implementing a specific topology. By following the right PCB grounding techniques, you can minimize the likelihood of compromising your PCB’s power integrity and signal integrity. But before we discuss the best strategies to ensure proper grounding, we should address what PCB grounding techniques actually are and why they are important for PCB development.
What are PCB Grounding Techniques?
Grounding techniques can be generally divided into two areas:
- Placing power and ground planes in your layer stack.
- Arranging components in order to provide short, reliable return paths to ground.
The first of these areas is very important as layer counts in multilayer PCBs continue increasing; and the arrangement of signal, power, and ground layers has important consequences on signal integrity and power integrity. Even with two-layer PCBs; although difficult, it is a good idea to dedicate one layer to a solid ground plane in order to minimize loop areas throughout your board, if possible.
The second of these two points relates directly to the first point, as you will need to place ground planes such that you minimize the loop area for signals in your board. There are some simple, easily manufacturable optimized layer stacks that will help you satisfy both of these requirements.
PCB Grounding Techniques and Your Stackup
In a multilayer PCB, your layer stack forms the foundation for your device, and choosing the right arrangement of power, ground, and signal layers is crucial. Most cost-minded designers who are not creating extremely complex devices, or aren’t using HDI design techniques, usually opt for a four- or six-layer stackup in their board.
In these lower layer count boards, the stackup you choose can have a major effect on signal integrity. Your ground plane arrangement will also affect your routing choices, as certain layers will be occupied by power and ground planes. With a four-layer board, it is a common practice to place power and ground planes in the inner two layers, while the surface layers are used for component placement and signal routing.
This arrangement can also be reversed if your board will be deployed in an electrically noisy environment. Placing power and ground planes on the outer layers of a four-layer board with signals placed on the inner layers will provide natural shielding for your signal against external EMI. However, there may still be problems from conducted EMI resulting from noisy power supplies (e.g., a switching power supply running at high current).
With these higher layer count boards, you can take advantage of surface layers and interior layers for routing signals without sacrificing ground connectivity or shielding. With high enough layer counts, you can surround each signal layer with a pair of ground planes, ensuring that you have tight coupling between signal traces and your ground planes. Be sure to connect these ground planes with vias, and make sure to design your vias based on the tolerances and specifications of your contract manufacturer (CM).
Finally, you should consider how your current return path from your board reaches a grounding point, in order to provide a consistent ground potential and prevent ground loops. If you are designing a device to run off of battery power, you’ll want to take advantage of a chassis ground or a central grounding point. As such, you should connect a single point in a ground plane to the chassis ground point. Additionally, don’t use multiple returns back to your mains or grid-connected external power supply as this creates the potential for ground loops.
Arranging Components to Ensure Proper Grounding
Generally, if you place components on the surface layer, you’ll want to place components on the signal layer directly above the ground layer. This allows you to route return signals directly to ground and ensures that traces remain tightly coupled to your ground layer. Placing components and signals directly above the ground layer also minimizes the loop area, which reduces susceptibility to EMI.
If you are designing a device that will interface with the analog world using sensors, such as an IoT device, your grounding and component arrangement strategy will become very important. Unless your board will be connected to a data acquisition module or a PC, you will need to use mixed some mixed-signal design strategies to ensure proper grounding.
Some PCB designers recommend that you use two different ground planes on the same layer: one for digital components/signals, and the other for analog components/signals. They will then tell you to connect the two ground planes with a ferrite bead or a capacitor. At low speeds and signal frequencies, you can get away with this practice without incurring power integrity or signal integrity problems.
In reality, given that IoT and many other devices operate at high switching speed and with signal frequencies in the MHz or higher range, using split ground planes creates more EMI problems than it solves. Instead, you should use a continuous ground plane that is partitioned into digital and analog sections. The return path to the power supply should be placed at the bridge between the analog and digital ground plane sections.
Make sure you use the right ground plane arrangement
Note that, based on the above image, you should not route any traces over the split in the ground plane. However, this split is useful as you can place mixed-signal components (for example, ADCs or DACs) over the gap in the ground plane. This simple ground plane arrangement is easy to fabricate in a board with low layer count and is the best grounding choice for mixed-signal devices.
Decoupling and Bypassing to Ensure Power Integrity
Judicious use of decoupling and capacitors in concert with the right grounding strategy can also improve power integrity throughout your board. Placing a bypass capacitor directly from the power pin on a component to its ground plane will dampen any voltage fluctuations in the power plane, ensuring that a steady DC voltage reaches the component. This underscores the need to place the ground plane close to the surface layer as this minimizes the loop area associated with these circuits.
A decoupling capacitor provides the same function, although this generally refers to a capacitor that is placed directly between the power and ground plane upstream from a component. This also dampens any fluctuations in the source voltage by allowing these fluctuations to pass directly to the ground plane. This works best when your power plane and ground plane nicely overlap.
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