I have never been afraid to fly, and in truth, I enjoy air travel. However, like many other people, I feel a great sense of relief when the plane lands. Having both feet planted firmly on the ground is a compelling urge for most people and necessary for many industries like construction. Ground is also an essential part of electronics, and its design and configuration are critical to the success of PCB design.
“Ground” can serve many purposes in electronic design, including a physical connection to the earth, a reference point in a circuit for measuring voltages, and a common return path for signals and power. In PCB layout, the way ground is conducted through the board is critical for power and signal integrity. Designers must follow some specific guidelines to ensure the best performance of the completed board. To help, here is a guide to PCB grounding techniques for your next circuit board design.
Grounding in a PCB Design
Grounding schemes in PCB designs will change depending on the board’s requirements, size, and technology. A small single-sided circuit board without high-speed circuitry on it may not require as elaborate a ground scheme as other boards do and will use routed traces for its ground network. Large, complicated PCBs like motherboards or backplanes will probably have multiple internal ground planes designed into them as part of their overall power delivery network (PDN).
When developing the grounding for a circuit board, designers can use single-point grounding or multipoint. Single point, or “star” grounding, is usually used in low-frequency designs and has one point where all the grounds are tied together. Different areas of circuitry, such as digital and analog ground, will be split and tied together at one point. A multipoint ground is used for circuit boards that operate at higher frequencies, and all grounding is managed on one continuous plane. This grounding methodology helps reduce EMI and provides a natural reference plane for signal return paths between the different circuitry areas.
Most circuit boards today are operating in higher frequency ranges and typically use multipoint grounding, providing several advantages:
- A single continuous reference plane allows for clear signal return paths throughout the board, even for signals that cross between different circuitry areas.
- The ground plane will help shield sensitive circuits from EMI’s effects and prevent board-generated EMI from affecting nearby devices.
- A ground plane layer will aid in separating signals, preventing broadside coupling, or crosstalk, between layers of sensitive signals.
- The large ground plane area controls some of the noise of digital circuits that can pulse through routed ground traces and influence others into false switching.
- Large ground planes provide heat dissipation through thermal vias for components that are running hot.
Another advantage to using a large ground plane is its ability to help control ground bounce. Multiple high-speed digital devices that are switching states at ultra-fast rates can fail to return to their reference level and create a new reference level instead. In some cases, a signal that should be at a high state is received, but it is misinterpreted as a low state instead because of the bounce.
Next, we will look at some PCB layout techniques for creating a robust ground on a circuit board.
Guide to PCB Grounding Techniques in Layout
When routing ground traces, whether, on a single layer board or a multilayer board, it is always good to use wider traces than what is used for regular routing. However, even two-layer boards are good candidates for developing some ground plane on one of the layers. The more metal you can use for a plane, the better. Remember, when connecting surface mount pins to ground planes, only use traces wide enough for the current needs. You still need to design the board to be manufacturable, and too much metal on a small surface-mount part can lead to thermal unbalancing between the pins during soldering.
The most important rule for most boards using a multipoint ground system is NOT to split the ground planes. As we have said, this can cause a plethora of undesirable signal integrity problems in the design. Additionally, make sure to use thermal relief pads for thru-hole components that connect to any plane. You must ensure that the relief pad provides enough metal to conduct the current, but not so much that it will pull all the heat away from the pad during soldering.
Here are some additional ground plane layout recommendations to help with signal and power integrity:
- Bypass capacitors: Place the caps as close as possible to each power supply pin of each device on the board. This placement will help reduce the impact of current spikes during switching for controlling ground bounce.
- Bypass capacitor connections: Use larger traces and vias to connect the caps to reduce inductance and conduct increased current flow. Also, do not daisy chain the bypass ground connections together, which can increase their inductance. Connect each ground pin to the plane individually.
- Routing high-speed signals: Do not route these lines across voids in the ground plane. You will ruin the clear path of the returning signal, which wants to couple with the outgoing trace.
- Stay on one routing layer: As much as possible, keep your sensitive routing on one layer to help maintain the clear return path on the plane. If you have to transition layers, use a ground transfer via beside the signal via so the clear return path of the signal is not interrupted.
Designing your circuit board with a strong PDN and an effective ground plane requires a lot of care for success. Fortunately, resources are available to help.
Additional PCB Design Resources for Grounding Techniques
You can find a lot of information on PCB grounding techniques through online training offered by industry professionals and CAD vendors. Blogs, like this one, also cover a lot of information, and there are many white papers posted on the subject. Design conferences will often have classes devoted to PCB grounding techniques taught by those with years of industry experience. Another resource you should investigate is the PCB contract manufacturer that will be building your circuit boards.
At VSE, we have been partnering with technical customers from a wide range of the electronics industry for over 30 years. We understand the ground requirements of power supplies, analog, digital, mixed-signal, and high-frequency designs, and our engineers are ready to help you with your next design project.