In casual usage, most people consider the term “grounded” a compliment, denoting composure beyond superficiality and short-sightedness. While working with electricity, being grounded doesn’t always carry this positive connotation – it can be deadly! – but for most work with electronics, it is a benefit that protects the devices from rogue shorts. On a deeper design level, ground is a necessity for a high level of signal integrity, minimizing crosstalk, EMI, and other detrimental effects that can severely undermine expected performance.
Ground can come in many forms in a circuit board, but the ground planes are the most substantial, almost entirely flooding signal layers. PCB ground planes usually have one or more metal layers within the layer stackup of the circuit board or, in some cases, metal areas on the external board layers. These layers are referred to by several names, including area fills, copper pours, and negative planes, in addition to the standard “ground plane.” Not only do they supply the zero-reference voltage for the circuit board, but they also provide signal return paths for most of the circuitry in the design.
Types of PCB Ground Plane Connections
|Signal ground (i.e., digital/analog ground)
|Indicates signal reference for systems that use distinct grounds to isolate analog components from their faster-switching analog counterparts. These “two” grounds need at least one connection point.
|It connects a metal enclosure and the board to Earth ground.
|Analogous to a neutral line in three-wire AC electrical cabling, it carries the return signal.
|Also known as true ground; a literal connection between the Earth and the electronic system.
Author’s note: Ground is ground is ground – all ground exists as a 0V reference. HOWEVER, board design can result in two “grounds” that have a potential difference, making one (or both) varyingly unreliable as a reference voltage. For safety and thorough board design, assume ground is NOT 0V (or within an acceptable margin of error, depending on the accuracy of the measuring tool) until proven otherwise!
The Purpose of PCB Ground Planes
Ground planes in a printed circuit board provide the return path for electrical current and are considered the zero-reference voltage for the design. In addition to their reference voltage, ground planes also serve many other functions in the design:
The ground plane protects the PCB’s circuitry from outside sources’ electromagnetic interference (EMI), affecting its regular operation. Simultaneously, it helps contain any EMI the board creates from radiating outward. Surface layer routing protected by a ground plane on the next adjacent layer is known as a microstrip configuration; stripline describes internal routing layers sandwiched between two ground planes for internal routing. Ground plane shielding is essential to preventing crosstalk and other noise problems in high-speed circuit boards that require good signal integrity.
Signal Return Paths
The ground planes in a PCB also serve as the most common return path for the signals on the board. Each signal conducted through a trace has to return to its source, and it will find the path of least resistance. The problem with many designs that don’t pass EMI testing is that the ground plane design does not facilitate a direct signal return path (usually due to some obstruction). As the signals wander around the board, seeking a different path back, they create a lot of noise that ruins the circuit board’s signal integrity. However, with a straightforward path on the reference plane, the return current will couple to the signal on the adjacent layer for the best noise performance of the board.
Circuit boards contain components that demand a lot of power, creating spikes that necessitate signal conditioning. At the same time, the continual switching of digital logic states can create issues. To counter these noise-causing scenarios, the large metal area of the ground plane helps steady the voltages across the board and provides a low inductance ground path that helps control the noise.
PCB Design Guidelines for Ground Planes and Connections
When working with ground in a PCB design, here are some layout recommendations that will help create the most robust ground system possible:
- Routing – The ground should have the widest possible trace for a single layer board where area fills are impractical. However, creating a ground plane to leverage the electrical benefits listed above on all other designs is best.
- Multilayer Designs – High-speed multilayer designs need their board layer stackups calculated for the correct configuration of ground layers, dielectric material, and layer widths for impedance-controlled routing. Depending on the fabricator’s needs, the CAD system can create the ground planes as positive or negative images. In many instances, additional area fills of ground will still be necessary throughout the design to balance copper distribution. If possible, avoid split ground planes, and designs that utilize split ground planes must avoid routing over the gaps to prevent significant EMI.
- Connections – Traces from SMT pads to ground vias should be wider to lower their inductance. At the same time, be careful not to use too wide of a trace on small discrete parts, as it can create a thermal imbalance and cause manufacturing problems. Thermal relief pads should be used on through-hole pins to make good solder joints, while smaller ground vias can be continuous copper instead.
PCB CAD systems have many features and functions to help designers create ground planes. The system often automatically creates negative planes as long as they are configured correctly before starting the layout. In contrast, positive planes can be adjusted on the fly as needed. Positive planes or area fills are “poured” by the designer manually or controlled by the precise placement of keep-out zones and routing outlines. Thermal relief pads can usually be controlled individually or in batch mode by the design rules and constraints.
Your CM Can Assist in Ground Plane Design
One resource designers can turn to for help is the PCB contract manufacturer building the finished design. They have staff engineers who can help you with design and layout questions to get to the finish line quicker and with less expense and trouble. At VSE, we have been supporting designers in understanding PCB ground planes for many years, and we know how to navigate a challenging DFM board. Together with our manufacturing partners, we’ve been building life-saving and life-changing devices for over forty years.