There are few things in life more unnerving than an earthquake. As human beings, we rely on having a firm foundation under our feet, but when that foundation begins to shake, our confidence is usually left shaken as well. Electronic devices also rely on a solid foundation for operation, and their foundation is likewise based on the ground. The difference in a printed circuit board is that their foundation is usually a layer of metal known as the ground plane instead of the earth.
PCB ground planes are usually one or more metal layers within the layer stackup of the circuit board, or in some cases, areas of metal 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. That’s just the beginning, though, as there’s a lot more to understanding ground planes in PCB design yet to come.
Electrically Understanding Ground Planes in PCB Design
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:
Shielding
The ground plane protects from electromagnetic interference (EMI) from outside sources affecting the PCB’s circuitry’s normal operation. Simultaneously, it also helps contain any EMI that the board creates from radiating outward. Surface layer routing protected by a ground plane on the next adjacent layer is considered a microstrip configuration, while stripline describes those internal routing layers sandwiched between two ground planes. Ground plane shielding is essential to preventing crosstalk and other noise problems in high-speed circuit boards that require good signal integrity for their operation.
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 planes weren’t designed with clear signal return paths in mind, and the return paths are blocked. 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 clear path on the reference plane, the return path will couple with the signal on the adjacent layer for the best noise performance of the board.
Power integrity
Circuit boards are often populated with devices that demand a lot of power in their operation, creating power spikes that must be smoothed out. At the same time, the continual switching of digital signal states from low to high and then back to low again creates problems. 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.
There are some specific design guidelines that layout designers need to be aware of to get these benefits from the ground plane in a circuit board. We will look at these guidelines next.
PCB Design Guidelines for Ground Planes and Their Connections
When working with the ground in your PCB design, here are some layout recommendations that will help you to create the most robust ground system possible:
- Routing: The ground should be routed with the widest possible trace for a single layer board where area fills are not practical. However, it is best to create some sort of ground plane if at all possible to leverage the electrical benefits listed above on all other designs.
- 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. The ground planes themselves can be created in the CAD system as either positive or negative images, whichever is best for both the CAD system and the fabricator. In many instances, additional area fills of ground will still be needed throughout the design. Some designs will need split ground planes, but designers should avoid this as much as possible for signal integrity purposes.
- 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 you may create a thermal imbalance and cause manufacturing problems. Thermal relief pads should be used on thru-hole pins to aid in making good solder joints, while smaller ground vias can usually be connected with a solid connection instead.
There are a lot of design rules that the layout team will need to follow to create the most manufacturable board, but fortunately, your CAD tools can help you with these requirements.
Using Your PCB CAD System to Create Ground Planes
PCB CAD systems have many different features and functions within them 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 either 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.
One resource that you can turn to for help is the PCB contract manufacturer building your finished design. They have staff engineers that 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 like you in understanding ground planes in PCB design for many years, and we know the challenges you face. Bring us your questions, and let us help you design your printed circuit board for optimum performance and efficient manufacturing.
