Building upon a firm foundation is a staple of literature, poetry, and parables. Without a firm foundation, whatever is built on that foundation will be subject to failure. This applies equally well for construction concepts, career and education, relationships, and even mental health. The old parable is that a foolish person built their house on the sand, and it wasn’t able to withstand the wind and the rain, and great was its fall. It pays to lay a solid foundation in whatever it is that we are building.
Printed circuit boards certainly have a foundation in their board materials and how those materials are configured in the layer stackup. Another foundation that is equally important however to the PCB is its ground plane. Without careful attention to the ground plane the board may not perform as intended, and it may be subjected to all manner of electrical noise and interference. Let’s take a look at what a PCB ground plane is, and some tips for how to make sure that it serves as the firm foundation that your board needs.
Why is the PCB Ground Plane Important?
The ground plane on a printed circuit board is typically a large area of metal connected to the circuit ground. This area of metal is sometimes only a small portion of the board, or in a multi-layer design it may be one entire board layer. Depending on the needs of the design, it may even occupy multiple layers.
The ground plane serves three important purposes in a printed circuit board:
Voltage return: Most every component on the PCB will connect to a power net, and then the return voltage will come back through the ground net. On boards with only one or two layers, ground nets usually have to be routed using wider traces. By devoting an entire layer to the ground plane in a multi-layer board however, it simplifies connecting each component to the ground net.
Signal return: Regular signals also need to return, and with high speed designs it is very important that they have a clear return path on the ground plane. Without this clear return path these signals can generate a lot of interference for the rest of the PCB.
Reduce noise and interference: As signal speeds increase, there are more digital circuits switching states. This creates noise pulses through the ground circuit that may affect other parts of the circuit. A ground plane with its large conducting area helps reduce the amount of this disturbance because it has a lower impedance than if the ground net is routed through a trace.
By carefully planning out the layer stackup configuration of a multi-layer circuit board, the PCB designer can use ground planes to help control the electrical performance of the board. By using a ground plane between two active signal layers, the crosstalk between the signals on those layers can be eliminated. And by making sure that there is an uninterrupted signal return path on the ground plane, the signal integrity of high speed transmission lines can be improved. Ground planes are also often connected to components that get hot in order to help dissipate the heat.
Examples of thermal relief pads in a small ground plane
Important Elements of a Power or Ground Plane
As we said, a ground plane can either be a designated area of metal on a circuit board layer, or it could take up the entire layer itself. How this is done depends on the type of PCB design CAD system that you are using. Most CAD systems give you the ability to draw a plane so that it appears as a solid patch of metal, and you can designate any net for a plane whether it is ground, power, or something different. CAD systems used to use negative image planes, as that took less computing power, but most PCB designers prefer drawing positive planes today as the tools now have plenty of power for this.
The PCB designer will draw the area for the power or ground plane on the designated layer in the shape that they desire. On an inner layer of the board this is usually the entire layer, while on the exterior layers of the board it is often smaller areas to service specific components or group of components. In some cases the designer may split the plane on an inner layer. This can prove to be very useful when cutting back on how many layers the board will be made with. Usually a plane split is done for power nets however, while the ground plane remains a full layer to help improve signal integrity and eliminate noise and interference.
When the plane is drawn by the PCB designer, the CAD tools will automatically create a connection for the component pins that are within the contours of the plane. This connection is either a thermal relief, or a direct connection (flood). Thermal reliefs are small voids in the plane around the perimeter of hole used for a thru-hole pin. Without a thermal relief, the thru-hole pin would be directly connected to the plane which would act as a huge heat sink making soldering difficult. With the relief however, the part can be soldered and unsoldered much easier. All other holes, such as vias, will normally have a direct connection to the plane.
Thermal reliefs can be controlled for the ground plane in OrCAD PCB Designer
Using your PCB Design Tools to Effectively Create a Plane
The key to creating a good ground plane in your PCB design is to put the tools to work for you. Make sure that you go through and set up all of your design rules and constraints before you start working with plane layers. Most PCB design CAD systems will allow you to set up different connection parameters depending on the net, and the layer where the connection will be made. The rules and constraints of the PCB design system will also give you plenty of flexibility in what kind of clearances that you specify. This will also help you when you are creating your ground planes as the tools will do the work for you, all you have to do is tell it where you want your plane.
Another thing that will help you when you work with planes is to first make sure that your board layer stackup is fully set up. This way the positioning of your plane layers will already be decided, and you won’t have to worry about changing the location and shape of your planes later on during the design.
To get the most out of the tool capabilities as we have described, you should work with PCB design tools that are fully equipped for creating ground planes. Fortunately these tools are already available for you from Cadence. OrCAD PCB Designer has the ground plane creation capabilities that we’ve discussed here. Additionally, OrCAD PCB Designer also has a full set of design rules, constraints, and dynamic parameter menus so that you can set your design up the way you need it for your PCB ground planes.
If you’re looking to learn more about how Cadence has the solution for you, talk to us and our team of experts.
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