The Plane Facts of PCB Return Current Paths
The need to return home if you’ve become lost is a powerful emotion. Trying to find your way home while flying an airplane or out in a boat can be terrifying if you don’t know where you are. Even our entertainment capitalizes on this emotional imperative with shows like “Lost in Space,” or even simply “Lost.” Whatever your situation is, the drive to return to where you started from is so powerful that people will do anything possible to make it happen. It’s surprising then that with this powerful emotional need to always find a way to return back home, that many designers do not understand the equally important need for a clear return path on their PCB designs.
As a traveler who has set out on a journey will eventually want to return home, so do the signals that we send out in our electrical designs. To do this requires a clear return path for those signals to find their way back. And as a straight line is the most efficient way to travel from New York to Los Angeles, so is a straight path the most efficient way for signals to return on a PCB. Yet many designs end up looking like their signal return paths are a detour up around the Great Lakes through Wisconsin, and then down through Texas and getting stuck trying to cross the Grand Canyon in Arizona before they head home. Here are some ideas for how to create better PCB return current paths on your next design that will take the direct route home instead of getting lost going the wrong way around.
The Importance of a Clear PCB Return Current Path
When a signal is sent out from its source and arrives at its destination, it will then want to return back to the source. To do this it will try to find the easiest way back through the path of least resistance. This is a pretty basic rule of circuit design and has been a staple of our industry from the beginning. What is different is how the advancements of electronics in terms of high speed signals have increased the importance of how a clear return path is incorporated into the design.
Back in the days of placing thru-hole parts onto a board and then turning the primitive auto-routers lose on the design to route wherever they wanted to, a clear return path wasn’t as critical. Signal speeds were slow enough that the return path would find its way back no matter how you configured the placement and routing of your layout. With the higher speeds of today's designs however, that has changed. It is essential to establish a clear return path for your signals in order to reduce electromagnetic interference (EMI). This is usually done by carefully planning the power and ground layers in a PCB stackup adjacent to signal layers that will be available for return paths. Power integrity has to be considered along with high speed routing by the PCB designer in order to control the following effects:
Crosstalk: The use of ground planes between two signal layers will help to eliminate the broadside coupling effect that causes crosstalk as well providing a good return path for those signals.
Signal Integrity: To preserve signal integrity, good routing techniques should always be used. This includes providing uninterrupted return paths for the high speed signals both within the board and through interconnects to other system boards.
Controlled Impedance: The board materials that will be used, the dielectric layer thickness and constant (Dk), and the metal thickness of your traces will all be used to calculate your controlled impedance line widths. Another factor in controlled impedance routings is the configuration of your board layer stackup to provide plane layers for uninterrupted signal return paths.
With a carefully controlled layer stackup that provides adequate ground planes for signal return paths, a lot of the battle will already have been won. But there is another issue that must be considered as well in order to make sure that your design has the signal return paths that it needs.
Split planes are important for design consolidation, but can cause problems with return paths
Crossing the Grand Canyon; Beware of Those Split Planes
There are a lot of different power and ground nets on a PCB design today that ideally want their own devoted plane layer. Often there are more of these nets than are available plane layers in the stackup. To resolve this layer dilemma, designers will often split their power planes. Sometimes this is simply to provide a local area for one of the nets, and sometimes it is to snake a power net through a congested area. Whatever the reason is, these splits can cause problems with a signal’s return path.
When a high speed signal is routed across the board, it is expecting to find a return path on an adjacent ground plane. If the ground plane has been split however because a 12 volt supply was needed to be routed through that area, the signal’s return path is cut off. In the same way that the Grand Canyon effectively prevents automobile traffic from crossing it, a split plane will halt a signal return in its tracks. This will lead to all kinds of EMI problems that can ruin the signal integrity performance of your design.
This doesn’t mean that you shouldn’t use split planes, it only means that you should be careful about how you manage those splits in your board layer stackups. Floorplan your placement and routing ahead of time so you know where the sensitive high speed routing needs to go, and plan your split planes accordingly to avoid those areas. This may require some delicate re-thinking of your component placement, but it is critical that you preserve an uninterrupted return path for those high speed signals.
Tuned high speed routing on a solid plane for an uninterrupted PCB return current path
Use Your PCB Design CAD Tools to Help You Navigate Your Way Home
In order to help yourself design to the requirements needed for high speed routing and PCB return current paths, you need design tools that have the capabilities in them for this level of advanced design. You need to be able to control your PCB layer stackup and calculate the types and thicknesses of materials that you need for your design. You need to be able set up and control the design constraints needed for the high speed routing that you will be doing. And you need a design tool that will give you complete control over how you create and manage your power and ground plane layers.
The PCB design systems from Cadence have all of these functions and features built into them that will help you to create the best high speed signal return paths in your next PCB design. With its power and versatility, OrCAD PCB Designer has what you are looking for.
If you’re looking to learn more about how Cadence has the solution for you, talk to us and our team of experts.