Grounding and Voltage Routing Considerations in PCB Design

Key Takeaways: 

• Learn about grounding and voltage routing across printed circuit board layout and design
• Discover what unique software offerings from Allegro PCB Designer can aid in your grounding and voltage routing
• Implement best practices for power and ground planes in your PCB designs

And if you've still got more to learn, watch our video on Allegro Route Vision to see what more Allegro PCB Designer can do to make your routing masterful. 

It is so easy to plug a power cord into a wall socket, or flip on a light switch, that you might think connecting the components on a printed circuit board into power or ground would seem to be easy too. And to tell the truth, that used to be the case with PCB design. On boards where signal and power integrity wasn’t as much of a concern, you could simply drop a via into a power or ground plane and forget about it.

With the design requirements of today’s electronics however, there is a lot more to managing your power distribution network than simply throwing some vias into the design. You need to consider the effects of your PDN on the rest of the board, while still ensuring that your devices have access to the appropriate power and ground nets. This requires some finesse in the grounding and voltage routing of your board, and we have a few ideas here which can help.

Grounding and Voltage Routing on Printed Circuit Boards

Even though dense multi-layer boards are used extensively for advanced electronic devices today, there is still a need for the inexpensive two layer board. For devices that don’t require much circuitry, such as toys or other simple consumer products, two layer boards are still preferred to reduce manufacturing time and costs. At the same time though, the performance of these electronics continues to grow, which requires more diligence in the design of the board’s power delivery network.

With only two layers to work with you won’t have internal layers that you can use for power and ground planes, so you will have to route the power instead. It is recommended that in most applications that designers use the smallest trace widths possible that are still manufacturable at a low price. This usually ends up being a 6 mil trace for signals, and a 20 mil trace for power. Keep in mind that trace width for power is proportional to current flow —  trace width increases if the current increases and vice versa.  

As you route your board, you should keep your signal and power routing on the top layer, and reserve the bottom layer for the return paths. The simplest way to do this is to dedicate the bottom layer as a solid ground plane. You may end up having to use some of the bottom layer for signal routing, but if you do make sure that you maintain clear paths for signal returns.

Routing voltages along with your signal traces takes careful planning

Using the bottom layer for ground will also help you with noise and other signal integrity issues, but it will also consume a lot of space. Therefore, it is important that you carefully plan out your power routing on the top layer to make sure that you have power well distributed throughout the board. 

You will also need to plan your signal routing so that sensitive traces don’t get too close to the power traces. Remember that just because a two layer board is less expensive to manufacture, doesn’t make it any easier to design. In fact, you may find out that designing a two layer board with routed power traces is more of a challenge than you expected.

If you are looking to prepare more for the greater routing considerations of your board layout, look through our great E-book on PCB routing. It offers insights on the following:

• Net management strategies for signal or critical area nets

• Via usage

• Managing stackup considerations

Along with much else, you might want to refresh or reconsider if you’re working with particularly dense or challenging board layouts. 

Making the Connection From Components to Power and Ground

Whether your power and ground are routed using traces, power signals through star connections, or conducted through solid planes, you still need to connect your components to it. Although connections to ground for signal return paths don’t require any more metal than a regular signal trace, the connections that are conducting high currents need a lot more. The flip side is that metal acts as a heat sink, and could, therefore, cause soldering problems during manufacturing. To counter this, it is important to use thermal reliefs when connecting component pins to power and ground.

A thermal relief is where a portion of the metal is removed from the connection to the pin in order to lessen its ability to conduct heat. Without a thermal relief, the heat needed to solder the pin will easily be absorbed by the metal plane instead of staying on the pin where it is required. PCB design tools like Cadence Allegro gives designers the ability they need to manipulate the size and shape of thermal relief as shown in the picture below:

Managing the thermal reliefs of a PCB design using Cadence Allegro’s parameters menu

In PCB design you will see variations of thermal reliefs on both surface mount and thru-hole component pins:

SMT pins: 

If SMT pins are connected to power or ground with large areas of metal, it can cause a thermal imbalance between them and pins with less metal attached. In small two-pin discrete parts, this imbalance may lead to a condition known as “tombstoning.” This is where the solder on one pin will reflow faster than the other, and pull the part up and away from the other pin.

When connecting discrete SMT pins to grounding or voltage routing, it is best to use a trace width just wide enough to satisfy the current needs in order to provide thermal relief. This will help keep the two pins of the part thermally balanced. 

Another problem with small discrete parts is when one pin is placed onto a large area of metal. Although this gives the best electrical performance, it also behaves as a massive heat sink that creates a lot of thermal imbalance with the other pin. The best practice here to satisfy the needs of both the electrical design and PCB manufacturing is to connect the SMT pin with multiple traces or “ties.” This gives the SMT pin the thermal relief it needs for soldering.

Thru-hole pins: 

Connecting a thru-hole pin to a power and ground trace is usually done as any other trace with a direct connection from the trace to the pin’s pad. With a trace that is wider than the pad though, or with an area of metal fill such as a power or ground plane, you will want to use a thermal relief pad as shown in the picture below. 

These thermal reliefs give an adequate amount of metal to conduct the current but decreases the amount of heat that would be pulled from the pin by the metal plane. With PCB design tools like Cadence Allegro, you can control the width of the ties and spaces in the thermal relief in order to give the pad enough metal for its needs.

Thermal reliefs for thru-hole pins in a ground plane

The Pros and Cons of Power and Ground Planes

If you are designing a multi-layer circuit board, you will probably have the board layer stackup configured for dedicated power and ground planes. The big advantage to working with planes is that it offers an easy way to connect your components to power and ground, without having to route power with wide traces as on a two layer board. Using ground planes in your design also gives you a lot of other benefits including the following:

• Return paths: Signals will travel from their source to a destination, and then they will need to return back to their source. If there isn’t a clear return path for them, they will create a lot of noise as they wander around that could affect other circuits. A ground plane will provide that easy return path.

• Shielding: Ground planes will help to shield your sensitive circuits from the effects of external electromagnetic interference (EMI), as well as keeping internally created EMI from affecting other devices. Additionally, using ground planes between active signal layers in your design will help reduce the possibility of broadside coupling, or crosstalk, between layers.

• Reduce noise: As the digital circuits switch states, they will create noise pulsing through the ground circuit which could create false switching in other circuits. The large area of a ground plane will help to reduce the effect because it has a lower impedance than if the ground was routed through a trace.

• Heat dissipation: Ground planes also make for a good heatsink for components that run hot. By connecting these components with holes through the board to the ground plane, the heat can be spread evenly around the board.

On the other hand, there can be some downsides when working with power and ground planes that need to be mentioned as well. Planes will add to the layer count of the board, which will increase the manufacturing costs. Different areas of circuitry, such as digital and analog, will need to have their grounds carefully managed so that noise from one doesn’t adversely affect the other. And care must be taken when working with planes that are split to accommodate multiple power or ground nets. This is especially critical for signal return paths as a split plane could inadvertently ruin or block what should have been a clear path.

All of these issues, though, are part of the PCB design process, and they can be expertly navigated by using advanced PCB design tools.

To design complex power delivery networks, you need powerful PCB design systems

How Advanced PCB Design Tools Can Make a Difference

Working with the grounding and voltage routing of your PCB design will be a major portion of your design process. Fortunately, you have an ally in this with the capabilities and functions of your PCB design tools. CAD systems like Cadence Allegro have many features that are designed to help you with the design of your power delivery network including:

• A full set of comprehensive design rules and constraints to set up your power and ground nets with the width and spacing requirements that you need.

• Full routing and plane creation utilities to give you complete control over your power and ground design including the creation of split planes and control over thermal relief pads.

• Simulation and analysis tools to monitor how your design will work before you go to manufacturing.

You need the best PCB design system to help you design a clean PDN for your printed circuit board, and Allegro PCB Designer has all of the capabilities that you are looking for. With all of the features that we’ve been discussing, plus many more as well, Allegro is the right tool for the job.

If you’re looking to learn more about how Cadence has the solution for you, talk to us and our team of experts.