Your PDN Design Guide

October 12, 2020 Cadence PCB Solutions

Key Takeaways

  • The necessity of a good power delivery network on your circuit board design.

  • How the PDN of your board affects both its signal integrity and manufacturing requirements.

  • How the features in your PCB design tools can help you design a good PDN.

Too many power cords plugged into a power socket not following PDN design guide best practices

Too many plugs can create a bad power delivery network (PDN) in your home.

Every year, my family watches the movie “A Christmas Story,” and every year, I cringe when I see how many appliances and lights the dad in the story tries to plug in at once. Sparks, smoke, and a blackout proceed those immortal words, “Don’t anybody move—a fuse is out!” Fortunately, our homes today usually have a much better power system built into them than what they had available 40 years ago. That scene, however, serves as an effective reminder of the importance of designing a good power delivery network (PDN) into our printed circuit boards.

Although I have seen a few circuit boards spark and smoke in my time, the majority of the problems from a bad PDN are usually not that dramatic. They are, however, no less damaging, as an incorrectly designed PDN can cause intermittent problems to outright failures of the circuit board. Let’s dive into the importance of how we design the power delivery network of our circuit board with this—your PDN design guide.

The Necessity of a Good Power Delivery Network on Your PCB

All of the active electronic components on a circuit board will need power to operate, and to do that, the PCB needs a well-designed power delivery network (PDN). At one time, the integrated circuit devices on a PCB only had a single power and ground pin and could be easily connected with a simple wide trace. Then, as circuit density grew, it became easier to connect them with power and ground planes in a multi-layer board. But as pin-counts rose and the power requirements of ICs got more complex, circuit boards started experiencing some of the following problems:

  • Electromagnetic interference (EMI): The faster the circuitry became, the more sensitive it could be to EMI from both internal and external sources. One way to guard against EMI problems is to configure the power and ground planes to shield against both incoming and outgoing interference.

  • Ground bounce: When a lot of digital signals are switching states at the same time, it can produce an effect known as simultaneous switching noise (SSN) or ground bounce. This can be seen in memory or data buses, and if the rapid switching causes the signals to not return to their reference ground level, they will bounce above it. This effect can produce undesirable noise in the circuits that could potentially create false switching and disrupt the operation of the device. A well designed PDN can help control the reference ground level.

  • Power ripples: Switching may also be caused by a power supply, which will create noise, or ripples, in the circuitry. These ripples can manifest themselves as crosstalk in other circuitry, which can adversely affect the signals in those circuits.

Along with the faster switching speeds of modern circuitry, there are many more power and ground connections on an IC that have to be managed. With a large processor chip in a ball-grid array (BGA) package, there can be hundreds of power and ground pins at various reference levels. These pins can also pull a lot of current to power the processor, and making sure that the power is clean without any spikes, ripples, or noise requires a carefully designed PDN. In addition to providing clean power, however, the PDN also has some other important electrical tasks that it needs to do.

A ground plane in a circuit board showing a line of vias blocking that area of the plane

Vias can create an effective barrier on a ground plane for a clear signal return path.

The Many Roles of Power and Ground

While it may seem that the successful design of a PDN should be solely focused on providing clean power to its processor chips and other large-scale ICs, there is actually a lot more that the PDN needs to do. One of these tasks is to provide clear return paths on the ground planes for high-speed, sensitive signals, but there are some potential problems to be aware of. 

With the need to provide power and ground to so many pins on high-density parts, the planes can get filled up with vias, as shown in the picture above. These create obstacles in the plane that can make it difficult for the signal returns to find a clear path back to their source. As these return signals wander around trying to find their way home, they can create noise and ruin the signal integrity of the design. The challenge to the designer, then, is to provide adequate power and ground to all the active devices while at the same time ensuring that the return paths are not blocked off.

Another task for the ground planes is to provide microstrip or stripline layer configurations for controlled impedance routing of high-speed transmission lines. By routing specific widths of traces adjacent to or between two ground planes where the thickness and dielectric constant (Dk) of the insulating material between them are carefully controlled, the traces will operate at a specific impedance level. This will help to eliminate any signal reflections from these traces and is another important part of controlling the signal integrity of the board. The challenge here is to configure the planes in such a way as to satisfy both the power delivery requirements and the microstrip or stripline configuration requirements.

Successful PDN design requires the even distribution of the many different power and ground nets to the different devices on the board. While it would be convenient to have a separate plane layer for each required voltage, the number of these nets usually exceeds the available layers in the board stackup. To solve this, PCB designers will often split the planes so that they can route multiple power or ground nets on a signal plane layer. The challenge, though, is that while the split plane does efficiently distribute the different power and ground nets, it also creates additional barriers for the signal return paths. Therefore, the PCB designer needs to design their split planes to evenly deliver all the power and ground while at the same time preserving the clear return paths needed for sensitive high-speed signals.

With all the different design requirements associated with creating a good PDN, it is helpful to have a good design rules and constraint management system like what is available in Allegro PCB Designer. As you can see in the picture below, with a constraint management system, you can set up your power and ground nets with the width and spacing rules that you need for your power net classifications

There’s still more to what is required of a good PDN on a circuit board. Next, we will look at some of the manufacturing requirements that PCB PDN designers need to be aware of.

The Cadence Allegro PCB Designer constraint manager menu showing the power nets

Using a design rules and constraint manager to set up the routing width of a power net.

PCB Manufacturing: Another Aspect of Your PDN Design Guide

Although a good PDN design may require the use of a non-symmetrical board layer stackup, some manufacturers will have a problem with that configuration. PCB fabricators usually prefer for the top layers of the board layer stackup to be mirrored in the bottom layers to create a symmetrical layer stackup. With the temperatures and high pressures of the fabrication process, the combination of an uneven layer stackup can lead to warpage of the board. The larger the board, the more pronounced this effect can be. Board warpage can put stress on the thin metal traces used in long, high-speed transmission line routing as well as on solder joints.

To avoid these problems, circuit board designers should consider the following:

  • Keep the power and ground plane layers symmetrical in the board layer stackup, if possible to do without compromising signal integrity.

  • Make sure that the prepreg and core layers are also symmetrical in their thickness throughout the board layer stackup.

  • Try to keep the densest copper layers of the board in the center of the stackup. Again, this has to be done in conjunction with good signal integrity practices.

  • Use the same copper weight for the different plane layers.

  • Consider adding metal fills (copper pours) in areas of the board that don’t already have much metal on them.

  • Most importantly, check with your PCB manufacturer on your proposed board layer stackup before you commit to it in order to ensure that they will be able to fabricate it.

In addition to what we’ve already discussed for good PDN guidelines, don’t forget some of the basic rules of working with large areas of metal on a printed circuit board. First, use thermal heat relief pads for your components to aid in soldering. Those large areas of metal will act as a giant heatsink and pull the heat and the solder along with them. Second, for surface mount parts soldered to exterior planes, don’t forget to manage their connections as well. Again, the solid areas of metal will act as a heatsink and cause a thermal imbalance. This imbalance could cause small two-pin passive parts to stand up on end during solder reflow just like a tombstone.

There is a lot to know about in order to design a good PDN on your circuit board, and fortunately, there is plenty of help available to you in your design tools.

The IR Drop Vision in Cadence Allegro PCB design software

Tools such as the IR Drop Vision in Cadence Allegro can help plan out an effective PDN.

Leveraging Your PCB Design Tools to Create the Best PCB PDN

There are many different features in PCB design tools that can help with the creation of a good PDN for your circuit board design. One such tool from Cadence is the IR Drop Vision integrated analysis tool available in Allegro PCB Designer. This tool lets you see the power integrity status of your PCB design in real-time. It will show you pins that are not receiving a sufficient amount of voltage on a power net so that you can make immediate corrections to your layout.

Along with powerful shape editing features to configure your power and ground planes the way you need them, plus that advanced constraint manager, Allegro PCB Designer offers you a truly powerful platform for creating a well-performing PDN. You can find out more information on how to create an effective PDN by looking at this E-book.

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

 

About the Author

Cadence PCB solutions is a complete front to back design tool to enable fast and efficient product creation. Cadence enables users accurately shorten design cycles to hand off to manufacturing through modern, IPC-2581 industry standard.

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