Using ESD Grounding Techniques to Protect Your PCB from Electrical Damage

Whether you're repairing a computer or you're handling your PCB, no one wants to have their electronic devices ruined by ESD. Think back to the grounding wristband you would use when installing parts in a computer; the connection between your body and a large chunk of metal in the chassis should be your clue as to how you can protect your devices from ESD events. The function of that connection is to provide a low-impedance path to a large charge reservoir for any ESD pulse that passes into your device.

Any conductor that you use to absorb an ESD event functions as a safety ground, and the most important aspect of ESD protection is implementing a grounding strategy in your PCB layout and your enclosure. The enclosure and PCB layout must work together to provide a preferable path for any ESD pulse away from your components, and that means you need to create the PCB layout around your grounding connection and strategy. In this article, I'll outline the important grounding techniques used to ensure your board can withstand an ESD event.

Grounding Methods for ESD Protection

There are a few important methods that can be used to help protect your design against ESD. We typically associate ESD with large dielectric breakdown events that can produce arcing. However, even an ESD event that produces an electric shock, such as discharge from a human finger, could reach kV levels. At these voltages, taking advantage of ground to divert ESD away from your components is the most effective way to ensure protection. This is essentially the strategy that is used in most systems that could experience ESD and be put at risk of damage.

To keep your electronics protected from ESD, there are three important guidelines to follow in your PCB layout:

1. Use ground planes
2. Use a chassis ground if available
3. Use ESD protection components

Use Ground Planes

The major function of safety conductors used for ESD protection is to provide a path for the current from ESD to flow so that it does not flow into your components. This is also the function of protection components as described below. A large ground plane in your PCB can provide just such a protection mechanism, particularly when used with ESD protection components. Ground planes are also important more generally; most designs should use ground planes on the layer adjacent to your components as signals, such as in this standard 4-layer stackup.

Some devices will only have a ground plane as their protection mechanism; mobile devices are a great example. However, some devices will have access to an additional safety ground region in the PCBA chassis.

Use a Chassis Ground

This is where the computer tower case grounding strategy should become obvious.

Enclosures can be used as a ground when the have some built-in conductors. These enclosures serve a dual role of providing a mounting point for a board through mounting holes, as well as serving as a safety ground for the system. By "safety ground" we mean that the enclosure can receive an ESD event and withstand the received energy without failing or delivering that energy to the components. The best way to ensure this in systems with a chassis ground is to include two low-impedance connections:

• A direct connection between the chassis ground and earth
• A low-impedance connection between the system ground and chassis

The first case applies when a device is being powered from a utility outlet (could be AC or DC), or in power distribution equipment that can run a connection direct to the earth (e.g., from a utility pole). Read more about this first case here. The second cace applies regardless of the presence of an earth connection.

The final important point here is how the chassis ground is integrated into the should not be used to carry signal return currents, it should only be used to shield the system from radiated EMI and ESD from external sources. Some systems like desktop computers will use a chassis ground region as a piece of metal in the enclosure, but the user will not interact with the chassis ground because this ground is concealed from the user by an insulating material (e.g., the plastic casing in the enclosure). In this case, the chassis ground could carry a return current without creating a safety issue for the end user, but in this case a ground plane on the board will often be the lowest-impedance, most direct path to the bridged earth/chassis connection point.

Use ESD Protection Components

Ground is by far the most important part of providing ESD protection, but there is still a possibility for exposed signal lines and pins to receive strong ESD that can damage components. To protect these vulnerable lines, ESD components are used. An example with TVS diodes is outlined in this blog.

There are three broad classes of ESD protection components that are suitable for different protection voltage ranges and response times: TVS diodes (unidirectional and bidirectional), relays (normally open or normally closed), and gas discharge tubes. These three component categories are outlined and summarized in the following table.

The response time here is one of the most important specifications. One of the characteristics that makes ESD protection components operate and provide protection is their ability to switch into a conducting state very quickly, which then can divert the current to ground through a low-impedance connection.

During the rising edge of the ESD pulse, there is still a possibility that the pulse damages the component before the protection component can divert the pulse. This is why a faster response time is preferable, as well as lower clamping voltage when available. Another strategy if there is an expectation of strong ESD is to use multiple components in parallel on the vulnerable conductors. For example, gas discharge tubes can be used in parallel with TVS diodes, such as on an RS-485 interface.

With the complete schematic capture and PCB layout tools in Altium Designer^®, you can design your ESD grounding strategy and place components to ensure your device will operate reliably in tough conditions. When you’ve finished your design, and you want to release files to your manufacturer, the Altium 365^™ platform makes it easy to collaborate and share your projects.

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