I am designing a keyboard that connects through USB-C to a computer and has Aluminum chassis. I am dealing with ESD protection for the enclosure itself and some exposed components (that have a "shielding" pin for this purpose).

As I can't guarantee the keyboard to be connected to the computer with a shielded cable, I suppose I need to dissipate the ESD into GND. My thought is I need to do this near the USB connector through an ESD capacitor. So having separate shielding traces or layer that connects the exposed parts and the ESD capacitor seems logical.

However, I am getting confused because many people are recommending having an earth ground for this purpose that connects just to the connector shielding and not the GND pins on the connector. And I have never seen such "ESD" traces or layer wired through a board just to be connected to a normal GND near the board connector.

Can someone clarify?

  • 1
    \$\begingroup\$ As you say, separate ESD traces are almost unheard of. Normal course of action is to have a local "earth", which if you don't have access to ground is for example created locally with Y-capacitors to a good enough reference for things like EMI and ESD. Have you tried to just sink ESD via TVS or similar to your local GND as-is? If you passed without issues, it's job done for the unpowered state. \$\endgroup\$
    – winny
    Jul 25 at 14:28

3 Answers 3


As I can't guarantee the keyboard to be connected to the computer with a shielded cable, I suppose I need to dissipate the ESD into GND.

The ESD current will need to go somewhere, trace it through your design. So if the unit is plugged into the wall or a computer, if the user has a high voltage an initiates an ESD event, the current will travel from wherever the user touched and through the ground and out through the USB. ESD can also jump through plastic and cable insulation to reach conductors so also keep that in mind

It could also work the other way around if the user touched the cable, but the cable is shielded so the most likely path to ground will be down the shield and back to the charging computer device.

What you need to do as a designer is shunt ESD away from your device, especially any sensitive electronics. If you don't have a chassis you can use ground but this could create problems as common mode voltages on ground could exceed the voltage of a microprocessor (momentarily) and create latchups or errors in your device. With a USB device, the best path is to shunt ESD on a chassis and out through the shield of the connector. If there are other cables they should do a similar pathway. You can create a chassis ground on your PCB that is separate from regular ground, also this will need to be compatible with FCC considerations.

It really depends on what electronics are accessible (touchable) to the user and what routes the ESD will take to get to ground.

For example a device with a screen may need some kind of conductive gasket tied to ground or chassis ground (in this case the so the ESD gets shunted to ground and not down the digital lines to the screen).

  • \$\begingroup\$ Can you please elaborate on the part "create a chassis ground on your PCB that is separate from regular ground, also this will need to be compatible with FCC considerations"? I have a metal chassis so shunting the ESD away from electronics is not a problem. However, the cable between the computer and the keyboard might not be shielded. So the question is, how to make the chassis GND and how to properly connect it to the real GND. My assumption would be an ESD capacitor and 1M resistor near the USB connector that would connect the chassis GND and normal GND. Im also interested in the FCC. \$\endgroup\$
    – Marek
    Aug 1 at 14:41
  • \$\begingroup\$ If the cable is not shielded if you device is an unintentional radiator the cable could become an antenna. It really depends on what the device has in terms of clocks/PWM that could generate radio in the FCC bands. I usually prefer a direct path to ground, you can rent ESD guns and test you device against ESD susceptibility \$\endgroup\$
    – Voltage Spike
    Aug 1 at 17:44

ESD occurs when a body have a differential potential to a device. It can occur with a grounded or not device (connected or not to your computer). The problem with ESD is not that it elevates your rail, it is that the discharge is high voltage and pass trough sensitive component. For ESD challenge, you should ask yourself where the ESD discharge can come from? Then on every of those point you need to figure if it's an issue or not. Typically, anything enclosed is not at risk (shielded or not). If your enclosure is conductive, you can select where the point of contact is (usually the GND) so it's not an issue and if your case is non conductive, well the ESD discharge won't pass.

Where issues comes in is when you can't cover the access. This happens typically near switches, displays and connectors. At those entry point you need to place ESD protection.

For the bare board, basically an ESD discharge can occur everywhere so protecting them is almost impossible.

  • \$\begingroup\$ My question is more about not having the shielded cable. Yes, I have a chassis and I try to shunt the ESD away from the sensitive parts. However, sooner or later, I need to connect the chassis to the USB GND as well, not just the shield. I had some guy advise me to keep clearance of 1.5mm (consumer) or 3mm (industrial) between GND and shielding. Which I am not sure how am I supposed to do on a small USBC connector, however, he didn't tell me why, just that is a standard clearance. \$\endgroup\$
    – Marek
    Aug 1 at 14:46
  • \$\begingroup\$ So you just answer the first part, where this ESD discharge come from (essentially, the end of your shielded USB cable). The next question is, is it an issue if an ESD discharge occurs here. If you are not protected, you will have a great differential of potential between two signal (yes it is an issue). From here, you can take two approach, either preventing the discharge (shielding) or protecting against it (use of TVS). The first might be hard to implement because of the connector definition. The second is to add TVS diode on those lines. This is the generally used solution for that issue. \$\endgroup\$
    – Julien
    Aug 1 at 15:40
  • \$\begingroup\$ You won't be able to always shield yourself of ESD. The first step is always to add a shield (as mentioned, bare board have so many entry point that protecting it is almost impossible). The enclosure acts as this shield (conductive or not). Then, in most case, you will have openings. All these opening are subject to ESD discharge. Some of these discharge will be prevented (using tapes or modification of the enclosure) other will be protected against a discharge. If, for any reason, none of these can be applied, you might have warnings telling you that this port is not ESD protected (Like VNA) \$\endgroup\$
    – Julien
    Aug 1 at 15:45

As I can't guarantee the keyboard to be connected to the computer with a shielded cable

This is a root problem.

Add in the product manual that a compliant USB cable is required, and operation without such is not guaranteed to function, nor warrantied in case of failure.

As for routing, ESD can't be "wired" anywhere because it is a very fast signal. Shunting it into a wire merely turns the wire into an aggressive antenna. If that wire is a trace over ground plane, its characteristic impedance couples ESD into ground anyway (as well as anything else along/nearby the trace).

It's all going to ground anyway, so put it there first and foremost, don't beat around the bush. Surround your circuit with ground. Whether that means extending the ground plane outward somewhat beyond the component area (with top/bottom ground exposed in case of direct strike), or extending it further as shield cans over parts of the circuit, or a whole enclosure.

If ESD is shunted around the board entirely (by flowing along contiguous / bonded panels), then it can only enter on explicit connections made through it -- connectors.

A wave analogy is very apt for this: ESD has a risetime of mere nanoseconds, so the wavefront really does "wash" over mechanical features in real time, like waves crashing around a lighthouse. For a smaller device (10s cm), it may be more like a rising tide than a crashing wave, but the overall effect is no different: the electric field can be 100s of V/cm along a conductor.

Whatever the case, as the chassis gets energized by the propagating wave, wave energy diverts down connectors and cables, radiates off back into space, bounces around between cables (standing waves), etc. Where it transitions onto cables, the equal-and-opposite-reaction carries some fraction of ESD energy into the connector, thus the connector (and signals within) must be bonded to the shield, whether by filters and ESD diodes (unshielded cable) or direct connection (shielded). Or it must be isolated so well that it draws no current in response to the incident wave, but this is very difficult indeed to implement. (Ethernet for example has the best shot at this, being a transformer-isolated interface -- but for a variety of reasons, the shunt strategy was chosen anyway.)

Other approaches -- I have seen designs with, for example, a mostly-isolated loop around the periphery of a PCB, tied common at one point, intended as an ESD sink. I haven't seen any measurements or modeling to suggest that this guard-ring strategy is actually useful, or in what circumstances. (If you [reader] have some, feel free to comment!) Conversely, I haven't done any tests/measurements myself to prove it is actively harmful. I do suspect it is overall more harmful than helpful, but not by enough that it will be easily uncovered as such. That is: more a matter of degree, than an outright fail.

So these are the land of opinion rather than fact. Unfortunately, opinion and superstition are altogether too common in the field of EMC. The reality is: while these things can be modeled and measured, it is often at great expense (e.g. PCB/enclosure full-field simulation, or painstaking setup and testing in the lab), and when project budget and timeline doesn't allow for such, us engineers must fall back on secondary information -- say, application notes instead of research papers; or word-of-mouth and opinion entirely(!). There is good work out there (e.g. Henry Ott's books), but one must take the time to digest them. EMC is a complex topic, requiring holistic evaluation of a system; one should not expect to solve a problem without significant research and understanding. (Or a whole lot of luck.)

USB itself is unfortunately a frequent victim of opinion and word-of-mouth. The simple fact is this: >99% of applications don't need (or can even make use of!) galvanic isolation between signal/power ground and shield, and ground and shield can be tied directly to ground plane at the connector. The connector housing must be tied with the enclosure, if a metallic enclosure / shield is present, and this shield connection causes incident ESD waves to bounce off and wash around the shield, preventing them from entering the board and signals (more precisely: greatly attenuating, perhaps in the 40-80dB range, depending on shielding effectiveness). When a metal enclosure (with EMI fingers bonding it to the connector) is present, the PCB GND/connector shield connection can be made at some distance (mounting screws, say), or through bypass capacitors (to afford some galvanic isolation / DC offset, particularly in multi-card assemblies, like PCs). When a metal enclosure is absent (bare board in space -- plastic enclosures are transparent for EMC purposes), a high quality AC ground connection must be made to the shield. (Again: bypass caps are acceptable, if many are used in parallel, distributed around the connector. Most likely, this isn't necessary, and it can be direct grounded.)

I don't know if this particular issue [miscommunication / misinformation on USB grounding] has affected your design, but it seems to happen often enough to be worth including here anyway.

There are also USB HMI devices, which use a low-baud mode, and can be carried on unshielded cable. In this case, adequate filtering and protection must be provided to divert ESD around the interface device(s), as all wires in the cable shall be considered suspect for purposes of ESD. A plastic enclosure is recommended, to prevent direct ESD strikes in the first place. If metal is chosen, it should be grounded to circuit ground, as an extension of the PCB ground plane.

  • \$\begingroup\$ My approach was close to the "isolated-loop around PCB" you mentioned. If I understand this correctly, you are suggesting connecting the chassis to GND near the USB-C connector (GND and Shield of the connector get connected together). So that the ESD sinks to the GND or cable shield (if available), but doesn't affect the electronics components as much as it would if it would be tied all around the board or at another point? \$\endgroup\$
    – Marek
    Aug 1 at 14:59
  • \$\begingroup\$ Yes, and in contrast to the "loop around PCB" method, notice the enclosure surrounds the board (or at least has few gaps/openings, and preferably at some distance from the PCB), so ESD is diverted along its outside surface (Faraday cage); whereas a wire (or trace) loop is fully exposed as an antenna. \$\endgroup\$ Aug 1 at 15:04
  • \$\begingroup\$ My thought was to make the "loop" (1.5mm clearance from GND) for on-PCB components that are exposed through the metal chassis (and have a shield pin). And tie this loop (or "ESD trace"), the chassis, and the GND near the connector. Though I am not sure if I should make the connection directly or through an ESD capacitor and 1M resistor in parallel. \$\endgroup\$
    – Marek
    Aug 1 at 15:17
  • \$\begingroup\$ I would have to see a diagram of what the exposure and components are like. \$\endgroup\$ Aug 1 at 21:07

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