I have a board design inside a metal case (video processing MCU), and it has one external USB port with ESD protection on all pins. It's battery powered and handheld. The exterior metal case is not tied to the ground of the internal circuit board. In fact the board essentially floats inside the metal housing on plastic standoffs, with the processor pushed up against the metal through a gap pad for a heat sink. I don't have the option to go to a plastic case.

The problem is that occasionally when someone touches the unit, or hands it to another user it goes bonkers. I suspect ESD, but I'm not sure of the right path to fix it. If it were a plugged in device I'd do ESD protection to ground. But in a portable case I don't see where I can redirect the energy to. I'm left thinking about using an ESD protection coating that greatly increases the resistance of the outer case.

But is there another option I'm not thinking of?

  • \$\begingroup\$ I thought what you do is to connect the ground of the circuit to the case so it stays at a fixed potential relative to the circuit so the RFI from the shock doesn't muck around with. But I've never tested it. I'd like a solid answer to this too. The solution might be to paint or powdercoat it with something non-conductive or dissipative. Of course, it's also possible that if the spark is large enough it's actually jumping from your enclosure to somewhere on the board bypassing the ESD protection. \$\endgroup\$ – DKNguyen Mar 14 '19 at 18:55
  • \$\begingroup\$ Unclear if your unit is connected via USB cable to a host, or not. Or to some "video" unit, since it is "video processing". Please clarify. \$\endgroup\$ – Ale..chenski Mar 14 '19 at 19:12
  • \$\begingroup\$ @Toor Yes I also think it might be jumping across the enclosure. No it is not connected to any cable at time of failure. I was only trying to explain that there were some openings to the case as well. \$\endgroup\$ – confused Mar 14 '19 at 20:25
  • \$\begingroup\$ It's easy enough to try the coating. Get silicone conformal coating spray and coat the inside or outside of the enclosure, or the PCB (which would also protect the PCB at the expense of making it more difficult to modify although you can solder through silicone conformal coatings). If you chose to coat the inside and it still messes up then that means the RFI is screwing up your board rather than a spark jumping to the board. If you coat the outside you won't be able to differentiate. \$\endgroup\$ – DKNguyen Mar 14 '19 at 20:39
  • \$\begingroup\$ You might also try connecting the enclosure to the ground of the circuit through A TVS diode (maybe the power rail as well?) That should give it somewhere to dissipate. I think that might help dissipate the spark if it is jumping from the enclosure to the PCB. \$\endgroup\$ – DKNguyen Mar 14 '19 at 20:39

Whatever your doing, if the device is in handheld mode or plugged into the USB, and ESD currents should be redirected from the PCB on the inside. A metal case is better than a plastic one, and the problem is most likely due to the inputs.

A PCB surrounded by a metal case is in a very good condition, as the case acts like a faraday cage. An ESD event or spurious voltages will met the conductor of the case and be redirected. The problem is most likely due to the inputs. Any inputs to the PCB should be protected with diodes, to shunt any high voltages to the chassis of the device. The device chassis could be considered a ground while floating or in 'handheld mode', we do this with cars all the time.

You should have some kind of ESD protection on the USB end also and on the video end. The shield of the USB should be connected to the chassis. The 'video end' most likely has shielding, if it does these should also be connected to the chassis. That way if ESD hits the chassis with USB or video connected, the current pathway would be through the chassis and out through the shield.

You may also want to preform an experiment. Get aluminum foil or copper tape and wrap the device ports (video and USB) (but not the chassis). Then handle the device in a similar manner when the failures were experienced. If the device fails, then it is probably not due to ESD and may be something else. My thoughts are that the device would be fine in this situation and will operate properly.

It may also be advantageous to experiment with the chassis tied to the PCB ground at a reasonable point.

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  • \$\begingroup\$ Would tying the chassis to ground just cause the entire circuit to float up to the chassis potential when shocked? Or would you still blow something due to finite time it takes for the everything to propagate through the PCB all the way to the positive rail (though I assume a TVS diode between GND and PWR would protect against that)? That's why I was wondering if connected to ground through a TVS diode wouldn't be more effective and also give the energy somewhere to dissipate. \$\endgroup\$ – DKNguyen Mar 14 '19 at 20:57
  • \$\begingroup\$ I can't really say without knowing more about your setup. If your device is being knocked out from ESD, the most likely mechanism is ESD currents traveling across the PCB itself, and causing a spike on the ground or VCC and causing metastabilty in the electronics. I can also only speak for ESD principals and the things I have learned. You want the inside of the case and the PCB ground to be the same potential, which minimizes capacitance between the case and the PCB. (Most any two conductors separated by distance form a capacitor) \$\endgroup\$ – Voltage Spike Mar 14 '19 at 21:13

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