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Suppose you have a static/ESD shielding grey bag housing electronics sensitive to ESD. I believe that this type of bag has a static dissipative surface and a conductor on the inside serving as a Faraday cage. While these bags offer a high level of protection, they don't shield the contents 100% from static electricity.

Now suppose you have a setup where you have a static dissipative mat, you are connected by a static dissipative wrist strap to the mat, and the bag is placed by you onto the mat while wearing the wrist-strap. However, the mat is not grounded externally to a true ground, but connected to a metal object, e.g. a metal computer case. As I understand it, after being placed on the mat, you, the metal object, and the bag are at the same voltage/potential (not zero, and so different from the question in How should I use a static-shielding bag prior to opening, and why?)

My question is: is it safe from an ESD standpoint to then remove the ESD sensitive electronics from the bag (possibly handling them via a grounding plane on the electronics)? In other words, are the contents of the bag at the same potential as your hands or the mat? If it's not safe, is there a safe way to do this without potentially causing damage, or does the mat necessarily have to drain charge to a true ground?

The reason I ask is that I see this ESA setup recommended here and there in this and other stack exchanges (https://superuser.com/questions/975427/how-to-properly-use-an-antistatic-wrist-strap-when-working-on-a-desktop-pc), and the ANSI standard S20.20 talks about equipotential bonding.

I am wondering if someone would be able to answer with the appropriate theory, whether or not charges between one's hands and the contents of the bag will equilibrate/equipotential bond when handling a static shielding bag or when it is placed on a static dissipating surface.

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  • \$\begingroup\$ I believe the "local" air itself will quickly (if it hasn't beforehand) bring the board/chip to nearly the same potential by simply leaking charges through the air. If picky, just wait a few seconds at most for that after opening the bag. (You should have the near optimal humidity of 50%, though, if possible. That helps a lot for this effect.) Of course, if you place your setup at one end's potental of a Tesla coil, then all bets are off when you open the package! \$\endgroup\$
    – jonk
    Jul 12, 2020 at 4:26
  • \$\begingroup\$ When a device is contained in a casing, only protected I/O is exposed to ESD, and all circuitry are safe without needing to earth anything. In your case, the equipotential conditions matter, not the earthing. HOWEVER, when a passer-by even just points at a chip on your board, that person must be at the same potential, and that is best achieved by earthing everyone and everything throughout the lab. \$\endgroup\$
    – P2000
    Jul 12, 2020 at 5:57
  • \$\begingroup\$ I tend to "pour" bag contents onto an ESD mat which I am in safe equilibrium with. You get any protection (however much that may be) that is mentioned by others from holding the bag and if there is difference in potential between contents and your system then the high resistance mat surface provides a gradual equalisation path. \$\endgroup\$
    – Russell McMahon
    Jul 12, 2020 at 11:45

2 Answers 2

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I've successfully used this mindset --- get the person, the bag, and the sensitive circuit to be at the same potential --- with NO ESD damage, on numerous sensitive projects.

I guide people with ---- find the GROUND pin on the PCB, and first touch that.

Now you can remove or insert whatever you want from or into the PCB, safely.

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I tend to "pour" or slide ESD bag contents onto an ESD mat which I am in safe equilibrium with. You get any protection (however much that may be) that is mentioned by others from holding the bag and if there is a difference in potential between contents and your system then the high resistance mat surface provides a gradual equalisation path.

Ideal anti-ESD mats have high resistance per square / low conductivity.
For many decades I used mats made from "reject" Carbon loaded Butyl Rubber intended for roofing applications. When a BR roofing pack is transported they usually have outer "cover sheets" of the same material that is sold at low cost due to cosmetic damage.
This material has quite high conductivity when sharp pointed probes are pushed into it due to the carbon loading, but light surface contact tends to have substantially higher resistance.

One may expect that such higher conductivity core mats might run a greater risk of causing ESD damage during charge transfer due to higher discharge rates. However, despite this material seeming to be less than ideal I have had no ESD problems with it.

When working with a somewhat conductive surface one is advised to not place operating PCBAs on it directly, especially not those employing high voltage :-).

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  • \$\begingroup\$ "one is advised to not place operating PCBAs on it directly", and, as I learned, one is advised not to solder on it either. \$\endgroup\$
    – P2000
    Jul 12, 2020 at 19:13

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