Without any connection to ground, the ability of the ESD wrist strap to bleed off static charge is limited. One thing you will see in older computer manuals is a direction to clip the ESD strap onto the metal chassis of the DUT. The reasoning is that as you bring your arm inside the case, you have entered a kind of Faraday cage where charges will be sucked out of your body and into the chassis (as the charge inside a metal shell always goes towards zero). It's better than nothing. The same effect can't really apply to an ESD mat as it does not surround your hands, but I guess you would bring the mat and body to the same voltage. You would still not be able to safely use grounded tools like soldering irons.
I think I got most of that but not sure that I understood this:
“You would still not be able to safely use grounded tools like soldering irons.“
Please say more on this. Is this comment a reference to part safety or operator safety (or both)? And is this comment in reference to my question about using the ESD mat without the wired connection to PE (or is the added risk with a soldering iron intended to mean with the mat wired to PE)?
The last thing I want to do is add an ESD mat to the bench and decrease operator safety.
I think he's referring to ESD safety, i.e. part safety, in that the point of a grounded ESD mat is to reduce to zero the potential difference (voltage) between the part and ground. A mat which isn't connected to ground (floating) has no way to equalize the potential. What I think helius meant is that while the mat may succeed in bringing your hands to the same potential as the part (because of incidental contact with it, or via a wrist strap), once you bring in your iron, which is at ground potential and not mat-and-electrofan potential, there's the risk that current will flow through the part, i.e. an ESD event.
The resistor(s) in ESD grounding serve to limit current, so that a) charges dissipate slowly rather than as energetic bursts, reducing risk of component damage, and b) to protect the operator, since the operator isn't floating any more. For example, if you were not grounded in any way (no ESD, on an insulating floor and insulated work area), and you accidentally touched a 230V live wire, nothing would happen to because there's no circuit. Conversely, suppose you were connected to ground
without an inline resistor, and then you touched the 230V live wire: you'd get a shock as the 230V flowed through your body from live to ground. Putting the 1+ megohms of resistance in series means no more than 230µA of current would flow, far too little to perceive, and far too little to harm you.
In essence, the resistor allows your ESD workspace to be a
de-facto insulator from an operator safety standpoint, while being a conductor from an electrostatic charge standpoint.
The last thing I want to do is add an ESD mat to the bench and decrease operator safety. I’ve gone decades (forever) with zero static glitches...
…that you know of. The insidious thing about ESD damage is that it's often subtle, leaving the component working, but weakened, such that it might misbehave in a subtle way, or simply fail sooner than it would have without ESD damage. (But long enough in the future that you don't make the connection with the ESD event, if you even had a way to detect the ESD event when it happened.)
So between the damage often being invisible, and the fact that ESD events nearly always go completely undetected since they're practically always invisible, the
only way to deal with ESD is comprehensive prevention.
Helius, if I've gotten any of this wrong, please correct me ruthlessly!
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