Is it generally safe to rest a low voltage powered up circuit board on an anti-static bag when debugging? A simple meter reading suggests it is a good insulator, but it must conduct to some extent?
Ditto on anti-static mat on the workbench
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asked Nov 2, 2015 at 10:55
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\$\begingroup\$ Why would anyone do this? \$\endgroup\$– Andy akaNov 2, 2015 at 11:17
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1\$\begingroup\$ Working on the desk with a PC \$\endgroup\$– Dirk BruereNov 2, 2015 at 12:14
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4\$\begingroup\$ When I was a kid, hard drives arrived in metalized bags printed with "do NOT run the disk in or on this bag". \$\endgroup\$– Agent_LNov 2, 2015 at 15:45
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2\$\begingroup\$ I once powered a circuit (before I learned anything about electronics) sitting on anti static foam. It made "poof", there was a hole in the foam and I turned the power off very quickly (was running directly from 230 V mains). I learned my lesson that day :D \$\endgroup\$– ArsenalNov 2, 2015 at 16:26
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2\$\begingroup\$ Our company tried storing a PCB that had a coin-cell battery soldered to it in a static bag. We found the batteries were becoming discharged while sitting on the shelf in the bag. \$\endgroup\$– SteveNov 2, 2015 at 18:55
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2 Answers
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There are different bags available. Some of them are insulators (nearly, see below) and only prevent the build-up of static charge. Others are conducting (grey-metallic ones typically or black ones).
I wouldn't place any powered electronics on conducting anti-static bags or mats. Even the fairly high resistance can have a big influence on the electronics (depending on function and design).
Low powered stuff often works with fairly high resistance values, so the effect of parallel resistance there is quite profound.
I often use a sheet of paper to put my circuit boards on (note it can be charged, so make sure to wipe the charges off). Or just a holder where you clamp it from both sides so the main area has no contact at all (like when it is installed somewhere).
Update:
I'm not an expert on ESD and anti-static materials, let me try anyway:
Materials are divided into different classes depending on their sheet resistance (Ohms per square):
- \$10^{13}\$ and greater: insulators and basic plastics
- \$10^{9}\$-\$10^{12}\$: anti-static
- \$10^{5}\$-\$10^{9}\$: dissipative
- \$10^{3}\$-\$10^{5}\$: conductive
Everything below that doesn't belong in the field of ESD anymore. So the insulators and basic plastics are not able in any way to let charges move on their surface or through them. If they get charged, the charge stays there until it is dissipated in some way (discharge on you or an IC, in humid air it will decrease over time).
Anti-static materials have still a very high resistivity, so that you probably are not able to measure it with a normal multimeter. They are just a little bit conducting, so that the surface charge cannot build up. The triboelectric effect (charging through rubbing something etc.) is prevented. This gives a very basic protection. You can easily zap through a bag with a static discharge. These bags are usually pink in color, they are made of a pink polyethylene.
This is what I had in mind when I wrote "Some of them are insulators and only prevent the build-up of static charge." So it's not true. It's just that the resistance is very very high (gigaohm range) and unlikely to cause a problem, but if the resistivity is at the lower end of the range, it might.
I recommend watching the two videos from EEVblog: #247 and #250. Dave actually shows that the mats are not conductive enough to make problems with measurements.
One thing which is mentioned in the research is that the materials used do not behave as a simple Ohmic element, so the resistivity can be voltage dependent. The stuff gets usually tested at 500V to take that into account, but you never know.
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1\$\begingroup\$ I normally use a sheet of paper, but right now I have it sitting on the bag it arrived in. \$\endgroup\$ Nov 2, 2015 at 12:16
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\$\begingroup\$ I think the pink bags are the preventive insulator ones; the silvery or black ones, and ones with the grid pattern are conductive to some degree and I would NEVER run any electronics on them. \$\endgroup\$– Doktor JNov 2, 2015 at 17:54
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\$\begingroup\$ @Arsenal: Can you please explain how an insulating bag works to "prevent the build-up of static charge"? \$\endgroup\$– JS.Nov 2, 2015 at 18:06
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\$\begingroup\$ @JS. I updated my answer to be a bit more detailed and more correct (hopefully). \$\endgroup\$– ArsenalNov 2, 2015 at 20:25
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I would strongly suggest getting out of the habit of thinking of anti-static material as "safe" insulators. While that may be true enough for some materials at low voltages, all anti-static materials are conductors at higher voltages. That's what makes them anti-static. You don't know precisely what voltage below which they become safe to consider an effective insulator, so they aren't safe to use as such. Period.
We had a fire in one of our labs caused by this exact problem. People got in the habit of treating an anti-static mat as a safe place to terminate exposed banana plugs, and it was close enough for several years. Then they started testing a product for a European market with voltages 10 volts higher, and it allowed just enough current to heat up just enough overnight to start a fire. It's just not worth it when it's so easy to avoid.
