First time workbench / ESD mat / grounding questions

[mannyace]

Hi folks,

I’ve got a project (some audio equipment, through-hole components, nothing high powered) at home that’s going to require some soldering. It’s been years since I’ve done any soldering work and at the time, I didn’t have to worry about setting up the workbench or buying the equipment, so that’s something that I had to research about and that’s how I ran into EEVB. I sure am glad I did.

I’ve already got a workbench but up until now, it’s been for regular household stuff, not electronics. I’ve got some plastic storage containers with sliding drawers (holding everything from screws to light bulbs), and also a steel toolbox with wrenches, screwdrivers, etc.

My main questions are: Would it be bad if I bought a big ESD mat and put the plastic storage and steel toolbox on the mat? Or would it be better to get a smaller mat?

I’m not concerned with storage of lots of ESD sensitive parts. I’ll use antistatic bags for any parts that I might have on hand, but there won’t be a lot of them anyway. I’m mostly concerned with not having static discharge while actively working. I want the components/board I’m handling to be safe and of course for me and anyone else around to be safe.

That brings me to my second set of questions: If the bench has an ESD mat, hooked up to electrical ground, is there any danger to anyone in the room as they interact with the mat? If I’m grounded, is there anything I need to be careful about? Since the room is a shared space, is there any danger for anyone else who isn’t grounded (for example, if they lean against the bench while no work is happening?) If anyone puts down or picks up any non-ESD-sensitive items (for example, a hammer), is there any danger related to the mat being grounded?

Thanks,
Manny

[Matje]

[...]
My main questions are: Would it be bad if I bought a big ESD mat and put the plastic storage and steel toolbox on the mat? Or would it be better to get a smaller mat?

No problem. All that might happen is that heavy parts will make indents/depressions in the mat.

You might consider getting a smaller mat because they are quite pricey for just having something sit on top of it permanently.

[...]
That brings me to my second set of questions: If the bench has an ESD mat, hooked up to electrical ground, is there any danger to anyone in the room as they interact with the mat? If I’m grounded, is there anything I need to be careful about? Since the room is a shared space, is there any danger for anyone else who isn’t grounded (for example, if they lean against the bench while no work is happening?) If anyone puts down or picks up any non-ESD-sensitive items (for example, a hammer), is there any danger related to the mat being grounded?

No danger. The mat is not exactly what one would call conductive (just a tiny little bit) and the grounding lead will have a large resistor (order 1 MOhm) in it precisely for security reasons.

Take care to get a proper silicone rubber mat, not the cheap PVC stuff that can be easily burned (raising a nasty stink) with a soldering iron.

[mannyace]

Thanks. I'll be sure to pick up a rubber mat.

My bench isn't that big but I do want to work on 19" rack equipment so I can either get one that is just big enough, but then I won't be able to spread out, or I can get one that is bigger but will have things sitting on it. The pre-sizing isn't great also as I don't want the tool box sitting half on the bench and half on the mat, as it might wobble, or the drawers might slide on their own. I figure if I go big, I'll never have to buy another, and at least the mat won't slide around with the weight on it.

Thanks again,
Manny

[Fsck]

actually you shouldn't find a resistor in the grounding lead of an esd mat - the mat itself has such high resistance that adding 1M would be a negligible increase. only ESD wristbands have a resistor in their grounding leads.

[mannyace]

Thanks for the heads up. The mat I'm looking at comes with a grounding cord that doesn't have a resistor in it. I was going to purchase another cord separately but what you said makes sense and now I don't need to buy an extra one.

The cord it does come with doesn't come with banana jacks though. Is that a deal breaker? Or is it safe to use a wrist strap and another snap connector in the mat, or a wrist strap that has an alligator clip and have clip it to the mats edge?

Thanks,
Manny

[timb]

You should be able to find a grounding lead with a snap connector that has a (pair) of banana jacks on it. Personally, I don't trust connecting the wrist strap to the snap on the mat, I'd much rather it connect to the same grounding point as the mat (via the above mentioned grounding lead with banana jacks), that way I know there's a direct conductive path.

That's another tip, be sure to periodically do a resistance test between the metal plate on your wrist strap and *the ground of an outlet* to make sure it really is earthed. It should read in the megaohms range. (Wires periodically break, things go wrong. It's a good idea to test so you know it's working.)


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[mannyace]

Thanks for the info. Sounds like I've got some (online) shopping to do.

Manny

[Matje]

actually you shouldn't find a resistor in the grounding lead of an esd mat - the mat itself has such high resistance that adding 1M would be a negligible increase. only ESD wristbands have a resistor in their grounding leads.

Hmm, that may differ by make and model. The grounding leads I have (with either a snap fastener or a banana plug at the end) all show 1 MOhm resistance. It may be to guard against people doing strange setups, after all it doesn't cost much and doesn't impair function either.

[nanofrog]

Would it be bad if I bought a big ESD mat and put the plastic storage and steel toolbox on the mat? Or would it be better to get a smaller mat?

Not at all. Leaving such items off of the matting and using a smaller mat is just less expensive.

Personally, I'd recommend getting such items off of the desk surface entirely (physical desktop real estate is too precious). This is what shelves are good for. 

If the bench has an ESD mat, hooked up to electrical ground, is there any danger to anyone in the room as they interact with the mat? If I’m grounded, is there anything I need to be careful about? Since the room is a shared space, is there any danger for anyone else who isn’t grounded (for example, if they lean against the bench while no work is happening?) If anyone puts down or picks up any non-ESD-sensitive items (for example, a hammer), is there any danger related to the mat being grounded?

Resistance in the ground path slows the discharge rate, so if you or someone else in the room comes up that's carrying a static charge and touches the mat or wrist strap, it will reduce the intensity of the shock sensation, or possibly even eliminate it. Which is why you want to "strap in" before touching anything. 

As always, be careful as to what you're making contact with. A proper bench mat and wrist strap may reduce the intensity of the shock, but that doesn't mean it won't hurt like hell when you make a mistake (mains, but more commonly IME, are charged capacitors). So use due caution around either (discharge caps when possible). If you're not working on anything, shut the power down (eliminates the potential for shock; this includes covering up or discharging capacitors). As long as any tool carrying a static charge is placed on the mat first and/or picked up by a grounded operator (conductive part of tool), it's charge will be conducted to ground without being able to cause any damage to sensitive parts.

Now as per your location... If you go into your user profile and enter in your country, it's flag will display beneath your userID. Makes things easier for everyone (you don't get the endless "where are you located" questions, and others can post relevant links).

For example, you indicate the standard sizing is a bit of an issue. But if you're in the US or CAN, there's a seller on eBay that will do custom sizes if you contact him (seller is located in Canada). Pricing is decent as well, though you can do better on the right sale or closeout (i.e. all-spec.com).

actually you shouldn't find a resistor in the grounding lead of an esd mat - the mat itself has such high resistance that adding 1M would be a negligible increase. only ESD wristbands have a resistor in their grounding leads.

FWIW, most kits come with mat ground wires that do contain 1M resistors (some have a plastic block w/ 2 banana jacks for wrist straps, others don't but both types contain the 1M resistor). Easiest way to spot them is by color (wire = black, then it's sporting a 1M resistor in the path). Green w/ yellow stripe = no resistor.

Keep in mind, that the top dissipative layer is highly resistive (typically in the 10^8 Ohm range), while the bottom layer is conductive (typically 10^5 Ohm). And the top layer bleeds to the bottom layer, which is then conducted to ground through that cable. Now keep in mind, that if you end up paralleling the wrist strap at 1M to the conductive layer of that mat at 20k (quick measurement on one of mine), you've only got ~19.6k between you and safety earth.

Don't know about you, but I'd rather have ~500k in that instance by using 1M resistors on the mat as well as the wrist strap. 

[mannyace]

Thanks for the info. I ended up getting a mat, a cord and wrist strap, both with a resistor each, from HMC. I went with Desco branded stuff since that's what we use here at work.

If I have more questions once I get it all delivered, I'll post again.

Thanks for the help everyone.

Manny