Work surface material & ESD hazard, what material is safe and which is not?

[David_]

Hello.

I wonder what kind of surfaces is to consider safe for electronic circuits, on my desktop where I work I have a wristband connected to a ESD mat connected to PE-ground and then my soldering station and some other things is connected to the mat so everything is at one and the same potential level or what its called(lack the language?).

But then sometimes I want to work with a microcontroller evaluation board while sitting with my laptop on a wood table covered with a glass sheet and I always wonder if glass is safe?

Then I am pondering buying a new desktop made out of cork, you know the same material often used to seal wine bottles.

And I don't really understand what it is that makes some surfaces worse than others and I think it is obvious that just because a material does not conduct electricity as such it don't mean it does not pose a potential hazard risk for ESD sensitive IC's, now I can't defend that statement but plastic is not a viable conductor jet putting a IC in plastic does not mean it is safe, not at all.

I have read about ESD safety online but that has not given me a understanding to know these things, is there someone who have any understanding about these things who feel to elaborate or explain I would appreciate it very much.

Regards

 

[andre_luis]

In laboratory for electronics it is usual to cover the bench with a rubber canvas stuffed with a metalic mesh for grounding which provides protection at the whole surface. Its terminals must be grounded in the PE, as well as the user must use an anti-static bracelet conected at another terminal. This not only protects the electronic components from damage, as well safeguard the person himself against harmful discharges.

 

[BradtheRad]

In regard to static charge, I find I am the worst carrier. For some years my workbench was in a carpeted room. Before handling electronic parts I got in the habit of touching a metal file cabinet. Sometimes I got a spark, indicating I had picked up high voltage (which is easy when walking across a carpet).

A wristband and earth ground are more effective, of course, than an ungrounded metal cabinet. The cabinet was merely sharing some of my electrical charge, not carrying it away.

Some plastic items (bags, etc.) generate high voltage. Keep them away from your workbench. And who knows, this may apply to plastic desktops, writing boards, etc.

An easy-to-build static detector can be made from a jfet.

http://amasci.com/emotor/chargdet.html

 

[ads-ee]

Anything over 600V of static charge may destroy a part. That's at a level that you still can't even feel it. We don't feel static charge till it reaches something around 3000V (which is when you see and feel a spark). You want to keep ESD well below 300V. You really don't even want your wife/kids to come over and point to your electronics on the bench, just the act of pointing at the thing from a number of inches away can induce static potentials that are over 300V.

For ESD the bad stuff is:
1) conductive materials are bad.
2) insulative materials are bad.
For 1 the discharge rate is so high it can destroy parts, when the discharge occurs.
for 2 there is no discharge so you build up charge, which can eventually destroy a part.

Static dissipative materials are good. These are materials which have quite a bit of resistance, but still conduct somewhat. They are usually over 1M ohm to something like 10Mohm.
The mats slowly discharge static charge to ground at a low rate so there isn't a sudden large discharge (spark).

Best practice is to place the anti-static shielded bag with the electronic parts on the grounded workbench ESD mat first. Then put on the grounded wrist band (minimum, best is to have a anti-static lab coat and ground it don't forget that you should be grounded through a 1Mohm resistance!). Now that you are grounded you can open the bag (not up in the air or over the carpet, which you shouldn't even have in the "electronics lab" area of your house), keeping it in contact with the grounded anti-static surface. You are trying to equalize potentials here.

Paper/fur(fake/real)/Styrofoam/pink poly bags/tape/glass/plastics (plastic cling wraps, etc) should never be used around exposed ICs all of them are ESD generators. Metal hand tools etc aren't an issue as when you handle them they discharge through you through the 1Mohm grounded wrist strap. Just make sure the ESD strap has good contact with your skin and that the ESD mat is clean. A dirty mat is probably not doing a very good job of static dissipation.

 

[D.A.(Tony)Stewart]

Metal counter is the worst due to the high currents from a static discharge.

Wood is good as long as not plastic coated and absorbs moisture to dissipate charge.

Glass is ok but bad with certain fabric and creates a surface charge with fur.

Carpets are generally bad but can be sprayed weekly to reduce ESD discharge levels.

Static mats are best because of the carbon particles embedded in the plastic to dissipate charge.

Wrist straps have a 1M series R to limit current of discharges.

high speed FETs can be damaged easily
LEDs are only rated for -5V but it takes a ground return for the static discharge.

I had one factory experience 0.1% failure rate on LEDs until I flew down and reviewed their storage, handling, soldering and injection molding process and found more than a dozen reasons for them to fail. Now it is 1ppm failure rate from process handling.

The best ESD protection is between your ears with awareness of causes and potential silent killers and wounding latent failures. ( e.g. touch fingers before handing a board to someone. )

Most wise factories have EOS/ESD, shoes, flooring and mats with irons insulated with 1MOhm to ground.

 

[Audioguru]

There are areas in the world where it is humid all the time and the people there have never experienced electrostatic discharge.
But here in Canada when it is cold outside in winter the humidity of the air is low inside when a furnace heats it so static sparks are frequent.

My office had a synthetic carpet and I bought the latest shoes that had excellent "treadwear". The soles were plastic. Boy oh boy I could build up tens of thousands of volts of static and create very long loud sparks. I did not wear a wristband but I was careful to discharge myself before touching many electronic parts and circuit boards. Then nothing ever got destroyed.

 

[andre_luis]

just the act of pointing at the thing from a number of inches away can induce static potentials that are over 300V.

I was not aware of that before, but it is an interesting observation.

After measuring the mat that covers the workbench of the lab where I work ( between its surface and the wrist strap grounding pin, near it ) although having a small dielectric characteristic ~3nF I would expect a higher damping effect with a small resistivity, but it have ~80 MOhms. In short, no other measures completelly eliminates the need of the grounded anti-static pulse bracelet.

 

[D.A.(Tony)Stewart]

When I implemented ESD/EOS safeguards in our factory in the early 80's our factory concrete floor had a vapor barrier and thus dry while the warehouses did not thus We used conductive epoxy paint for the production floor and the warehouse had free excellent anti-stat. The office had nylon carpet and easy to generate many kV. for very low capacitance devices with low ESD failure thresholds, it was a real risk. Tek differential FET probes were damaged often by young engineers who did not heed the warnings to touch ground before tip and connecting the probe.

We had a static charge measurement device in the office and just by walking on the carpet, ground yourself, measure voltage =0 then raise one leg, we could generate 200V due to V=Q/C by change the insulated body capacitance to ground with a fixed charge.

 

[David_]

Thank you for all answers, I wonder if this is a viable protection:
I have some laminate wood furniture on top of which I wanted to place boards and other electronics stuff so I took one of those big ESD shielded bags with bobbles in the as impact protection, I cut it open and placed it with the pink poly side down so that the shield became the new surface then I took some copper wire and connected one end to the ESD mat connector and the other one I heated and melted into the protection bag as well as a few
staples to form a connection between the copper and the shield.

Will this offer any protection?

Regards