ESD work mat selection guidelines help, please

[Michael Weston]

I thoroughly searched the Web for guidelines about selecting an ESD work mat, and there appears to be no authoritative source without a vested interest in selling mats or specification documents.  Further, specifications among different brands and models varies widely in both parameter values and what is specified.  So, I have some ESD mat questions whose answers I hope will eliminate any confusion and serve as an accurate guide for myself and others.

While these questions are mostly directed to Mr. Dave Jones, I also invite the input of others who do not sell ESD related items.

Thanks in advance for any help with these questions.

For the purpose of this post, the following definitions apply:

• RTT is Resistance Top to Top, also known as resistance surface to surface.
• RTG is Resistance Top to Ground, also known as resistance surface to ground.

Part 1
General ESD work mat details:

• Which is better for the entirety of the assembled mat "sandwich", including the dissipative layer(s) and the conductive layer combined: high electrical resistance or low electrical resistance?
• Which is better for the dissipative layer: high electrical resistance or low electrical resistance?
• Which is better for the conductive layer: high electrical resistance or low electrical resistance?

Part 2
Regarding the entirety of the assembled mat "sandwich", including the dissipative layer(s) and the conductive layer combined:

• What should be the minimum RTT?
• What should be the maximum RTT?
• What should be the minimum RTG?
• What should be the maximum RTG?

Part 3
Regarding the mat's dissipative layer(s):

• What should be the minimum RTT?
• What should be the maximum RTT?
• What should be the minimum RTG?
• What should be the maximum RTG?

Part 4
Regarding the mat's conductive layer:

• What should be the minimum RTT?
• What should be the maximum RTT?
• What should be the minimum RTG?
• What should be the maximum RTG?

Part 5

• Charge Decay (5kV to 50V) as seconds

Are any of the questions above unimportant? If so, which ones?


Specifically for Mr. Dave Jones:
I have some questions about the ESD work mat you reference in "EEVblog #168 - How To Set Up An Electronics Lab" at 25:07:

• Do you depend on this mat for critical work where very expensive items could be damaged?
• Who makes this mat?
• What is the model number of this mat?
• Is this mat still the one we see in your recent videos?

[robrenz]

This does not answer any of your questions but I think its important.  Make sure you get a high temp rubber based mat not a vinyl mat.

[Michael Weston]

This does not answer any of your questions but I think its important.  Make sure you get a high temp rubber based mat not a vinyl mat.

Thank you for that good advice.

Even though I already know a rubber mat is better for high temperatures, I'm glad you brought that detail up so others can lean.

[gregariz]

Well you've done more research on it than me. I generally leave it to a reputable manufacturer to make sure the resistances are appropriate. I'm not really sure how important ESD is, its been a long time since I've fried a component. It seems everyone's skin is a little different. I have a workmate who is regularly frying stuff. However, I would recommend a thinner mat (1.5 or 2mm), something like 2 layer rubber mats, which is what I use. The reason is that with the thicker (3 and 4mm) 2 and 3 layer spongy mat I find that surface mount components bounce around on it. Otherwise if you are on the hunt for benches you can buy ESD laminate already on them so you don't need mats anymore.

[Michael Weston]

I'm not really sure how important ESD is...

I think it depends at least in part how critical the device is that is being worked on.  I don't think ESD protection would be particularly important for a circuit board that blinks lights for a toy, and at the same time I think ESD protection is of paramount importance if one is working on medical life support devices.  With life support, even latent failures could cost a person their life.

[IanB]

I think you have mostly picked the wrong questions. From an electrical point of view, all mats from reputable manufacturers/suppliers, when used correctly, will protect against ESD damage as intended.

What will make a huge difference are the mechanical properties. Temperature resistance as already pointed out, but also scratch/cut resistance, abrasion resistance, chemical resistance (liquid flux, circuit board cleaner, other solvents), softness/hardness of the work surface, grip/slide resistance, etc.

It is the non-electrical properties where you get to make your choices.

[T4P]

Just don't get the cheap ones,

it should be 2 layer rubber with the dissipative layer being 10M-1Gohm and the conductive layer 100k-10M

But yeah, i bought 2 $39 600(L)x900mm(W) from Element14 branded "Multicomp" and they all fit most categories,
but the most depressing thing is that it's hard to clean and there are imperfections on the mat, not a problem though 

[Michael Weston]

I think you have mostly picked the wrong questions.

"Mostly" means I got 1 or more questions correct; so, please point out which were the correct question(s).

From an electrical point of view, all mats from reputable manufacturers/suppliers, when used correctly, will protect against ESD damage as intended.

Have you never bought a product from a "reputable" manufacturer, later to discover that the item you bought was garbage?  I know there are people who have.  For example, at one time Hewlett-Packard Company made excellent products that were built like a brick.  Since Compaq bough HP, their product quality degraded considerably.  I used to be a big fan of HP, and now I won't buy anything from them; probably ever again.

What about high quality products from manufacturers who have yet to be recognized as "reputable"?  How can you recognize a bargain from a yet to be recognized company if you do not understand what you should be seeking (like RTT and RTG specifications)?

[Michael Weston]

Just don't get the cheap ones

What do you define as cheap?

it should be 2 layer rubber with the dissipative layer being 10M-1Gohm and the conductive layer 100k-10M

Thank you for being very specific.  Can you suggest some ESD mats that fit these criteria (manufacturer and model number please)?

[T4P]

The ones that are 1 layer ..

This should give you a idea
http://sg.element14.com/jsp/search/productdetail.jsp?SKU=1687904

[JohnS_AZ]

Surface resistivity is not all THAT important. What it will tell you is the condition of the mat. Most importantly, if someone has cleaned it with inappropriate cleansers, or it has been contaminated by dirt, insulative oil films or other materials that would harm its effectiveness. Surface resistivity is measured in "Ohms per Square" and special testers and/or fixtures are needed to conduct this test (ie. Concentric Ring Electrode )

Resistance top to top tests the performance of the material by measuring the resistance between several widely spaced points on the mat. This shows if the mat has equal conductance across it's surface. RTT needs to be greater than 10^6 ohms, but lower than 10^12 ohms.

Resistance top to ground is the most important test. This measurement is taken from several points on the mats surface to the snap. This tells you if the mat is actually working.  RTG should be greater than 10^6 ohms, but lower than 10^9 ohms.

Important note .. NONE of these tests can be done accurately with a DMM. Standards call for an instrument that can apply an open test circuit voltage of at least 100VDC (a megger) and five pound conductive electrodes to make test contact with the surface material.

(this is all based on EOS/ESD Standard S4.1, and ASTM-F150)

(A good basic article about "Ohms per Square" can be read here: http://www.esdjournal.com/techpapr/ohmmtr/ohm.htm )

[SeanB]

All I did was to clean an offcut of conveyor belting and use it. 2M per square ( measured with insulation tester at 500V over a 20mm square) and around the same top to bottom. Free as well, i ordered a belt for work, and when I went to fetch it I asked and went through the scrap bin to get a lot of offcut strips to use as door protectors ( staff and trolleys..........) and this was a piece from that pile. At 5mm thick it could be overthick, but is perfect for me, solder proof, hard to cut by accident and resilient.

[JuKu]

Also, single colour and NOT gray. You don't want the small parts play camouflage on you.

[T4P]

Also, single colour and NOT gray. You don't want the small parts play camouflage on you.

Camo? They aren't playing camo on me!
Top to Bottom should be some other colour on the top and black on the bottom

[Michael Weston]

Surface resistivity is not all THAT important. What it will tell you is the condition of the mat. Most importantly, if someone has cleaned it with inappropriate cleansers, or it has been contaminated by dirt, insulative oil films or other materials that would harm its effectiveness. Surface resistivity is measured in "Ohms per Square" and special testers and/or fixtures are needed to conduct this test (ie. Concentric Ring Electrode )

Resistance top to top tests the performance of the material by measuring the resistance between several widely spaced points on the mat. This shows if the mat has equal conductance across it's surface. RTT needs to be greater than 10^6 ohms, but lower than 10^12 ohms.

Resistance top to ground is the most important test. This measurement is taken from several points on the mats surface to the snap. This tells you if the mat is actually working.  RTG should be greater than 10^6 ohms, but lower than 10^9 ohms.

Important note .. NONE of these tests can be done accurately with a DMM. Standards call for an instrument that can apply an open test circuit voltage of at least 100VDC (a megger) and five pound conductive electrodes to make test contact with the surface material.

(this is all based on EOS/ESD Standard S4.1, and ASTM-F150)

(A good basic article about "Ohms per Square" can be read here: http://www.esdjournal.com/techpapr/ohmmtr/ohm.htm )

That was a totally awesome reply!    Thank you very, very much for everything you wrote.

I suspected inappropriate cleansers and contaminants might degrade the ESD effectiveness of the mat.  Thank you very much for confirming my suspicion.
Thank you for the very interesting link clarifying the meaning of "Ohms per Square".

NONE of these tests can be done accurately with a DMM.

How can one test if their new mat is proper, or if their existing mat is performing correctly, without spending a big wad of cash?

[Michael Weston]

The ones that are 1 layer ..

This should give you a idea
http://sg.element14.com/jsp/search/productdetail.jsp?SKU=1687904

I read the web page from your link, and it describes the mat as having 2 layers.  The PDF specifications linked from the product description page confirms the mat as having 2 layers.  Did I miss something? 

[T4P]

Split my post up ... the cheap ones refer to the 1 layer mats

[Michael Weston]

Also, single colour and NOT gray. You don't want the small parts play camouflage on you.

I had essentially the same thought.  I can easily imagine hunting for a SMD resistor on a grey or black mat, then discovering it is immediately in front of me.

[Michael Weston]

What should be the specifications for Charge Decay time (5kV to 50V)?

[JohnS_AZ]

Test instruments depends on your situation.

If you're talking about mats on a whole bunch of benches in a production environment, you really should have good gear. On the lower end of this range is the 3M analog megger: http://www.all-spec.com/products/TM701NIST.html?gclid=COWLjdO74rECFYIaQgodY08Akw 

On this high end is the Prostat : http://prostatcorp.com/Resistance-System-Set    This is the instrument I have. The set cost around $2,000US a number of years ago.

If you're in a professional/commercial setting and only have one bench to worry about, you can get away with using something like this: http://www.all-spec.com/products/Test_and_Measurement%7CESD_Test_and_Measurement%7CESD-01/SCC-625.html

If you're a hobbiest with little or no budget, it gets tougher. You can TRY using the high-ohms setting on your DMM (or let it auto-range). If it reads anything (less than 10^12 ohms) odds are good your mat is working. If you can come up with a few 1% 1Meg resistors, you could actually test your DMM and see how accurate it is in those ranges.

I hesitate to include this since I've never seen one (or evaluated it), but there are some VERY cheap meggers on eBay from HongKong. It could easily be a box of shit, but I really don't know: http://www.ebay.com/itm/VC60B-Digital-Insulation-Tester-Megger-MegOhm-Meter-/251078809099?pt=LH_DefaultDomain_0&hash=item3a7576960b#ht_7652wt_1190

Also, about decay time ... a decent rule of thumb is that a charge should take MORE than 1 second to dissipate, and less than 10 seconds.

[Michael Weston]

Test instruments depends on your situation.

If you're talking about mats on a whole bunch of benches in a production environment, you really should have good gear. On the lower end of this range is the 3M analog megger: http://www.all-spec.com/products/TM701NIST.html?gclid=COWLjdO74rECFYIaQgodY08Akw 

On this high end is the Prostat : http://prostatcorp.com/Resistance-System-Set    This is the instrument I have. The set cost around $2,000US a number of years ago.

If you're in a professional/commercial setting and only have one bench to worry about, you can get away with using something like this: http://www.all-spec.com/products/Test_and_Measurement%7CESD_Test_and_Measurement%7CESD-01/SCC-625.html

If you're a hobbyist with little or no budget, it gets tougher. You can TRY using the high-ohms setting on your DMM (or let it auto-range). If it reads anything (less than 10^12 ohms) odds are good your mat is working. If you can come up with a few 1% 1Meg resistors, you could actually test your DMM and see how accurate it is in those ranges.

I hesitate to include this since I've never seen one (or evaluated it), but there are some VERY cheap meggers on eBay from Hong Kong. It could easily be a box of shit, but I really don't know: http://www.ebay.com/itm/VC60B-Digital-Insulation-Tester-Megger-MegOhm-Meter-/251078809099?pt=LH_DefaultDomain_0&hash=item3a7576960b#ht_7652wt_1190

Also, about decay time ... a decent rule of thumb is that a charge should take MORE than 1 second to dissipate, and less than 10 seconds.

Thank you very, very much for another excellent post. 

I discovered I can get a continuous ESD monitor for about the same cost as a Megger (insulation tester).  If you were working on a critical medical device, and you could get only one a continuous ESD monitor, or a megger, and not both, which would you choose and why?

[Joshua]

When you end up buying one, look at allspec.com   They seem to have the best prices on rubber mats I've seen. I've got one and have been happy with it. No affiliation, just a happy customer.

[Michael Weston]

When you end up buying one, look at allspec.com   They seem to have the best prices on rubber mats I've seen. I've got one and have been happy with it. No affiliation, just a happy customer.

Thank you for the tip.    I noticed they also have some ESD contant monitors.  I need to get some soldering iron tips from All-Spec, so buying the mat and constant monitor from them would be convenient.

Which mat did you buy?

I read that shipping ESD mats is quite expensive.  So, I am curious about the shipping cost.  If you are in the USA, how much did the shipping cost and what state were the mats shipped to?

[SteveyG]

I use these on my benches at home: http://uk.farnell.com/vermason/228116/bench-mat-blue-1-2x0-6m-4x10mm/dp/1833415

Seem to be pretty tough, tests out OK using the test instrument I borrowed from work. I also have one of those active monitors which attach to the mat for real time monitoring.

[tramjoe]

When you end up buying one, look at allspec.com   They seem to have the best prices on rubber mats I've seen. I've got one and have been happy with it. No affiliation, just a happy customer.

I just took a look at them, wow, they are darn expensive IMHO, how is that cheap ? First price is like $70 or more, even for multi-layer vinyl mats ? I mean, for high end rubber mats, this is about the market price, nothing special there (still think this is expensive, but these things seen to be overpriced everywhere), but you can find vinyl mats around for like half that price.