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ESD Grounding

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harlista

harlista

Electrical
Mar 5, 2006
5
I have an issue that is eluding me regarding ESD grounding of carts. We have metal carts in our facility that we use for material movement between work stations (and some workers use for storage). In our previous building we had a regular linoleum tile floor which had ESD wax applied once a quarter. Because the wheels of the metal carts are not ESD grounded I used a drag chain attached to the bottom of every cart which has worked well for years. Testing with our ESD tester by putting one 5 lb electrode on the shelf of the cart and one on the floor. The carts measured in the dissipative range. Fast forward to the present. We bought a new building and spent a ton of money putting in static dissipative flooring in the production area. We grounded all our workstation ESD mats and everything passes when measured from the mat to the floor. However, I cannot get my carts to pass. I verify the top shelf is electrically connected to the bottom shelf and both are connected to the drag chain but I get an insulative reading on my meter when testing from the cart to the floor.

Additionally, using my static meter I see no indication of a static charge built up on the carts. Why am I not getting any continuity to the floor through the drag chain (I have even tried a much larger and heavier chain than the jack chain used in the old building)? Am I wasting my time trying to ground a metal cart that doesn't show any static charge? Curiously, I use my static meter on a PET disposable water bottle which are strictly banned from the production floor and see no static charge but the company provided and sanctioned covered mugs generate a ton of static. I talked with the flooring manufacturer and didn't get anywhere.

Any insight would be appreciated.

Regards
 
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I think it depends from what you call "continuity". Ten to hundreds megohms are OK to bleed away static charge. But above 20 Mohms usually reads as insulation on most DMMs. I think that you are fine.

Gunnar Englund
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.
 
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A more cogent question might whether this cart is the only preventing ESD damage to whatever it is that you are transporting. We typically ESD bag anything that we transport, anywhere. This allows us to transport by simply carrying things from place to place.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert! faq731-376 forum1529 Entire Forum list
 
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When I said "continuity" I meant a dead short. All the cart's components (with the sole exception of the wheels) and the drag chain are electrically shorted (zero ohms) to each other.

I am trying to prevent ESD damage to parts and or boards being transported within the same ESD safe area. We do use ESD trays to move PCAs on the carts as well as ESD totes to carry other parts. I was trying to tie the carts to the dissipative floor so as not no generate any additional charges.

 
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Not to nitpick, per se, but almost nothing is ever "zero ohms," even in MIL-STD-464, electrical bonding for EMI, etc., is specified at 2.5 milliohms.

according to and a static dissipative flooring has between 1 megohm to 1 gigohm of resistance to ground. That said, there may be some voltage dependency that your meter is unable to overcome.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert! faq731-376 forum1529 Entire Forum list
 
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Make sure the cleaning crew is using a recommended floor wax on your ESD floor. Nothing like a good shiny wax coating to insulate your expensive conductive floor.
 
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I can easily imagine the anti-static floor having a radically different surface resistance to the non-static floor covered in anti-static wax. Anti-static means "conductive". However as Skogs pointed out that can be all over the place from 1kΩ to 100MΩ and so your chain scheme could easily fail if your new floor is 100MΩ stuff and the old wax was 100kΩ stuff.

Put a 6x6" metal plate on the floor and hook it to the bottom of your cart with an alligator jumper and see what happens. Step slowly on the plate and see what happens. It may be the floor not doing it's job at all. Perhaps it's over concrete that is somehow not grounded at all.

Keith Cress
kcress -
 
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IRstuff, I realize bonding all my carts components together does not yield zero ohms but for all intents and purposes it is essentially zero. I use a Desco 19784 Analog Surface Resistance tester and measure the new floor as 'dissipative' somewhere in the 10E6 to 10E9 region. The tester uses a 5 lb electrode to contact the floor whereas the chain is much lighter. However, the very same scheme in our old building yielded much different results. The floor still tested between 10E6 and 10E9 but the drag chains seemed to make better contact to the floor and gave me dissipative readings between 10E6 and 10E9 with one electrode on the cart and one on the floor. In the new building with the same cart, same drag chain, same test methodology of one electrode on the cart and one on the floor I get a reading in excess of 10E11

Comokid. The cleaning crew isn't waxing the floor. It is cleaned per the floor manufacturers specification and tested using the Desco meter described above. The floor tests well within the dissipative scale at various point throughout the production area. It is the drag chain not contacting the ESD floor as well as it contacted the ESD waxed floor

itsmoked. If I touch the cart and performm the test the cart measures good. If I step on the 6 x 6 metal plate I am doing the same thing. I'm considerably heavier than the 5 lb electrode from the tester and I also wear ESD shoes at work.
 
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Getting rather personal aren't we. lol

I thought of the chain cleanliness so I cleaned it to no avail Then I tried a brand new much heavier chain in an attempt to get better contact with the floor. I think I'm going to have to face the fact that I cannot ground (yes, I mean in a static dissipative sense) like I did in the old building and we will have to employ other ESD safeguards like bags and totes on the cart.
 
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It may all be fine, even if you cannot measure it the same way. The obvious test would be to actually charge up the cart and see if moving across the floor dissipates the charge.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert! faq731-376 forum1529 Entire Forum list
 
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Would a tinsel bar or brush be worth a try? 25+ years ago I did a very short stint in a catalog printing plant that made extensive use of tinsel bars and brushes for all manner of paper handling apparatus to keep static under control, particularly during the dry winter months. Not quite the same level of criticality as electronic components, for sure, but I remember it as being very effective. Just a thought.

It is better to have enough ideas for some of them to be wrong, than to be always right by having no ideas at all.
 
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