ESD Control Program

[soupy1957]

I'm a QC Tech for a Communications Equipment Assembly plant in Connecticut, responsible for the ESD Training Program, which I've been teaching for a while now.
Recently a question was raised that begged an answer that I am unsure of:

Employees who are working at a properly designed ESD Workstation, who are wearing their wrist straps and ankle straps, can still accumulate electrical charges on their body?

Although I tend to agree that additional ESD potential can still be possible:

1. Doesn't this imply that our wrist straps and ankle
straps are inferior, or at the very least insufficient
with regard to truly "grounding" the employee?

2. Wouldn't this be a good justification for Ionizers
even if they ARE $800.00 a pop?

3. Shouldn't there be an additional effort to ground
any of the computers, analyzers, meters, electric
screwdrivers, etc..beyond just having them plugged
into the common ground?

Suggestions, ideas, imput??????
-soupy1957

 

[IRstuff]

It's a trade between safety and utility. While there can be charges generated, the assumption is that the bleed resistor in the strap is low enough to bleed down the charge while still protecting the operator from electric shock danger. Also, most ICs are fairly robust from an ESD perspective.

Ionizers are probably good for ensuring that charge buildup is further minimized.

User equipment tends to be more robust than unpackaged circuit boards, since there's usually much more capacitance to spread the charge across, thereby limiting the ESD voltage.

TTFN

 

[soupy1957]

Thanks for your posted response "TTFN"

 

[felixc]

Depending on where you live, one big variable is the air humidity. If you can manage to control the humidity to be over 30% and that the people wear cotton clothes, there will be no need for ionizers.

Seats that are grounded, and that are using leather instaad of synthetic materials is a good thing to do. As with cotton, leather will not generate ESD with humidity levels over 30%.

In Canada, during cold winter days the humidity drops to very low levels. A humidifier has been added to the forced air heating system.

 

[BrianG]

Another method that we use is to issue employees with special workshop coats which have a very fine metal thread grid woven in to the material(about 3/8 inch square). The coat also has a pop stud, like the wrist strap, to connect the grid to the ESD system ground. The coat thus ensures minimal surface charge can accumulate from the operator's clothing and body and is much less expensive than ionisers. With the company logo embroidered on them they look smart too!

 

[sreid]

1. Have a dedicated ground wire from the work station that clamps to the AC ground conduit or water pipe. Don't trust an AC plug ground.

2. Nylon lab coats (etc.) are the worse.

3. Buy the Electronic Assembly Hand Lotion. And buy the disposible assembly (Cotton?) gloves.

4. Buy good wrist straps. etc. (3M). There is some real junk available for the unwairy.

5. Add the humifier to the HVAC first before Ionizers. Some people are really sensitive to Ozone.

 

[zeitghost]

<1. Have a dedicated ground wire from the work station that clamps to the AC ground conduit or water pipe. >

There's a safety issue here: we are trying to bleed charge away not provide a solid ground.

Solid grounds are dangerous in the event of a fault ocurring that makes something in the workstation live...

 

[soupy1957]

Thanks to all who have contributed your thoughts to this string.........It's much appreciated.
Currently, our arsenol of safety gear consists of the Blue Nylon Coats; gloves; wrist straps; ankle straps or ESD shoes; Blue mats on the ESD Table, (grounded of course) and in some cases floor mats; Carts with chains dragging on an ESD paint floor; Conductive cream; Constant monitors; Test stations for testing the Ankle straps and wrist straps (daily)......
I'm in Connecticut, as I indicated, so although we have an A/C control to our facility, it is constantly compromised, and we DO get some fairly significant humidity during the year.
"Thanks" again, as I stated......
-Soupy1957