ESD Mitigation - Floor mats or conductive floor?

[im871345]

Every cart has a metal drag strap and is made for electronics handling.

Becareful with drag chains because typically they are not heavy enough to make good contact on the floor or if they are heavy enough they are not long enough to make good contact on the floor. Also if they are multiple level carts do you loose connection where it is joined at each level? I learned the hard way... Customer pointed it out!

 

[jeffrey_Chang]

Becareful with drag chains because typically they are not heavy enough to make good contact on the floor or if they are heavy enough they are not long enough to make good contact on the floor. Also if they are multiple level carts do you loose connection where it is joined at each level? I learned the hard way... Customer pointed it out!

For drag chains, the problem is that it will accumulate dirt easily upon frequent movement, in a typical class 10K environment, it will only take approx. 1 month for the chain to get real dirty and resulted in intermittent poor grounding. Of course, it will be much better in a class 100 or 1000 environment.
For multiple level carts, to ensure connection between each level, conductive clips or collars are installed at each corners of the joints. This will give you a good grounding path.
I would strongly suggest to avoid using drag chains but instead go for conductive castor wheels for mobile carts or trolleys. All the trolleys and mobile carts in my factory are with conductive wheels and so far even when it accumulates dust, it doesn't fail. I've been using it for easily 2 years with NO cleaning of the wheels and yet it works fine. Setback is conductive wheels are a lot more costly.
thks.
jeffrey.

 

[don small]

Basically,the purpose of esd control is to protect esd sensitive device from being damaged by ESD.
The source of ESD damage comes from people, machine and materials that the esd sensitive come into contact with.
As to protect static damage from people, we need to wear anti-static wrist strap whenever we handle esd sensitive component in seated position and esd shoes whenever we handle parts in standing position. This is to drain the static charges through the wrist strap and esd shoes. However, esd shoes only work well when it is worn with the presence of static control flooring,i.e esd floor mat or conductive flooring.
The resistance to ground of the system ( man, esd shoes and flooring) should be less than 35M in order to have voltage build up on the person to be less than 100Volt.

As more material where the esd sensitive may come into contact with or placed on, the material worksurface to ground should be less tahn 1x10E9 ohm.This is to ensure the static charge is drained to ground at rate which is not too fast or too slow.

For grounding and bonding system : all the esd material for example esd mat, wrist strap outlet or plug, esd floor mat etc should be bonded at the same common point. The resistance from this point to the equipment ac ground should be less than 1 ohm. And impedance less than 1 ohm from ac equipment ground outlet to the neutral bond at main service entrance.
This less than 1ohm requirement is to ensure good bonding system existence so that charges flow to ground through the bonding system quickly.

More of the requirement can be download for FREE at http://www.esda.org.

As for cart, the chain would not serves much purpose as the good contact usually comes from the caster conductive wheels.


Training and periodic audit is important to ensure people in the factory understand the impact that ESD has on the product. Visual aids on the esd control may be a good idea to impart the controls that you want to set.
Hope that helps.

 

[Icy Mountain]

We use a metal wire rack type cart. Our original carts were all put together with the standard plastic holders on the corners. We tore them all down some years ago and replaced one holder on each shelf with a metal holder. I have tested every cart to make sure that every shelf is connected to ground (i.e. our conductive floor).

As for cart, the chain would not serves much purpose as the good contact usually comes from the caster conductive wheels.

The best part about the Cove is that we get a rebuttal to statements like this.
We made our drag chains. They consist of your basic hardware store dog chain, fastened to the bottom shelf with a bolt, with another bolt fastened to the loose end of the chain to provide good contact with the floor. The chains are long enough to drag multiple links but short enough to stay out from under the wheels. The bolt on the end of the chain tends to move around such that the same side of the chain isn't always dragging. I believe that this eliminates any dirt build-up, based on testing. I've retested the carts several times over the years, without manipulating the drag chains in any way, and they have always remained connected to ground. The conductive wheels for these things are just outrageously expensive. In addition, they are DEFINITELY susceptible to dirt build up that ends up insulating them from the conductive floor, unless you clean them regularly. I have 5 years of ESD equipment testing records that disprove your statement.

 

[Quality Queen]

You can buy antistatic spray for the chairs. An ESD program never seems to end.

I'm trying to implement an ESD program and I am finding just that: The ESD program never seems to end. And since ESD damage is hard to detect I'm having a hard time justifying all these new requirements to our quality system.

So first question is do we need this program? I believe the answer is yes. We manufacture medical devices that contain circuit boards, and EPROMs.

Next question is how much is enough? If we have a program is it required to meet EN 61340-5-2? Do we have to measure and control humidity levels? Are wrist straps enough? Or am I telling management that the $2 components that we use require $2000 worth of new equipment despite the fact that nonconforming product has never been traced to problems caused by ESD damage...

Any help/input anyone may have would be greatly appreciated!

 

[Al Rosen]

I'm trying to implement an ESD program and I am finding just that: The ESD program never seems to end. And since ESD damage is hard to detect I'm having a hard time justifying all these new requirements to our quality system.

So first question is do we need this program? I believe the answer is yes. We manufacture medical devices that contain circuit boards, and EPROMs.

Next question is how much is enough? If we have a program is it required to meet EN 61340-5-2? Do we have to measure and control humidity levels? Are wrist straps enough? Or am I telling management that the $2 components that we use require $2000 worth of new equipment despite the fact that nonconforming product has never been traced to problems caused by ESD damage...

Any help/input anyone may have would be greatly appreciated!

This is one of my biggest headaches as well. I feel your pain. Yes humidity levels are important. Other ways to mitigate low humidity is to use ionizers. I am not familiar with the standard you cited, but with ANSI/ESD S20.20. I attached it in the thread, ESD Control Certification by NARTE, along with a few other documents. You may also want to look at the ESDA web site as well.

 

[Quality Queen]

Thanks Al! That ESDA link is a huge help.

 

[don small]

ESD sensitive components are getting more sensitive to ESD damage in the coming years.

Definitely there is always a need to have a static control program.
Working as an ESD coordinator myself, our plant has been certified to ANSI/ESD S20.20 for more than 5 years now.

ANSI/ESD S20.20 standard is widely used by many companies and you can download for FREE at http://www.esda.org.
This standard will give you a guideline what is required when you set up your static control program. It is just like when we have ISO 9000 certification.
Certify your facility against ANSI/ESD S20.20 would give your customer more confidence ; you're telling your customer that you're serious about ESD.