ESD Prevention Questions

[frankporter]

I am new to the world of ESD prevention. My company has a mobile laboratory (40' trailer) that is going to be used to clean printed circuit boards. Of course, one of the main concerns with PCBs is ESD. The the lab has stainless steel (SS) flooring, SS tables, SS cabinets, etc. Basically everything you come in contact with is a conductive material that is all connected the same ground as the trailer shell.

My thought is to have the workers wear heel-strap ESD devices to bleed off their static charge while walking in the trailer through the ss floor. They will also use wrist straps tied to the tables. Both devices will have at least 1M resistance to prevent death from accidental high voltage contact.

There may be flaws in my thinking of course, but one of my main questions is whether the trailer will have to be earth grounded in order for the ESD prevention to be effective? If the trailer ground is at some random potential associate with the on-board portable generators, is it possible that the static charges may not bleed off?

Am I just totally wrong in all this?? Ha Ha! I hope not!

Thanks!

 

[VEBill]

The trailer should be grounded anyway for basic electrical safety reasons ( since you're probably going to have AC power). But it's hard to imagine how grounding the trailer or not would affect ESD at the workstations fully within the trailer.

By way of explanation, what's planet Earth grounded to?

 

[VEBill]

And I assume the PCBs are populated with components? Circuit Card Assemblies, not just the empty PCB itself?

 

[IRstuff]

The vehicle does not need to be grounded. Just imagine the environment inside an ESD pouch. It protects without grounding, no?

The main issue is to understand what you are trying to do, which is to prevent voltage differences from occuring between different parts of the PCB. Since the metal of the vehicle is an equipotential surface, there is no ESD issue with that part.

The biggest source of ESD is usually the people, since they move, and wear ESD-generating clothing. Ideally, when they are working in the facility, they should be wearing ESD dissipative labcoats and they should be wrist-strapped or equivalent. You might also need to look at humidity control, and/or air ionizers for particularly dry days.

Don't forget that most of the surfaces that people can come into contact with should not be hard grounded, to ensure that electrical shock hazards don't kill the workers outright.

TTFN

FAQ731-376

 

[MacGyverS2000]

A mobile PCB cleaning lab? Talk about an environmentalist's nightmare!

Dan - Owner

http://www.Hi-TecDesigns.com

 

[frankporter]

Hey everyone,

Thanks for your replies. Yes, we are cleaning PCBs that have already been in use and were contaminated by some flood/fire event. This is mostly for equipment that is proprietary and irreplaceable. It's part of the manufacturing process, so there are little "environmentally unfriendly" chemicals to worry about.

I can see now that the trailer does not need to be earth grounded for ESD prevention reasons. As far as grounding it for safety reasons, as long as we are running equipment that is only powered by the portable generators and and also grounded to the trailer frame/thus generator frame, we should be in compliance with OSHA guideline 1926.404(f)(3) if we do not take the extra step of earth grounding.

Do you guy/gals know of any other rules/specifications that deal with this that say otherwise?

Also, IRstuff, when you say the workers should not be hard grounded, do you think that adding the 1M resistance to their grounding straps is a step in the right direction towards preventing this?

Thanks!

Frank

 

[zappedagain]

I would think you would want the trailer grounded to eliminate the potential for ESD events when someone enters or exits the trailer (think about the zap you get from your car door on a winter day). Ideally the PCBs should be in static safe transfer cases at this point. If they are only in static dissipative transfers you may still have issues; we have parts that are sensitive to 20V. That's a lot of dissipation to knock 8KV down to 20V.

John D

 

[frankporter]

Thanks John,

That was one of my original thoughts and one of the reasons I asked the question in the first place. After doing more research on ESD prevention, I see that some components can be affected by as little as 10V!! Wow! Don't they design some kind of ESD protection around devices that are that sensitive?

I found out today that we have some kind of "static charge tester". Not sure of it's type or function yet, but I'm gonna go check it out and see what kind of readings I get. Also gonna run some of the equipment and overhead A/C units to see what the humidity levels average.

Thanks for everyone's suggestions!

Frank

 

[IRstuff]

Yes, they did. However, the protection devices, which were, at one point in time, protecting up to 25kV, basically consist of zeners and SCRs, which presented GIGANTIC capacitive loads, and were not compatible with the high frequency signals in use today, so the protection ahd to downgraded or removed.

re: resistance. Your typical wrist straps come with a built-in 1Mohm resistor, and per standard ESD rules, the wrist straps, etc., need to be checked twice a day.

TTFN

FAQ731-376

 

[VEBill]

This is not an ESD issue, but well worth mentioning:

"...if we do not take the extra step of earth grounding."

You had better impose an outright ban on any extension cords running from the trailer or its generator to the outside world. Outside outlets. Accessible generator outlets. Cords running out doors or windows.

It only requires a relatively common type of fault (Hot shorted to real world earth) that would result in the real earth ground being hot relative to the trailer ground. And if you've ever seen the sort of home-made extension cords used by Plant Maintenance folks, you know that this sort of fault could happen.

With a proper (common) earth ground, such faults would open the applicable circuit breaker. Lacking such a common ground, the fault will remain undetected until the first person joins the grounds while entering or exiting the trailer and gets a very nasty shock. Then everyone else comes to help their downed co-worker, and you end up with a huge, heaping pile of smoldering employees.

Lest you think such issues are fantastically-extremely rare, within the past two weeks I've seen almost the exact same sort of 'hot chassis' fault (but thankfully at low voltage).

It'd be much easier to ground the trailer to real world earth ground.

 

[biff44]

You have to bring the boards into the trailer, so if the trailer is sitting there at 10 KV, and you walk onto the steps holding the boards, poof. It is a big common misconception that these "antistatic bags" are some sort of magic shield for ESD energy. Take any standard ESD meter, put it inside the bag, and wave a piece of scotch tape around on the outside and watch the meter!

You need clothing that is esd dissipative, heel straps or wrist straps, in inside environment without any static generating things (like the dreaded scotch tape dispener!, equipement to twice a day test the antistatic stuff (wrist strap testers and a log for workers to sign for each test, etc).

http://www.MaguffinMicrowave.com

Maguffin Microwave wireless design consulting

 

[biff44]

Remember, grounding the trailer can probably be done by hanging two conductive rubber strips down from the chassis!

http://www.MaguffinMicrowave.com

Maguffin Microwave wireless design consulting

 

[Leitrim]

There are available dissipative shoes and sneakers that are very effective, particularly if you have a well grounded floor surface (likely not much help if you are walking on carpet). They are somewhat expensive but in the right environment work very well. Also, I don't recall seeing dissipative work mats mentioned in this thread, I consider these a given, must need.

 

[IRstuff]

While absolute value of potential on the bag may vary, that, in itself, will not kill the boards. It's only if there's a potential difference between parts of the board that can sustain a certain level of current will there be actual damage sustained. Thus, so long as bag is equilibrated with the interior of the van, there should be no issue.

TTFN

FAQ731-376