ESD Prevention Questions
ESD Prevention Questions
(OP)
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!
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!





RE: ESD Prevention Questions
By way of explanation, what's planet Earth grounded to?
RE: ESD Prevention Questions
RE: ESD Prevention Questions
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: Eng-Tips.com Forum Policies
RE: ESD Prevention Questions
Dan - Owner

http://www.Hi-TecDesigns.com
RE: ESD Prevention Questions
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
RE: ESD Prevention Questions
John D
RE: ESD Prevention Questions
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
RE: ESD Prevention Questions
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: Eng-Tips.com Forum Policies
RE: ESD Prevention Questions
"...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.
RE: ESD Prevention Questions
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).
www.MaguffinMicrowave.com
Maguffin Microwave wireless design consulting
RE: ESD Prevention Questions
www.MaguffinMicrowave.com
Maguffin Microwave wireless design consulting
RE: ESD Prevention Questions
RE: ESD Prevention Questions
TTFN
FAQ731-376: Eng-Tips.com Forum Policies