Author Topic: 'Anti-Static' white coats  (Read 8661 times)

0 Members and 1 Guest are viewing this topic.

Offline Thermal RunawayTopic starter

  • Contributor
  • Posts: 48
'Anti-Static' white coats
« on: January 30, 2010, 06:05:28 pm »
At my place of work, everyone has to wear a white coat if they're on the shop floor.  Now, I've always questioned the reasoning behind this, and to be honest I reckon the only reason we're really forced to wear these on the shop floor is because management think it looks more professional when they have customers walking around if everyone is dressed in a white coat.  That's all well and good, but I just wish they'd come clean and tell us that's the reason.  Instead, they insist upon justifying the white coats for ESD reasons.  Are they really any good for this purpose? Well, I don't actually know.  But one day...

I was invited to an ESD training session.  During the training session we were shown an instrument that basically measured static charge, and then we were asked to rub our clothes together and take measurements to prove that our clothes were generating a significant charge.
And I accept that they were - this was already apparent to me because I've noticed it when pulling off jumpers and such like - some of my jumpers can really generate quite a significant charge.  In the night time you can actually see the mini-lightning effect that is generated!

But... what does my white coat generate, I wonder? It's *supposed* to be an anti-static coat, which I take to mean that it won't generate any static charge itself (although whether it shields against static generated by your clothes underneath it is another matter).  But I rubbed my white coat together with itself, and the instrument was still measuring quite a significant charge.  Not as much as on some people's jumpers (particularly the Ladies fluffy jumpers), but it seemed to me that it was significant enough to totally bust the idea (Adam Savage style) that wearing white coats protects equipment on the shop floor from static charges generated by a person's clothes rubbing together.  It's far more likely that our static foot-straps that we also have to wear, are conducting any charge away.  I'm just not convinced by the whole white coat thing.

So what do you say Dave? Is the white coat thing a valid method for preventing static build up on your clothes or what? I wanted to take my test one step further by rubbing my jumper together with itself, underneath the white coat, and seeing if the coat actually provided any kind of shielding against it or not.  But the general consensus amongst the group was that I should stop mucking about with my silly experiments and let the trainer get on with it, so that's what I did.

Perhaps the EEVblog can provide a definitive answer?

Brian
« Last Edit: January 30, 2010, 06:15:01 pm by Brian Hoskins »
--------------------
Electronics Engineer, Land Rover enthusiast, Amiga Computer fan and general GEEK
--------------------
 

Offline desolatordan

  • Regular Contributor
  • *
  • Posts: 65
  • Country: 00
Re: 'Anti-Static' white coats
« Reply #1 on: January 30, 2010, 08:49:17 pm »
There are about 3 EEVblog episodes relating to this topic.

http://www.eevblog.com/2009/07/26/eevblog-20-the-unusual-oscilloscope-phenomenon/
http://www.eevblog.com/2009/07/28/eevblog-21-the-unusual-oscilloscope-phenomenon-part3/

I remember Dave donning a lab coat for a demonstration like this, but I can't seem to find the episode.
 

Online EEVblog

  • Administrator
  • *****
  • Posts: 38055
  • Country: au
    • EEVblog
Re: 'Anti-Static' white coats
« Reply #2 on: January 30, 2010, 09:29:56 pm »
There are about 3 EEVblog episodes relating to this topic.

http://www.eevblog.com/2009/07/26/eevblog-20-the-unusual-oscilloscope-phenomenon/
http://www.eevblog.com/2009/07/28/eevblog-21-the-unusual-oscilloscope-phenomenon-part3/

I remember Dave donning a lab coat for a demonstration like this, but I can't seem to find the episode.

It's here:
http://www.eevblog.com/2009/04/12/eevblog-3-anti-static-myths-busted/

But with hindsight I should have busted much more than the one myth about "anti-static" bags.

As you've seen in my oscilloscope phenomenon video, the anti-static coat and wrist strap didn't do much under those conditions.
Good anti-stat coats have carbon fibers to help better disperse any static buildup generated, and the material is coated in an anti-static chemical which I believe does wear off with time. So if the material didn't build up a charge ever, there would be no need for the carbon fibre threads.
"Anti-static" means that the material is supposed to be incapable of holding a charge, not necessarily incapable of building up a charge for a brief period under all conditions.

To do static protection properly you need the whole gamut of protection. Ionizer, wrist strap, floor mat, proper chair, desk mat, coat, shielding packaging, sprays etc. It's all about minimization, not necessarily total elimination.

A more in-depth look at all this might make an interesting blog...

Dave.
 

Offline Ferroto

  • Frequent Contributor
  • **
  • Posts: 289
  • Country: ca
Re: 'Anti-Static' white coats
« Reply #3 on: January 31, 2010, 03:42:55 am »
Just grab a large metallic grounded object, such as the chassis of a PC (not the painted part, the unpainted part inside). Honestly, I have never lost anything due to ESD, I'm starting to think it's total BS.
 

GeekGirl

  • Guest
Re: 'Anti-Static' white coats
« Reply #4 on: January 31, 2010, 11:35:12 am »
Just grab a large metallic grounded object, such as the chassis of a PC (not the painted part, the unpainted part inside). Honestly, I have never lost anything due to ESD, I'm starting to think it's total BS.

Ummm, how long have you been doing electronics ?

When I used to board stuff (did it for 1 year to make some cash) one of the people fitting chips to sockets did not use a static strap, EVERY board worked, but 6-12 months latter FAILED with very strange problems. We ended up having to REPLACE EVERY chip in EVERY product this guy had installed. This was back in the MID 90's.

Yes these days chips are better at withstanding static, BUT DO NOT RELY on this.

Static in high speed logic or high impedance is VERY REAL.

Regards,

Kat.

PS I refrained from swearing, as I have seen many times how real static is.
 

Online EEVblog

  • Administrator
  • *****
  • Posts: 38055
  • Country: au
    • EEVblog
Re: 'Anti-Static' white coats
« Reply #5 on: January 31, 2010, 11:46:29 am »
At one of my previous jobs working on very expensive high end military boards (that were often one-off's), static was such an issue that entering the lab without a labcoat was instant dismissal. As was touching a board without a wrist strap.
The same company also failed Farnell on an ESD audit and black listed them (very annoying!). And Farnell are pretty darn good with their ESD precautions.

Yes, as geek girl says, static can cause big problems with long term chip reliability.

Modern ones are much better at handling it, but for critical apps you never take the risk.

Dave.
 

Offline Thermal RunawayTopic starter

  • Contributor
  • Posts: 48
Re: 'Anti-Static' white coats
« Reply #6 on: January 31, 2010, 01:06:46 pm »
I've actually destroyed a chip on purpose (as an experiment) to see for myself the real consequences of zapping a device when you're carrying a static charge.  It was just a basic counter circuit that I knocked up with a CMOS chip in it.  I turned on a CRT based monitor, rubbed my right hand on the screen, and then with my left hand touched the chip.  The circuit stopped working instantly, and the chip was a gonner.

But as Kat said, the worst cases are where you cause damage to the device that is not quite enough to cause immediate failure, but manifests itself and causes failures later on - when the product is in the hands of your customers.

I am totally convinced by the static wrist straps (and foot straps), dissipative flooring and static dissipative matting.  That much I am sold on.  But the white coats? I dunno... I still need convincing about that.
Maybe it's just that our coats are not very good.  Dave mentioned that they are coated in a chemical that dissipates any build up of charge, and that this can wear off over time.  I can tell you that the only time our lab coats are replaced is if they become physically worn with age.  There's no system to replace them after a pre-defined time to ensure that the anti-static agents on the coat are still working.

So maybe that's why our coats seemed to fail my quick test....

Might look into that!

Brian
--------------------
Electronics Engineer, Land Rover enthusiast, Amiga Computer fan and general GEEK
--------------------
 

GeekGirl

  • Guest
Re: 'Anti-Static' white coats
« Reply #7 on: January 31, 2010, 01:12:18 pm »

Maybe it's just that our coats are not very good.  Dave mentioned that they are coated in a chemical that dissipates any build up of charge, and that this can wear off over time.  I can tell you that the only time our lab coats are replaced is if they become physically worn with age.  There's no system to replace them after a pre-defined time to ensure that the anti-static agents on the coat are still working.

So maybe that's why our coats seemed to fail my quick test....

Might look into that!

Brian

I think RS or Farnell sell anti static spray ? Somewhere that I can not remember here in Au you could buy a chemical that you soak the coats in after they have been washed (IIRC SpotLess Linen used to do this after washing Lab Coats (they are a hospital, hotel and industrial linen company (you buy or rent the product and factored in the price is the pickup, washing and return of the product))

 

Offline Thermal RunawayTopic starter

  • Contributor
  • Posts: 48
Re: 'Anti-Static' white coats
« Reply #8 on: January 31, 2010, 03:04:11 pm »
Thanks for that Kat.

I think this might warrant some tests.  I have three labcoats, of which all are at least a couple of years old but some are older than others.  All have been washed regularly by myself, at home, in my washing machine in the regular way.  I should do some tests with these coats to see if I can measure their relative static dissipation performance.  I'm betting they'll all perform pretty much the same (useless) after all this time.

Then I should try to get them soaked in the chemicals you described (I'll look into that) and test them again to see if they are any better.

Watch this space!!!
--------------------
Electronics Engineer, Land Rover enthusiast, Amiga Computer fan and general GEEK
--------------------
 

Offline Ferroto

  • Frequent Contributor
  • **
  • Posts: 289
  • Country: ca
Re: 'Anti-Static' white coats
« Reply #9 on: January 31, 2010, 04:14:50 pm »
Just grab a large metallic grounded object, such as the chassis of a PC (not the painted part, the unpainted part inside). Honestly, I have never lost anything due to ESD, I'm starting to think it's total BS.

Ummm, how long have you been doing electronics ?

When I used to board stuff (did it for 1 year to make some cash) one of the people fitting chips to sockets did not use a static strap, EVERY board worked, but 6-12 months latter FAILED with very strange problems. We ended up having to REPLACE EVERY chip in EVERY product this guy had installed. This was back in the MID 90's.

Yes these days chips are better at withstanding static, BUT DO NOT RELY on this.

Static in high speed logic or high impedance is VERY REAL.

Regards,

Kat.

PS I refrained from swearing, as I have seen many times how real static is.
Alright you can stop with the public shameing  ;D

I'm not completely ignorant of ESD, I do hold PCB's by the edges, avoid touching the contacts, and ground myself. I've just never used the static wrist bands, or ESD mat, and never had problems. I've been building custom PC's for almost 14 years, and have never lost a board to ESD, maybe I'm just lucky :D
« Last Edit: January 31, 2010, 04:16:45 pm by Ferroto »
 

GeekGirl

  • Guest
Re: 'Anti-Static' white coats
« Reply #10 on: January 31, 2010, 04:38:39 pm »
It is not public shaming, but trying to prevent miss information.

an assembled PCB is LESS suseptable than a bare chip.

Also remember that a mosfet suffers from ESD, if you charge yourself and touch the gate pin you can punch through the oxide and destroy the mosfet. This was actually done in TAFE (where I did my servicing apprenticeship) to prove to us the importance of ESD precautions.

You maybe lucky, or you may have customer who think you use cheap parts as they failed 6+ months down the track.

The precautions people take are in line with the cost of the product (a $2 product in china may have no ESD protection in a factory assemble environment, but a $20K product will have every precaution taken)


You can remove the need for ESD protection, buy controlling the atmosphere, clothing worn (down to underwear and socks) floor material, work bench surfaces and chairs. BUT this is all very expensive. it is cheaper to use wrist straps, and bonding of the product to the same earth (when working with very static sensative equipment I ground the ground plane to mains earth, my ESD strap is from the same earth via a 10M high voltage resistor (rated at 2.5Kv as I work with equipment up to 415Vac)

 

Offline charliex

  • Frequent Contributor
  • **
  • Posts: 342
  • Country: 00
  • Car Hacker
Re: 'Anti-Static' white coats
« Reply #11 on: January 31, 2010, 11:28:29 pm »
Definitely luck :) i know people who just seem to have some sort of magical properties for not damaging chips, then others who can just look at it and kill it. We used to have a regular customer who'd bet money he could cause damage.

i've heard about magnetic personailities, but esd personality never.
 

Offline Ferroto

  • Frequent Contributor
  • **
  • Posts: 289
  • Country: ca
Re: 'Anti-Static' white coats
« Reply #12 on: February 01, 2010, 12:19:32 am »
Peter Griffin knows a thing or two about ESD.

 


Share me

Digg  Facebook  SlashDot  Delicious  Technorati  Twitter  Google  Yahoo
Smf