ESD question

[engmechs]

Hi all,
I am a mech. engr. & just received a project to deal with ESD issue. A charger/cradle device had a cutout at the back side for program/ testing purposes. But this caused the ESD problem. Right now, a rubber seal is applied to block the gap but didn't meet ESD req't. Testing results found the min. distance between PCB board & test probe is at least 8mm ! But physically I saw the PCB is actually about 3 mm away from the plastic surface.

The rubber seal idea works, when the PCB sits far away from cutout. But since the plastic tooling had been made, modifications to the tooling is costly.

Is there any way to resolve this issue, from a quick fix point of view? Any sites to do research on this topic?

My initial thoughts:
Make the rubber seal larger, to meet the 8mm distance req't.
Metallic/ conductive coating be a possible solution? (Applied to inside cover surface)

I really want to know on this topic, as this kind of issues may come up more often. Any comments are welcome.

Thanks in advance.

engmechs

 

[itsmoked]

ESD not EMI right?! Very different.

Can you change the rubber seal to a metal seal? This would short any ESD to the chassis as apposed to the board -however close it gets!

If the rubber seal is actually needed to prevent water entry as compared to finger, bugs, and dirt then add a stick on rubber label or plastic label over the metal plug.

Another possibility is just get a standard polycarbonate overlay/label made that has a metal foil layer inside.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[engmechs]

Hi Keith,
Thanks for your suggestions. Yes, it is ESD, not EMI.
I am interested to your last suggestion, which is a standard polycarbonate overlay/ label with a metal foil inside it. Is this something custom made, or I can get from off the shelf? A quick trial & test would be needed. Is there any companies making it?

Please advise, & thanks for your comments.


engmechs

 

[itsmoked]

I'm talking the standard overlays on everything you see. The keyboard overlays used on coffee machines, copy machines, electronic instruments, ovens, washing machines, etc, etc.

They are generally multiple layers of thin polycarbonate(a good insulator).

Steven Label is a huge label maker I wouldn't use as they consider small fry to be barely worth the time. Then they charge a fortune. They quoted me $600 on a job that I went local on for about $200. I would look to a small local company. They will usually bend over backwards to help you and charge less. But Steven Label has an educational web site so rummage around on it for a quick gain of knowledge.

You can get virtually anything. Any kind of adhesives. Any layering, shape, materials, whatever.

http://www.stevenlabel.com/Index.htm

These are just a few random ones off the web. Hunt down a local company. They will come over and discuss everything and dump samples on you.

http://www.melrose-nl.com/overlay.html?source=googleadwords&gclid=CNnuhsKmyoYCFRiACwodggnA4g

http://www.bwind.com/inst.a.screen.htm

http://www.millerdial.com/

http://www.ssi-electronics.com/

http://www.switchwizard.com/products.php

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[engmechs]

Hi Keith,
Thanks for your prompt reply again! Greatly appreciated. I will take a look at the website. It has tons of info there.

Regards,

Edward

 

[engmechs]

Hi Keith,
One more Question: what is the chance of applying ESD gasket around the cutout area as an option? As gasket would provide a means to short out the discharge, & prevent from hitting the PCB directly.

Any thoughts?

engmechs

 

[itsmoked]

Hi engmechs!.

Any chance you can take a picture and post it? You could crop it so all we see is the area of interest. That might generate more suggestions as I am at the limits of my imagination, (of what you're attempting), at this point.

[red]"applying ESD gasket around the cutout area"[/red] is not computing.... But then the fact that is 3:25AM may also be a contributing factor.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[ScottyUK]

Chomerics have a range of conductive gasket materials, primarily aimed at EMI control rather than ESD. They do not understand the concept of 'low cost' for their products, but they are effective.

http://www.chomerics.com

----------------------------------

I don't suffer from insanity. I enjoy it...

 

[itsmoked]

Boy isn't that the truth! I looked up some of those aligned elastomeric elements for LCD mounting... I was shocked. Thinking maybeee $0.20 but it was like $5.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[logbook]

I don't quite see the problem either but it seems like you are talking about metalising the inside of the case as an option. Typically the case would be earthed to dissipate the ESD event. If the case is plastic without metalisation anywhere then I don't think itsmoked's idea of the label will work. If the label transferred the charge to the case proper that would be fine. It sounds like the case is all insulating so the charge will just accumulate and arc over.

If ESD is an issue then metalise the case. Now you need to divert charge from the rubber seal to this metalisation. I think since the seal is not touching the pcb you will get away with it as is, especially as charge doesn’t like to go inside screened enclosures.

 

[itsmoked]

Yeah log that's a possibility. (plastic case) Not clear on that. But then a polycarbonate label sans metal would probably still do the trick if the rest of the case protects from ESD via raw insulation ability.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[engmechs]

Hi Keith & logbook,

Let me provide more info:
The case is plastic. It hosts the PCB inside. Rubber seal sits very close to the PCB surface right now.

Test results suggested to keep testing probe 8mm away from PCB, right now is around half way only. (4mm)

If I understand the idea right, I need to do the following:

Metalize the case
Still use rubber seal + metal foil inside (to discharge static to the case)

I know I can paint conductive paint inside the case ($$$), or any other means? Since this is not a metallic chassis, it is not easy to add a grounding wire to it, or can I provide a wire from the cutout area to the pcb's grounding position? Am I making senses?... Any ideas?

Comments are appreciated.

engmechs

 

[itsmoked]

My question would be does the case provide ESD protection now? (excluding the pesky hole)

If it does, then a better insulator than the existing rubber plug may be all the solution you need. (see my previous suggestions)

If the existing case does not provide protection then you will need to do something to the whole case.

Which is it?

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[engmechs]

Hi Keith,
The fact is that the case doesn't provide protection right now. Ground zero!


engmechs

 

[itsmoked]

Okay the first thing I would try is just plain silver spray paint. Read the label and make sure it has a high aluminum content. Try it on a piece of plastic and measure it with an ohm meter after it's dry. See what happens.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[engmechs]

Hi Keith,
Thanks for your advice. I will see on it.
Regards,

engmechs