[SI-LIST] Re: EMI Ring in PCB

Hi Bill and the group,

Just have a few minutes here in a Holiday Inn on the way to Newport 
tomorrow.

Yes, it is a good idea to use a 1 meg resistor between the face plate 
and the board ground to prevent static buildup. In addition, the metal 
  face plate should be closer to circuit ground than any other board 
feature to prevent a spark from hitting a component, a spark gap might 
be a good description.

As between circuit ground and the chassis, if possible I connect them 
as robustly as possible in many places. On average this is closer to 
the end point for ESD and EMI, but there is a lot of considerations on 
how to accomplish this. Trying to use single point ground can cause 
problems at high frequencies. Some articles from my site on this:

http://emcesd.com/tt2002/tt040102.htm

http://emcesd.com/tt2002/tt050102.htm (an ESD example)

http://emcesd.com/tt2002/tt100102.htm

http://emcesd.com/tt2004/tt030104.htm

More later,

Doug

Bill Wurst wrote:

> Hi Doug,
> 
> Thanks for reminding us how ESD protection has evolved since Henry Ott 
> published the second edition of his book almost 20 years ago.  What was 
> good practice then has been greatly improved upon over the years, 
> especially with regard to the section on board layout!
> 
> Is it still good practice to provide a very high impedance (>1MegOhm) 
> discharge path between circuit ground and the chassis, to bleed off any 
> accumulated charge before the PCB is inserted into the backplane?  Or, 
> if the PCB no longer connects the metal faceplate to the card edges, 
> which contact the metal card guides (also attached to the chassis), is 
> it better to allow circuit ground to extend to the card edge so as to 
> contact the metal card guides, both dissipating any charge and reducing EMI?
> 
> This also raises questions about the best way to handle items on the 
> faceplate:  I/O cabling, switches, and LEDs.  LEDs can be isolated 
> through the use of light pipes, but what about cabling and switches? 
> Should one still follow the other ideas put forth by Ott in chapter 12?
> 
> As you said, it would take pages to adequately explain how to handle ESD 
> currents.  Perhaps you will have some time to give us your thoughts on 
> current best practices when you return from teaching.
> 
> Best regards,
>      -Bill
> 
> =================================================================
> Doug Smith wrote:
> 
>>Hi Bill and the group,
>>
>>In general, an ESD ring causes as many problems as it attempts to fix. 
>>I would not recommend using them and certainly not connecting them to 
>>chassis ground through a backplane connector. That just inserts lots 
>>of ESD noise into the sensitive signals in the backplane.
>>
>>It would take a couple of pages of typing to adequately discuss how to 
>>handle ESD currents, here is a couple of major points (individual 
>>exceptions may apply):
>>
>>-Isolating circuit boards from the chassis except through one point, 
>>generally increases ESD problems and forms a great tuned circuit 
>>(parallel plate capacitor from board and chassis and inductance of the 
>>signal connection). There are a few articles on my site at 
>>http://www.dsmith.org on this.
>>
>>-Keep the ESD off the board entirely either through insulation or a 
>>metal faceplate robustly connected to chassis.
>>
>>-ESD can generate hundreds of Volts/inch as it flows on a wire or 
>>circuit board trace. Even a tiny fraction of the coupled into nearby 
>>paths can spell disaster.
>>
>>Lots more, but out of time. I have to finish loading up the equipment 
>>into the minivan for my teaching next week .
>>
>>Doug
>>
>>Bill Wurst wrote:
>>
>>
>>
>>>Jongbae,
>>>
>>>I believe you may have confused ESD with EMI.  If that is the case, I 
>>>would refer you to Chapter 12 in "Noise Reduction Techniques in 
>>>Electronic Systems" (2nd edition) by Henry W. Ott, and in particular, 
>>>Figure 12-17 on p.343.
>>>
>>>Best regards,
>>>    -Bill
>>>
>>>
>>>      /************************************
>>>     /         billw@xxxxxxxxxxx         /
>>>    /                                   /
>>>   / Advanced Electronic Concepts, LLC /
>>>  /           www.aec-lab.com         /
>>>  ************************************
>>>=======================================================
>>>jbtera77@xxxxxxxxx wrote:
>>>
>>>
>>>
>>>>Hi, all
>>>>Do anybody know about EMI ring and its effect?
>>>>
>>>>If someone recomment any document or paper which demonstrates EMI ring in
>>>>PCB, it will be very helpful.
>>>>
>>>>Thanks in advance,
>>>>Jongbae Park.
>>>>
> 
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