Ray,
I really can't see trace width having a lot of bearing here. You are dealing with an EM field that is terminating at the reference plane. The relationship to height above this plane is the important thing. The fact that the trace impedance is related to both width and height means that it's credible to use trace width as a basis for this, but height is the real player.
I've usually worked with at least 2 x the height above the plane as a rule of thumb in these instances. More is of course better..... That's why you are getting various 'rules of thumb' from different people depending on their own experiences. Andy Kowalewski's 5 x rule would be fairly safe. I don't know if any of the field solver programs that are available can give you some better information, but may be worth looking into.
IPC-D-317 is the closest IPC spec I've found dealing with these sort of problems, yet it doesn't seem to address your particular question.
regards,
Phil Dutton C.I.D.
-----Original Message-----
From: Ray Dwyer [mailto:[log in to unmask]]
Sent: Friday, 6 February 2004 9:09 AM
To: [log in to unmask]
Subject: Re: [DC] copper pour stand-off
Greetings DC,
Lets see, I got these suggestions; match trace width, in this case that
would be 10 mills, and three times trace width and 5 or 6 times trace width.
The dielectric thickness on this would be about 7 mills so any of these
would be thicker than that.
What of you engineer types, what do you guys say? And does IPC say anything
about this?
In the past I have either matched trace width or for stronger signals
doubled trace width. But either way I have never come across a clear study
of how to make a best judgment.
> -----Original Message-----
> From: Ray Dwyer
> Sent: Thursday, February 05, 2004 11:58 AM
> To: [log in to unmask]
> Subject: copper pour stand-off
>
>
> Greetings,
> I am concerned about rules for stand-off distance for copper pour on an
> outer layer to provide added isolation to a pair of sensitive low current
> traces. There are other traces on the top layer as well. The basic info
> is 50 ohm impedance, 10 mil wide microstrip, digital TTL signal current as
> low as in 10s of mili-amps, FR-4 material, 1 oz copper.
> Ray Dwyer
> 972-371-4046
> Maxim
> 4401 Beltwood Parkway
> Dallas, Tx. 75244
>
>
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