Re: [SI-LIST] : Heat sink and radiated emissions

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From: Bill Owsley (owsley@cisco.com)
Date: Wed Jul 26 2000 - 07:11:45 PDT


I have had EMI concerns with most heatsinks - proper grounding has resolved
those.
Proper grounding includes a conductive path. The black stuff on aluminum
heatsinks is an insulator.
In a forced air cooling system, black has very little effect on cooling (<
2% say the thermal guys)
Thus, well grounded plain aluminum heatsinks in a forced air system works
for EMI and thermal.
You'll be challenged to find the proper grounding...
- Bill

At 05:37 PM 07/25/2000 -0700, Doug McKean wrote:
>Wayne Gibson wrote:
> >
> > Hello,
> >
> > I have an EMC oriented question. Currently, investigating the use of
> > aluminum, ceramic and enhanced plastic heat sinks for possible use on a
> > TBPA package. Does anyone have information regarding different heat
> > sink material with respect to radiated emission levels? As well as heat
> > dissipation for different material.
>
>Wayne,
>
>Apparently some have misunderstood my previous post regarding
>thermal data.
>
>Thermalwise - your last comment regarding heat dissipation for
>different materials - doesn't really matter with regard to the
>material surface-to-air interface as long as it's black in color.
>The material of the heatsink does matter with regards to heat
>flow from what it is sinking determined by thermal resistances.
>You want to keep your thermal resistances as low as possible.
>
>EMIwise - you've put an ungrounded structure very close to a noise
>source. It will matter with regard to radiation if the heatsink
>material is electrically nonconductive.
>
>Both of these points opens up the type of material you can
>actually use. If in fact there are EMI problems that result
>and you're using electrically conductive heatsink material?
>Consider grounding if possible. I've never had a heatsink
>give me an EMI problem.
>
>I have had a problem with a heatsink. It's size was such
>that it was able to introduce random errors through simple
>coupling of the noise by it's proximity to other circuits.
>That's more of a signal integrity issue than anything else.
>
>Our chip experts here on the list are going to have note
>if a heatsink does introduce a form of coupling within
>the chip that's unacceptable. I can't speak for that.
>
>Sorry for the misunderstanding.
>
>Regards, Doug McKean
>
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----------------------------
Bill Owsley
TMBU Compliance
919) 392-8341
owsley@cisco.com

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