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

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From: Doug McKean (dmckean@corp.auspex.com)
Date: Tue Jul 25 2000 - 17:37:45 PDT


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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