Kim -

The terminals are a significant heat sink for embedded resistors, but
they can be in second place because of area considerations.  Imagine
that you lengthen an embedded resistor, keeping the width and the total
power the same.  Nothing changes with the terminals - they keep their
same Cu and contact geometry.  But, the resistor has more FR4 in
contact with it along its length until this becomes the dominant heat
transfer mechanism.  Yes, Cu is the better conductor by far, but it can
have a very small share of the contact area, and just on the very ends.

That trimming effect is interesting.  Do you guys try to do this when
you trim?

- Rick


On Jun 23, 2004, at 7:53 PM, Kim Fjeldsted wrote:

> Rick,
>
> Good summary of a poorly discussed area.
>
> For the long thin resistors to have better thermal dissipation, doesn't
> this imply that the FR4 has better heat conduction (and heat capacity)
> than the Cu terminals?  I had seen some modeling studies that showed
> significant thermal transfer at the terminals (Clouser, IPC EXPO'03?),
> to a point where trim cuts close to the terminals do not create hot
> spots.
>
> Am I missing something?
>
> Kim Fjeldsted
> 503.671.5218  Office
> 503.819.2176  Cell
> -----Original Message-----
> From: EmbeddedNet [mailto:[log in to unmask]] On Behalf Of Richard
> Ulrich
> Sent: Tuesday, June 22, 2004 1:33 PM
> To: [log in to unmask]
> Subject: [EM] CircuiTree Column
>
> As usual, I'd like to show the draft of my upcoming column to you EP
> experts, so please let me know what needs to be added or changed.  I've
> got a couple of weeks before it's due.
>
> Also, does anyone have a nice picture having to do with the thermal
> effects of embedded resistors that I could submit along with this
> article?
>
> - Rick
>
>
******************************************************
Dr. Richard Ulrich
Professor
Dept. of Chemical Engineering
3202 Bell Center
Univ. of Arkansas
Fayetteville, AR   72701

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(479) 575-5645
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