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From: "Spehro Pefhany"
Subject: Re: Thermal resistance--does this sound right?
References: <3D781177.firstname.lastname@example.org> <3D78ECA4.4FAAE397@SpamMeSenseless.us.ibm.com>
User-Agent: tin/pre-1.4-19990216 ("Styrofoam") (UNIX) (Linux/2.2.14 (i586))
Date: Sat, 07 Sep 2002 11:58:23 GMT
NNTP-Posting-Date: Sat, 07 Sep 2002 07:58:23 EDT
The renowned Phil Hobbs wrote:
> It's fortunate that this is so, because the simple calculations people
> use (based on 1-D thermal resistances) become seriously inaccurate at
> small thicknesses. There's a poorly understood "interfacial thermal
> resistance" that becomes dominant for thicknesses less than 10 microns
> or so--it can easily be 0.1 K*cm**2/W. It's sensitive to the history of
> the surfaces and to the exact makeup of the joint materials. This is
> very inconvenient sometimes--I ran into it trying to figure out why
> multistage Peltier coolers aren't better than they are.
If I remember the app notes for serious power semiconductors, they tend to
talk in terms of thermal resistance at a given pressure rather than some
meaningless thickness. Though, if the surfaces were surface-ground and
polished (diamond buffed) to an A-1 (<1 microinch RMS) finish, you might
be able to use the calculations.. but typically roughness and flatness on
those surfaces are not very good.
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