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From: Phil Hobbs
Subject: Re: Thermal resistance--does this sound right?
Date: Fri, 06 Sep 2002 13:57:56 -0400
Organization: IBM T. J. Watson Research Center
NNTP-Posting-Date: 6 Sep 2002 17:57:58 GMT
X-Mailer: Mozilla 4.61 [en] (OS/2; U)
Bob Wilson wrote:
> Nearly ANYTHING will give you less than 1 degree per Watt, for the large
> area you are referring to (assuming the heat is evenly distributed). Even no
> grease at all (just air) will do it. So if you want to get a value of (say)
> less than 1 degree/Watt, use anything you want; grease, sil-pads
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.
Thermal transfer materials suppliers also peddle a lot of snake oil.
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