On Sun, 27 May 2007 18:41:33 +0100, Gibbo
wrote:

John Larkin wrote:

Besides, filled adhesives aren't all that much better than unfilled
ones, even if the filler is diamond.


I made some measurements on Araldite filled with copper filings to quite
a high density, basically as much copper as I could get in there and
keep it workable. The heat transfer through it was amazing. I was quite
impressed.

Then my techie did the same tests with plain Araldite and the results
were a few percent worse. It actually made hardly any difference. So
you're bang on.

Yeah. Apparently random particles packed into an adhesive don't
actually contact very well, so the base material still dominates.

The key to good heat sinking is to keep the gap small, which means
very flat surfaces, and using a material that squeezes out as thin as
possible, with appropriate pressure. In that situation, sil-pads,
phase-change materials, and some filled compounds can be much worse
than plain grease, which can squeeze down into the microinches. Your
copper filings probably increased the gap more than they helped.

John