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August 16th 04, 07:11 PM

In article ,
says...
Keith wrote:

On Sat, 14 Aug 2004 18:24:14 +0100, John Woodgate wrote:

I read in sci.electronics.design that Keith wrote (in
) about 'Error in textbook?',
on Sat, 14 Aug 2004:

Unfortunately, boiling the CFC also distilled it, leaving any
contamination on the chips.

Sulfur hexafluoride might have been better, but it's probably more
costly than CFCs were.

Cost? Dunno. I do know that we were paying $50/qt (IIRC) for the stuff
in '75. I wouldn't think a sulfur/fluoride brew could be made any
"cleaner" than CFCs. THe problem was the contaminants.

Yeah, and whether you're working with a liquid or gas, it's the contaminants
that make it dirty. I'd think a gas could be cleaner already just by not
dissolving a bunch of solids, although the solids in suspension can be
even smaller than in a liquid. But those aren't going to condense out.
They might collect as dust, but if that's your problem, then you have
a lot worse problem than contaminated fluid!

The issue was that during the phase change the contaminants were left
behind. They were soluble in the liquid, but have a far higher BP than
85Cish so they're distilled *onto* the chips.

I guess my point is, it wasn't the CFC's fault that it was contaminated.

I didn't blame the CFC. I don't "blame" inanimate objects. ;-) No
matter how clean the CFCs any trace contaminate deposited on the parts
that were boiling, exactly where you don't want the contamination.

And, FYI, AIUI, SF6 is really quite inert. You make it sound like fire
and brimstone, for heaven's sakes! ;-)

I tried to look up SF6's properties, but got nowhere. I don't think it
could have replaced CFCs. The particular CFC was chosen for its
precise boiling point.

But it has raised a good question - which makes a better heat transfer
agent? Helium or SF6? (gases only, of course ;-) )

Note that the He was aided by pistons contacting the back of the chip
(C-4 mounted). It was a much different and more complicated package
than the LEM.

--
Keith