"Dave D" wrote in message
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"Arfa Daily" wrote in message
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"CitizenRuth" wrote in message



If it's white, and gets all over your trousers as soon as you walk in the
same room as it, then it's silicon grease, also known as thermal grease,
or heatsink compound.


Sorry if this seems pedantic, but silicone (not silicon) grease and
heatsink compound are not the same thing at all. The former is a clear
grease, often used for HV insulation and sometimes for lubricating plastic
gears. The latter is a silicone *based* compound with additives for
conducting heat efficiently.

Contrary to what you believe, the surface tension exhibited by this stuff
over a large area, causes it to stick surfaces together like the
proverbial, and it is often necessary to get a screwdriver in between the
two sheets of metal to get some air in between to ' break ' the bond.

That is true.

Thermal adhesive, on the other hand, is just that. It sticks heatsinks
directly to the tops of ICs, with a thermal conductivity similar to that
of silicon grease. The difference is that once it has done this, you
would need a crowbar to separate them again.

Provided there is still a reasonably even film of grease on both
surfaces, and it has not dried out and gone ' chalky ', then you can go
ahead and just reassemble with the old grease.

I would advise against this. It is by far the best course of action to
thoroughly clean both surfaces of old heatsink compound and reapply new.
It is after all rather cheap and readily available. Semiconductors OTOH
can be pricey, difficult to obtain and can cause widespread damage when
they short ;-)

Dave

Hi Dave

Yeah Yeah - you're right ! The missing ' e ' on the end of silicone was a
typo. I probably should have put it the other way around like " It's
heatsink compound, otherwise known as thermal grease or silicone grease ". I
know that it's not actually the same stuff as silicone grease - I have a big
tub of that which I use for all sorts of plastic lubricating. But I think
that you've gotta agree that for as long as any of us in the business can
remember, this hateful stuff has been known generically ( albeit
erroneously ) as ' Silicone Grease '. ( probably because it's based on
silicone, and greasy ... )

As far as replacing the compound goes, I would absolutely, definitely and
without question, agree with you to always always always replace compound
between semiconductors and their heatsinks, first cleaning off all traces of
the old, chemically. However, where the compound is between a true heatsink,
and a secondary ' belt and braces ' piece of metal such as a back panel,
like the OP seems to be suggesting, then in my experience, there is no need
to cover yourself, the kids and next door's cat ( for if any of them are
within 30 yards of you, they will get covered ... !! ) in the stuff, just
for the hell of cleaning it off and replacing it. Compound in this sort of
position doesn't normally get to a high enough temperature to dry out and go
powdery. : -))

Arfa