Why so very small amount of thermal compound?

I have read posts that suggest minimal thermal compound. The Arctic Silver
instructions also suggest a very small amount.
http://www.arcticsilver.com/arctic_s...structions.htm

I understand this advice if the concern is that too much compound may
contaminate the pins and cause electrical problems. But, I don't understand
it if the concern is heat transfer.
I realize that the objective is to only fill the microscopic gaps between
the two surfaces.

After the heat sink is mounted, the force pushing the two surfaces together
is so high that any extra compound will be squeezed out of the sides. The
compound is a fluid after all. There will be no compound between the peaks
(only in the valleys). Is that not true?
Am I missing something?

Thanks,

Navid

I understand this advice if the concern is that too much compound may
contaminate the pins and cause electrical problems. But, I don't understand
it if the concern is heat transfer.

The thermal conductivity of any fluid or suspension, however
well-designed, will never be as good as the thermal conductivity of a
metal. This is due to the way in which heat flows - heat is a
consequence of the random motion of atoms and is transmitted more
efficiently if the atoms are bonded in a regular (dense) structure, with
lots of (relatively) weak bonds to transmit the random vibrations along.

Hence, you want to apply as little as possible, only filling the valleys
between the surfaces (removing air which does not conduct heat well),
but not adding any more layers (of less thermal conductivity than the
heatsink) than is absolutely necessary between the chip and the sink.

The ultimate 'thermal compound' would be liquid metal that gets poured
between the chip and sink and forms a perfect seal upon cooling.
However, this would destroy your chip...

I realize that the objective is to only fill the microscopic gaps between
the two surfaces.

This is because the thermal conductivity of your Arctic Silver (or even
generic thermal grease) is greater than that of air, so you remove all
air pockets.

After the heat sink is mounted, the force pushing the two surfaces together
is so high that any extra compound will be squeezed out of the sides. The
compound is a fluid after all. There will be no compound between the peaks
(only in the valleys). Is that not true?

There will always be a small amount of thermal compound between the
peaks, however little, as it will be nigh-on impossible to remove all of
it purely by pressure. However, the separation between the peaks will be
much less than that between valleys, as the force is indeed rather large.

After the heat sink is mounted, the force pushing the two surfaces together
is so high that any extra compound will be squeezed out of the sides. The
compound is a fluid after all. There will be no compound between the peaks
(only in the valleys). Is that not true?

Well, the same theory could be applied to water under a car's tires, and
yet cars DO hydroplane.

You are right the force is great, but some small amount will remain.

Navid wrote:
I have read posts that suggest minimal thermal compound. The Arctic Silver
instructions also suggest a very small amount.
http://www.arcticsilver.com/arctic_s...structions.htm

I understand this advice if the concern is that too much compound may
contaminate the pins and cause electrical problems. But, I don't understand
it if the concern is heat transfer.
I realize that the objective is to only fill the microscopic gaps between
the two surfaces.

After the heat sink is mounted, the force pushing the two surfaces together
is so high that any extra compound will be squeezed out of the sides. The
compound is a fluid after all. There will be no compound between the peaks
(only in the valleys). Is that not true?
Am I missing something?

The force isn't as high as you think and the thermal compound doesn't flow
as easily as you think.


Thanks,

Navid

"David Besack" wrote in message
...
After the heat sink is mounted, the force pushing the two surfaces
together
is so high that any extra compound will be squeezed out of the sides.
The
compound is a fluid after all. There will be no compound between the
peaks
(only in the valleys). Is that not true?

Well, the same theory could be applied to water under a car's tires, and
yet cars DO hydroplane.

You are right the force is great, but some small amount will remain.

OK.
So, the pressure is not enough to get rid of all the compound between the
peaks, and a small layer remains.
Thanks for correcting me.

"Navid" wrote in message
. com...
I have read posts that suggest minimal thermal compound. The Arctic
Silver
instructions also suggest a very small amount.
http://www.arcticsilver.com/arctic_s...structions.htm

I understand this advice if the concern is that too much compound may
contaminate the pins and cause electrical problems. But, I don't
understand
it if the concern is heat transfer.
I realize that the objective is to only fill the microscopic gaps between
the two surfaces.

After the heat sink is mounted, the force pushing the two surfaces
together
is so high that any extra compound will be squeezed out of the sides. The
compound is a fluid after all. There will be no compound between the
peaks
(only in the valleys). Is that not true?
Am I missing something?

A layer will always remain, impeding heat transfer.

That said, you can glop on as much compound as you like, and the pressure of
the heatsink will push out enough to always give you half-decent
temperatures.

Unless you're a mad overclocker who wants every single megahertz and every
single 0.1 degree, you shouldn't worry about it. I certainly don't, and I
overclock, AND I've never had heat issues.

On Thu, 17 Jun 2004 01:17:45 -0400, "Mercury"
wrote:


"Navid" wrote in message
.com...
I have read posts that suggest minimal thermal compound. The Arctic
Silver
instructions also suggest a very small amount.
http://www.arcticsilver.com/arctic_s...structions.htm


A layer will always remain, impeding heat transfer.

That said, you can glop on as much compound as you like, and the pressure of
the heatsink will push out enough to always give you half-decent
temperatures.

Unless you're a mad overclocker who wants every single megahertz and every
single 0.1 degree, you shouldn't worry about it. I certainly don't, and I
overclock, AND I've never had heat issues.



A wonderful approach.

The moron's guide to unending stupidity.

...of course, those with IQ's over 40 understand it's easier to just
use less.

but... the morons will always be with us.

Let's have a hand for the moron. And let me say it's always been such
a pleasure working with people like you.

A layer will always remain, impeding heat transfer.

True, but it won't impede heat transfer as much as air will - that's why
you apply the stuff in the first place. Adding too much could cause the
stuff to glop out and bridge 2 pins. Some thermal compounds are
conductive, so you have just b0rked your system by doing that. AS is
slightly capacitive, so it could do nasty things to system stability if
it were to flow across 2 pins, and others are electrically neutral (like
their Ceramique compound) so you could smear your whole board in it and
suffer no ill effects.

Plus your syringe of AS will last longer if you use the tiny amounts
prescribed.

Lech Staniewicz wrote:

A layer will always remain, impeding heat transfer.


True, but it won't impede heat transfer as much as air will

Not necessarily true. With the proper amount there is contact with only the
voids filled but with too much there is a layer of the thermal compound
between the two surfaces, separating them. Where, before, with 'dry, you
had contact with some air, too much has thermal compound between all of it
and that layer can be worse than the smaller amount of air in the dry mating.

See he

http://power.ece.uiuc.edu/Balog/imag...lectronics.pdf

The principles are the same for processor heatsinks.



- that's why
you apply the stuff in the first place. Adding too much could cause the
stuff to glop out and bridge 2 pins. Some thermal compounds are
conductive, so you have just b0rked your system by doing that. AS is
slightly capacitive, so it could do nasty things to system stability if
it were to flow across 2 pins, and others are electrically neutral (like
their Ceramique compound) so you could smear your whole board in it and
suffer no ill effects.

Plus your syringe of AS will last longer if you use the tiny amounts
prescribed.

wrote in message
...

Let's have a hand for the moron.


Maybe instead of calling me a moron you could explain what it is I said that
you consider wrong.

Obviously glopping on heatsink compound is wasteful and runs the risk of
causing electrical problems.

My point was that one shouldn't worry about getting the
impossibly-ultra-thin layer that is mentined in the AS3 instructions.

And it's not "easier to use less". I tried the "half-grain of rice", and I
don't have the patience or dexterity to work with it. I put on enough to get
a nice even layer using a credit card. Like buttering toast.

If you don't like it, do it your own way, and don't me a moron.