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Jeff Liebermann Jeff Liebermann is offline
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Default Heat sink grease

On Thu, 12 Apr 2018 12:05:53 -0700 (PDT), John-Del
wrote:

On Thursday, April 12, 2018 at 1:17:00 PM UTC-4, Jeff Liebermann wrote:


You get sort of a feel for how big of a daub or bead to apply.
Ideally it should stick slightly out the sides of the device,
that assures full coverage. You don't want a huge amount of
excess. Also in some cases it is a good idea to push the device
on to the heatsink by hand with as much force as possible.
Screws strip in aluminum and some of them have a spring type
hold down.


Sigh.


Not sure where you disagree Jeff.


Sigh 2.0

Not speaking for the other Jeff (jurb), but I agree with him.


That makes three Jeff's. Nobody ever agrees with me, so you must be
referring to Jeff 1.0.

If you get too much compound out of the sides, you've wasted it.
If none appears you may not have put enough.

The idea is to squeeze as much out as is possible ensuring that
only enough to fill gaps is left, on that we all agree. Forcing
the device down by external pressure helps make sure any excess
is forced out without relying on the devices own hardware to
accomplish.

The center daub verses the even spread is debatable either way.
RCA back in the late 60s told us to use the center daub and let
it spread out on it's own. Their reason was to avoid air pockets.
On small devices like transistors, that's what I do. On larger
devices, I don't think enough pressure can be exerted to ensure
a large area can force enough compound out of the joint, so
I tend to carefully spread as thin a layer as I can accomplish.


As usual, I beg to differ:

1. If you apply enough pressure to make the thermal goo ooooze out
from the sandwich, you will either bend the device (as in a TO-3 or RF
power xsistor) or simply not be able to apply enough pressure with the
mounting screws and springs. I was going to calculate or measure the
pressure applied based on the recommended mounting screw torque for
various heat sinks, but don't have the time. I might have some time
this weekend to sandwich some Dow Corning 340 between two metal slabs
in an arbor press with a load cell and see what it really takes to
squeeze out the thermal goo. As you mention, I also suspect it will
be rather high pressure, far beyond what can be done with small
fasteners. Offhand, I would guess that this method was invented by a
thermal goo salesman in order to inspire users to consume more of his
company's thermal goo.

2. The problem with forming bubbles (voids) in the thermal goo was
originally mentioned in the literature in reference to removing the
heat sink after the thermal goo was applied. That will certainly
create bubbles. Somehow, someone extended that to the initial
application of thermal goo, which is not correct. I've never bothered
to prove this, so if you have time, try a blob of your favorite
thermal goo between two glass plates and look at it under a
microscope. If there are any bubbles, they should be obvious. If you
want, I can do this Friday as I have the necessary equipment:
http://802.11junk.com/jeffl/pics/microscopes/Olympus%20BH/slides/Olympus%20BH.html
3. Thermal goo is fairly cheap. However, it does make a mess when
the excess is squeezed out of the heat sink sandwich. I prefer not
cleaning up the mess.

My method of applying thermal go is fairly simple. Find the direction
on the heat sink in which the machining marks and gouges run. Apply a
tiny amount of thermal goo to the heat sink. Use a plastic razor
blade:
https://www.google.com/search?q=plastic+razor+blade&tbm=isch
to smear the thermal goo in the direction of ACROSS the machine marks.
The idea is to push the thermal goo into the grooves, crevasses, and
gouges in the heat sink. If the mating part of the sandwich also has
machine marks, do the same thing. The initial blob of thermal goo is
intentionally insufficient to cover the entire heat sink. Add small
blobs and continue to smear until the surface is covered. You will
see quite a bit of metal. That's good as you want metal to metal
contact. If there's any excess, wipe it off with the plastic razor.
When done, clamp it together and DON'T take it apart for "inspection".
If you do, start over by wiping the heat sink clean with alcohol, let
dry, and do the blob thing again. Thermal goo tends to harden as the
carrier evaporates. It will not flow into the cracks easily and will
resist compression. Best to start from scratch.



--
Jeff Liebermann
150 Felker St #D
http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558