View Single Post
  #36   Report Post  
Posted to sci.electronics.repair
Jeff Liebermann Jeff Liebermann is offline
external usenet poster
 
Posts: 4,045
Default Heat sink grease

On Fri, 13 Apr 2018 15:57:55 -0700 (PDT), whit3rd
wrote:

That greasy stuff (also available in waxy and rubbery forms) is thermal transfer compound;
like a grease, but it is NOT grease.


Well, it's called thermal grease, silicon grease, high viscosity
semi-solid, or maybe magic grease. While not a lubricating
petrochemical compound, it is a type of grease.

It has to stay put in small crevices in order to work.


It also has to have a grain size small enough to fit in the crevices.
If you use a 'grease' formula instead of a 'thermal compound' formula, it might only
work for a few hours, then flow away.


Actually, it's worse than that. When heated, the viscosity of
commodity silicon grease is lowered, making it flow easier. It also
expands when heated. If I built a sandwich with a spring loaded heat
sink, some thermal goo, and a CPU, the expansion and easier flow will
cause the thermal goo to sloooowly ooooooze out of the sandwich. The
spring tension (as found in some CPU coolers) will help prevent air
gaps and thermal goo losses. Setting the spring pressure correctly is
tricky. If too little, the assembly will eventually rattle. If too
much pressure, and if too much thermal goo was added, it could
potentially make a mess. Fortunately, as the thermal goo slowly
ooooozes out from the sandwich with every thermal cycle, the gap
between the heatsink and the CPU slowly decreases, causing an
improvement in heat sink performance. That's why Arctic Silver and
others mention that you should see lower temperatures after the
thermal goo has had time to "break in" [1].
https://archive.techarp.com/showarticle600c.html

If you try to add particles with high thermal conductivity to grease, you might
just be making spacers to keep the metal parts at a distance (it's distance times
resistance-to-heat-trasfer that you want to minimize). Don't fall for the 'better
conductivity' argument, it's conductivity DIVIDED BY GAP DISTANCE you care about.


Yep. Actually, energy distribution is by the inverse square of the
distance, but at tiny distances, it might as well be linear. Having
too large a grain size will certainly ruin the thermal conductivity
but not because they don't fit in the cracks. It's because large
particles offer fewer points of contact between adjacent particles
than smaller (nano) particles:
Particle Radius Thermal Conductivity
50 micro meters 0.8 W/mK
1 micro meter 1.1 W/mK
0.003 micro meters 2.4 W/mK
I don't have numbers handy for the grain size used in commodity
thermal goo.


[1] http://www.arcticsilver.com/as5.htm
Due to the unique shape and sizes of the particles in Arctic Silver
5's conductive matrix, it will take a up to 200 hours and several
thermal cycles to achieve maximum particle to particle thermal
conduction and for the heatsink to CPU interface to reach maximum
conductivity. (This period will be longer in a system without a fan on
the heatsink or with a low speed fan on the heatsink.) On systems
measuring actual internal core temperatures via the CPU's internal
diode, the measured temperature will often drop 2C to 5C over this
"break-in" period. This break-in will occur during the normal use of
the computer as long as the computer is turned off from time to time
and the interface is allowed to cool to room temperature. Once the
break-in is complete, the computer can be left on if desired.




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