Just a suggestion: As good as the compound is, it's still not as
conductive as the copper heat spreader.
The directions that should have come with the compound say to use as
little as possible (paraphrased). So by thinning the application, but
still maintaining 100% coverage, you should get even better heat transfer.
Heat transfer compounds have come a log way in just the past couple of
years.
73
Roger (K8RI)
On 4/10/2015 1:08 PM, Mike Waters wrote:
Manfred,
Thank you for this advice. I've been thinking about it ever since you sent
it. Today I got to see for myself the difference in thermal greases.
To make a long story short, replacing the original generic silver grease
(probably from Asia) between the CPU and its heat sink greatly lowered the
CPU core temps and eliminated all the problems I was having due to the CPU
overheating.
The thermal grease I used contains boron nitride flakes. It's *Arctic
Silver Premium Ceramic Polysynthetic Thermal Compound*. It's kind of an
off-white color. I even put in on thicker than I should have.
Before replacing that grease, it was not uncommon to see the CPU core
temperature over 190 degrees F. I now cannot get it over 120 F, even with
both cores at 99% for an extended period of time.
Thanks again!
73, Mike
www.w0btu.com
On Thu, Feb 5, 2015 at 11:38 AM, Manfred Mornhinweg <manfred@ludens.cl>
wrote:
Mike,
the thermal conductivity of such a paste depends more on the granulometry
of the solids in it, than on the actual material.
Thermal compound is usually a mixture of finely ground thermally
conductive material, and some sort of oil or other thick liquid. The
problem is that all suitable liquids have very low thermal conductivity,
compared to even a modest ceramic material, let alone metals. So, whether
the granules are alumina with a thermal conductivity of 18 W/(m*K), or
silver with 419 W/(m*K), makes little difference when the voids between the
granules are filled with an oil having a thermal conductivity of 0.21
W/(m*K)!
And between silver and copper (372 W/(m*K)) there is essentially no
difference, when the voids are filled with that oil.
But it makes a lot of difference whether all granules are balls of the
same size (worst), or flakes of widely varying sizes having flat sides
(that would be a good filler). With those flakes, there is far less space
between particles that has to be filled with the liquid, and this strongly
improves the thermal conductivity of the compound.
I would think that manufacturers of premium thermal compounds have their
own technologies to make filler powder with carefully optimized particle
size and shape. It should be such that under pressure the particles arrange
to squeeze out as much liquid as possible, so that the final compound layer
is mostly solid, with only very small spaces filled by the liquid.
Also it would help a lot to have liquids with higher thermal conductivity,
that are stable enough to use them in thermal compounds. Water (0.6
W/(m*K)) is much better than most other liquids, but of course it dries off
quickly, so it's useless in thermal compounds.
Cheap, lightly loaded thermal compounds typically have a thermal
conductivity of only around 0.3 W/(m*K), so it's only slightly better than
using unloaded plain grease. At least these are easy to apply and spread
out, producing a thin layer. Premium thermal compounds fal into the range
of 3 to 5 W/(m*K) - some manufacturers may claim even slightly more, and
it's up to teh user to believe that. But these premium compounds have lots
of solids and little liquids, so they are harder to spread out in a fine
enough layer. Well applied, they are indeed much better than the cheap
compounds, but used in a casual way they might not bring much improvement,
because the layer ends up thicker.
Instead if you solder down the parts to the heatsink, you get a thin
layer, along with the thermal conductivity of solder, around 50 W/(m*K).
This is very much better than any other mounting system allows, but it's
not always possible to do, it gives no electrical insulation, and there can
be problems coming from stress cracking due to different thermal expansion
of the parts.
Manfred
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
---
This email has been checked for viruses by Avast antivirus software.
http://www.avast.com
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
|