[Amps] Arctic Silver with and without silver

[Roger (K8RI)]

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 at 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
>>
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