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 Carbon Black   
Black is Beautiful
(Review by MS, October 7, 2003)
Methodological Considerations

One thing that needs to be made very clear from the beginning is that the values that we show are by no means accurate in terms of absolute temperatures. Each mainboard, within certain margins will translate the read-out from either the thermal sensors or else from the CPU's own thermal diode in a somewhat different way. In addition, we know that different BIOS revisions will change the read-out as well even if the hardware remains identical. In so far, we are stuck here really with comparing apples and apples or rather the same apple under different lights.

Likewise, there is no way to actually substitute any external measurement for the read-out of the on-die diode. Admittedly, the sensitivity of the diode by far exceeds the actual temperatures as measured in the socket or at the base of the heatsink but that is exactly what we are looking for: a super-sensitive measuring device that blows up differences "out of proportions" but is capable of showing these differences - as long as they exist.

Pressure Considerations

Carbon Black performance is strongly dependent on the pressure applied on the material. Below 100 psi, the compression is not sufficient to enable optimal conductivity of the carbon particles. We measured the amount of pressure required to disengage the bracket of the Thermalright SLK800 to be around 45 lbs, which translates into a pressure of roughly 90 lbs of the heatsink onto the processor's die of 108 mm2. Corresponding metrics are 42 kg/cm2 or ~4.1 MegaPascal or ~ 600 PSI. To the best of our knowledge, performance of AS is not pressure-dependent as long as there is enough pressure to warrant good contact.

Layer Thickness and Surface

Unless both surfaces are perfectly mirror polished, there will be grooves and "striations" from the machining process that, as established in numerous lapping articles, are the main source of insufficient transfer - unless good thermal interface material is used.

The Thermalright SLK800 does not fall into the mirror-polished category but for practical purposes, this is just what we need for a proof of concept.



Top row: Arctic Silver Céramique imprint on both CPU die and bottom of the cooler. In reality, the residual film with the crescent-shaped striations is only a haze, however, the oblique angle of the flash brings out the coverage and makes it look much thicker than it really is.

Bottom row: Arctic Silver Original forms a relatively thick TIM layer which is a known issue that has been resolved with ASIII. AS - Carbon Black emulgate (a bottle screw top is a perfect mixing bowl) note the "inhomogeneous" consistency of the emulgate. Right: "print" on the heatsink after disassembly.

The "Céramique" photographs show rather clear what is going on. The striations result in different thickness of the TIM meaning that both thermal conductance AND spreadability will play a major role. In addition, the ridges between the grooves are posing enough mechanical resistance to prevent a high pressure build-up in the material within the grooves, that is, the highly fluid Carbon Black will either escape to the sides or else just reside as a filler, albeit, without the pressure required to build up enough conductance and contact with the "host" surfaces.

Arctic Silver, on the other hand is simply too viscous to allow optimal spreadability of the TIM between the two surfaces. The emulgate on the other hand will have a two-fold effect, which is first, to block the "channels" and therefore prevent the leakage of Carbon Black, second, in areas of increased TIM thickness, the higher thermal conductance of AS will bridge the distance more sucessfully than Carbon Black by itself. Conversely, suspension of AS particles within the Carbon Black carrier, reduces the AS viscosity. and allows the formation of a thinner transfer layer.

One additional effect of the suspension could be that the elastic and compressible Carbon Black provides a better filling also in between AS particles.

"Céramique" by Arctic Silver still gave the overall best performance which is consistent with an extremely fine grain compound with high thermal conductivity. Notwithstanding these findings, Carbon Black turned out to provide similar performance as Arctic Silver for a fraction of the price. On the other hand, a heterogeneous suspension of highly conductive metal particles in Carbon Black may improve the thermal transfer between uneven surfaces.

If we had used two mirror polished surfaces against each other, we would have predicted a better performance of Carbon Black.

Stay tuned for Carbon Black "Part 2"

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