Thermal Interface Materials 101: Class is
in session…
Your goal is to use just
enough material to fill the voids. Any more becomes a bottleneck for heat
dissipation. So, you should apply a paper thin layer on the heatsink before you
install it. If you’re very conscious about it you may not want to use your
finger for this because. Again, don't use too much - the thinner, the better.
Before you secure the
heatsink to the CPU (or vice-versa), work the heatsink against the CPU with a
twisting motion to force any air bubbles from between the surfaces. Thermal
compounds can be very nasty substances. Most silicone based thermal goop is
sticky and is difficult to remove from your fingers and clothes.
Some high-end compounds
are electrically conductive, so be very careful in these cases to keep it away
from your computer circuitry, CPU pins, etc. Thermal compound is very
chemically stable, it normally does not get hard or dry and it will usually stay
sticky for years. If you’re really budget-minded, you could even reuse it when
you buy a new CPU. Now please
keep in mind that I absolutely DO NOT recommend this. But, most silicone thermal
compounds are so chemically stable that you could re-use it if you need a
temporary option until you get something else.
What is in that stuff?
Thermal compounds consist of two components: 1) Silicone or a similar binder
and 2) The conductive material itself. The conductive material usually falls in
one of two categories: Metal filled or metal oxide filled. The difference
between these is that the oxides aren’t electrically conductive while still
maintaining the original thermal conductive properties of the metal before it
oxidized.
To keep everything on an even playing field
I’m going to use the following method as the measuring stick for comparison’s
sake. The performance of thermal compound is measured in W/m*K. Various
materials conduct heat differently. The thermal conductivity, k,
of a material provides a measure of the ability of the material to conduct heat.
The thermal conductivity may change with temperature (i.e. k at room temperature
may be different than at 500 degrees), but the variation is often slight, and
for our purposes we can assume that k is constant.
Standard fare silicon/zinc oxide thermal
pastes have average thermal conductivity of approximately 0.7 and 0.9 W/m*K and
at the other end of the scale a high-end compound can average upwards of 5.5 - 6
W/m*K. Thermal conductivity is the
main criteria we’ll be looking at but the compound’s consistency should also be
very smooth. A compound that is either grainy or too hard or thick hinders its
ability be applied in a really thin layer. On the other hand, the thermal
compound shouldn’t be excessively fluid or runny either. In
evaluating the thermal interface materials in this review I’ll be looking at
thermal resistance measured in W/m*K. Remember… The higher the conductivity
rating, the better. To get an overall picture of other materials’ thermal
conductivity, check out this
guide.
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Page: The contenders
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