Compatherm Paste

Compatherm Paste beats top-range interface materials

The conventional wisdom that the thinnest films of the highest conductivity materials provide best thermal performance is challenged by materials with 10x better performance than 2–5x higher conductivity graphite TIM’s.

Major global OEM’s in the telecommunications industry are wholesale replacing their graphite TIM’s with paste, such as the Compatherm Paste 9543. Why are they doing this? What is the benefit of using paste vs. graphite?

Graphite materials use the high conductivity of the graphite flake to create a sheet that has a very high conductivity in-plane, up to several hundred W/(m·K). When the manufacturing process is adjusted to not conform the orientation of the graphene flakes very strictly with the plane of the material, some of this potential is leveraged as through-plane conductivity. These sheets then have through-plane conductivities of between approximately 5 and 15 W/(m·K), which are respectably high numbers for thermal interface materials.

These TIM sheets are manufactured for use as “thin bondline” materials – materials that give the thinnest films for use in challenging applications that demand the very best thermal performances, because they provide the shortest heat conduction path. Thus they are usually found in the 100µm thickness range – typically between 60 and 200µm.

 

principles

 

Several factors affect

It is therefore natural to expect that with such thin bondlines and high conductivities, these materials would be the very best materials imaginable for challenging applications – and looking at the simple formula for thermal resistance, that would indeed seem to be the case.

However, this expectation overlooks the fact that several other factors affect a TIM’s thermal resistance. The conductivity is merely one, and not always even the most significant, material parameter with an influence on the final performance.

Graphite materials are by nature very hard; something akin to soft paper or cardboard. They are also completely dry. The material’s ability to displace air from the thermal interface – any TIM’s primary function – is therefore entirely dependent upon enough pressure being applied to force the material to conform to adjoining geometries. The ability of components to withstand such pressure therefore becomes a limiting factor.

 

Compatherm Paste graph

 

Superior thermal resistance

The graphs show the thermal resistance vs. pressure performance of thermal interface materials. Lower thermal resistances denote better performance; and good performance at lower pressure is generally desirable. Thus, the closer to the origin a material comes, the better it may be considered to be.

Into the graphs, 21 different 100µm-range graphite materials, from seven manufacturers, have been plotted. In the same graphs are also Nolato’s eight Compatherm Paste range thin bondline TIM’s.

The graphs tell us that in spite of having much lower thermal conductivities than the graphite materials – by as much as a factor five – they still provide superior thermal resistance over the whole range, most particularly at low pressures – by a factor ten and more!

 

Compatherm Paste graph 

 

The importance of contact

Why is this? How can materials with so much lower conductivities show that much better thermal performances?

One part of this is the fact that these pastes generally allow much thinner bondlines – a few tens of microns, vs. the 100 microns of the graphite. But this is not entirely significant – some of the graphite TIM’s are as thin as 45µm.

More important than this is the contact resistance. The pastes wet the surfaces of the heat sink and the heat source, thoroughly displacing all the air. This requires basically no pressure at all. By contrast, the graphite materials depend solely on high pressure to overcome the contact resistance.

 

A vastly superior solution

From these results, it is very easy to determine that:

  • When 5x lower thermal conductivities still can provide 10x better performance, thermal conductivity cannot be relied upon as the determining metric for thermal interface materials.
  • For the very most demanding thermal challenges, and/or the most pressure sensitive high-power components Compatherm Paste will offer a vastly superior solution to any known graphite TIM.