Choosing The Right Paste: More Than A Matter Of Price
Because thermal paste is a high-margin product, the market is crowded. While the exact composition of most products is a well-kept secret, a Google search makes it pretty easy to get a list of typical ingredients. The upper temperature limit is typically 150 °C, though some pastes claim to withstand up to 300 °C or more.
The composition of a paste determines its thermal conductivity, its electrical conductivity, its viscosity, and its durability. But what is a paste really made of? Basic compounds consist of zinc oxide and silicone as a binding agent. However, such simple combinations are barely sold anymore. Most vendors start with these ingredients and add other materials, like aluminum. Case in point, the Prolimatech PK1 sports 60-85% aluminum content, 15-25% zinc oxide, and 12-20% silicone oil, as well as an anti-oxidation agent. Some ingredient lists are more mysterious. For instance, the one printed on be quiet!'s DC 1 syringe ambiguously specifies 60% metal oxide, 30% zinc oxide (wait a second; since when is zinc not a metal?), and 10% silicone.
Some pastes, like Arctic Silver 5, even contain silver. Other pastes are based on graphite, like the professional-grade WLPG 10 by Fischer Elektronik, forego the silicone, and claim very high thermal conductivity (10.5 W / m·K), but they are more difficult to apply and are typically electrically conductive. There are also pastes that employ carbon nanoparticles, though they're not suitable for most enthusiasts due to their electrical conductivity and price. The number of copper-based pastes on the market has shrunk, but if you search, you can still find a few.
I leave the more exotic thermal coupling solutions like liquid metal and metal pads for the second part of our tutorial. Applying these electrically conductive products is not without risk, and I don’t want to confuse anyone with a detailed discussion of them at this point. Let’s just say that they're for expert use only, and you'll want to satisfy a few prerequisites prior to applying them.
All pastes share one thing in common: regardless of their composition or price, they all fall short of heat sinks and spreaders with regard to thermal conductivity. Thus, a thermal paste is always the weakest link in the cooling chain, regardless of its price!