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Cova Scientific By Cova Scientific • September 24, 2015

Silicone and Synthetic Greases As Thermal Interface Materials



Thermal greases are non-reactive materials that provide increased thermal transfer between substrates. These thermal interface materials are usually either a silicone or synthetic fluid that is filled with various thermally conductive materials, from alumina to diamond particles. This blog post is all about how thermal greases work, why they are useful and the different types of greases on the market today.

Why are greases used as thermal interface materials?

In electronics today heat sinks are used to pull thermal energy away from high heat producing components such as CPUs. Heat sinks are generally aluminum alloys or other types of metals that exhibit high levels of thermal conductivity (aluminum alloy 1050A has a thermal conductivity value of 229 W/mk). Unfortunately the interface between a heat sink and a CPU surface is never perfectly flush. Air pockets along the interface create thermal resistance. Thermal interface materials, such as thermal greases, are significantly more thermally conductive than air and are used to display the air in these gaps. Filling these air voids along the interface significantly improve the efficiency of the entire heat sink system.

Silicone vs. synthetic fluid, whats the difference?

Thermal greases can come either as a silicone grease or as a synthetic fluid. Silicone based products are the traditional choice, however, they have some drawbacks including bleed and silicone migration. Basically, silicone greases like to get everywhere and tend to contaminate nearby surfaces. Synthetic fluids are easier to control and are basically a cleaner alternative.

What are the different types of filler materials?

Both traditional and synthetic materials can be filled with a number of different particle materials. Choice of filler determines two major aspects of a thermal grease. It determines:

  1. the thermal conductivity of the compound and...
  2. the minimum bond line thickness - bond line thickness is normally limited by the filler material particle size
Both are important for the ultimate efficiency of the heat sink system. Common filler materials include zinc oxide, boron nitride, and diamond. The three increase in thermal conductivity in the same order, zinc oxide offering the least thermal conductivity and diamond the most. Each type of filler material can come in a number of different partical sizes.