Thermal Compound Selection Guide

Thermal Interface Material (TIM) is any material used to enhance thermal coupling between two parts. TIM may be known by any number of names including thermal compound, thermal grease, thermal gel, heat sink compound, CPU grease, gap filler, and thermal paste. Each formulation provides an advantage in specific use conditions.

Mechanics of Heat Transfer

Choosing the best thermal compound requires some understanding of the mechanics of heat transfer and how the thickness of the thermal compound layer, the bond line thickness, influences product choice.

Bond line can be divided into three categories:

• Low, at less than 75 μm
• Medium, from 75 to 250 μm
• High, at greater than 250 μm
  
  

There are two critical thermal performance characteristics: Thermal Conductivity (TC) and Thermal Resistance (TR). In low bond line applications, thermal resistance dominates performance. In high bond line applications, thermal conductivity dominates performance. In medium bond line there is a blended influence.

Thermal Conductivity (TC)

TC is a measurement of heat transfer between Material 1 and Material 2, expressed in units of W/mK (see Figure 1). The thicker the layer of thermal compound, the greater the influence of thermal conductivity. Examples: copper 385, steel 50.4, glass 0.80, TIM 0.6-8.0, and wood <0.12.

Figure 1. Thermal Conductivity. How thermal compound (interface material) creates an
uninterrupted, thermally-conductive path between two materials.

Thermal Resistance (TR)

TR is a measurement of temperature drop across an interface of materials, expressed as °C/W. Thermal compounds that have the best wetting and filler structure can have exceptionally low thermal resistance with moderate thermal conductivity. In low and medium thickness applications this lower thermal resistance can greatly enhance heat transfer because thermal coupling is more efficient.

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