When Does a Thermal Interface Material Need Potting?

The interface question

The working principle behind interface materials is stated plainly in the thermal management documentation: any two component surfaces that appear very smooth will have gaps when they come into contact, and the air in those gaps is a poor conductor of heat. Once an interface material is added, the air at the interface is occupied by a material with good thermal conductivity.

That is the entire job of a thermal interface material. It does not have to hold anything together and it does not have to survive as a structural element.

• Thermal grease: paste, non-flowing, non-curing; excellent thermal conductivity and electrical insulation; good high and low temperature stability; low oil separation and low volatility.
• Thermally conductive gel: low stress and compressible, with good electrical insulation, good high and low temperature stability and excellent damping and shock absorption.

When potting is also required

Potting becomes necessary when the assembly also needs mechanical support, environmental sealing or dielectric protection in addition to heat transfer. At that point a non-curing interface material cannot do the job, because it will not stay in place or carry load.

Thermally conductive potting compounds in this range are described as low viscosity with excellent leveling, able to cure deeply into an elastomer, and combining good thermal conductivity with flame retardancy.

Both at once is normal

It is common to use an interface material on the component to board contact and a potting compound to fill the surrounding housing. The two decisions are independent and should be specified independently.

For inverters and power supply modules the stated recommendations combine potting compounds such as GMX-8152/11 and GMX-8152/08M with thermal grease GD-6941 series in the same assembly.