Why Is Thermal Grease Still Needed When Metal Surfaces Are in Direct Contact?

Many engineers wonder: if a heat sink is bolted directly onto a metal component, why is thermal grease still needed? The answer lies in the microscopic reality of metal surfaces. Even two apparently flat, polished metal surfaces touch only at a tiny fraction of their apparent area, and the trapped air pockets in between block heat flow. This article explains why direct metal-to-metal contact is never truly direct - and why thermal paste remains essential.

Metal Surfaces Are Not Flat at the Microscopic Level

Grinding, milling and lapping leave surface roughness of 0.4 to 3.2 µm Ra or more. When two surfaces are clamped together, contact occurs only at the highest peaks. Studies show that the real contact area between clamped metal surfaces is often less than 2-5% of the nominal area. The remaining 95% is trapped air.

Air Is an Excellent Insulator

Air conducts heat at roughly 0.026 W/m·K - about 10,000 times worse than aluminum (237 W/m·K). Even a very thin air layer creates a large temperature drop, because heat must squeeze through the few touching peaks.

What Thermal Grease Does Between Metal Surfaces

• Fills the valleys with a material conducting 1.0-6.0 W/m·K;
• Expels trapped air during assembly under mounting pressure;
• Creates hundreds of parallel heat paths across the whole interface;
• Reduces contact thermal resistance by up to an order of magnitude;
• Prevents oxidation and fretting at the joint over time.

When Is Thermal Grease Most Important?

Is It Possible to Improve Direct Contact Without Grease?

Options such as polishing, lapping or soldering can improve direct contact, but they are expensive and impractical in mass production. Thermal grease delivers most of the benefit at a fraction of the cost. Compared with rigid soldered joints, grease also tolerates differential thermal expansion because it stays soft and elastic.

Common Misconceptions

"Tightening harder removes the need for grease"

Higher pressure increases contact area but can warp components or crack dies. Grease achieves low resistance without excessive clamping force.

"Liquid metal is always better"

Liquid metal alloys conduct extremely well but are electrically conductive, corrosive to aluminum and difficult to handle. Ceramic-filled silicone grease is safer for most applications.

How to Verify the Benefit in Your Own Product

1. Mount the heat sink without grease and record the steady-state component temperature;
2. Reassemble with a controlled 20-50 µm grease layer;
3. Measure the temperature again under the same power and airflow;
4. Typical improvement is 5-20°C depending on power density and surface quality.

Surface Roughness, Pressure and Grease Selection

The rougher the surfaces, the thicker the grease layer needed to fill the valleys - but thicker layers also add resistance. In practice, manufacturers specify a controlled bond line of 20-80 µm and a mounting pressure of 20-60 N/cm² for spring-clip or screw assemblies. The grease viscosity should be matched to the assembly process: low-viscosity grease self-levels under light pressure, while higher-viscosity grease resists pump-out in vibrating equipment.

FAQ

Do I need thermal grease on anodized aluminum surfaces?

Yes. Anodized layers are rough and porous; grease fills the pores and improves heat transfer significantly.

Can thermal grease be applied too thick?

Yes. Excess grease acts as an insulator. Use the minimum thickness that gives full coverage - typically a thin, translucent layer.

Conclusion

Direct metal contact without thermal grease is a myth in practical engineering: trapped air always blocks heat. A high-quality thermally conductive silicone grease fills the microscopic gaps, cuts contact resistance and keeps components cool for years. For bulk thermal paste supply and OEM formulation, contact Hanast for samples and thermal testing support.