Engineering guide
Ceramics for Thermal Management
Moving heat while keeping the circuit isolated. Aluminum nitride is usually the answer, but the interface and the flatness matter as much as the material.
Requirement
Two requirements at once
Most thermal solutions fail because they solve one of the two requirements and add an interface for the other.
| Requirement | Common solution | Cost of that solution |
|---|---|---|
| Remove heat | Metal spreader | Electrically conductive |
| Isolate | Separate insulator | Adds an interface and a contact resistance |
| Both | Aluminum nitride | Higher material cost, one fewer interface |
Contact resistance beats material conductivity
A better material with a poor interface performs worse than a moderate material with a good one.
- Flatness of the thermal face controls how much of it touches
- Surface roughness controls the contact area at the interface
- Cleanliness after grinding, since residue adds resistance
- Interface material thickness, which is set by the surface conformity
- Clamping pressure and its stability over time
What each ceramic does in a thermal path
| Material | Behaviour |
|---|---|
| Aluminum nitride (AlN) | Conducts heat and insulates; the default choice |
| Silicon carbide (SiC) | Conducts heat; often electrically conductive |
| Alumina (Al2O3) | Insulates but conducts heat poorly; used where flux is moderate |
| Boron nitride (BN) | Machinable thermal components and non-wetting surfaces |
The property set behind the aluminum nitride row, and the grades it is supplied in, are on the aluminum nitride material page. The geometry these thermal parts take is covered by the ceramic substrates family.
Aluminum nitride in semiconductor equipment
In semiconductor process equipment the part often sits inside a chamber, and the requirement does not change: heat has to leave through the part while the part keeps the circuit isolated. Aluminum nitride is chosen for that combination of thermal conductivity and electrical insulation here for the same reason it is chosen in the assemblies above. What changes is the environment. A chamber adds cleanliness and particle-generation constraints, and those reach back into how the thermal face is finished and cleaned rather than into the choice of material.
The equipment context is covered under semiconductor equipment components, and the additional requirements a chamber imposes are collected under vacuum and semiconductor components.
How to write the requirement on the drawing
Parts from the batch
Frequently asked
Why not just use copper?
Copper conducts heat well and electricity equally well. If the assembly needs isolation, a ceramic has to be added anyway.
How flat does a thermal face need to be?
Flat enough for the interface material in use. The requirement comes from the thermal design and should be stated on the drawing.
Does surface finish really affect heat transfer?
It controls the contact area and therefore the interface resistance. On a critical path it is worth specifying separately.
Can a ceramic both insulate and conduct heat?
Aluminum nitride does. That combination is why it is used for power substrates instead of alumina.
Have a drawing that this guide applies to?
Send it with the material and the quantities. The review confirms what is achievable before any price is issued.