Aluminium Nitride Moves Heat Like a Metal and Still Blocks Current

Alumina is the default ceramic substrate for a reason. It is cheap, strong, and easy to metallize. It is also a poor heat pipe. A 96% Al₂O₃ board sits around 20–30 W/m·K. Aluminium nitride, done properly, lands in the 170–200 W/m·K range, sometimes a bit over 200. Theory for a perfect single crystal is near 320. Nobody ships that. What we ship is a sintered insulator that can take die heat out of a power module without becoming a conductor.

That combination is the job. Metals conduct heat and current. Most ceramics block current and trap heat. Aluminum nitride does heat and insulation at the same time, with a CTE around 4.5 ppm/°C — close to silicon. Alumina expands more like 6.5–7.5. On a high-power IGBT or SiC device that mismatch shows up as solder fatigue and a peeling copper foil. BeO conducts even better and we still avoid it when we can. Toxicity is not a datasheet footnote.

Oxygen is the impurity that ruins the conductivity story. Lattice oxygen creates aluminum vacancies. Phonons scatter. Powder with a couple percent oxygen sinters to something that looks like AlN and measures closer to 100 W/m·K. Low-oxygen powder plus a sintering aid that pulls oxygen into a grain-boundary phase — yttria is the usual one — is how you get the 170-plus grade. Density has to be high. A residual pore is another scattering site. So “AlN substrate” on a PO is not a grade. Ask for thermal conductivity, not just chemistry.

Powder handling is where shops get surprised. Fine AlN hydrolyzes. Leave it in humid air and you grow a hydroxide skin and raise the oxygen you just paid to remove. Dry storage, short open time, sometimes a surface treatment. That is not fussiness. It is how the k value survives until the kiln.

We use aluminum nitride where the thermal stack actually needs it: DBC and DPC power boards, RF packages, laser and LED submounts, some semiconductor process parts. If the die is a few watts and the heat sink is the bottleneck, alumina still wins on price. If you are dumping tens of watts through a thin ceramic into copper, the substrate drop on alumina is no longer free. The ceramic costs more. The module that does not throttle or delaminate is the return.

Machining and metallization need the same honesty. AlN is not as casual as 96% alumina. Edges chip. Copper bonding has to match the expansion. A beautiful 180 W/m·K disk with a bad interface is just an expensive insulator. Specify k, thickness, surface, and the metal system together. Aluminium nitride is a heat path that happens to be a ceramic. Treat it that way and it earns the premium. Treat it like white alumina with a better brochure and it will not.