As SiC devices push power densities and junction temperatures higher, the substrate — not the chip — is often the weak link. Here is how the three common ceramics compare.
| Property | Al₂O₃ (Alumina) | AlN | Si₃N₄ |
|---|---|---|---|
| Bending strength (MPa) | ~300 | ~300 | ≥ 600 |
| Thermal conductivity (W/m·K) | ~24 | ~170 | ≥ 80 |
| Thermal expansion (×10⁻⁶/K) | ~7.2 | ~4.5 | ~3.0 |
| Thermal-cycle reliability | Fair | Good | Excellent |
| Best for | Cost-sensitive | Max heat removal | Harsh cycling |
In a vehicle, the power module sees thousands of thermal cycles per year. Al₂O₃ is cheap but cracks; AlN conducts heat best but is brittle and shocks easily; Si₃N₄ trades some peak conductivity for far higher strength and the best resistance to thermal cycling — which is why AMB (active metal brazed) Si₃N₄ substrates are now the mainstream choice for 800 V SiC traction inverters.
If your bottleneck is pure heat flux (e.g. high-current industrial converters with stable mounting), AlN's ~170 W/m·K still wins on conduction. Choose Si₃N₄ when mechanical shock, vibration and cycling dominate.
For automotive and rugged power modules, yes — Si₃N₄ survives more thermal cycles due to higher strength and lower thermal expansion mismatch with the SiC chip. AlN wins only on raw thermal conductivity.
Active Metal Brazing: a titanium-containing braze bonds copper directly to the ceramic, giving high-current, high-reliability substrates used in SiC modules.
