Silicon nitride is not a better ceramic. It is the only one left standing at a specific crossing point: 800 V, thick copper, high current, and several thousand cycles between −40 and +175 °C. Here are the numbers, and the edges.
China's GB/T 45767-2025 divides silicon nitride substrate into four grades. The numbering is not marketing — it is a specification you can hold a supplier to.
| Grade | Thermal conductivity | Bending strength | Where it is specified |
|---|---|---|---|
| SN60 | ≥ 60 W/(m·K) | ≥ 650 MPa | Strongest of the four. Selected when the part is structural as much as thermal |
| SN80 | ≥ 80 W/(m·K) | High | The common middle grade for power modules |
| SN100 | ≥ 100 W/(m·K) | High | Higher heat flux, smaller die area |
| SN120 | ≥ 120 W/(m·K) | ≥ 550 MPa | Best heat path, weakest of the four |
Plot the two numbers that decide survival: how well it moves heat, and how hard it is to crack. Most grades win one axis and lose the other.
Beryllia and aluminium nitride move more heat. Alumina is cheaper and easier to get. Only silicon nitride sits high on both axes at once — and toughness, not conductivity, is what decides whether a substrate survives a power cycle without cracking.
Silicon nitride conducts roughly half as much heat per millimetre. It also runs at half the thickness.
One-dimensional thermal resistance:
| Configuration | Thickness | k | R, 10×10 mm die |
|---|---|---|---|
| Si₃N₄ | 0.32 mm | 85 W/(m·K) | 0.038 °C/W |
| AlN | 0.635 mm | 180 W/(m·K) | 0.035 °C/W |
Half the thickness, almost the same thermal resistance. That is why 0.32 mm became the standard silicon nitride substrate thickness, and why 0.20 mm ultra-thin grades exist.
Same rows, four materials. This is the table a selection meeting should be arguing over.
| Property | Si₃N₄ | AlN | Al₂O₃ 96% | BeO |
|---|---|---|---|---|
| Thermal conductivity | 70–100 (up to 120–130) | 170–220 | 20–30 | 250–300 |
| Bending strength | 600–900 MPa | 300–400 MPa | 300–380 MPa | 200–250 MPa |
| Fracture toughness | 6–8 MPa·m1/2 | 2.5–3.5 | 3.5–4.0 | 2–3 |
| CTE, 10⁻⁶/K | 2.7–3.2 | 4.5–5.3 | 7–8 | 7–8 |
| Thermal shock | Good, ΔT 600–900 °C | Moderate | Poor | Moderate |
| Cycles at −40/+250 °C | > 3,000 | 500–1,500 | 200–500 | — |
| Max copper thickness, AMB | 800–1,200 µm | 600–800 µm | — | — |
| Toxicity | None | None | None | Severe, as dust |
| Cost | High | Medium-high | Low | Very high |
Published comparisons put the gap in plainer terms: in −45 to 150 °C cycling, silicon nitride AMB lasts an order of magnitude longer than aluminium nitride and two orders longer than alumina. If your program is 800 V and the module has to live through field thermal cycling, the material question is already answered — what is left is who can hold the tolerance.
This section exists because a supplier who only lists advantages has not made the part yet.
| Boundary | Number | What it costs you |
|---|---|---|
| Conducts less than AlN | 40–50% of AlN | Only recovered by thinning, which makes warp harder |
| Conductivity and strength trade off | SN60 650 MPa → SN120 550 MPa | You must choose; there is no top-of-both grade |
| Large plates are limited | 138 × 90 mm is the largest public domestic size | High-power modules need panelisation, which adds interfaces |
| Warp is hard to hold | ≤ 0.2–0.4% typical spec | Warp drives die-attach pressure uniformity and bond-line thickness |
| Thin plates chip at holes | Small holes in thin substrate | Hole layout is constrained; laser parameters are the know-how |
| Long lead time, high cost | Overseas up to 24 weeks; domestic as low as 2 | Cost sits in powder, sintering, diamond grinding and AMB yield, multiplied |
| Hydrofluoric acid attacks it | 100% HF, strong attack | Semiconductor wet processes must avoid fluoride media |
| It is a flat plate | Not a 3D shape | Differentiation is only tolerance, copper thickness, pattern accuracy and yield |
Thermal cycling, warp, dielectric strength or cost. The answer changes the grade, the thickness and the grinding sequence — and it is a shorter conversation than sending drawings first.
Get a quote