A 0.63 mm aluminum nitride substrate looks like a plain white square on the bench. Put it through die attach and a thousand thermal cycles, and the differences between suppliers show up quickly: a crack at the corner, a pad that has warped, a leakage path the datasheet never mentioned. Most of that variation is settled long before packing, in the tape caster and in one furnace run at 1850°C.
We cast and sinter aluminum nitride substrates on our own line in Ningbo. Questions from buyers keep coming back to that furnace, so here is what happens inside it, and which numbers are worth demanding from any supplier you shortlist.
AlN arrives as powder, gets dispersed into a slurry, and is cast into a green sheet of controlled thickness. Thickness tolerance is won or lost at this stage, not at grinding. Our line covers finished substrates from 0.25 mm to 3.0 mm, and thinner work down to 0.1 mm with dimensional accuracy held within ±0.5%.
Green sheets shrink during firing, and that shrinkage has to be predictable. A 0.63 mm design that lands at 0.60 mm will not sit correctly in a module's thermal stack, no matter how good the ceramic is. Sheet count and lamination pressure are set per thickness. Loosen that discipline and no amount of furnace tuning rescues the batch.
Aluminum nitride only densifies properly at high temperature. We sinter at 1850°C in a nitrogen atmosphere. The nitrogen is not decoration. At that temperature AlN is unstable in air, and an oxygen-bearing atmosphere drives the surface toward alumina and leaves grain boundaries in poor shape. Nitrogen keeps the chemistry where it belongs, which is what gives the fired body its consistent crystalline structure and its predictable conductivity.
What leaves the furnace measures 170 to 230 W/mK thermal conductivity and dielectric strength above 15 kV/mm. Volume resistivity stays over 1014 Ω·cm, so leakage across an isolated baseplate remains negligible even at elevated temperature.
Thermal conductivity alone is a weak buying criterion. A part can hit 170 W/mK and still fail on flatness or surface finish. Four figures carry most of the risk. Conductivity, 170 to 230 W/mK, and ask whether it is a batch value or a best sample. Dielectric strength, above 15 kV/mm. Surface roughness, Ra 0.1 μm when you bond directly to copper. Thickness, 0.25 to 3.0 mm across the range, with 0.63 mm the usual pick for power modules.
Then ask how the supplier qualifies a batch. Ours goes through thermal shock from -55°C to 150°C for 1000 cycles, plus 85°C at 85% RH for 1000 hours. More than twenty checks run on every lot, covering roughness, dielectric constant and conductivity. The substrate range sits under ISO9001 and IATF16949 systems, with UL and RoHS documentation available on request.
High-power LED packages and RF modules are the obvious users, because heat has nowhere else to go. IGBT modules for electric vehicles are the harder case. The ceramic matches silicon's expansion closely enough that the die joint survives daily swings from -40°C to 150°C, and it does so without the toxic filler chemistry that beryllia once forced on designers.
For a first evaluation, sample pieces cut to your outline are usually enough to run one thermal cycle and a dielectric check. Send the drawing with thickness, surface finish and any metallization pattern, and our engineers will quote against it. Full specifications sit on the aluminum nitride ceramic substrate page, alongside the wider functional ceramic substrates range.
