Aluminium Nitride Substrate: Thermal Performance and Sourcing

Why Aluminium Nitride

Aluminium nitride is a ceramic that behaves like a metal in one respect and like an insulator in another. Its thermal conductivity is around 170 watts per metre kelvin, roughly seven times that of alumina and comparable to some aluminium alloys, while its electrical resistivity is that of a good insulator. That combination is what makes it the substrate of choice where a circuit has to carry power and heat at the same time, and where an ordinary board or an alumina substrate would run too hot.

The material also has a thermal expansion coefficient close to that of silicon, which reduces the stress on a die or a large semiconductor attached to it, and it is mechanically stiff and chemically stable. The costs are that it is expensive, hard to machine, and supplied in smaller formats than a laminate, so it is used for the small hot area of a circuit rather than for a whole assembly.

Where It Is Used

• Power modules. Insulated gate bipolar transistor and silicon carbide modules, where the die has to be cooled through the substrate.
• High power LEDs. CoB arrays and UV emitters, where the junction temperature sets the light output and the life.
• Radio frequency and microwave. Amplifiers and filters, where the low loss and the stable dielectric constant matter as much as the cooling.
• Automotive power electronics. Traction inverters, on-board chargers and DC to DC converters.
• Sensor and detector packages. Where the die has to be isolated and cooled at the same time.

AlN Against Alumina and Other Ceramics

Alumina (Al2O3) is the workhorse ceramic. It is cheap, widely available and adequate for low and medium power. Its thermal conductivity, around 24 to 30 watts per metre kelvin, is the limiting factor, and where a design needs more cooling it has to grow the substrate area or add a heat sink.

Aluminium nitride gives the same electrical performance with roughly seven times the thermal conductivity, so the same die runs cooler or the substrate shrinks. It is typically three to ten times the price of alumina and available in fewer standard sizes.

Silicon nitride is tougher and is used where mechanical cycling dominates, and beryllia conducts better than AlN but is toxic and avoided where a substitute will do. For most power and LED work the real choice is between alumina and AlN, and the decision is made on the watts that have to cross the substrate.

Metallisation

A bare ceramic cannot be soldered. The circuit is formed by bonding or depositing a metal layer on it, and the technique chosen determines the current capacity, the accuracy and the cost.

Direct bonded copper (DBC) uses a copper foil bonded to the ceramic at high temperature, producing a thick conductor with a strong bond. It carries heavy current and is the standard for power modules. The copper can be etched into the circuit pattern after bonding, and the resulting substrate tolerates thermal cycling well.

Active metal brazing uses a brazing alloy to attach copper, giving a very strong bond and thick copper, at a higher cost.

Thick film screens a metal paste onto the ceramic and fires it, which is cheaper and finer in resolution but much thinner, so it suits control and sensor circuits rather than power paths.

Thin film deposits a very fine metal layer for microwave circuits, where dimensional accuracy and low loss matter more than current.

Our notes on energy PCBA describe how these substrates are used in power conversion, and the assembly side is covered in our notes on PCB assembly.

Design Considerations

Size and thickness. Standard thicknesses run from about 0.25 to 1.0 millimetres. A thin substrate has a lower thermal resistance but is more fragile and harder to handle, so the mechanical fixture matters as much as the electrical design.

Thermal path. The substrate cools the die into the copper and then into the heat sink under it. If the interface between the substrate and the heat sink is poor, the advantage of the AlN is lost, so the mounting, the flatness and the thermal interface material are part of the thermal design.

Power cycling. Different materials expand by different amounts, and a DBC substrate flexes slightly with each thermal cycle. The copper thickness, the pattern geometry and the solder layer are all chosen so that the accumulated strain stays inside the limit for the required cycle count.

Isolation. The ceramic is the isolation between the live circuit and the heat sink, so its thickness and its dielectric strength set the working voltage. High voltage modules use a thicker substrate, which costs thermal performance, so the two requirements have to be balanced.

Metallisation pattern. The minimum feature size depends on the metallisation technique. A DBC pattern can be etched to a fraction of a millimetre; a thick film pattern is coarser. The layout has to respect the process that will make it.

Sourcing the Substrate

The market is smaller and more specialised than the laminate market, which changes how a supplier is chosen. Four questions decide most of the outcome.

Material quality. The thermal conductivity depends on the purity and the oxygen content of the ceramic, and it is quoted for a grade rather than for a batch. Ask for the measured value and the test method, not just the nominal figure, and ask about surface flatness, which affects how well a die or a heat sink contacts the substrate.

Metallisation capability. Ask which metallisation techniques the supplier offers, the copper thickness it can bond, and the finest pattern it can etch. A supplier who can do DBC but not thin film cannot serve a microwave design.

Certifications and cleanliness. Automotive and medical work requires a documented quality system and, often, a controlled environment for handling, because a particle on the substrate becomes a defect in the module.

Sample first. Buy a small quantity, attach the real die with the real process, and measure the thermal resistance and the shear strength of the joint. A substrate that passes a datasheet check can still fail after soldering, and the failure appears as a cracked ceramic or a delaminated copper edge.

Cost, MOQ and Lead Time

Indicative prices run from around 1.50 to 3.00 US dollars per piece for a standard substrates in moderate quantity, and up to 10 dollars or more for large formats, thick metallisation or tight tolerances. The price is driven by the ceramic area, the metallisation process and the quantity, and the fixed cost of tooling for a patterned DBC substrate is recovered across the order.

Minimum order quantities are the practical obstacle for small projects. Some suppliers ask for 100 to 200 pieces and quote three to six weeks, which is difficult for a development programme. Suppliers that serve prototyping will start at 10 to 20 pieces with a lead time of one to two weeks, at a higher unit price, and that is usually the right entry point. Our notes on PCB manufacturing describe how a ceramic substrate is processed alongside conventional boards.

Risks in a Global Supply Chain

Three problems dominate international sourcing. Quality varies between suppliers and even between batches, so an incoming inspection that measures flatness, thickness and thermal conductivity is worth the effort. Language and time zone differences slow down the technical exchanges that a substrate project needs, so a supplier with documented drawings and an English speaking engineering contact is a real advantage. And freight adds risk, because ceramic breaks: pack it for shock, use a tracked service, and allow for customs time in the schedule rather than treating it as a surprise.

FAQ

What is the thermal conductivity of an AlN substrate? Around 170 watts per metre kelvin for standard grades, roughly seven times alumina and far above any laminate.

Can AlN be soldered directly? Not bare. The ceramic needs a metallisation layer, usually a direct bonded copper foil, a brazed copper layer or a fired thick film, before a solder joint can be made.

How does AlN compare with alumina on cost? AlN typically costs several times more than alumina of the same size, so it is chosen when the thermal requirement cannot be met on alumina.

What is the typical lead time? One to two weeks for prototype quantities from a prototyping supplier, and three to six weeks for volume from a specialist ceramic house.

Conclusion

Aluminium nitride sits between a laminate and a metal core board in the thermal range it can handle, and it does so while providing full electrical isolation. Choose it when the power crossing the substrate cannot be carried on alumina, specify the metallisation technique from the current and the frequency, and verify the thermal and mechanical performance on real samples before committing to a production order. Our notes on quality management describe the incoming inspection that catches the batches that do not meet the specification.