Aluminium Nitride: Design Rules and Process Limits

Alumina has been the default ceramic substrate for decades because it is inexpensive, well understood and adequate for most applications. Aluminium nitride exists because alumina’s thermal conductivity is modest, and a substrate that insulates electrically while conducting heat is exactly what a power module needs. The choice between them is a trade of thermal performance against cost, process maturity and mechanical behaviour.

This article compares the two materials on the properties that matter for a substrate, and describes the applications where each is the better answer.

The comparison is worth making carefully because the thermal advantage of aluminium nitride is real but it is not always realised in a finished assembly, for reasons that have as much to do with the metalisation and the mounting as with the ceramic.

Thermal Conductivity

Alumina conducts heat at roughly twenty to thirty watts per metre kelvin depending on purity. Aluminium nitride conducts at between one hundred and seventy and two hundred, an improvement of nearly an order of magnitude. For a device dissipating tens of watts, the difference is the difference between needing an externally bonded heat spreader and not needing one.

The advantage is realised only if the heat can enter and leave the ceramic efficiently. The metalisation layers, the solder or sintered joint and the mounting interface all add thermal resistance in series, and if any of them dominates, the benefit of the better ceramic is lost. The general structure of a ceramic substrate and the way it is metalised determines how much of the material’s conductivity reaches the application.

Expansion And Mechanical Behaviour

Alumina is stiff and hard, with a coefficient of thermal expansion of about seven parts per million per kelvin, close to that of silicon and of many compound semiconductors. That closeness is a significant part of its popularity, because a small CTE mismatch between the die and the substrate keeps the strain in a soldered joint low and extends its fatigue life.

Aluminium nitride has a similar expansion coefficient to alumina but a much lower mechanical strength, and it is more brittle. It also reacts with moisture in certain conditions, forming a surface hydroxide that degrades the metalisation, which is why the surface treatment and the packaging are important. Both materials should be qualified in the intended environment rather than assumed equivalent.

Electrical Properties

Both are excellent insulators with high dielectric strength, and both are used for assemblies that must withstand high voltages. Their dielectric constants differ: alumina is around nine and aluminium nitride around eight, which matters where the substrate forms part of an RF structure and where the parasitics of a switching node are significant.

The dielectric strength per unit thickness allows a very thin substrate to hold off a large voltage, which is one reason ceramic substrates are used in power modules. The surface creepage distance still has to be designed, and the creepage and clearance rules apply to the metallised pattern as much as they do to a printed board.

Manufacturing And Cost

Alumina is available from many suppliers in a wide range of thicknesses with well established metalisation processes. Aluminium nitride is produced by fewer suppliers, is more difficult to fire and to metalise, and costs several times as much for the same area. The cost difference is a substantial fraction of the bill of materials for a power module.

The manufacturing route also differs. Direct bonded copper and active metal brazing are both used, and each has its own limits on copper thickness and on feature size. Choosing the material without checking which metalisation processes are available for it in the required thickness is a common cause of schedule delay.

Supply chain reality also enters the decision. A material that is technically superior but available from two suppliers with long lead times introduces a risk that a project cannot absorb, particularly where the product has a long manufacturing life. The procurement decision and the technical decision should be made together.

Where Each Material Fits

Alumina suits applications with moderate power density: sensor substrates, RF circuits, high voltage isolation and small modules where the thermal path is not the limiting factor. Its cost, availability and mechanical strength make it the default choice where the thermal requirement is met.

Aluminium nitride suits high power density: traction inverters, laser diode carriers, power modules with a small footprint and any design where the junction temperature cannot be held with the lower conductivity material. The decision is usually made by the thermal calculation rather than by preference. Where the module also has a thermal path through the board, the principles of thermal placement and relief apply to the copper pattern on the ceramic as well.

Designing With A Ceramic Substrate

The metallised pattern on a ceramic substrate is designed like a single layer board with a very good dielectric underneath. The considerations that differ are the adhesion of the metal to the ceramic, the ability to form fine features, and the stress at the edges of a metal feature where the expansion mismatch is concentrated.

The last of these is the reason metal features are usually rounded rather than square and the reason large areas of copper are avoided. The same reasoning applies to the solder joint between a die and the substrate, where the joint thickness and the alloy are chosen to absorb the remaining mismatch rather than to minimise resistance.

Testing is the final arbiter. A coupon with the intended metalisation, joint and die attach, subjected to the same thermal cycling as the application, will show whether the chosen material survives. The test is more convincing than a property comparison, because it includes the interfaces that the properties describe only indirectly.

Comparing On The Right Basis

A fair comparison between the two materials uses the thermal resistance of the whole path, including the metalisation and the joint, at the operating conditions. Comparing conductivity figures alone favours aluminium nitride by more than the application will realise, and comparing cost alone favours alumina by more than the thermal solution will cost.

The practical approach is to build the thermal model with both materials and to see whether alumina meets the requirement with an acceptable margin. Where it does, the lower cost and the more mature supply chain decide the question; where it does not, aluminium nitride is justified by the calculation rather than by assumption.

Where the design is between the two materials, a useful compromise is to use aluminium nitride only under the highest dissipating device and alumina elsewhere. The substrate then becomes a composite of two materials, joined either mechanically or as separate circuits, which recovers most of the cost while keeping the thermal performance where it is needed.

FAQ

Can aluminium nitride be used without a protective coating? Its surface can react with moisture, so the metalisation and the package usually protect it. Where the substrate is exposed, the humidity resistance of the specific process should be confirmed by the supplier.

Is a thicker ceramic better for thermal performance? Thicker ceramic adds thermal resistance in proportion to the thickness, so thinner is better thermally, subject to the mechanical strength and the dielectric voltage requirement.

How is the copper bonded to the ceramic? By direct bonding at high temperature or by active metal brazing, and the choice determines the achievable copper thickness and the feature resolution. It should be selected together with the circuit pattern.