Aluminum Nitride Substrate PCB

When Alumina Is Not Enough

Ceramic substrates have long been the answer where a laminate cannot take the temperature or the heat flux. Alumina is the traditional choice because it is inexpensive, well understood and electrically excellent, but its thermal conductivity is modest. As power densities have risen in power modules, radio frequency amplifiers and light emitting diodes, that limitation has become the constraint, and aluminum nitride has moved from a specialty material to a mainstream one.

The reason is simple: aluminum nitride conducts heat many times better than alumina while remaining an electrical insulator, which allows devices to be packed more tightly and run harder without exceeding their junction temperature.

What the Material Is

Aluminum nitride is produced by sintering high purity powder at high temperature, giving a hexagonal crystal structure that combines a high thermal conductivity with a low electrical conductivity. Typical commercial substrates have a purity between ninety six and ninety nine percent, a thermal conductivity of one hundred and seventy to two hundred watts per metre kelvin and a coefficient of thermal expansion of roughly four and a half to five and a half parts per million per degree Celsius.

That expansion figure is close to that of silicon, which is one of the reasons the material suits power semiconductor packaging so well: the mismatch between the device and the substrate is small, so the stresses in the joint are lower and the thermal cycling life is longer. Substrates are commonly available between a quarter of a millimetre and one millimetre thick, with size, flatness and surface roughness tailored to the application.

Key Properties in Practice

Thermally, the material conducts heat at roughly eight to ten times the rate of alumina, which lowers the junction temperature of a device for the same dissipation and therefore allows a higher power density. It also has good resistance to thermal shock, so it tolerates the rapid temperature changes of a switching application.

Electrically, it has a high dielectric strength of more than twelve kilovolts per millimetre and a stable dielectric constant of around eight and a half, which keeps losses low at radio and millimetre wave frequencies. Mechanically it is stiff and strong, with low warpage, which matters for large area heat spreading. It is also chemically stable, resisting moisture and corrosion over a long service life.

Metallisation

A substrate is only useful once it can be electrically connected, and aluminum nitride supports the common metallisation methods. Thick film silver and gold pastes are used for co-fired and post-fired circuits, copper can be applied by sputtering or plating for direct bonded and active metal brazed constructions, and nickel plating is used to improve solderability and joint strength.

Metallisation adhesion is the critical parameter, because the joint has to survive the thermal cycling of the application. A metallisation that looks adequate at the start but loses adhesion after a few hundred temperature cycles will fail in the field, so the qualification testing has to be done on the production process rather than on a sample made on the bench.

Where It Is Used

Power electronics is the largest application: insulated gate bipolar transistor and MOSFET modules, DC to DC and AC to DC converters, and the inverters used in motor drives. Light emitting diode and laser packaging uses it for high brightness and ultraviolet devices where the heat flux at the die is extreme. Radio frequency and microwave circuits use it for power amplifier modules, antenna structures and automotive radar, where the dielectric performance and the thermal path both matter.

In automotive and electric vehicle systems it appears in on board chargers, battery management electronics and motor controllers built around silicon carbide devices. It is also used in micro electromechanical systems, in chip carriers and thermal isolation substrates, and in precision temperature sensor packaging.

Comparison With Alternatives

Against alumina, aluminum nitride offers far better heat conduction at a higher material cost, which is the trade that has moved so many power designs onto it. Against silicon nitride, it conducts heat better but has lower mechanical strength, so silicon nitride remains the choice where toughness under thermal cycling is the dominant requirement, such as in some traction inverter designs.

Against copper based direct bonded and active metal brazed substrates, aluminum nitride provides electrical insulation and environmental tolerance that the copper constructions cannot, while the metal based substrates carry higher currents. The choice is normally made on the balance of thermal performance, current and insulation requirement. For related reading, see our notes on ceramic substrate manufacturing in PCB capabilities.

Manufacturing Challenges

Aluminum nitride is hard and brittle, so machining, dicing and hole forming demand tooling and processes developed for the material rather than borrowed from laminate work. Sintering is energy intensive and the process window for achieving the target purity and conductivity is narrow. Polishing to the flatness and roughness a thin film circuit requires pushes the surface preparation equipment to its limit.

Where the substrate is metallised, the patterning of the metal layer has to be precise enough that the conductor geometry and spacing meet the electrical requirement, and the finished part has to be inspected for voids, cracks and adhesion defects that would not be visible in a functional test. Our PCB manufacturing group handles this class of work.

Reliability Testing

The qualification programme follows the standards applicable to the application, commonly drawing on the military, IEC and JEDEC families. It typically includes thermal cycling across a wide temperature range, high temperature storage, and damp heat exposure at eighty five degrees Celsius and eighty five percent relative humidity.

Because the failure mechanism of interest is usually the degradation of the joint or the metallisation rather than the ceramic itself, the electrical measurements are taken before and after each stress, and the cross sections confirm that the interface is intact. Our notes on PCBA testing describe how the test plan is built.

Selection Criteria

The specification starts with the required thermal conductivity, which is usually quoted in one of a few standard grades, and continues with the metallisation system and its adhesion and solderability, the dimensional tolerance and surface roughness, the maximum operating temperature and the dielectric strength required by the design, and finally the cost balance between all of them.

Specifying a higher conductivity grade than the design needs, or a tighter tolerance than the circuit requires, adds cost without benefit, so the requirements should come from the thermal simulation rather than from a wish for the best available material. Our notes on quality management describe how the specification is held through production.

Cost

The price of an aluminum nitride substrate is set by the purity and thermal conductivity grade, the thickness, the metallisation method and the order quantity. A bare substrate is the least expensive form, metallised substrates cost several times more because of the added processing, and custom substrates for high power modules cost more again.

The manufacturing difficulty is the reason the material costs more than alumina: the raw material is more expensive, the sintering is more demanding, the machining is harder and the yields are lower. Those costs fall with volume, but they do not disappear, so the decision to use aluminum nitride should follow from a thermal requirement that alumina genuinely cannot meet.

Trends

As devices built on silicon carbide and gallium nitride become more common, and as power densities continue to rise in electric vehicles, renewable energy converters and radio frequency systems, demand for high conductivity ceramic substrates is growing. Improved metallisation processes that survive more thermal cycles, thinner substrates that lower the thermal resistance further, and larger substrates that spread heat over a wider area are the directions the technology is moving in.

FAQ

Is aluminum nitride better than alumina? For thermal performance, yes. Its thermal conductivity is several times higher, which is why it is used in high power and high heat flux designs. Alumina remains cheaper and adequate for lower power work.

What thermal conductivity should be expected? Between one hundred and seventy and two hundred watts per metre kelvin, depending on the purity and the sintering process.

Why is it more expensive? The raw material purity, the high temperature sintering, the difficult machining and the lower yields all contribute.

Does it suit automotive power modules? Yes. Its thermal performance and its expansion match to silicon make it well suited to silicon carbide and insulated gate bipolar transistor modules in electric vehicle systems.

Conclusion

An aluminum nitride substrate is the answer when a design needs electrical insulation and a high thermal conductivity at the same time. Its combination of heat conduction, matched expansion and dielectric stability allows power and radio frequency devices to run harder and longer, provided that the metallisation and the manufacturing process are qualified for the stresses the application will apply.