Ceramic PCB Materials: Alumina, AlN and Their Applications

Ceramic substrates are used where the electrical and the thermal requirements cannot both be met by an organic laminate. They offer a dielectric constant that does not drift, a thermal conductivity that an epoxy cannot approach and a coefficient of thermal expansion that is close enough to silicon to be useful.

They are not one material but a family. Alumina dominates by volume, aluminium nitride appears where heat is the limiting factor and a small number of other ceramics fill niches where a specific property is decisive.

Why Use Ceramic at All

The first reason is thermal. A power device mounted on a ceramic substrate dissipates its heat through the substrate rather than through a thermal via path, and the junction temperature that follows is lower than an equivalent organic construction can achieve.

The second reason is dimensional and electrical stability. The dielectric constant of a ceramic changes little with temperature and frequency, and the substrate does not absorb moisture, so a filter or a matching network built on it holds its response across the operating range.

Alumina: The Default Choice

Alumina, or aluminium oxide, is the workhorse of the family. It is available in several purities, usually 96 percent or 99.6 percent, and the higher purity grades have a lower loss and a smoother surface at a higher price.

Its thermal conductivity is roughly 20 to 30 watts per metre kelvin, which is far below a metal but far above an organic laminate. Its dielectric constant is around 9 to 10, so traces on it are narrow, and its expansion is around 7 parts per million per degree, which is closer to silicon than most packaging materials.

Aluminum Nitride

Aluminium nitride conducts heat at roughly five to eight times the rate of alumina, which makes it the material of choice for a high power device on a small substrate. Its dielectric constant is similar to alumina, so the electrical design rules are comparable.

The difficulty is processing. Aluminium nitride reacts with moisture and with some plating chemistries, so the surface preparation and the metallisation have to be controlled, and it is more expensive than alumina by a significant margin.

Silicon Carbide and Beryllia

Silicon carbide offers high thermal conductivity and a very high stiffness, and it is used in optical and high power applications where those properties matter. Its dielectric constant is high, which restricts its use in high frequency circuits, and its colour makes optical inspection more difficult.

Beryllia has excellent thermal conductivity and electrical insulation, but its dust is toxic and its processing requires strict controls. It is now used only where no other material will do, and many designs that once used it have moved to aluminium nitride.

Metallisation and Thick Film

Ceramic circuits are metallised by one of three routes. Thick film printing deposits a metal paste that is fired onto the surface, thin film deposition gives finer geometry and better adhesion, and direct bonded copper bonds a copper sheet to the ceramic at high temperature to form a substrate for power devices.

The metallisation determines the current capability and the attachment method. A thick film conductor is printed and fired, so it is comparatively thick and carries current well, while a thin film conductor is finer and better suited to high frequency structures and to wire bonding.

Thermal and Electrical Comparison

Comparing the family on a single number is misleading. Alumina is adequate for a moderate power density with a good thermal path, aluminium nitride is needed where the heat flux is high, and silicon carbide suits a large rigid substrate that also has to dissipate.

Electrically, alumina and aluminium nitride behave similarly, so the choice between them is usually thermal and economic. Silicon carbide and beryllia have higher dielectric constants and require a different design approach for the same impedance.

Mechanical Behaviour and Assembly

Ceramic is stiff and brittle. It does not flex to accommodate a mismatch, so the mounting and the attachment have to be designed to avoid applying a bending load, and the edges are usually chamfered to reduce the chance of a crack starting.

Components are attached by soldering, by sintering or by wire bonding, depending on the metallisation. A ceramic substrate with a large die is usually attached with a sintered or eutectic process rather than with solder, because the solder layer introduces a thermal resistance and a creep mechanism that the design may not tolerate.

Selection Rules

Start from the heat flux. If the device can be cooled through an organic substrate with thermal vias, a ceramic is probably unnecessary. If the heat flux requires the substrate itself to be the thermal path, the choice within the ceramic family follows from the conductivity and the cost.

Then check the electrical requirement, the size limit imposed by the process and the way the module will be mounted. gopcb supports ceramic filled laminates and metal backed constructions, and can advise whether a given thermal and electrical target needs a true ceramic substrate or whether a filled organic material will meet it at lower cost.

When a Ceramic PCB Is the Wrong Answer

A ceramic PCB solves a thermal and stability problem and creates a mechanical and cost one. It is brittle, it cannot be scored or snapped, it is limited in size and it costs several times more per unit area than a laminate. Where the heat can be removed with thermal vias, a metal backed construction or an aluminium heat spreader bonded to a conventional board, those routes usually deliver the required junction temperature at a fraction of the price.

The decision is therefore made from the heat flux at the device, not from the total power of the product. A single small device dissipating two watts through a one centimetre square is a high flux problem that favours ceramic, while the same two watts spread over a large area is easily handled by an organic substrate with a copper pour and a few thermal vias.

Designing the Circuit on a Ceramic Substrate

The design rules on a ceramic PCB differ from those on a laminate in several ways. The dielectric constant is higher, so a fifty ohm line is narrower for the same substrate thickness, which reduces the achievable impedance tolerance and increases the conductor loss per unit length. The metal is deposited or bonded rather than etched from a foil, so the thickness and the edge profile follow the metallisation route.

Drilling is not the only way to make a connection between the two faces. A ceramic PCB may use edge metallisation, wrap around conductors or a co-fired construction in which the vias are formed in the green tape before firing. Each of those has its own design rules, and the metallisation drawing must state which one the design assumes rather than leaving the fabricator to infer it from the artwork.

FAQ

Which ceramic is the most common? Alumina, by a wide margin. It is available in many purities, it processes predictably and it is the least expensive of the family.

When is aluminum nitride worth the cost? When the heat flux through the substrate is the limiting factor and alumina would give an unacceptably high junction temperature. Its advantage is entirely thermal.

Can a ceramic substrate be large? The size is limited by the firing process and by the brittleness of the material. Large circuits are usually divided into modules and mounted on a carrier.

Related reading: PTFE and ceramic substrates, high temperature PCB materials, aluminium and FR4 selection, and metal core PCB thickness.