Ceramic Substrate Or Aluminium: How To Choose

When a circuit has to get heat out of a small area, the choice of substrate becomes one of the first decisions rather than one of the last. Aluminium and ceramic boards both solve the problem, but they solve it differently, and the difference shows up in the achievable power density, the voltage the board can withstand and what the board costs.

This article compares the two constructions, explains where each one is limited, and gives the questions that decide between them.

Neither is better in the abstract. The right answer depends on how much heat has to cross a small area, how much voltage is involved, and how the devices will be attached.

Two Ways To Build An Insulated Substrate

An aluminium board is a laminate with a metal base. It has a copper circuit layer on top, an insulating layer beneath it, and an aluminium plate forming the base. The dielectric is a filled polymer, and it is the component that both insulates the circuit from the base and conducts heat into it.

A ceramic substrate removes the polymer from the path. Either copper is bonded directly to a ceramic sheet, a process known as direct bonded copper, or a metal layer is attached by an active metal braze. Because the ceramic is itself an insulator, there is no separate dielectric layer: the heat crosses the ceramic, and the copper on the far side is bonded to it. That changes both the thermal performance and the voltage capability, and it also changes the mechanical behaviour of the board.

Thermal Conductivity Compared

The thermal conductivity of the insulating layer is what limits an aluminium board. The filled polymer dielectrics used on the better substrates conduct a few watts per metre kelvin, and the best of them reach into the higher part of that range. Thinner layers reduce the thermal resistance, but at the cost of dielectric strength.

A ceramic substrate starts at a much higher figure. Alumina, the common and cheaper material, conducts in the range of twenty to thirty watts per metre kelvin, and aluminium nitride reaches well over a hundred, which is comparable with some metals. The practical consequence is that a ceramic substrate can move far more heat through the same area, or the same heat through a much smaller one, which is what makes it possible to run a power module at a high current density without the die overheating.

Dielectric Strength And Voltage

The second difference is the voltage the substrate can hold. On an aluminium board the dielectric layer has to be thick enough for the working voltage, and there is a direct trade against thermal performance. A higher voltage rating means a thicker layer, which means a higher thermal resistance, and the designer has to accept one or the other.

On a ceramic substrate the insulating material is the bulk ceramic, so the dielectric strength is a property of the material rather than of a thin coating. Alumina withstands a high field, and the thickness can be chosen for mechanical and thermal reasons without the same penalty. That is why ceramic substrates dominate in high voltage power modules and in applications where the isolation requirement is severe.

Expansion Matching

The third difference is the CTE, the coefficient of thermal expansion, and how closely it matches the devices attached to it. Ceramic materials expand far less than aluminium, and alumina is closer to silicon than any metal, which reduces the stress that temperature cycling puts on a die or a solder joint. Aluminium nitride is a little higher but still in the same region.

A large mismatch is what causes solder joints and wire bonds to fatigue over repeated cycles, so a closer match means a longer life under thermal cycling. That matters most where the temperature swing is large and the number of cycles is high, which is the normal condition in a power module. Sizing the conductors for the current gets the electrical design right, but the mechanical life is decided by the expansion match.

Size, Assembly And Cost

Aluminium boards are made in large panels and can be processed much like a conventional board, which makes them inexpensive and easy to produce in volume. They can be populated with surface mount parts on standard equipment, and the metal base can be shaped, drilled and mounted to a heatsink or a chassis without difficulty. Ceramic substrates are produced in much smaller panels, are brittle, and need care in handling and mounting.

The assembly processes also differ. A direct bonded copper substrate can carry heavy copper and can be wire bonded as well as soldered, which suits power modules; an aluminium board is normally populated by surface mount assembly. Where holes, cut outs and mounting features are needed, the mechanical outline of the board has to be designed with the brittleness of the ceramic or the softness of the aluminium in mind, since the two behave very differently under a screw.

Choosing Between Them

The decision usually follows from the power density and the voltage. Where a moderate amount of heat has to leave a reasonable area, and the voltage is modest, an aluminium board is the economical answer and can be produced in quantity on ordinary equipment. Where the heat flux is high, the area is small, or the isolation requirement is severe, a ceramic substrate is the only practical option, and its cost is justified by the performance it makes possible.

A useful way to decide is to calculate the junction temperature twice, once for each substrate, using the thermal resistance of the insulating layer in each case, and to see which one closes. If both do, the cheaper one wins. If only the ceramic closes, the choice is made. gopcb works with both constructions and can advise on the layer thickness and dielectric that a given design needs, rather than offering the most capable option by default.

Process Control and Verification

On a design of this kind, direct bonded copper is the item that decides how the rest of the board is arranged. Where the requirement is not written down, the shop supplies its own default, and the default is chosen for the process rather than for the design. The measurements that matter are the repeatable ones: conductor width and spacing, annular ring, finished hole size, plating thickness and surface finish are all verifiable on a coupon that travels with the panel.

Related reading: our fabrication notes, board quality and design release notes cover the same ground.

FAQ

Is a ceramic substrate always more expensive? Per unit area, generally yes, because the material and the panel size are more constrained. It can still be the cheaper overall answer where it removes a heatsink or allows a smaller package.

Can an aluminium board handle high voltage? It can, up to the rating of the dielectric layer, but the thickness required for a high voltage also raises the thermal resistance, which is the trade that ceramic avoids.

Which one has the better thermal performance? Ceramic, by a wide margin, because there is no polymer layer in the heat path. How much that matters depends on the heat flux the design has to handle.