Metal Core PCB and Ceramic Substrates: Thermal Paths Compared

When a design has to remove heat from a component, two substrate families are usually considered: a metal core construction, in which an aluminium or copper base acts as the heat spreader, and a ceramic substrate, in which the material itself conducts the heat away.

Both solve the same problem in different ways, and the comparison is decided by the heat flux, the voltage, the size of the circuit and the cost that the product can bear. Treating them as interchangeable alternatives is a common mistake.

The Thermal Problem Being Solved

The heat generated by a device has to travel from the junction to the ambient, and every layer in that path contributes a thermal resistance. The substrate is one of those layers, and it is usually the one the designer can change most easily.

The relevant quantity is not the thermal conductivity of the material alone but the thermal resistance of the path through it, which depends on the thickness and the area as well. A thin layer of a moderate conductor can outperform a thick layer of a good one.

Metal Core Construction

A metal core board consists of a copper foil, a thermally conductive but electrically insulating dielectric and a metal base, usually aluminium. The dielectric is typically 60 to 150 micrometres thick, and it is the component that dominates the thermal resistance through the stack.

The base spreads heat laterally, so a small source can be cooled by the whole area of the board. That lateral spreading is the main advantage over an organic substrate with thermal vias, because the vias conduct heat downwards without spreading it sideways.

Ceramic Substrate Construction

A ceramic substrate conducts heat through its own thickness with no separate dielectric layer, and its thermal conductivity is a material property rather than a thin film. That is why a ceramic can handle a much higher heat flux in a small area.

The metallisation carries the current and the attachment method carries the heat from the device into the ceramic. A direct bonded copper substrate, with a thick copper layer bonded to the ceramic, distributes the current well and allows a large die to be attached over a small area.

Comparing Thermal Resistance

For a given area the ceramic route usually has the lower thermal resistance, because there is no organic dielectric in the path. The metal core route compensates by spreading heat over a much larger area at a lower cost per unit area.

The practical consequence is a difference in application. A single high power device with a small footprint favours ceramic, while a large array of moderate power devices distributed over an area, such as an LED panel, favours a metal core board.

Electrical Insulation and Voltage

The dielectric in a metal core board has a breakdown rating, typically quoted in kilovolts, and it sets the maximum working voltage between the circuit and the metal base. The thickness is therefore chosen for the insulation requirement as well as for the thermal one, and the two requirements pull in opposite directions.

A ceramic substrate is an insulator in its own right and withstands much higher voltages, which is one reason it appears in high voltage power modules. The creepage distances on the surface still apply, and the metallisation pattern has to respect them.

Mechanical and Manufacturing Differences

A metal core board is milled rather than scored or punched, because the metal base does not shear cleanly, and the hole sizes and spacings follow the machining capability. It is stiff, it can carry a thread and it frequently forms part of the enclosure.

A ceramic substrate is brittle and cannot be machined after firing. The shape is defined by the green tape or by laser cutting, the edges are chamfered and the assembly has to avoid applying any bending load. That restricts the size and the mounting arrangement.

Cost and Application Fit

A metal core board costs more than a laminate and considerably less than a ceramic, and it is available in larger sizes with a well established process. A ceramic substrate costs more, is limited in area and requires a specialised assembly flow.

Both are chosen from the heat flux. Below a few watts per square centimetre an organic board with thermal vias and a copper pour is usually adequate, between that and the ceramic threshold a metal core board is the economical answer, and above it the ceramic becomes the only practical route.

Selection Rules and Design Checklist

Calculate the thermal resistance of the whole path rather than of the substrate alone, confirm the breakdown rating against the working voltage, check the achievable outline and hole size for the construction and confirm that the assembly method will not apply a bending load to a brittle substrate.

gopcb produces metal core boards and works with ceramic filled and ceramic substrate materials, and can compare the two constructions against a given dissipation and working voltage before the design is committed.

Thermal Interface and Attachment

Neither substrate solves the problem on its own, because the device has to be attached to it. A solder or sintered layer, a thermal interface material and the mounting of the substrate to a heat sink each add a thermal resistance that can be larger than the substrate itself. In a metal core assembly the interface between the board and the heat sink is often the largest single term in the path, and its quality depends on the flatness, the mounting pressure and the thickness of the interface material.

The attachment method also affects the dielectric breakdown requirement. A device attached with a large area of solder spreads the heat well but also applies mechanical stress to the dielectric during thermal cycling, so the material has to tolerate both the voltage and the strain. Where the voltage is high, the dielectric is made thicker, which raises the thermal resistance and forces the design to compensate elsewhere in the path.

Testing and Qualification

Qualification follows the failure modes of the construction. A metal core board is tested for dielectric breakdown at the working voltage plus a margin, then thermal cycled and tested again, because the dielectric can degrade at the interface between the copper and the base. A ceramic substrate is tested for the same electrical properties and additionally for cracking, using a thermal shock test and, where the application demands it, a mechanical test on the mounted assembly.

The measurements that matter for the thermal design are the thermal resistance before and after the cycling rather than only the electrical pass or fail. A dielectric that has delaminated locally still insulates but stops conducting heat, and the first sign is a junction temperature that has risen without any change in the electrical behaviour of the circuit.

FAQ

Is a metal core board always cooler than a ceramic? No. For a small, hot device the ceramic usually wins because the thermal path is shorter. For a large area with a moderate flux the metal core spreads heat more cheaply.

Can a metal core board be multilayer? Yes, but the construction becomes more complex and the benefit narrows, because the additional dielectric layers add thermal resistance. It is usually used where the circuit needs more than one routing layer.

Why does the dielectric thickness matter so much? Because it is the dominant thermal resistance in a metal core board and also the insulation between the circuit and the base. Thinner is better thermally and worse electrically, so the value follows from both requirements.

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