Metal Core PCB Thermal Design

A metal core board replaces the usual glass reinforced laminate with a metal base, normally aluminium, covered by a thin dielectric layer and the copper circuit. The construction takes the heat that a component generates and spreads it across the whole board area, which lowers the temperature of the device without a heat sink. It is the standard approach for high power LED arrays, for power conversion modules and for any circuit where the heat has to leave through the board.

How the Heat Path Works

In a conventional board the heat from a component passes through a thin copper pad, into a small area of laminate and then into the air through the surface. The laminate has a thermal conductivity that is a fraction of that of copper, so the path is the limiting factor.

In a metal core board the heat passes through a dielectric layer that is a fraction of a millimetre thick into a base whose thermal conductivity is hundreds of times higher than a laminate. The base then spreads the heat sideways and passes it to a heat sink or to the ambient through its whole area.

The result is a much lower junction temperature for the same power, or the same temperature for a much higher power. That is what makes the construction attractive for an LED array, where the light output and the life both depend on the junction temperature.

Base Materials

Aluminium is the most common base. It is light, it conducts heat well, it is available in a range of thicknesses and it can be machined and formed into shapes that include bends and mounting features.

Copper is used where the thermal conductivity has to be higher or where the base has to carry current. It is heavier and more expensive, and it is chosen when the aluminium option cannot move the heat quickly enough.

Other options include steel for a magnetic application and a composite base, but the choice is usually between aluminium and copper, with the thickness of the base set by the spreading area and the mounting rather than by the conductivity alone.

The Dielectric Layer

The dielectric is the critical component of the construction. It has to insulate the circuit from the metal base, which may be at a very different potential, and it has to conduct heat. Those two requirements pull in opposite directions because a thin layer conducts better and insulates less.

The layer thickness is usually between about 50 and 150 microns. A thin layer gives the best thermal performance and a lower breakdown voltage, and the choice has to satisfy the safety requirement of the product rather than the thermal requirement alone.

The dielectric is usually a filled epoxy or a polyimide with a ceramic filler, and the filler content sets both the conductivity and the dielectric strength. The specification should state both, because a supplier who changes the filler can change the thermal performance without changing the thickness.

Electrical Isolation and Safety

The isolation between the circuit and the base is the parameter that has to satisfy the safety standard. The relevant figure is the dielectric strength, expressed as a voltage per unit thickness or as a breakdown voltage for the layer.

The base is normally connected to a heat sink or to a chassis, and that connection has to be defined in the design. A base that is connected to the protective earth of the product is a different safety case from one that is floating, and the two need different creepage distances on the board surface.

The creepage and the clearance at the edge of the board and around the mounting holes also matter. A screw that passes through the board near a track reduces the effective isolation, and the layout should keep a defined distance from any edge or fixing feature.

Design and Layout Rules

The copper thickness on a metal core board is usually thicker than on a conventional board, because it carries the current of the power circuit and because it helps to spread the heat laterally before it passes into the dielectric.

The dielectric is often patterned, so that the copper circuit sits directly on the base in the areas where no isolation is needed. The pattern has to be defined in the layout and produced by the fabricator, and it is not something that can be added to a standard design without a change to the artwork.

The thermal path should be short and wide. A large pad under the device, connected to the plane with many short links, gives a lower thermal resistance than a narrow trace of the same copper area, because the spreading resistance is what limits the transfer into the dielectric.

Fabrication Considerations

A metal core board cannot be drilled and plated in the same way as a laminate board. The metal base is conductive, so a plated through hole must be isolated from it, and the process involves a pre drilled or a punched hole with an insulating sleeve or a filled dielectric.

The board can be machined after the circuit is made, which allows cut outs, mounting holes and bends that a laminate cannot accept. The machining can also be used to create a pedestal that brings the base directly under a component.

The surface finish and the solder mask have to suit the application. A board that will be exposed to the weather needs a finish and a mask that tolerate ultraviolet light and moisture, which is common for an outdoor lighting product.

Reliability and Testing

The failure modes are thermal cycling of the dielectric, the delamination of the layer from the base and the degradation of the solder joints under the repeated expansion of the aluminium. The expansion coefficient of aluminium is much higher than that of the laminate, which makes the joint between the base and the dielectric the critical interface.

A thermal shock test on a sample is the standard qualification for the construction, and the sample should include the largest component and the thinnest dielectric in the product. The test is more informative than a steady state temperature measurement, because it exercises the interface.

The isolation should be tested at the production level with a dielectric withstand test between the circuit and the base. A test that is too fast can pass a layer with a weak point, and the test parameters should follow the standard rather than an internal preference.

Where It Is Chosen

The construction is chosen where a conventional board cannot move the heat and where a heat sink is not acceptable because of the size, the cost or the ingress protection requirement.

It is not always the cheapest answer to a thermal problem. A conventional board with a thermal via array and an external heat sink can be cheaper where the space allows, and a thick copper board is an intermediate option that offers some of the heat spreading without the weight and the cost of the metal base.

The decision should be made with a measurement of the junction temperature in the intended enclosure rather than with a calculation alone. A metal core board that is mounted with a poor interface to the heat sink can perform worse than a conventional board that is mounted well.

Practical Rules

Measure the thermal path from the device to the base, keep it short and wide, and specify both the thermal conductivity and the dielectric strength. Define the potential of the base and the creepage around every fixing.

Record the dielectric data and the thermal measurements with the build records and the dimensional stability data, and review the metal core comparison and the thermal design rules when the construction is selected.

FAQ

Why does a metal core board run cooler? The heat passes through a thin dielectric into a metal base with a thermal conductivity hundreds of times higher than a laminate, which spreads it across the whole board area.

What limits the dielectric thickness? The isolation requirement. A thinner layer conducts heat better and has a lower breakdown voltage, so the safety standard sets the minimum.

Can a metal core board have plated through holes? Not in the usual way. The base is conductive, so a hole has to be isolated from it with a sleeve or a filled dielectric.