Aluminium Base PCB: Substrate, Dielectric and Copper Foil

An aluminium base PCB is a sandwich of three materials with very different properties, and its performance is decided by the thinnest layer in the stack. The metal core provides stiffness and heat spreading, the copper carries the circuit, and the dielectric between them determines how much heat can cross and how much voltage the board can withstand.

The Metal Core

The base is usually an aluminium alloy chosen for thermal conductivity and for formability. Standard alloys conduct heat at around two hundred watts per metre per kelvin, and the plate is typically one to three millimetres thick.

The alloy also determines whether the board can be machined, tapped or bent. Some alloys form well and are used where the board has to follow a curve, while others are chosen for flatness and stiffness. The surface treatment before lamination matters, because the dielectric bonds to the metal and a contaminated surface produces a weak interface that fails under thermal cycling.

The Dielectric Layer

The dielectric is a polymer filled with a ceramic powder to raise its thermal conductivity, supplied as a sheet and laminated onto the metal. Its conductivity is typically one to three watts per metre per kelvin, and its thickness is usually between seventy five and a hundred and fifty micrometres.

Those two numbers set the thermal resistance of the whole board. Halving the thickness halves the resistance, and doubling the conductivity does the same, which is why the choice of this layer matters more than anything else in the construction. The same layer also sets the electrical isolation, and the trade between thermal and dielectric performance is the central decision in specifying an aluminium base PCB. The current carrying side of the design is covered in trace width and current calculation, and the two requirements should be evaluated on the same drawing rather than in separate reviews.

Thermal Resistance in Use

The thermal resistance of the dielectric is quoted per unit area, and the total is that figure divided by the area through which the heat passes. That is why a large thermal pad under a component is worth more than a thicker aluminium plate: spreading the heat over a bigger area reduces the resistance through the dielectric.

The metal then spreads the heat laterally and conducts it to the mounting surface. Where the board is clamped to a heat sink, the interface between them adds its own resistance, and the quality of that interface often dominates the calculation once the dielectric has been chosen carefully.

Copper Foil

The circuit is formed in copper foil bonded to the dielectric. One ounce copper, thirty five micrometres, is typical, with two or three ounces used where the current is high or where more lateral spreading under the components is wanted.

Heavier copper reduces the spreading resistance directly above the heat source and allows wider conductors for the same etch tolerance, but it also raises the minimum gap and increases the etching time. On an aluminium board the copper also has to survive the thermal cycling without lifting at the edges of a large pad, so the adhesion of the foil to the dielectric is part of the specification rather than a process detail.

Breakdown Voltage and Safety

The dielectric has to isolate the circuit from the metal base, and the breakdown voltage falls as the layer becomes thinner. Typical materials offer several kilovolts for a hundred micrometre layer, which is ample for a low voltage lighting product and marginal for a mains powered one.

Where the board carries mains voltage, the thickness is increased and the thermal performance is accepted as worse. Creepage and clearance along the surface have to be satisfied independently of the dielectric, because a thin layer does nothing for a surface path. This is the same reasoning that applies on any high voltage board, and the protection measures used afterwards are described in conformal coating and board protection.

Surface Finish and Assembly

The finish on an aluminium base board has to be solderable and compatible with the thermal pad. Hot air levelling is common on low cost lighting boards, while immersion tin and gold finishes suit finer pitch parts and longer storage.

Assembly is affected by the thermal mass of the metal. The board draws heat out of the joints during reflow, so the profile needs a longer soak or a higher peak than a thin laminate, and the paste has to be selected for the profile. The alloy question is the same as on any board, and the trade offs are set out in lead-free versus leaded solder.

Mechanical Considerations

The metal base is stiff and heavier than a laminate of the same area. Where weight matters, as in portable products, that is a disadvantage; where the board also serves as the mounting plate, it removes parts from the assembly.

Machining introduces burrs that must be removed, and the metal edge is conductive, so copper must be kept back from the outline. Where the board is used as a heat sink against a chassis, the flatness of the base and the mounting method determine how well the two surfaces make contact, which is another tolerance to plan for rather than to discover.

Choosing the Right Construction

Start from the thermal requirement. Estimate the power dissipated in the hottest component, the maximum junction temperature and the ambient in the enclosure, and calculate the resistance available. That determines whether the dielectric layer is adequate and whether the board needs to be clamped to a heat sink.

Then check the electrical requirement, which may force a thicker dielectric and therefore a different thermal answer. The two constraints have to be satisfied together, and a design that meets one and not the other will appear to work on the bench and fail in the enclosure. Where the two cannot be reconciled, the usual resolution is to move the isolation requirement away from the board, using a separate barrier component instead of the dielectric layer.

Common Mistakes

The most common is to assume that the metal base alone solves the thermal problem, without checking the dielectric resistance or the path from the board to the ambient. The second is to specify a dielectric for its thermal conductivity and forget its breakdown voltage, which appears later as a safety issue.

The third is to design the copper as if the board were FR4. Heavier copper and larger thermal pads change the etch tolerance and the solder joint formation, and the layout rules should come from the supplier rather than from the previous design on a different material.

FAQ

Is the aluminium base connected to the circuit? It is normally isolated by the dielectric and left floating, or connected to ground deliberately. The choice affects EMC and should be made explicitly.

Can an aluminium base board be multilayer? It can, with an additional laminate layer on top of the metal core, which allows a second copper layer while keeping the thermal path. The extra lamination adds cost and thickness.

What thickness should the dielectric be? From the thermal requirement and the isolation requirement together. A thinner layer is better thermally and worse for breakdown voltage.