Double Sided Aluminium Core Board Structure And Use

An aluminium core board replaces the usual insulating laminate with a metal base, and a double sided version sandwiches that base between two copper layers. The arrangement solves a specific problem. Circuits that carry significant current generate heat that has to leave the board through the metal beneath it, and a conventional laminate is a poor conductor of heat in exactly the direction the heat needs to travel.

The structure is not a drop in replacement for a laminate, because the conductive base changes the electrical environment as well as the thermal one. The sections below describe the construction, what it does well, and the constraints that come with it.

What The Sandwiched Structure Looks Like

The base is an aluminium plate, typically between 0.8 and 3 millimetres thick, and it provides both the mechanical stiffness and the thermal path. On each side sits a thin dielectric layer, often a filled epoxy or a polyimide, and on top of that a copper foil that is patterned into the circuit. The result is a copper, dielectric, aluminium, dielectric, copper stack that can route circuits on both faces.

The thicknesses are asymmetric by design. The dielectric is kept thin, commonly 50 to 150 micrometres, so that heat crosses it with little resistance, while the aluminium is thick enough to spread the heat laterally. The copper is usually one or two ounces for power circuits, and heavier foil is used where the current density demands it.

Thermal Conductivity And Spreading

The advantage of the structure is the combination of a short path and a wide one. Heat crosses the thin dielectric quickly, then spreads through the aluminium, whose thermal conductivity is measured in the hundreds of watts per metre kelvin, compared with less than one for a polymer laminate. A small component with a high power density therefore sees a much larger effective heat sink than its own footprint provides.

The spreading matters as much as the conduction. A hot spot that would raise the local temperature of a laminate by a large margin causes a much smaller rise when the heat is spread over the whole plate, and the temperature of the component falls with it. This is why the structure appears in LED lighting and in motor drives, where a small number of devices dissipate a large fraction of the total power. Sizing the copper to carry the current without excess drop is a separate exercise, and the usual starting point is a trace width calculation based on the allowable temperature rise.

Dielectric Isolation And Voltage Rating

The dielectric layer has two jobs, and they compete. It must conduct heat, which favours a thin layer and a high filler content, and it must isolate the circuit from the metal base, which favours a thicker layer and a material with high breakdown strength. The voltage rating of the finished board is set by the dielectric, not by the aluminium.

Dielectric isolation has to be evaluated for the working voltage, the transient that the circuit can produce, and the pollution degree of the environment. A layer that withstands a hipot test on the bench may still fail after thermal cycling if a void or a delamination forms at the interface. Partial discharge testing is more revealing than a simple breakdown test, because it detects the small voids that precede a failure. Automotive and industrial standards also impose a minimum creepage across the surface, which the metal base does not change.

<img src="https://www.gopcba.com/wp-content/uploads/2020/12/project_image_10.jpg" alt="Power module mounted on an aluminium core board during thermal test” />

Impedance Control On A Metal Core

A metal base is a ground plane with a very low impedance, which is a benefit for high frequency circuits and also a complication. A microstrip referenced to the aluminium sees a different effective dielectric constant from one referenced to a copper plane on a laminate, because the dielectric is thin and its properties are set by the filler loading rather than by glass weave. The trace geometry has to be recalculated rather than copied.

Impedance control across a production batch then depends on the tolerance of the dielectric thickness and of its dielectric constant. A layer that varies by ten percent in thickness produces a corresponding variation in impedance, and at high frequency that variation consumes part of the design margin. Suppliers who offer the material for radio frequency work publish the tolerance they can hold, and that figure should be part of the specification. The fabrication process also affects the result, because the lamination pressure determines how uniform the thin dielectric becomes.

Where The Structure Fits: Power Modules And LED Drivers

The clearest application is a power module, where several devices are mounted on a common metal base that also serves as the heat spreader and sometimes as the mounting surface for the whole assembly. The board and the heat sink become the same component, which removes a thermal interface and simplifies the mechanical design. The same logic applies to an LED driver, where the light emitting diodes need a low thermal resistance path to the housing.

The structure also appears in motor drives and in power supplies for industrial equipment, where the board is bolted to a chassis. In each of these cases the electrical isolation between the circuit and the chassis is a safety requirement, and it is the reason the dielectric layer is specified so carefully. Where the environment adds moisture or condensation, the assembly may also need protection at the surface, and the considerations in board level protection apply in addition to the isolation provided internally.

Fabrication Constraints

Machining is the first constraint. The board is cut, drilled and routed with tooling that suits aluminium, and the debris has to be removed completely because a metal particle left across two conductors is a short. Vias through the dielectric require an insulated wall or an isolated pad, since the metal base is always present as a reference.

Component attachment is the second. The whole plate acts as a heat sink during soldering, so the assembly may need more heat than a laminate and a slower profile to bring the joint to temperature. A large board can pull heat away from a small pad faster than the iron or the reflow oven can supply it. Reflow on a metal core board is usually done with a longer soak and a higher preheat set point, and both should be validated on a test panel rather than assumed from the profile used for a laminate.

FAQ

Is an aluminium core board electrically connected to the circuit? It is normally isolated by the dielectric layer bonded to it. The isolation is a designed feature tested at the working and transient voltages, not an incidental property of the metal.

Can a double sided aluminium core board carry controlled impedance traces? Yes, but the geometry must be calculated for the specific dielectric and its tolerance. Copying dimensions from a laminate design does not give the same impedance.

Why not use a thinner aluminium base to save weight? A thinner base spreads heat less effectively and is more difficult to keep flat, so the thermal benefit that justifies the structure is reduced. The base thickness is a thermal and mechanical decision.