Metal Core PCB: Structure, Types And Where Each Fits

A metal core board replaces the usual glass reinforced laminate with a metal plate, usually aluminium, covered by a thin dielectric layer and a copper circuit. It exists for one reason: to move heat away from a component that would otherwise overheat on a conventional board. Where that requirement is severe, the metal core is the cheapest effective answer.

This article describes the construction, the variations that matter, and the situations in which a metal core board is the right choice and where it is not.

The technology sits between a normal printed circuit board and a ceramic substrate, and understanding where it fits requires knowing what each layer contributes.

The Three Layers And What Each Does

The base is a metal plate, most often aluminium because it is light, cheap and conducts heat reasonably well. Copper and steel are used where a specific property is needed: copper where the thermal performance must be the best available, steel where magnetic shielding or a very stiff base is required.

Above the base sits the dielectric layer of the insulated metal substrate, which provides the electrical insulation and bonds the copper foil to the metal. It is thin, typically between fifty and a hundred and fifty micrometres, and its thermal conductivity is the parameter that determines whether the board performs as intended. The top layer is ordinary copper foil, patterned into the circuit in the usual way.

Why The Dielectric Dominates

Heat flows from the component through the copper, then through the dielectric, then into the metal base and out to the heat sink. The metal base conducts extremely well and the copper is not the limiting element, so almost all of the thermal resistance in the stack is in the dielectric layer.

That is why two metal core boards with the same aluminium base can differ by a factor of several in performance. The relevant figure is the thermal conductivity of the dielectric, and the range available from different suppliers is wide. A board specified only by its base metal has not specified its thermal performance at all. The interaction between the insulation and the thermal path is discussed in the guidance on metal core insulation and thermal paths.

Types Of Metal Core Board

The simplest is the single sided board, with a circuit on one face and the metal base on the other. It is the most common and the cheapest, and it suits LED lighting and power conversion where the components are on one side and the heat sink is the base itself.

Double sided metal core boards exist but are less common, because the base plate has to be drilled and insulated at every through hole, and the second layer of dielectric adds thermal resistance. Where a double sided circuit is required, the designer should consider whether the thermal benefit still justifies the additional complexity.

Thermal Vias And Their Limits

The dielectric layer is an electrical insulator, so a component cannot be connected directly to the metal base. Where the thermal path has to be improved further, the usual technique is a thermal via through the dielectric, filled with a thermally conductive material and connected to a pad on the base side.

These vias are expensive and they are not equivalent to a via in a conventional board. Their cross section is small, their filling is critical, and their contribution has to be modelled rather than assumed. Where a large amount of heat must be moved, it is often better to use a larger copper area under the component than to add vias. The general thermal design practice for placement and copper area applies here with more emphasis on the plane than on the via.

Where The Technology Fits

LED lighting is the classic application, because the heat is generated at the junction and the board is also the mechanical structure. Power supplies, motor drives and automotive electronics follow, wherever a component dissipates enough power that a conventional board would need a heat sink bonded to a small copper area.

It does not fit where the circuit is dense. The dielectric layer cannot support fine features as well as a laminate, because it is bonded to a rigid base that expands at a different rate, and the layer count is limited. A high density digital design with a hot processor is better served by a conventional board with a thermal solution than by a metal core board.

Mechanical And Process Considerations

The metal base expands and contracts with temperature, and it does so differently from the copper above it. On a large board this causes stress at the dielectric interface and can lead to delamination over thermal cycles, which is why the dielectric formulation and the cure are important and why the board should be qualified rather than assumed.

The base also carries the board’s mechanical function. It can be used as the mounting surface, and it can be bent or extruded into a shape that a laminate could not take. Where a design uses that capability, the circuit pattern has to avoid the regions that will be formed, because the dielectric will not tolerate the deformation indefinitely.

Cost And Availability

Metal core boards cost more than a conventional board of the same size, and the difference is largest for small quantities because the material is bought in panels. The additional cost is usually justified by the elimination of a separate heat sink and of the thermal interface material and assembly steps that go with it.

Availability is a practical constraint. Not every fabricator processes metal core material, and those that do may stock a limited range of dielectric grades. Choosing the board before checking what is available leads to a design that has to be adapted, so the material survey should come first. Where the current is also high, the copper thickness interacts with the thermal design and the principles behind current capacity and trace width should be applied to the copper layer.

Designing The Circuit Layer

The circuit on top of a metal core board is designed like any single sided board, with one important difference: the proximity of a large metal plane below the copper makes every trace a capacitor to the base. The capacitance is small per unit area but it is present everywhere, and on a switching node it adds a loss path that has to be accounted for in the efficiency calculation.

Spacing rules also have to respect the dielectric’s breakdown voltage rather than the air clearance. Where the board carries mains voltage, the creepage along the surface and the clearance through the dielectric are both part of the safety analysis, and the metal base must be treated as a conductive part that has to be earthed or isolated.

FAQ

Can a metal core board be used for high frequency circuits? The metal base makes every trace capacitive to it, and the dielectric is usually lossy. It is not the right choice for RF work unless the design accounts for the change in the environment.

Is aluminium always the base material? No. Copper is used where the thermal performance is the priority, and steel where stiffness or magnetic properties matter. Each has a different expansion coefficient, which affects the dielectric selection.

How is the thermal performance specified? By the thermal conductivity and the thickness of the dielectric layer, and by the copper area above it. Stating the base metal alone leaves the performance undefined.