Metal Core PCB Design and Fabrication Basics
When a board has to move heat rather than carry signals, ordinary FR-4 stops being the right material. A metal core PCB replaces the usual insulating base with aluminium or copper, separated from the circuit by a thin dielectric that conducts heat but not current. The result is a board that behaves thermally like a heatsink and electrically like a conventional circuit.
The Three Layer Structure
The base is a metal plate, usually aluminium because it is cheap, light and easy to machine, or copper where the highest thermal performance is needed. Above it sits the dielectric layer, a thin sheet of resin filled with ceramic particles.
On top of that is the copper foil that carries the circuit. The whole assembly is laminated under heat and pressure, and the dielectric thickness is typically a fraction of the thickness used in an ordinary board, because thermal resistance falls as the layer gets thinner.
How Heat Leaves the Board
Heat flows from the component through the solder joint, into the copper pad, through the dielectric and into the metal base, from where it spreads laterally and passes into whatever the board is mounted against.
The thermal conductivity of the dielectric is the limiting element in that path, not the metal. Values vary between suppliers, and the quoted figure is often measured under conditions that differ from the application, which is why the datasheet number should be treated as a comparison rather than a prediction.
Design Rules That Change
Several ordinary layout habits do not apply. The dielectric cannot be drilled and plated in the same way as a multilayer board, so plated through holes are rare and usually replaced by thermal vias only where the supplier supports them.
Single sided construction is the norm, because the metal base is solid and offers no room for a second circuit layer without an additional insulated layer. That forces all routing onto one side, which changes the layout entirely: no crossover without a link, and no ground plane unless the base itself is used as one.
Using the Base as a Conductor
Where the circuit allows, the aluminium base can be connected to ground, which gives a large plane with excellent thermal mass and a useful shielding effect. That connection must be made at a point that the design can tolerate, and it must be compatible with how the product is mounted.
If the product chassis is also connected to that node, the mounting screws provide the connection and no extra feature is needed. If it is not, mounting must be isolated, and that isolation has to survive the voltage the product will see. The electrical and mechanical requirements interact here more than on any conventional board.
Fabrication Constraints
The metal base is cut, punched or routed after lamination, and the tools must handle a soft metal rather than a fibre reinforced resin. Features such as mounting holes and board outlines are usually formed in this step, and tight tolerances cost more than they do in FR-4.
Solder mask and surface finish are applied to the copper in the usual way, so the assembly process is largely unchanged. The board is heavier and stiffer, which affects how it is handled and how it is supported through reflow; a heavy panel that sags in the oven will produce placement defects that never appear on a lighter board. Our fabrication notes describe the equivalent lamination steps for multilayer work.
Choosing Between Metal Core and Thermal Vias
A conventional board with a dense array of thermal vias and thick copper can dissipate a surprising amount of heat, and it offers much more routing freedom. Where the power density is moderate, that approach often wins on cost and flexibility.
The metal core option becomes compelling when the heat flux is high and local, as it is under a power LED or a large semiconductor, and when the board must also be mechanically rigid. Comparing the two on thermal resistance per unit area, rather than on material price, is what produces a defensible decision. Test and inspection considerations for either choice are covered in our electrical test coverage article.
Solder Mask, Finish and the Dielectric Surface
The dielectric surface accepts solder mask and finishes in much the same way as FR-4, so the assembly process is familiar. What differs is the thermal path beneath the pad and the way the substrate expands.
Avoiding soldermask on thermal pads matters more here than on a conventional board, because the mask adds a layer of thermal resistance directly in the path the heat is supposed to follow. The pad geometry and the paste volume should be chosen together with the thermal requirement in mind, rather than copied from a standard land pattern. Our stencil requirements notes explain how the aperture and the deposit interact.
Assembly Considerations
A metal-backed board is heavy and stiff, and it will not flex to accommodate a panel that is not supported properly. Support pins, a carrier or a fixture are usually needed through printing, placement and reflow.
Because the base conducts heat readily, the board reaches reflow temperature faster than an equivalent FR-4 board but also loses heat faster after the zone. Profiles are therefore retuned rather than reused, and the thermocouple placement needs to include a point on the metal base to see what is actually happening. The aluminium substrate does not insulate the joint the way a laminate does.
Where Metal Core Is Not the Answer
Signals are the other half of the problem. A metal core board is single sided in most practical constructions, so a design with moderate routing complexity will not fit, no matter how good the thermal performance is.
Where the power dissipation is a few watts spread across the board rather than concentrated in one device, a conventional board with thermal vias and internal copper planes is usually cheaper and far easier to route. The metal core solution should be reserved for the cases it genuinely solves, and the decision documented so that it is not applied by habit to the next product. Our component tolerance and reliability notes cover the alternative approach.
Thermal Measurement and Verification
The thermal performance claimed for a metal core board is a property of the whole stack, not of the base metal. Measuring the actual case temperature of the device under load is the only way to know whether the design works.
Attach the thermocouple directly beneath the device on the metal base, run the product at its worst case load and let it reach steady state. If the result is higher than the calculation predicted, the dielectric or the mounting is the likely cause, and both are easier to change before the tooling is committed.
Our reliability notes describe how the thermal result feeds into the life calculation for the product.
FAQ
Why is aluminium used instead of copper as the base? Aluminium is lighter, cheaper and easier to machine, and its thermal performance is adequate for most lighting and power applications. Copper is used where the thermal requirement is extreme.
Can a metal core PCB have two circuit layers? It can, by adding a second insulated layer, but the additional dielectric adds thermal resistance and cost, and the simpler answer is usually a metal core board with careful single sided routing.
Is the metal base always connected to ground? No. It can be connected when the design and the mounting permit it, but it must be isolated where the product requires the base to float.
