Copper Core PCB Thermal Conductivity and Manufacturing
A copper core PCB is chosen for one reason above all others: heat. Where a circuit dissipates enough power that an ordinary laminate cannot spread it away fast enough, replacing the base material with a metal plate turns a thermal management exercise into a substrate selection. Copper is the best of the common metals for that job, and the numbers behind the choice are worth understanding before the material is specified.
Why Copper Is Used as a Core
A metal core board is built like a sandwich. A metal plate forms the base, a thin dielectric layer is bonded to it, and the copper foil that carries the circuit sits on top. Heat travels from the components into the copper, through the dielectric, into the metal plate, and out to whatever the plate is mounted against. Each layer adds resistance to that path, so the finished performance depends on all of them rather than on the base metal alone.
Both aluminium and copper are used as the base. Aluminium is lighter and cheaper and handles most LED lighting work. Copper is roughly sixty percent denser and several times more expensive, and it is chosen where the extra conductivity is the difference between a part that runs within its ratings and one that does not.
Thermal Conductivity Numbers
Thermal conductivity is expressed in watts per metre per kelvin, and the ranking of the common materials puts the range in perspective. Diamond leads at roughly 1,300 to 2,400, silicon follows near 611, then silver near 429, copper near 401, gold near 317, beryllium near 250 and aluminium near 240. Further down are aluminium nitride near 200, tungsten near 180, zinc near 116, nickel near 91 and iron between about 84 and 90.
Two conclusions follow. Copper is close to the best metal that can be used economically, and it is markedly better than aluminium. That gap is what justifies a copper base in a high power design, and it is also why copper is the reference against which a cheaper alternative has to be judged rather than the other way round.
The Dielectric Layer Is the Bottleneck
The metal plate dominates the discussion, but the dielectric layer in the middle of the sandwich usually dominates the result. That layer is thin, electrically necessary and thermally far worse than either the copper above it or the plate below. A typical metal core laminate has a dielectric conductivity in the range of about 1 to 8 watts per metre per kelvin depending on the formulation and thickness, against roughly 401 for the copper surrounding it.
That asymmetry is the design lever. Reducing the dielectric thickness lowers the thermal resistance of the path directly, because resistance scales with thickness and inversely with conductivity and area. Increasing the area over which heat crosses helps in the same way, which is why a large copper pour under a power device outperforms a narrow trace even when the trace is generously wide. Where the layout can spread heat laterally in the top copper before it crosses the dielectric, the whole path becomes shorter. How that spreading copper should be shaped is discussed in copper flooding: mesh or solid.
Thermal Resistance and Breakdown Voltage
A datasheet figure for conductivity is not the same as the performance of a finished board. The number that describes the finished part is thermal resistance, in kelvin per watt, and it combines the dielectric thickness, its conductivity and the area through which the heat passes. Two laminates with the same conductivity can have very different thermal resistance if one is supplied in a thinner grade.
The second specification is breakdown voltage, and it pulls in the opposite direction. Making the dielectric thinner improves heat transfer and reduces the voltage the layer will withstand. A high power design therefore has a voltage isolation requirement that sets a minimum thickness, and the thermal path has to be designed within that constraint rather than around it. Reading a conductivity figure without the thickness and the voltage rating beside it says almost nothing.
Current-carrying copper follows the same pattern of trade-offs, and the width needed for a given current at an acceptable temperature rise is worked out in trace width and current calculation.
Thermally Separated Copper Substrates
A variant worth understanding is the thermally separated construction, in which the copper that generates heat and the copper that carries current are not the same layer. The thermal path is provided by a thick copper feature bonded to the base, while the circuit copper sits separately. The reported conductivity for that thermal path is around 398 watts per metre per kelvin, against roughly 1 to 8 for the dielectric of an ordinary metal core board.
The trade is manufacturability. The construction is more specialised, the process window is narrower, and the finished board costs more. It is used where the thermal demand is high enough that the ordinary dielectric path cannot meet it, such as dense LED arrays and power modules that must run at a high junction temperature without a fan.
Fabrication Notes
Producing a copper base board differs from producing an FR-4 board in ways that reach back into the design. The metal plate does not drill and rout like a laminate, so hole counts and edge features should be kept within what the process can hold. The plate is conductive, so the electrical isolation of every hole and every edge feature has to be verified rather than assumed, and the solder mask has to bond to a copper surface rather than to a laminate.
Thermal design does not end at the substrate. Where the board is mounted on a heat sink, the interface material between them is part of the path, and both its conductivity and the flatness of the plate matter. Where the finished assembly will see moisture or condensation, the coating options are compared in conformal coating for board protection. The substrate is one link in a chain, and the weakest link sets the junction temperature.
Specifying the Material
A copper base board is specified with four numbers, and leaving any of them out hands the decision to the fabricator. State the thermal conductivity of the dielectric, because that figure sets the resistance in the middle of the sandwich. State its thickness and the breakdown voltage it has to withstand, since those two move together and one cannot be chosen without the other. State the copper thickness on the circuit side, because it controls how far heat spreads laterally before it crosses into the base. And state the flatness of the plate, because a base that does not sit flat against a heat sink has an air gap in the thermal path that no amount of conductivity in the metal will recover.
It is also worth specifying what the board will be mounted on. A copper base used with a proper interface and a heat sink behaves very differently from the same board standing in still air, and a specification written for one case can be badly wrong for the other.
FAQ
Why choose copper rather than aluminium as a core? Because its thermal conductivity is near 401 against roughly 240 for aluminium. The gap is decisive in high power designs; where it is not decisive, aluminium is lighter and cheaper.
What actually limits the thermal path? Usually the dielectric layer. Its conductivity is in the range of about 1 to 8 watts per metre per kelvin, so it adds far more resistance than either the copper above it or the plate below.
Does a thinner dielectric always help? Thermally yes, but it reduces breakdown voltage. The isolation requirement sets a minimum thickness, and the thermal design has to work inside that limit.
What is a thermally separated copper substrate? A construction in which a thick copper thermal path is separate from the circuit copper. Its conductivity is reported around 398 watts per metre per kelvin, it is more specialised to build, and it suits dense LED arrays and power modules.
