LED Aluminum Substrate Thermal Conductivity Explained

An LED aluminium substrate is judged on three numbers: thermal conductivity, thermal resistance and dielectric strength. The aluminium plate is not the limiting factor, because aluminium conducts heat well. The thin insulating layer between the copper circuit and the metal base decides almost everything, and that layer is where the design effort has to go. This article explains what the thermal conductivity figure means, how it is measured, and how to choose a laminate for a lighting product.

What the Thermal Conductivity Figure Means

Thermal conductivity describes how much heat passes through a material of a given thickness and area for a given temperature difference, expressed in W/m·K. Aluminium sits near 200 W/m·K, copper near 400 W/m·K, and the polymer dielectric used on most metal core boards falls between 1 and 3 W/m·K for standard grades and up to about 8 W/m·K for filled high performance materials.

Because the dielectric is only 75 to 150 µm thick, its absolute thermal resistance is small even though its conductivity looks low next to metal. That combination of low conductivity and small thickness is why the layer can still dominate the thermal path, and why a small improvement in its conductivity changes the junction temperature noticeably.

The Insulating Layer Decides the Performance

Three dielectric families are used. A standard prepreg based laminate gives the lowest cost and the lowest conductivity. A filled epoxy with ceramic particles improves conductivity at moderate extra cost. A thermal adhesive film gives the best performance and is the usual choice for high power lighting, although it costs more and narrows the process window.

The datasheet figure should be read with the thickness beside it. A material quoted at 3 W/m·K in a 150 µm layer may perform worse than one quoted at 1.5 W/m·K in a 75 µm layer, because thermal resistance scales with thickness and inversely with conductivity.

<img src="https://www.gopcba.com/wp-content/uploads/2021/02/h1_image_c3.jpg" alt="LED metal core PCB with aluminium base plate and white solder mask” />

Thermal Resistance and Junction Temperature

Thermal resistance is measured in °C per watt and is the figure that can be used directly in a calculation. Add the resistance of the dielectric, the resistance through the aluminium base, the resistance of the thermal interface material and the resistance of the heat sink, then multiply the total by the dissipated power to estimate the temperature rise above ambient.

In practice the interface material and the heat sink usually contribute as much as the board itself, so improving only the laminate while leaving a thick, poorly applied thermal pad in place produces little benefit. Measuring the case temperature of the LED and the temperature of the base plate under load shows where the bottleneck really is.

Dielectric Strength and Safety

The same thin layer that carries heat must also isolate the circuit from the metal base. Dielectric strength is quoted in kV per millimetre or as a breakdown voltage for the finished thickness, and it has to satisfy the creepage and clearance requirements of the applicable safety standard, not just the material rating.

Mechanical stress reduces the margin. A burr on the aluminium, an air pocket during lamination or a screw that deforms the plate can all lower the breakdown voltage locally, so the isolation test should be performed on the finished board rather than on a sample of raw laminate.

How the Value Is Measured

Steady state methods such as the guarded hot plate give the most accurate result but take time and are applied to flat samples. Transient methods such as the laser flash technique are faster and are what most laminate suppliers use for their datasheets. The two do not always agree, so comparisons should be made between figures produced by the same method.

For a real board, the useful measurement is the thermal resistance from the LED pad to the base plate, taken on a finished assembly. That value includes the copper spreading, the dielectric and the interface between the dielectric and the metal, and it is the number that predicts field performance.

Design Rules That Reduce Temperature

Spread the heat before it reaches the dielectric. A large copper pad under the LED, connected to a surrounding pour, lowers the local flux and uses more of the insulating layer’s area. Thermal vias through the dielectric into the base are used on some designs, but they add process steps and must be filled to avoid solder wicking.

Keep the copper balanced across the panel so the thin dielectric is not stressed during lamination, and avoid placing heavy copper and fine traces next to each other where the etch tolerance can create a thin spot. Where the design allows, use the trace width and current calculation methods to size the conductors that also carry heat away from the source.

Choosing a Substrate for a Lighting Product

Start with the power per LED and the maximum junction temperature, then work backwards to the required thermal resistance. Select the dielectric thickness first, because it has the strongest effect, and the conductivity second. Confirm the dielectric strength against the safety standard before optimising anything else.

Then check the practical items: the aluminium alloy and thickness for mechanical stiffness, the surface finish for the assembly process, the panel size the supplier can handle and whether the base will be machined or bent after fabrication. Heat spreading mastics and gap fillers used at final assembly are described in the notes on potting and dispensing adhesives, and they often matter more than a marginal laminate upgrade.

Reading a Laminate Datasheet

Supplier data is rarely presented in a form that allows a direct comparison. Conductivity may be quoted for the resin alone, for the filled compound, or for the finished laminate, and the test method may differ between suppliers. Thickness may be nominal or a range. Ask for the thermal resistance of the finished dielectric at the thickness you intend to buy, and ask which method produced the number.

Check the dielectric strength on the same basis, and confirm the peel strength of the copper foil, because a lighting board that is screwed to a heat sink and cycled thermally will load that bond. Where the panel has to survive bending or machining after fabrication, confirm the flexibility of the laminate before it is ordered.

The rest of the documentation follows normal practice. A complete fabrication package with the stack, the aluminium alloy and thickness, the finish and the mechanical drawing removes the ambiguity that usually causes a rebuild, as described in the manufacturable design guidelines.

FAQ

Is a higher thermal conductivity always better? Not by itself. A thin layer with moderate conductivity can beat a thick layer with high conductivity, so the two values must be compared as a thermal resistance, which is thickness divided by conductivity.

How thin can the dielectric be? Common production thicknesses run from about 75 to 150 µm. Thinner layers improve heat transfer but reduce breakdown voltage and make the lamination more sensitive to burrs and contamination.

Do thermal vias help on a metal core board? They can, but the benefit is smaller than on a conventional FR-4 board because the dielectric is already thin. Use them where the local heat flux is highest and where the vias can be reliably filled.