LED Board Design and Thermal Layout
An LED Board Is a Thermal Design
An LED converts only part of its input into light; the rest becomes heat, and that heat has to leave the junction or the device will run hotter, produce less light and fail sooner. The board is the first element of the thermal path, and for many LED products it is the main one. The electrical design is usually simple, but the thermal design decides the lifetime, the colour stability and the light output. An LED board should therefore be designed from the thermal requirement backwards rather than from the circuit.
The Thermal Path
Heat leaves the junction through the die attach, the package, the solder joint and the board, and then into a heatsink or the enclosure. Each interface adds a resistance, and the total decides the junction temperature, which is the number that matters. The board’s contribution is the spreading through the copper and the conduction through the dielectric to the base, and it is dominated by the copper’s area and the dielectric’s thickness. A large copper area around the pad lowers the spreading resistance, and a thin, high conductivity dielectric lowers the through resistance. The design should estimate the total resistance before the layout, then verify it with a measurement.
Copper Area and Spreading
The copper under and around the LED is the spreading layer, and its effectiveness grows with its area and its thickness. A copper pour that is connected to the LED’s thermal pad and extends over the board gives the heat somewhere to go, and a thicker copper carries it further before the temperature drops. The pour should be connected to the thermal pad by a large area rather than by a few narrow traces, because a narrow connection is a bottleneck. Where the board has multiple LEDs, the pours should be arranged so that the heat from one does not add to the heat of its neighbour any more than necessary.
Spacing and Density
The spacing between the LEDs decides how much the heat from each adds to the others. A tight array has a higher thermal resistance per device because the neighbours share the same board area, so the junction temperature rises faster than the input power suggests. Where a high density is required, the board should be a metal core or a bonded to a heatsink, and the array should be designed so that the heat leaves through the base rather than spreading laterally. The spacing is therefore a design parameter and not only a lighting one.
The Dielectric and the Base
Where the board is a metal core, the dielectric is the controlling thermal element, and its thickness and conductivity should be specified deliberately. A thinner dielectric lowers the resistance but lowers the breakdown voltage, so the choice trades the thermal requirement against the isolation requirement. On a standard FR-4 board the laminate’s thermal conductivity is poor, so the heat has to be carried by the copper to a heatsink through the board’s edge or through thermal vias. The two approaches have different cost and different limits, and the choice follows the power density.
Optical and Mechanical Considerations
The board also affects the optics. A white solder mask reflects the light and is the usual choice for a lighting board, while a black mask absorbs it. The mask’s colour and its stability over time matter for the colour rendering and for the appearance of the product. The board’s flatness matters where the light is projected through a lens or a diffuser, and the mechanical mounting has to hold the board in contact with the heatsink without warping it. A board that warps during reflow or in service will open the thermal interface and lose the conduction path.
Driving and Protection
The electrical design has its own requirements. The LEDs are usually driven by a constant current, and the driver’s layout matters because it is a switching circuit with a current loop that has to be kept tight. Where the LEDs are in a string, a failed open LED takes the string out, so the design may need a bypass or a parallel arrangement. The protection against a reverse connection, an over voltage and a transient is part of the design, and the driver’s thermal dissipation is another heat source that has to be included in the thermal budget. The driver and the LEDs share the board, and their heats add.
Verification
The thermal design is verified by measurement rather than by calculation alone. The junction temperature can be measured by the forward voltage method, in which a known temperature coefficient is used to infer the junction’s temperature from the voltage at a low current, or by a thermal image of the board. The measurement should be made at the worst case ambient and the worst case drive, and it should be repeated after the board is mounted in its enclosure, because the enclosure changes the path to the air. Where the measured temperature is above the target, the design change is usually the copper area, the board’s construction or the heatsink’s interface.
FAQ
Why is an LED board a thermal design? Because the junction temperature decides the lifetime, the colour and the light output, and the board is the first element of the path.
How does copper help? A large, thick copper area spreads the heat away from the pad and lowers the spreading resistance.
Why does a dense array run hotter? The LEDs share the same board area, so each one’s heat raises the others’ temperature.
What dominates a metal core board’s resistance? The dielectric layer, so its thickness and conductivity set the thermal performance.
How is the junction temperature verified? By the forward voltage method or a thermal image, measured at the worst case and again in the enclosure.
Conclusion
An LED board is designed from the thermal path backwards, so size the copper and choose the construction for the junction temperature you need, then measure it in the enclosure. Optics and heat share the board. Thermal layout belongs to PCB design and layout, the construction choice is described under PCB capabilities, and the assembly sits in PCB assembly. LED boards are first built during prototype PCB assembly in 2026.
