Aluminium PCB for LED lighting: a design guide
Why LEDs belong on aluminium, how to lay out the board so that heat actually leaves it and what to tell your manufacturer.
Why LEDs need an aluminium board
An LED turns only part of its power into light; the rest becomes heat. The hotter the LED junction, the lower the light output and the shorter the life. On FR4, whose thermal conductivity is only about 0.3 W/m·K, the heat stays where it is made. An aluminium (IMS) board carries it through a thin dielectric into the aluminium base and on to a heatsink or housing.
How the board is built
An IMS board has three layers: a copper circuit, a thin thermally conductive but electrically insulating dielectric, and a solid aluminium base. The dielectric is typically 75 to 150 µm thick, with a thermal conductivity of about 1 to 8 W/m·K depending on the grade; the aluminium base is commonly 1.0 to 2.0 mm. There is normally one copper layer and no plated holes. Details are on the IMS aluminium PCB page.
Design rules
- Thermal pad: connect the thermal pad of the LED to a generous copper area; this area spreads the heat to the dielectric.
- Copper weight: 35 µm is the usual choice; use 70 µm where the current is high or the traces must stay narrow. See trace width and current.
- Spread the LEDs: do not pack all heat sources into one corner; an even layout keeps the temperature low.
- Single layer routing: where traces must cross, use zero-ohm links or jumper parts instead of a second layer.
- Holes: only non-plated mounting holes; a plated hole would short to the aluminium. Keep copper clear of the holes and of the board edge by a distance suited to the working voltage.
- White solder mask: it reflects light and is a common choice for LED boards; then the silkscreen must be black (see solder mask and silkscreen).
- Outline: aluminium boards are separated almost exclusively by routing, not by V-cut.
Mounting
The board only works as a heat path if it sits flat on the heatsink or housing with good thermal contact; use a thermal interface material and avoid air gaps. The aluminium base is often connected to ground or to the chassis, so the dielectric must hold the voltage — IMS boards at PCB-ELECTRA get an additional dielectric withstand test (see electrical test and AOI).
What to specify when you order
Base thickness, thermal conductivity of the dielectric, copper weight, mask and silkscreen colour, and the outline. The maximum size is 390 × 520 mm. For the decision between FR4 and aluminium read FR4 or aluminium. Configure your board in the price calculator or contact us.
Frequently asked questions
Can an aluminium board have two copper layers?
No. Our aluminium boards (IMS) have one copper layer on the aluminium base.
Why is insulation tested on aluminium boards?
The thin dielectric between copper and aluminium has to withstand the voltage of the application, so we run a hi-pot test on these boards.
Which surface finish do aluminium boards get?
Lead-free HASL, like our other boards.
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