Circular LED PCB Manufacturing

When the Fixture Is Round, So Is the Board

Many light fixtures are circular by design. Downlights, ceiling fittings, stage lamps, watch faces and vehicle lamp modules all present a round aperture, and a rectangular board inside a round housing wastes material and produces uneven light. A circular board fits the housing, allows the light sources to be arranged radially so that the illumination is symmetric, and removes the dead corners that a rectangular board would leave.

The shape is simple but the consequences are not. A round board has its own constraints on panel utilisation, on the routing of the current to each light source and on how the heat is spread and removed.

Layout for Even Light

Radial arrangement is the reason the shape works. Placing the light sources on one or more concentric circles gives a uniform distribution across the aperture, and it allows the copper to be routed outward from a centre point so that each device sees a similar electrical path. That similarity matters, because devices that see different trace resistances carry slightly different currents and therefore produce slightly different brightness and colour.

The routing should be symmetric for the same reason, and every source should have a comparable thermal path to the substrate so that the temperature across the board is even. A board with a hot side and a cool side will show a visible colour shift. Our notes on PCB manufacturing describe the fabrication.

Substrate Choice

FR-4 is adequate for low and medium power lighting, where the heat generated is modest and the light sources are not driven hard. For high brightness lighting it is the wrong choice, because the laminate’s thermal conductivity is far too low to keep the junction temperature under control.

A metal core board, usually aluminium, is the standard answer for higher power circular lighting. The dielectric layer between the copper circuit and the metal base is thin and thermally conductive, so the heat passes into the base and spreads across it, and the base can be attached to the fixture, which acts as the heatsink. The thermal conductivity of the dielectric is typically in the range of one to three watts per metre kelvin, which is far above a standard laminate.

Ceramic substrates, alumina or aluminium nitride, are used where the environment or the power density demands the best possible thermal performance and the cost can be justified. Our PCB capabilities page describes the range.

Copper and Thermal Design

Copper weight is chosen for two reasons at once. It carries the current to the light sources, and it spreads the heat away from them. Standard weight is adequate for low power products, and heavier copper is used where the current and the thermal load are higher.

Thermal vias through the substrate conduct heat from the copper directly beneath the light source into the metal base, and the copper area around each source should be sized to spread the heat laterally before it passes through the dielectric. The dielectric thickness is the controlling parameter: thinner is better thermally but more demanding to manufacture and to insulate. The design has to balance the thermal benefit against the electrical isolation the product requires.

Mechanical Features

A round board generally needs mounting features: screw holes, slots for the fixture, a central cutout for a driver or a connector, and sometimes an alignment feature to fix the rotational position. Those features have to be positioned so that they do not interrupt the thermal path or cut through the current-carrying copper, and they have to be dimensioned so that the board sits flat against the fixture, because a board that does not make contact over its whole area loses a large part of its thermal path.

Space clearances around the mounting points and around any mains-connected parts are also designed here, because the metal base changes the isolation geometry compared with a laminate board.

Assembly

The light sources are surface mount devices, placed accurately because their position affects the optical alignment as well as the electrical connection, and a misplaced device shows up as a dark or bright spot in the beam. Reflow has to be controlled because the metal base absorbs heat and the profile developed for a laminate board will not reproduce on a metal core one. Where the assembly includes connectors or driver components, those add a second process step.

Handling also matters, because the metal base is exposed on the underside and the finished assembly is often tested and packed individually. Our PCB assembly group handles this class of build.

Test and Inspection

Every board should be electrically tested, which for a lighting board means checking the continuity of the circuit and the isolation between the circuit and the metal base. A functional test verifies that the light sources illuminate and that the current and the colour are within specification, and an optical check confirms that all the devices are present and correctly oriented.

For boards intended for outdoor or automotive use, thermal cycling and humidity testing confirm that the assembly survives the environment and that the dielectric layer continues to insulate the circuit from the base. Our notes on PCBA testing and quality management describe how the results are recorded.

Cost

The cost of a circular board depends on the substrate, the copper weight, the diameter and the complexity of the layout, and on the quantity. FR-4 boards are inexpensive per unit, metal core boards cost more because the material and the processing cost more, and ceramic substrates cost more again. A small diameter board in high volume is one of the least expensive lighting boards available, while a large high power metal core board costs several times as much.

Panel utilisation is a specific cost consideration for round boards, because circles do not tile efficiently and a significant part of the panel becomes waste. Where the design allows, arranging the boards to share the panel with other parts of the same material reduces that loss.

Applications

Architectural lighting covers downlights, ceiling fittings and decorative fixtures. Automotive lighting includes headlamp modules, fog lamps and interior illumination. Stage and entertainment lighting uses high power colour mixing arrays on round boards. Wearables use small circular boards for watches and rings, and consumer products use them in lamps, camping lights and personal care devices.

FAQ

Why use a circular board instead of a rectangular one? Because it fits a round housing without wasted material and allows the light sources to be arranged radially for even illumination.

When is a metal core board necessary? When the light sources are driven hard enough that the junction temperature has to be kept low, which is beyond what a laminate can achieve.

What copper weight should be used? Standard weight for low power boards, heavier copper where the current and the thermal spreading requirement are higher.

Why does the mounting surface matter so much? Because the metal base conducts heat into the fixture, so poor contact between the board and the fixture removes a large part of the thermal path.

How is a lighting board tested? Electrical continuity and isolation, a functional check of the light output, and environmental testing for the applications that require it.

Conclusion

A circular LED board is a shape that serves the optics and the mechanical design at the same time. A radial layout for even illumination, a metal core substrate where the power demands it, copper sized for both current and heat, mounting features that preserve the thermal path and an assembly process tuned to the metal base are what produce a fixture that is bright, uniform and durable.