LED Assembly Thermal Design and Reflow

An LED assembly is a thermal design with a light attached. The junction temperature sets the light output, the colour and the life, and the board is the path that removes the heat.

Why the Junction Temperature Matters

The light output falls as the junction temperature rises, and the colour shifts with it. The life falls steeply, so a design that runs ten degrees cooler can last noticeably longer.

The junction temperature is not measurable directly, so it is calculated from a thermal resistance and a measured board or case temperature. Our thermal measurement notes describe how the measurement is taken.

The Path Through the Board

The heat leaves the package through a thermal pad on the underside and enters the board. From there it spreads laterally and leaves through the board surface, the housing or a heatsink.

The dielectric between the pad and the inner layers is the bottleneck, so the design uses thermal vias under the pad and a copper area around them. Our thermal via notes describe the arrangement.

Metal Core and Ceramic Boards

Where the heat is high, an aluminium or a copper core board removes the dielectric from the path and spreads the heat over the whole panel. The dielectric is a thin insulating layer instead of the laminate.

A ceramic substrate performs better still and it costs more and it is more brittle. The choice follows the power density rather than the cost per board alone. Our metal core notes describe the comparison.

Reflow of LED Assemblies

The package is sensitive to moisture and to thermal exposure, and the profile must stay inside the limits the supplier specifies rather than inside a general lead free window.

The board’s thermal mass delay also means the pad under the LED reaches the peak later than the rest of the board, so the measurement should be taken there. Our thermal mass notes describe the measurement.

Optical Considerations

The solder mask colour around an LED affects the light that is reflected back, and a white mask is used where the reflection matters. The finish and the mask both change the colour of the light that leaves the assembly.

Those are design decisions rather than process decisions, and they should be stated with the assembly rather than left to the fabricator.

Layout for Heat Spreading

The copper around the LED spreads the heat before it leaves the board, so the area matters as much as the via count. A large copper area at the same voltage potential is the usual arrangement.

Where the copper is split by the routing, the spreading is interrupted and the temperature rises locally. Our copper balance notes describe the patterns that spread heat without unbalancing the board.

Verification

The verification is the board temperature measured at the worst case drive current and ambient, converted to a junction temperature with the supplier’s thermal resistance.

The measurement should be taken in the finished housing, because the housing is usually part of the thermal path. Our heatsink notes describe the interface that connects them.

Process Control and Verification

A short note on the drawing about handling, storage or packaging is often worth more than an extra decimal place on a tolerance. Where a value sits close to a process limit, the drawing should say so, since the shop can then open the process window rather than working to a nominal figure that carries no tolerance.

A stack-up that is drawn rather than described removes most of the ambiguity from a quotation, and it lets the fabricator price the board against the dielectric and copper weights that will actually be used. Where the requirement is not written down, the shop supplies its own default, and the default is chosen for the process rather than for the design.

The measurements that matter are the repeatable ones: conductor width and spacing, annular ring, finished hole size, plating thickness and surface finish are all verifiable on a coupon that travels with the panel. Running a first article through the same checks as the production panel confirms that the two agree, and that comparison is the cheapest form of process control available at prototype stage.

Checks Before Release

The checks that matter are the ones performed on the product rather than on a sample kept for the purpose, because a coupon that travels with the panel is the only evidence about that panel. Where a requirement can be measured, it should be measured at the point of manufacture and recorded against the board or the lot it applies to.

A parameter that is set once and never re verified drifts, and the drift is usually discovered by a defect rather than by the record. The tooling, the material and the profile form one system, and a change to any of them should be assessed against the other two before it is released.

Where the process window is narrow, the measurement resolution has to be better than the window, or the data cannot distinguish a good part from a marginal one. A record that identifies the operator, the date and the settings is worth more than a record that identifies only the result.

The acceptance criteria should be written before the work starts, so that the decision is made by the specification rather than by the person inspecting.

Verification and Records

Where an operation cannot be verified afterwards, it has to be controlled during the operation, and that control has to be visible in the record. Sampling is a compromise between cost and confidence, and the sample size should follow from the failure rate that has to be detected.

The first article confirms that the setup matches the intent, and it is the cheapest point at which a wrong setup can still be corrected.

FAQ

Is a metal core board always better for LEDs? It is better where the power density is high and the board is a single layer. It is not suited to a dense control circuit on the same board.

Can the light output be used to estimate the junction temperature? It changes with the temperature and it also changes with the drive current and the age, so the thermal measurement is the more reliable route.

What does gopcb provide for LED assemblies? We provide thermal path design with vias and spreading copper, material selection between laminate, metal core and ceramic, profiles developed to the package limits, measurements taken at the package pad and in the housing, and layout that spreads heat without unbalancing the board.