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96 SMT Magazine • May 2014 LED THE SUNSHINE IN continues thermal conductivity of FR-4 is worse than IMS (typical values for FR-4 are 0.3 watts/mK and for IMS are 1–3 watt/mK), special precautions are needed for FR-4 substrates to be used in this tough thermal environment. These precautions relate to the use of ther- mal vias in the board. Here, vias are drilled below the footprint of the thermal pad of the LED packages to be mounted. These vias are then filled with copper or thermally conductive paste enabling heat transfer from the LED pack- ages to the heat diffuser that is connected to the bottom of the LED substrate. Placing LED Packages Since most LED lamps consist of an array of multiple LEDs, limitations relating to opti- cal and electrical properties of these LEDs ap- ply. For example: Significant differences in flux or color will be clearly visible to the customer. As a result, such products are rejected and the LED industry has introduced LED product classes. During produc- tion, LEDs will vary in color, flux and forward voltage, and this is also true for products from the same batch. The differences are significant and so LED properties are measured and delivered to the market in subclasses or groups called bins. This process of segre- gating LEDs is called binning. So a particular bin may con- tain LEDs, which emit light with- in a range of wavelengths, a range of flux values and also a range of forward voltages that can be ap- plied safely to the LEDs. If the LEDs aren't properly matched, they will produce a kind of rain- bow of different shades of white light on the wall. This problem doesn't exist with an incandes- cent lamp since it produces a wide range of frequencies. To minimize this potential problem, vendors provide binning charts. White LEDs are binned for correlated color temperature SMT TrenDS & TeChnologieS (CCT) or by x, y coordinates of the LED in the CIE 1931 chromaticity diagram (Figure 3). The x, y coordinates define the color point of any light source in the CIE diagram. New binning structures for white LEDs consider both CCT and x, y coordinates of the LED whilst binning. This is because even white LEDs with approxi- mately the same CCT can have different color appearances. Each lighting manufacturer has its own recipe (build plan) for LED array manu- facturing, indicating binning restrictions for neighboring LED components. These restrictions translate at the pick-and- place machine to so-called binning software. In the binning software, the remaining parts stock at the pick-and-place machine is compared to the manufacturing batch size. If the remaining parts stock is insufficient, the pick-and-place machine should instantly switch over to a new figure 3: CIe 1931 chromaticity diagram; points a–g refer to leD images a (top right) to g (bottom left).

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