LED ACADEMY by OMS Lighting, Ltd.

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High-emissive coatings are used to increase heat-transfer rate to surroundings. In general, the duller and blacker a materialis, the closer its emissivity is to 1. The more reflective a material is, its emissivity is closer to 0.Figure 3.7.3:Fin-shape housing.Printed circuit boardThe LEDs are mounted on multi-layer FR4 or on metal core printed circuit board (MCPCB). To ensure optimal operation,the thermal resistance of PCB should be kept as low as possible.FR4-based PCBFR4 is standard material for PCBs. Number of LEDs per each PCB depends on the LED input power, boundary conditions,etc. Thermal vias are the method to transfer the heat from the PCB to the cooling system. These thermal vias are platedthrough holes (PTH) that can be open, plugged, filled or filled and capped. The final thermal resistance is determined bythe number and the density of thermal vias, the copper plating thickness and PTH plating thickness.MCPCBFigure 3.7.4 shows structure of MCPCB. A MCPCB consists of Copper layer, dielectric layer and heat spreader, Aluminumor Copper plate. Increasing the copper thickness and using a thinner dielectric with higher thermal conductivity wouldlower the thermal resistance dramatically. Figure 3.7.5 exemplifies actual FR4- and metal core-based PCBsFigure 3.7.4:Cross section of MCPBC (not to scale).Solder maskCopper layerDielectric layerAlluminium/CopperFigure 3.7.5:(a) FR4-based PCB,(b) metal core-based PCB.back / frontfront / backFR4metal corea) b)RoughnessAttaching a heat sink to a semiconductor package requires that two solid surfaces be brought together into contact. Unfortunately,no matter how well-prepared, solid surfaces are never really flat or smooth enough to permit intimate contact.All surfaces have a certain roughness due to microscopic hills and valleys. Superimposed on this surface roughnessis a macroscopic non-planarity in the form of a concave, convex or twisted shape. As two such surfaces are brought intothe contact, only the hills of the surfaces come into physical contact. The valleys are separated and form air-filled gaps.Thermal interface materialThermal interfacial materials (TIMs) are thermally conductive materials, which are applied to increase thermal conductanceacross jointed solid surfaces, such as between the PCB and heat sink in order to increase thermal transfer efficiency.The gaps between surfaces which are in mechanical contact are filled with the air which is a very poor conductor (seeFigure 3.7.6).18LED ACADEMY/LED THERMAL
Figure 3.7.6:Air gaps between two surfaces: a)unfilled, b) filled by TIM.The most common is the white-colored paste or thermal grease, typically silicone oil filled with aluminum oxide, zincoxide, or boron nitride. Some brands of thermal interfaces use micronized or pulverized silver. Another type of TIM isthe phase-change materials. These are solid at room temperature but liquefy and behave like grease at operating temperatures.LED ACADEMYa) Interstitial air b) Thermal interface materialManufacturing methodsThe most common technique to take advantage of natural convection is to put holes in the top and bottom side of thehousing to allow for vertical air flow over the LED. Comparing an Aluminum die casting to an aluminum extrusion, theextrusion process inherently creates a product with greater density (less air voids inside the heat sink) than the die castingprocess (Figure 3.7.7). Assuming insulating properties of air and the conductive properties of aluminum, even the smallamounts of the air in the Aluminum die cast renders a significant reduction in thermal conductivity. Die-casted heatsinksare 20-30% less thermally conductive than extruded ones of the same shape and size.Figure 3.7.7:Heat sinks fabricated by extrusionmethod.Housing design and mounting methodLED housings should be designed to provide a conductive path from the backside of PCB to the housing. This is typicallyaccomplished by mounting the backside of the PCB directly to the LED housing such that they are contacting oneanother across the entire rear surface of the PCB (Figure 3.7.8).Figure 3.7.8:Mounting method.This mounting scheme can be improved by applying a thermal conductive pad between the PCB and the housing.Thermally conductive pad conforms to the features on the backside of the PCB and provides a larger contact area forconduction.Also, the most common technique to take advantage of natural convection is to put holes in the top and bottom sideof the housing to allow for vertical air flow over the LED light source (Figure 3.7.8).19
Page 1 and 2: C=42.Reflective cupEmitted lightMol
Page 3 and 4: Figure 1.2.1:Various light sources
Page 5 and 6: Figure 1.2.4:The black-body curve (
Page 7 and 8: 60º60º60º60º30ºC0 / C1800ºcd
Page 9 and 10: Figure 2.3.3:Examples of the most u
Page 11 and 12: Aluminium - it is the most common m
Page 13 and 14: ConductionWhen a temperature gradie
Page 15 and 16: 3.5. Thermal design of LED-based lu
Page 17: Figure 3.6.1:a) Passive coolingb) A
Page 21 and 22: Figure 4.2.1:Relative luminous flux
Page 23 and 24: Figure 4.3.3:LEDs in serial/paralle
Page 25 and 26: 4.5. LED driver additional features
Page 27 and 28: LED ACADEMY27
Page 29 and 30: Figure 5.4.1:Unlike traditional lig
Page 31 and 32: LED ACADEMY31
Page 33 and 34: Figure 6.2.1:LotusLotusAccent light
Page 35 and 36: Figure 6.4.1:Gacrux - microprism li
Page 37 and 38: 6.6. Architecture, facade, city mar
Page 39 and 40: 6.8. House, flat and living areaDes
Page 41 and 42: Figure 7.1.5:Solid state lighting m
Page 43: Figure 7.4.3:OMCVD FAB development

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