LED modules can offer significant benefits over filament and fluorescent lighting, but the right materials are crucial to proper heat dissipation.
LEDs (light-emitting diodes) may seem “cool” – at least to the touch – but they all produce heat. This is a particular design concern for high-brightness diodes, especially in LED clusters (Figure 1), and when they are contained within an airtight enclosure. Design challenges also occur in mounting LEDs on circuit boards along with other heat-generating devices. In such a case, insufficient thermal transfer with regard to one or more devices can impact the performance of LEDs and other components on the board.
For most applications, the answer in terms of dissipating heat within an LED assembly involves the selection and use of thermally conductive and (usually) electrically insulating materials. This process of thermal management is the sufficient transfer of heat generated by the LEDs to ensure optimum performance over time. Typical end-use products include automotive headlights, street lights, traffic signals, etc., all of which have a critical purpose and mandate both maximum brightness and longest possible life. And a key contributor in the selection, configuration, and application of materials is the materials converter.
LED diodes consist of a die of semiconductor material impregnated, or doped, with impurities to form a p-n junction (Figure 2). When the LED is switched on – in other words, when a forward bias is applied to the LED – current flows from the anode (“p” side) to the cathode (“n” side). At the junction, higher-energy electrons fill lower-energy “holes” in the atomic structure of the cathode material, due to the voltage difference across the electrodes.
The energy released by the electrons in filling the holes produces both light and heat. The light, in turn, is reflected upward by a cavity created for that purpose, while heat is transferred downward into the base of the LED, and ultimately through a torturous path to where it can be dissipated into the atmosphere by convection, usually with use of a heat sink (Figure 3).
The process of light emission is called electroluminescence, and the color of the light produced is determined by the energy gap of the semiconductor. Since a small change in voltage can cause a large change in the current, care must be taken to ensure both are within spec and are as constant as possible. Otherwise, the performance of the LED can become degraded over time, even to the point of failure.
Is heat really a problem with an LED? It definitely can be a problem, and sometimes a difficult one. As the temperature rises within the LED, the forward voltage drops and the current passing through the diode increases exponentially, thereby leading to even higher junction temperatures. (Figure 4 shows how a small change in voltage can cause a significant change in current.) While catastrophic failures probably are rare, an LED module’s light output will diminish over time (Figure 5); efficiency will drop, and the color of the light emitted may change, due to shifts in wavelength brought on by the temperature rise. Wavelengths typically rise from 0.3 to 0.13 nm per °C, depending on the die type. As a result, orange LEDs, for example, may appear to be red, and LEDs producing white light – such as automobile headlights and street lamps – may have a bluish tinge. Other effects include yellowing of the lens, breaking of the wire, and die-bond adhesive damage.
Figure 5 also suggests why white light created from RGB LEDs can appear to have a blue tinge as the junction temperature rises above its intended value. As the figure depicts, blue light falls off slightly less than green and much more so than red.
Proper thermal management in designing circuitry and modules containing LEDs is thus essential, and while various approaches are available, involving heat sinks, base plates, constant-current power supplies, and fans, the solution almost always encompasses the selection of materials for attachment, from thermally conductive adhesives to die cut pads, that are electrically isolating and thermally conductive (Figure 6). In most instances, a thermally-conductive/electrically-insulating pad will be the choice in transferring heat to either a heat spreader or a heat sink.
Designing an LED assembly – whether on a circuit board with other components or within an enclosure – first requires an assessment of the methods available for dissipating the heat to be generated by the assembly. Will the LEDs be through-hole or surface-mounted? Should a dielectric substrate be employed with thermal vias and a copper plate on the underside for absorbing and distributing the heat, or will the LEDs be mounted on a coated metal-core board that acts as a heat spreader? In other words, the initial effort is to determine how the heat is to be dissipated and the most efficient and effective heat path for transferring the heat.
Upfront design work for an LED assembly can be performed either in-house by the manufacturer, or with the assistance of an outside service, namely, a converter experienced in the dissipation of heat generated by electronic and optoelectronic components. In some cases, determining how best to dissipate the heat may benefit from in-depth thermal analyses using temperature modeling software for LED-based module designs.
Once the thermal path has been determined, the next step in designing an LED assembly is the selection and configuration of the thermal interface materials. Among these are liquid adhesives, die cut pads, etc. that provide the required thermal conductivity and electrical insulation. Such parameters as surface flatness of the substrate and heat sink, shape and metal used for the heat sink, applied mounting pressure, thickness of the interface, contact area, etc., may also be specified.
Various families of materials have been developed for thermal management in electronic and optoelectronic assemblies. Options include both off-the-shelf or custom formulations in specified thicknesses and configurations, as well as a variety of choices in types of material: conductive adhesives and greases, tapes, ceramic and metal-filled elastomers (also called “gap fillers”), coated fabrics, and phase-change materials.
Adhesives and greases have historically been the means of attaching a heat-generating device to a heat sink, and are relatively inexpensive. Pressure-sensitive tapes are also used for mounting components to heat sinks, as are elastomer gaskets, which can be coated with an adhesive on one or both sides, and can be die-cut into almost any shape. Thermal fabrics are typically fiberglass-reinforced, ceramic-filled polymer sheets that can provide both thermal conductivity and electrical isolation. Tapes, elastomer pads, and coated fabrics can be formulated to achieve specified performance values in terms of dielectric strength, thermal conductivity, and thermal impedance.
Phase-change materials change from a solid to a liquid during the process of absorbing heat at specified temperatures. The net result is the transfer of heat from a heat-generating device, such as a microprocessor, which is thermally coupled through the material to a heat sink.
Note that the role of a converter involves more than recommending the use of certain materials. For most requirements, the converter provides the finished part – for example, a die-cut gasket. Depending on the needs of the manufacturer, the converter should be able to perform the actual assembly work. While the requirement may typically involve thermal transfer, the binder, filler material, size and shape of the pad, type of adhesive, method of application, etc. are electrical insulation considerations. So too is EMI/RFI shielding, if needed. Then, too – again depending on the product – environmental sealing may also be required, since LED applications often entail operation under environmental extremes of temperature and weather, and even vibration. (Consider, for example, the vibration requirements for a sealed LED headlight.)
While LEDs seem cool to the touch, heat can be a significant problem, and could cause a product failure. Though excessive heat is not going to cause a color shift that results in a red stop light changing to green, the traffic signal could go out, or more likely, it could dim to the point of not being easily visible.
In designing LED-based products, the heat generated both by the LEDs and surrounding components, if any, requires serious consideration by the product designer, the materials engineer, and the converter contracted to provide a viable, cost-effective solution.